4
Power Train Technologies for Reducing Load-Specific Fuel Consumption

Technologies for reducing fuel consumption of medium-and heavy-duty vehicles depend on the power train type. For instance, replacing gasoline engines with diesel engines in medium-duty trucks is a very effective technology, but heavy-duty trucks are already more than 99 percent dieselized. This chapter discusses the energy balance for a typical diesel engine that leads to a resulting brake power or brake thermal efficiency. It presents technologies for improving the efficiency of diesel and gasoline engines (including fuels and emission systems) as well as technologies for transmissions and drive axles. It also discusses the role of hybrid power trains (electric and hydraulic) in reducing fuel consumption.

DIESEL ENGINE TECHNOLOGIES

Diesel engines use the high gas temperatures generated by compression as the ignition source. The timing of ignition is determined mainly by when the fuel is injected. These engines operate on the four-stroke-cycle principle and are arranged either in-line or in a “vee” configuration. Displacements range from 3.0 to 16.0 liters. These engines typically burn diesel fuel, and also some kerosene and some biodiesel blends. Some engines that were originally designed as diesel engines are converted to use spark ignition to take advantage of alternative fuels. These engines typically burn gaseous fuels such as compressed natural gas (CNG), liquefied natural gas (LNG), or propane, but other spark ignition fuels can also be used. Essentially all of the diesel engines used today in medium- and heavy-duty vehicles are turbocharged, direct fuel injected, and electronically controlled; most are intercooled or after cooled. In addition, they use exhaust gas recirculation (EGR) to limit in-cylinder formation of nitrogen oxides (NOx) and some form of exhaust aftertreatment (diesel oxidation catalyst [DOC] diesel particulate filter [DPF], or other system) to control particulate matter (PM) emissions. Starting in 2010, most diesel engines will add selective catalytic reduction systems (SCR) as a form of NOx aftertreatment to meet 2010 requirements. A typical diesel engine energy audit is shown in Figure 4-1, where the fuel energy is converted to brake power and the efficiency associated with the output power will be referred to as brake thermal efficiency. The accessory losses are for engine-driven pumps that are necessary to run the engine on a dynamometer or on the road (fuel, lubricating oil, cooling water). Auxiliary loads such as alternator, air compressor, and power steering pump will use a portion of the brake power.

The following material summarizes various technologies for reducing fuel consumption from diesel engines. Some of the engine technologies listed here are the products of participants in the multiagency, multicompany 21st Century Truck Partnership. The partnership’s goals for engines are to achieve 50 percent thermal efficiency, while meeting 2010 emissions standards, by 2010 and to develop technologies to achieve 55 percent thermal efficiency by 2013 (NRC, 2008).

Turbochargers

In a turbocharger the radial exhaust-driven turbine drives the radial compressor to increase the air density going into the engine. The turbochargers can have a fixed geometry or more commonly a variable geometry turbine, or they can have a “wastegated” turbine (a bypass). Improved efficiency of the compressor or turbine will improve fuel consumption. Higher pressure ratio radial compressors or axial compressors are emerging technologies. Improvements in compressor efficiency and/or turbine efficiency can contribute to improved fuel consumption. A presentation1 to the committee on Japan’s Top Runner fuel efficiency regulation estimated 0.3 to 0.5 percent improvement from increased supercharging efficiency. The TIAX investigation, by contract to the committee, put the improvement at up to 2 percent (TIAX,

1

Akihiko Hoshi, Ministry of Land, Infrastructure, Transport, and Tourism, “Japanese Fuel Efficiency Regulation,” presentation to the committee by teleconference, Washington, D.C., February 4, 2009.



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4 Power Train Technologies for Reducing Load-Specific Fuel Consumption Technologies for reducing fuel consumption of medium- typical diesel engine energy audit is shown in Figure 4-1, and heavy-duty vehicles depend on the power train type. where the fuel energy is converted to brake power and the For instance, replacing gasoline engines with diesel engines efficiency associated with the output power will be referred in medium-duty trucks is a very effective technology, but to as brake thermal efficiency. The accessory losses are for heavy-duty trucks are already more than 99 percent diesel- engine-driven pumps that are necessary to run the engine on ized. This chapter discusses the energy balance for a typical a dynamometer or on the road (fuel, lubricating oil, cooling diesel engine that leads to a resulting brake power or brake water). Auxiliary loads such as alternator, air compressor, thermal efficiency. It presents technologies for improving and power steering pump will use a portion of the brake the efficiency of diesel and gasoline engines (including power. fuels and emission systems) as well as technologies for The following material summarizes various technologies transmissions and drive axles. It also discusses the role of for reducing fuel consumption from diesel engines. Some hybrid power trains (electric and hydraulic) in reducing fuel of the engine technologies listed here are the products of consumption. participants in the multiagency, multicompany 21st Century Truck Partnership. The partnership’s goals for engines are to achieve 50 percent thermal efficiency, while meeting 2010 DIESEL ENGINE TECHNOLOGIES emissions standards, by 2010 and to develop technologies Diesel engines use the high gas temperatures gener- to achieve 55 percent thermal efficiency by 2013 (NRC, ated by compression as the ignition source. The timing of 2008). ignition is determined mainly by when the fuel is injected. These engines operate on the four-stroke-cycle principle Turbochargers and are arranged either in-line or in a “vee” configuration. Displacements range from 3.0 to 16.0 liters. These engines In a turbocharger the radial exhaust-driven turbine drives typically burn diesel fuel, and also some kerosene and some the radial compressor to increase the air density going into biodiesel blends. Some engines that were originally designed the engine. The turbochargers can have a fixed geometry or as diesel engines are converted to use spark ignition to take more commonly a variable geometry turbine, or they can advantage of alternative fuels. These engines typically burn have a “wastegated” turbine (a bypass). Improved efficiency gaseous fuels such as compressed natural gas (CNG), lique- of the compressor or turbine will improve fuel consumption. fied natural gas (LNG), or propane, but other spark ignition Higher pressure ratio radial compressors or axial compres - fuels can also be used. Essentially all of the diesel engines sors are emerging technologies. Improvements in compres- used today in medium- and heavy-duty vehicles are turbo- sor efficiency and/or turbine efficiency can contribute to im- proved fuel consumption. A presentation1 to the committee charged, direct fuel injected, and electronically controlled; most are intercooled or after cooled. In addition, they use on Japan’s Top Runner fuel efficiency regulation estimated exhaust gas recirculation (EGR) to limit in-cylinder forma- 0.3 to 0.5 percent improvement from increased supercharg- tion of nitrogen oxides (NOx) and some form of exhaust ing efficiency. The TIAX investigation, by contract to the aftertreatment (diesel oxidation catalyst [DOC] diesel par- committee, put the improvement at up to 2 percent (TIAX, ticulate filter [DPF], or other system) to control particulate matter (PM) emissions. Starting in 2010, most diesel engines 1Akihiko Hoshi, Ministry of Land, Infrastructure, Transport, and Tour- will add selective catalytic reduction systems (SCR) as a ism, “Japanese Fuel Efficiency Regulation,” presentation to the committee form of NOx aftertreatment to meet 2010 requirements. A by teleconference, Washington, D.C., February 4, 2009. 

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 TECHNOLOGIES AND APPROACHES TO REDUCING THE FUEL CONSUMPTION OF MEDIUM- AND HEAVY-DUTY VEHICLES FIGURE 4-1 Energy audit for a 4-1 Industrial Perspectives of the 21stst Century Tru.eps Figure typical diesel engine. SOURCE: Adapted from Vinod Duggal, Cummins, Inc., “Industrial Perspectives of the 21st Century Truck Partnership,” presentation to the bitmap committee, Dearborn, Mich., April 6, 2009, Slide 14 (and TIAX (2009), p. 4-3, Table 4-1). 2009, pp. 3-5 and 4-14). NESCCAF/ICCT (2009, p. 83) Conventional two-stage turbocharging employs two tur- estimates the fuel savings of an improved-efficiency single- bochargers working in series at all times. True sequential stage turbocharger at 1 percent. Another source projects turbocharging switches turbochargers in and out of use as that a high-pressure-ratio axial compressor will reduce fuel required, but they are normally connected in parallel. A mod- consumption by 1.1 to 3.6 percent.2 ulated two-stage system brings some of the benefits of each Almost all heavy-duty diesel engines sold in North of these two approaches. At low engine speeds it works as a America today use high-pressure loop EGR for control of two-stage system, delivering high-boost pressure despite the engine-out NOx levels. To get EGR to flow from the exhaust low engine speed. At high engine speeds it bypasses the small manifold to the intake manifold, the pressure in the exhaust high-pressure turbocharger, allowing the bigger, low pressure manifold must be higher than the pressure in the intake turbocharger to work on its own and produce the higher flows manifold. When the exhaust manifold pressure is higher than at high engine speeds. Modulated two-stage systems offer the the intake manifold pressure, this is called having a negative benefits of both high-boost pressure and wide-flow range, Δp, where Δp refers to the difference in pressure between the mainly due to the fact that two different-sized compressors intake and exhaust manifolds. High-efficiency turbochargers are used. Using two compressors replicates the effect of a naturally produce a positive Δp over much of their operat- variable compressor without the need for a complex housing. ing range, so turbocharger efficiency must be intentionally The modulated two-stage system can have a high-pressure compromised in order to facilitate EGR flow. If it is possible turbocharger far smaller than that of a conventional two-stage to produce adequate EGR flow without reducing turbo ef- system, improving transient performance by reducing the turbo lag that affects both drivability and emissions.3 ficiency, the overall engine efficiency will increase. Dual-stage turbocharging is used in production by Navi- star, Daimler Trucks, and Caterpillar in the United States and Dual-Stage Turbocharging with Intercooling by MAN and Mercedes in Europe. Ford has announced that Modern engines use high-pressure ratios, which limit the 2011 diesel engine used in its Class 2b to 7 trucks will the efficiency of turbochargers. Using two turbochargers in use a twin-compressor turbocharger (back-to-back compres- series with intercooling would allow higher turbocharger sors on the same shaft). Another source estimates a 2 to 5 percent reduction in fuel consumption.4 These benefits are efficiency, but this adds cost and packaging complexity and requires an EGR pump or other device such as a tur- only available if a way to provide the required EGR flow is bocompound system to facilitate EGR flow. Air-to-water available. intercooling is used after the first-stage compressor, in some applications, and air-to-air aftercooling is used after the second-stage compressor. 3 See www.cummins.com/turbos. 2 Personal communication between Steve Edmonds and David F. Merrion, 4 Private communication from S.M. Shahed to David F. Merrion, October committee member, September 2008. 1, 2009.

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 POWER TRAIN TECHNOLOGIES FOR REDUCING LOAD-SPECIFIC FUEL CONSUMPTION Mechanical Turbocompound cluded the benefits of electric accessories. Achieving the full benefit of electric turbocompound requires the electrification The base turbocharged engine remains unchanged and a of vehicle accessories, the addition of an electric motor to power turbine is added to the exhaust stream to extract ad- apply turbocompound energy to supplement engine output, ditional energy from the exhaust. The power turbine is con- and an electric storage system (battery) to store any energy nected to the crankshaft to supply additional power (NESC- from the power turbine that is not immediately required. CAF/ICCT, 2009, p. 81). Typically, the attachment includes Making all of these changes to the vehicle will pose signifi- a fluid coupling (to allow for speed variation and to protect cant development and cost challenges. the power turbine from engine torsional vibration) and a gear set to match power turbine speed to crankshaft speed. Pub- Variable Valve Actuation lished information on the fuel consumption reduction from mechanical turbocompounding varies, as evidenced by the Variable valve actuation (VVA), also called variable following: 3 percent, according to the Detroit Diesel Corpo- valve timing or discrete variable valve lift, allows the valve ration,5 which has a turbocompound engine in production; actuation to be adjusted independently from the crankshaft 2.5 to 3 percent (NESCCAF/ICCT, 2009, p. 54); 3 percent angle. There are many implementations of VVA. Some are (K.G. Duleep, Energy and Environmental Analysis)6 and 4 hydromechanical, such as the system used on some BMW to 5 percent (Kruiswyk, 2008, pp. 212-214); TIAX (2009, passenger car engines. Other designs use electromagnets pp. 4-17) used 2.5 to 3 percent. Some of these differences or high-pressure hydraulic systems. Some versions offer may depend on the operating condition or duty cycle that was “full authority,” or unlimited, control of valve timing and considered by the different researchers. The performance of lift, while other implementations offer limited control, such a turbocompound system tends to be highest at full load and as variable duration only, variable lift only, or even more much less or even zero at light load. limited control, such as with the system used on some Cat- erpillar engines to permit a Miller cycle to be used under Electric Turbocompound some operating conditions. VVA technology can also be used for cylinder deactivation. One of the primary drivers This approach is similar in concept to mechanical turbo- for introducing VVA in diesel engines is to facilitate the use compound, except that the power turbine drives an electrical of nonconventional combustion modes. According to several generator (NESCCAF/ICCT, 2009, p. 29). The electricity sources, variable valve timing can improve fuel consumption produced can be used to power an electrical motor supple- by about 1 percent when standard diesel combustion is used menting the engine output, to power electrified accessories, (NESCCAF/ICCT, 2009, p. 55). or to charge a hybrid system battery. Electric turbocompound is a technology that fits particularly well with a hybrid elec- Low-Temperature Exhaust Gas Recirculation (Also Called tric power train for long-haul applications where regenera- Advanced EGR Cooling) tive braking opportunities are limited. The benefits of electric turbocompound and an electric hybrid power train can be Most medium- and heavy-duty vehicle diesel engines additive. Energy and Environmenal Analyis7 has said that sold in the U.S. market today use cooled EGR, in which “electric turbo-compound is more efficient and possible as part of the exhaust gas is routed through a cooler (rejecting part of hybrid packages.” Fuel consumption reduction bene- energy to the engine coolant) before being returned to the fits as large as 10 percent are claimed. The NESCCAF/ICCT engine intake manifold. EGR is a technology employed to study (p. 54) modeled an electric turbocompound system and reduce peak combustion temperatures and thus NOx. Low- estimated benefits at 4.2 percent, including electrification temperature EGR uses a larger or secondary EGR cooler of accessories. Caterpillar, Inc., as part of Department of to achieve lower intake charge temperatures, which tend Energy (DOE) funded work, modeled a system that showed to further reduce NOx formation. If the NOx requirement is 3 to 5 percent improvement, while John Deere investigated unchanged, low-temperature EGR can allow changes such a system (off-highway) that offered 10 percent improvement as more advanced injection timing that will increase engine (Vuk, 2006; TIAX, 2009, p. A-10). None of these systems efficiency slightly more than 1 percent (NESCCAF/ICCT, have been demonstrated commercially. TIAX (2009, pp. 3-5) 2009, p. 62). Because low-temperature EGR reduces the used a range of 4 to 5 percent for its estimates, which in- engine’s exhaust temperature, it may not be compatible with exhaust energy recovery systems such as turbocompound or 5 DetroitDiesel Corporation, DD15 Brochure, DDC-EMC-BRO-0003- a bottoming cycle. 0408, April 2008. 6 K.G. Duleep, Energy and Environmental Analysis, “Heavy Duty Trucks Fuel Economy Technology,” presentation to the committee, Washington, Electrification of Engine-Driven Accessories D.C., December 5, 2008, slide 17. 7 K.G. Duleep, Energy and Environmental Analysis, “Heavy Duty Trucks Accessories that are traditionally gear or belt driven by Fuel Economy Technology,” presentation to the committee, Washington, a vehicle’s engine can be converted to electric power. Ex- D.C., December 5, 2008, slide 17.

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 TECHNOLOGIES AND APPROACHES TO REDUCING THE FUEL CONSUMPTION OF MEDIUM- AND HEAVY-DUTY VEHICLES amples include the engine water pump, the air compressor, make these alternative combustion modes work, and these the power-steering pump, cooling fans, and the vehicle’s modes cannot generally be used over the whole operating air-conditioning system. In many cases this can result in a range of the engine, nor have they demonstrated inherent fuel reduction in power demand, because electrically powered ac- consumption advantages (NRC, 2008, Finding 3-8, p. 42). cessories (such as the air compressor or power steering) oper- The primary purpose of alternative combustion cycles is to ate only when needed if they are electrically powered, but lower engine-out emissions. This can lead to either lower they impose a parasitic demand all the time if they are engine overall emissions or lower cost for exhaust aftertreatment. driven. In other cases, such as cooling fans or an engine’s water pump, electric power allows the accessory to run at Effects of DPF and SCR on Engine Efficiency speeds independent of engine speed, which can reduce power consumption. Electrification of accessories can individually The use of emissions control devices has an influence improve fuel consumption, but as a package on a hybrid on engine efficiency. This is true whether the emissions are vehicle it is estimated that 3 to 5 percent fuel consumption controlled on an in-cylinder basis or via the aftertreatment. In reduction is possible.8 The TIAX (2009, pp. 3-5) study used most cases, the effect of adding an emissions control device 2 to 4 percent fuel consumption improvement for accessory increases fuel consumption, either directly by reducing the electrification, with the understanding that electrification of efficiency of energy extraction from the combustion process accessories will have more effect in short-haul/urban appli- or indirectly by requiring the use of additional fuel to main- cations and less benefit in line-haul applications. tain the performance of an aftertreatment system. Engine Friction Reduction Improved SCR Conversion Efficiency Reduced friction in bearings, valve trains, and the piston- NOx is formed in a reaction that occurs naturally whenever to-liner interface will improve efficiency. Any friction nitrogen and oxygen are heated above a certain temperature. reduction must be carefully developed to avoid issues with The higher the temperature, the more rapid the NOx-forming durability or performance capability. An example would be reaction occurs. In-cylinder technologies to control NOx for- to develop heavy-duty diesel engines to run on 10W-30 oil mation in diesel engines are aimed at reducing the maximum instead of the current standard of 15W-40. The lower vis- temperature reached by the gases in the combustion cham- cosity oil would reduce friction, at the expense of bearing ber. The approaches used include retarded injection timing, capability. Fuel consumption improvement from one source9 multiple injection events and injection rate shaping, EGR, was 2 percent, whereas another source10 claims 1 to 1.5 charge air cooling, and alternative combustion modes (such percent. The use of a thermatic oil cooler (thermostatically as HCCI, PCCI, LTC). Some of these approaches leads to a controlled oil cooler) in conjunction with lower viscosity decrease in work output of the engine due to exhaust emis- lubricating oils could yield 1.5 percent improvement.11 The sions control (NRC, 2008), except charge air cooling. effect of friction reduction and oil temperature control will The DPF is used to eliminate PM on an aftertreatment be greatest during cold starts and under light load operation, basis. A DPF requires energy for regenerating the filter on where friction accounts for a larger portion of total energy a periodic basis. This energy most commonly comes from consumption. injecting diesel fuel into the exhaust stream. By definition, fuel injected into the exhaust stream will not contribute to crankshaft power and thus represents a decrease in efficiency. Alternative Combustion Cycles A DOC or other device oxidizes the fuel in the exhaust Alternatives to the standard diesel combustion cycle are stream, providing the heat required for DPF regeneration available, such as low-temperature combustion (LTC), ho- and increasing the fuel consumption of the vehicle. Another mogeneous charge compression ignition (HCCI), and premix method to provide the heat required for DPF regeneration is charge compression ignition (PCCI). These combustion to revise the air/fuel ratio of the engine to produce exhaust modes can be more efficient than standard diesel combustion constituents and heat that are used to regenerate the DPF. under some conditions, particularly when very low NOx is This approach also increases the fuel consumption of the a requirement. There are significant control requirements to vehicle. The SCR aftertreatment system for reducing NOx also requires a fluid, which is urea mixed with water (called Ad- 8Anthony Greszler, Volvo Powertrain, “Reducing Emissions in Heavy blue in Europe and DEF [Diesel Exhaust Fluid] in the United Vehicles,” presentation to the committee. Washington, D.C., December 5, States), to supply the reducing agent. The urea is made from 2008, slide 23. natural gas. The energy use of this fluid and/or its cost must 9Anthony Greszler, Volvo Powertrain, “Reducing Emissions in Heavy Vehicles,” presentation to the committee. Washington, D.C., December 4, be accounted for in the calculation of energy consumption. 2008, page 14. The use of SCR can allow a higher engine-out NOx level, 10 Site visit to Daimler/Detroit Diesel, April 7, 2009. which in turn can be used to reduce fuel consumption, but 11 Site visit to Daimler/Detroit Diesel, April 7, 2009.

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0.39  POWER TRAIN TECHNOLOGIES FOR REDUCING LOAD-SPECIFIC FUEL CONSUMPTION 0.2 g/hp-hr NOx 0.37 with SCR 1.2 g/hp-hr NOx 38% 0.35 2.5 g/hp-hr NOx 40% BSFC (lb/BHPH) 0.33 Consent Decree 6 g/hp-hr NOx 43% 0.31 11% Projection at 8 g/hp-hr NOx 0.29 46% Best BSFC low speed marine diesel 10% * 13 g/hp-hr Nox * Unlimited cooling capacity *2 stroke 0.27 50% * 95 RPM * Full Load * Unlimited space and weight * 51% Brake Thermal Efficiency 0.25 1975 1980 1985 1990 1995 1999 2001 2004 2007 2010 Year Brake Thermal Efficiency, % FIGURE 4-2 Historical trend of heavy-duty truck engine fuel consumption as a function of NOx requirement. SOURCE: Tony Greszler, Volvo, October 2009. igure 4-2 Historical trend of heavy-duty truck engine fuel.eps F bitmap this improvement must be weighed against the urea con- cooling factors that are limits faced in vehicle applications. sumption of the SCR aftertreatment system. The upcoming The 2010 model truck engines will suffer an 11 percent fuel 2010 heavy-duty emissions standards reduce the allowable consumption penalty for NOx control (compared with an NOx level by a factor of 6 from 1.2 g/hp-hr to 0.2 g/hp-hr, 8 g/hp-hr NOx engine, see Figure 4.2), which is less than which limits the ability of some manufacturers to use SCR the penalty of 2002 and 2007 engines. This penalty does to increase engine-out emissions. not include the energy content of DEF (urea) for the SCR There is a close relationship between emissions control system. requirements and fuel consumption. In particular, certain Most 2010 engines will use SCR aftertreatment to reduce technologies that are used to control NOx emissions have NOx emissions. Daimler’s Detroit Diesel estimates that, with the effect of increasing fuel consumption. See Figure 4-2 the installation of SCR in 2010, fuel consumption will be for an example of this trend. Figure 4-2 also compares truck reduced 3 to 4 percent by a combination of higher engine-out engines with the most efficient large marine engines, which NOx, controls and fuel system improvements, reduced DPF so far do not face any emissions constraints. The efficiency regeneration frequency, and other efficiency gains, while of large marine engines is due to several factors that cannot Volvo estimates the potential improvement of its 2010 en- be reproduced in vehicle applications. Marine engines are gines at 2 percent. As the SCR system conversion efficiency very large and heavy, they run at very low speeds, they have improves, it allows higher engine-out NOx emissions. Engine a source of unlimited cooling capacity (seawater), and they manufacturers can take advantage of this by making changes face (for the time being) no emissions constraints. All four in fuel injection timing or by using less EGR, in order to of these factors contribute to the high efficiency of marine reduce fuel consumption. engines. According to information provided by Volvo,12 the fuel consumption of truck diesel engines is about 10 percent Thermal Insulation of Ports and Manifolds higher than for marine engines due to the size, weight, and Thermal insulation would reduce heat rejection to the engine coolant (from exhaust ports) or to the ambient air 12Tony Greszler, Volvo, private communication with Thomas Reinhart, (from manifolds). The energy retained in the exhaust can October 2009.

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 TECHNOLOGIES AND APPROACHES TO REDUCING THE FUEL CONSUMPTION OF MEDIUM- AND HEAVY-DUTY VEHICLES be used by downstream devices such as a turbocompound based on site visits). These advances may be possible in system or bottoming cycle. Caterpillar Inc. made compo- the 2013 to 2015 time frame (TIAX, 2009). Fuel injection nents, such as air gap pistons and exhaust port liners as part systems were estimated on site visits to offer between 1 of the 21st Century Truck Partnership, but reported results and 4 percent improvement in fuel consumption; Vyas are not available (NRC, 2008, p. 30). The anticipated benefit et al. (2002) estimates fuel injection systems have the po - is small. tential to improve fuel consumption by 6 percent, although this estimate is now several years old, and considerable improvement in fuel systems has already been made since Improved Work Extraction from Combustion Process 2002. Real-time combustion control with start of combus - The compression ratio, expansion ratio, combustion tion sensors can also yield a fuel consumption reduction of 1 percent to 4 percent.14 chamber shape, injection spray pattern, injection pressure, injection timing, injection rate shaping, air/fuel mixing, peak cylinder pressure limit, air/fuel ratio, and EGR rate are all Engine Electronic Controller Calibration Management parameters that can be modified in an effort to reduce fuel consumption. Improved combustion chamber design allows Advanced engine controls will be enabled in part by for improved air management and mixing. Improved mate- the onboard diagnostic systems that are mandated for me- rials and structural design enable higher cylinder pressures. dium and heavy trucks beginning in 2010 on one family These enhancements allow more precise control of the tim- and across the board in 2013. Increasingly sophisticated ing and rate of heat release (combustion) as well as higher engine controls, particularly a transition to closed-loop combustion temperatures, both of which can improve ther- control approaches, will enable engine efficiency improve- mal efficiency. Unfortunately, higher combustion tempera- ments. Closed-loop controls will feed information about the tures also lead to higher NOx. Combustion chamber design engine’s operating regime and emissions back to the system enhancements may require more advanced materials and a controls. This improved feedback will allow manufactur- more complex machining process. More complex and expen- ers to optimize emissions and fuel consumption within the sive fuel systems allow greater control of injection pressure, constraints of emissions requirements across a variety of timing, and rate shaping. In addition, because higher cylinder operating conditions. pressures must not result in higher NOx, measures must be Better use of calibration tools to improve control of taken to allow the improved fuel consumption without creat- EGR, injection rate shapes, multiple injection events, and ing an increase in emissions. These measures may include increased injection pressure can yield 1 to 4 percent fuel improved NOx conversion efficiency by the aftertreatment, consumption reduction. These benefits are redundant with advanced fuel injection techniques (which enable more de- those described above for improved work extraction from tailed control of combustion), and improved engine controls. the combustion process. Another feature already in use on The efficiency benefit of these improvements is estimated at some long-haul trucks is adding 200 lb-ft of torque in the 1 to 3 percent.13 top two transmission gears, which manufacturers claim can Finely controlled, high-pressure fuel injection is a key give 2 percent reduction in fuel consumption by reducing enabler for more fuel efficient combustion and a cleaner, the need for downshifting on modest grades. With the next more consistent fuel burn. Current state-of-the-art systems generation electronic controller, using model-based controls, planned for deployment in 2010 engines include very high it is predicted that another 1 to 4 percent fuel consumption reduction will be achieved.15 Note that the reductions listed pressure (2,000 to 2,400 bar) common rail injection systems with advanced nozzle designs that are capable of finely in here may be repeats of reductions from previous sections, shaped and controlled spray, along with multiple injection and there may be some redundancy in the percentages quot- events per cycle. ed, but the concepts and percentages presented here came Potential future improvements will continue to improve from committee site visits where engineers talked about control, allow more accurate timing and metering of injec - these reductions. The overall approach of more sophisticated tion with combustion events, and further increase fuel in - control of the combustion process tends to include several jection pressure. Improved material properties and controls building blocks, such as upgraded fuel system capabilities, could enable pressures of up to 3,000 bar in the 2015 time sophisticated control algorithms, additional sensor inputs frame and perhaps 4,000 bar by 2020. Future systems will for feedback control, and technologies such as model-based also utilize increasingly sophisticated injection techniques controls. The benefits of this approach are often claimed by such as variable-spray nozzles, piezo-electric nozzles, or each of the individual building blocks, leading to redundant supercritical fuel injection (fuel changing instantaneously claims on the same fuel consumption benefit. from liquid state to supercritical gaseous state at injection 13 Presentations 14 Committee by and discussions with Cummins, Detroit Diesel, and site visit, Daimler/Detroit Diesel, April 8, 2009. 15 Committee Volvo. site visit, Daimler/Detroit Diesel, April 8, 2009.

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 POWER TRAIN TECHNOLOGIES FOR REDUCING LOAD-SPECIFIC FUEL CONSUMPTION Bottoming Cycle at packaging fuel-saving technologies for engines were in the DOE programs with a goal of demonstrating 50 percent A bottoming cycle is basically a secondary engine that thermal efficiency while meeting 2010 emissions. The uses exhaust energy or other heat sources from the primary National Research Council (2008) review of that program engine to develop additional power without using additional showed a baseline thermal efficiency of 42 percent with a fuel. The energy sources used by the bottoming cycle are goal of 50 percent, or a 19 percent improvement in thermal sources that normally go to waste in a conventional engine. A efficiency and a 16 percent fuel consumption reduction. typical bottoming cycle includes the following components: Three engine manufacturers—Caterpillar, Cummins, and a feed pump to drive the working fluid from the condenser Detroit Diesel—were funded at a level exceeding $116 to the evaporator (or boiler); the evaporator, which transfers million over five years with the following result, according waste heat energy from the primary engine to the working to the report: “These results show that none of the industry fluid; an expander, which takes energy from the working partners achieved the goal of measuring 50 percent ther- fluid to make mechanical power; and a condenser that rejects mal efficiency at 2010 emissions from a complete engine unused heat energy from the bottoming cycle working fluid system.” before starting a new cycle. The power generated by the Cummins has supplied the committee with the follow- expander can be used to make electricity, which in turn can ing research roadmaps for achieving 49.1 percent thermal power an electric motor supplementing the engine output, efficiency and 52.9 percent thermal efficiency. Figure 4-3 is power electrified accessories, or charge a hybrid system a research roadmap for 49.1 percent thermal efficiency by battery. Sources of energy to power a bottoming cycle can in- 2016, which is an improvement of 17 percent from the 42 clude the EGR stream, exhaust stream, charge air stream, and percent baseline (14.5 percent reduction in fuel consump- engine coolant circuit (NESCCAF/ICCT, 2009, pp. 85-88). tion). Figure 4-4 is a research roadmap for 52.9 percent Cummins, Inc. has shown a projected increase of thermal thermal efficiency by 2019, which is an improvement of 26 efficiency from 49.1 to 52.9 percent (7.2 percent decrease percent from the baseline 42 percent (20.6 percent reduction in fuel consumption) using an organic Rankine cycle. Cum- in fuel consumption). These roadmaps can be compared to mins also lists turbocompounding and a Brayton cycle as the baseline shown in Figure 4-1. Note that these are plans alternative methods of extracting work from unused energy and goals, not actual development results. Actual results that in the exhaust stream. Cummins reports recovering 2.5 will be achieved in development may vary from the planned thermal efficiency points from the exhaust and 1.3 thermal benefits. efficiency points from the coolant and EGR stream.16 The In its report for the committee, TIAX (2009, Tables 5-8 NESCCAF/ICCT report (2009, pp. 55-56) showed the effect and Table 5-9) summarized the diesel engine fuel consump- of a steam bottoming cycle to reduce fuel consumption by tion potential reductions by range of years and by application up to 10 percent. as shown in Table 4.1. Other Technologies GASOLINE ENGINE TECHNOLOGIES Other technologies for reducing the fuel consumption Gasoline engines operate with a premixed charge of fuel of diesel engines are discussed in the press almost every and air and use spark ignition to start the combustion process. day (e.g., Automotie News, Transport Topics, Diesel Fuel They are used in many Class 2b applications as well as Class News, DieselNet.com). Some are emerging technologies and 3 to 6 applications. Within the medium-duty truck sector, all may not become production feasible, including new diesel gasoline engines operate on the four-stroke-cycle principle engines of two-stroke-cycle design, split-cycle design, free- and are of an in-line or “vee” configuration. Displacements of piston design, rotary design and camless engines with digital these engines typically range from 6 to 8 liters. These engines valve control such as the Sturman Industries concept. The list normally burn gasoline, but with slight changes they can of potential technologies also includes oxygen injection into burn natural gas (compressed [CNG] or liquefied [LNG]), the intake air, hydrogen injection into the intake air, air injec- propane, hydrogen, ethanol, methanol, and so forth. tion from the air compressor to overcome turbo lag, or the The fundamental operating principle for gasoline en- use of fuel-borne catalysts such as platinum and cerium. gines used today relies on creating a well-mixed charge of gasoline and air at the time the spark plug fires. After the Diesel Engine Summary combustion process is over, a catalyst in the exhaust system is used to perform the final emissions cleanup. Emissions of In summary, to add up all these individual potential NOx, carbon monoxide, and unburned hydrocarbons are the reductions to arrive at an overall potential fuel consump- principal species being treated by the catalyst in the exhaust. tion reduction would not be correct because there would be The three-way catalyst treats all three of these emissions si- double counting of some effects. The best recent attempts multaneously; however, the three-way catalyst will function 16 Jeff Seger, Cummins, Inc., at committee site visit, May 15, 2009.

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 TECHNOLOGIES AND APPROACHES TO REDUCING THE FUEL CONSUMPTION OF MEDIUM- AND HEAVY-DUTY VEHICLES Advanced Combustion Emissions enabler Fuel Energy 10 0% Inj. Pressure, Multiple Injection CEGR Cooling Systems Increased Peak Cylinder Pressure Closed Loop Combustion Control Indicated Ef ficient Aftertreatment Exhaust Heat Transfer Power 28.2 % 19.2 % 52 .6 % Brake Power Gas Exchange Friction Accessories 2.0% 0.9% 0.6% 49.1% DOE Electrically assisted turbo EGR pump R & D Goal 50% Piston and rings Variable valve actuation Bearings Surface treatment FIGURE 4-3 Research roadmap 3 Roadmap for 49.1 percent thermal eProvided undery 20.eps Figure 4- for 49.1 percent thermal efficiency by 2016. SOURCE: fficiency b license by Cummins Inc. Copyright 4 small bitmaps, the rest is vector 2009 Cummins Inc. All rights reserved. properly only if the air/fuel ratio is carefully controlled to be be applied to gasoline engines to reduce fuel consumption chemically correct, or stoichiometric.17 are not used, primarily because of the need to maintain low Heavy-duty gasoline engines, and most heavy-duty en- NOx emissions. For example, lean gasoline direct injection gines using spark-ignited alternative fuels such as natural (GDI) has the potential to provide double-digit percentage gas, use a relatively simple emissions control strategy. The reductions in fuel consumption, but it is not used, because it engine operates at a stoichiometric air/fuel ratio across would result in higher NOx emissions. The NOx emissions most of the operating range, with relatively high engine- of a lean GDI engine could be much lower than those of an out emissions levels. A three-way catalyst is used both to unregulated engine, but engine makers have not been able to oxidize hydrocarbon and carbon monoxide emissions and to make lean GDI reach the very stringent U.S. NOx standards reduce NOx. The three-way catalyst can function properly applied to new cars and trucks today. GDI has been used in only if the air/fuel ratio is carefully controlled in order to Europe, where NOx is less stringently regulated. meet current and future emissions requirements. Additional One consequence of requiring a stoichiometric mixture of considerations for gasoline engine emissions control include air and fuel is that the intake airflow needs to be throttled for achieving rapid catalyst light-off on startup and controlling lighter load operation. Lighter loads necessitate a lower fuel evaporative emissions, but the basic emissions control tech- flow rate into the engine, and since the air/fuel mixture is to nology for spark-ignited engines is the relatively simple and be maintained in stoichiometric proportions, the airflow rate inexpensive three-way catalyst. needs to be reduced in proportion to the fuel flow rate. The There is a fuel consumption penalty that comes with the process of throttling the intake airflow results in significant three-way catalyst used on gasoline engines. Because the pumping losses that are not present in diesel engines that air/fuel ratio must be maintained at stoichiometric all the operate using traditional diesel combustion. This pumping way down to idle, the pumping losses from throttling are loss is one of the main reasons spark ignition engines are less large. Lean operation could provide significant fuel sav- efficient than diesel engines. The magnitude of the pumping ings but would not allow the NOx reduction function of the loss depends on the operating duty cycle of the engine. If three-way catalyst to work. Many technologies that could the engine spends most of its time in light load operation, its throttling losses will be higher than for an engine that spends most of its operation under heavier load. Also, the pumping 17 Stoichiometric refers to the chemically balanced reaction of air and fuel. Under stoichiometric conditions there is a certain amount of oxidant work will depend on the engine size relative to the vehicle. A (air) such that all of the carbon in the fuel could react to carbon dioxide smaller engine size in a given vehicle application will spend and all the hydrogen in the fuel could react to water, with no oxidant or a higher portion of its operation at a higher load, relative to a fuel left in the products.

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 POWER TRAIN TECHNOLOGIES FOR REDUCING LOAD-SPECIFIC FUEL CONSUMPTION Ram Airflow WHR Condenser Cooler CAC Engine Radiator NOT to scale Me chanical Fan Condenser Cooler Pump Condenser Air In EGR Fuel Energy Valve ISX Engine Comp 10 0% Intake Manifold Boiler Exh.Manifold Turb Recupe Superheate r Turbin e WHR Fe edpump Generator Power DMG Conditioning Power Out Exhaust Out Indicated Exhaust Heat Transfer Power 28.2 % 19.2 % 52 .6 % Waste Energy Recovery • Organic Rankine Cycle • Turbo compounding • Bray ton Cycle Exhaust Coolant & EGR Gas Exchange Friction Accessories Brake Power Source Source 2.0% 0.9% 0.6 % 52 .9 % (ORC ) (ORC ) 2.5% 1.3% DOE R &D Goal 55% FIGURE 4-4 Research roadmap for 52.9 percent thermal efficiency by 2019. SOURCE: Provided under license by Cummins Inc. Copyright Figure 4-4 Research rRoadmap for 52.9 -percent thermal effic.eps 2009 Cummins Inc. All rights reserved. 4-5 gives a qualitative partitioning of the fuel energy for a TABLE 4-1 Diesel Engine Fuel Consumption typical gasoline-fueled vehicle. This is analogous to Figure (percentage) by Years and Applications 4-1, which gives an energy partitioning for diesel-powered Application 2013-2015 2015-2020 vehicles. Figure 4-5 is illustrative in describing the tech- Tractor trailer 10.5 20 nologies being considered to reduce gasoline engine fuel Class 6 box truck 9 14 consumption. Class 6 bucket truck 7.2 11.2 The proportion of the fuel energy that gets converted Refuse truck 10.5 14 into indicated work is a direct measure of the engine’s fuel Urban bus 9 14 conversion efficiency. Factors that affect an engine’s fuel Motor coach 10.5 20 Class 2b pickup and van 14 23 conversion efficiency include irreversibilities18 in the com- bustion process, the amount of energy leaving the engine SOURCE: TIAX (2009). cylinder as heat transfer, and the energy remaining in the exhaust at the end of the expansion process. These losses represent fuel energy that did not get converted into useful shaft work. Not all of the energy that was converted into work in the combustion process makes it to the final shaft output. larger engine in the same vehicle, and consequently will have a lower pumping loss than the corresponding larger engine. As an approximate guide, pumping losses might range from 18 Irreversibility is a thermodynamic concept. It is used to describe and 2 to 5 percent of the fuel energy (Patton et al., 2002). quantify the degree of imperfection in any real process. In the context used Compared to diesel engines, spark ignition engines are here it describes the degradation of energy during the combustion process generally simpler and less expensive, they have more effec- into a form that is less capable of being converted into work. Theoretically it is possible to convert all of the chemical energy contained within the fuel tive and lower cost exhaust emissions aftertreatment systems, completely into work. Inherent in the chemical reaction of the actual com- and they have higher fuel consumption. bustion process are irreversibilities that render the resultant thermal energy The current emphasis in the development of spark ig- of the combustion products not completely available to be converted into nition engines is on reducing fuel consumption. Figure work, even though the quantity of energy is conserved.

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0 TECHNOLOGIES AND APPROACHES TO REDUCING THE FUEL CONSUMPTION OF MEDIUM- AND HEAVY-DUTY VEHICLES FIGURE 4-5 Partitioning of the fuel energy in a gasoline-fueled engine (proportions vary with vehicle design, type of engine, and operat - ing conditions). SOURCE: igure 4-5 Partitioning of the fuel energy...and operating.eps FNRC (1992). bitmap Some gets used to overcome friction, some is used to pump efficiency point, with minimal pumping work, over larger air and fuel into the engine, and the exhaust gases out of the portions of the duty cycle. engine, and some is used to power accessories. The work Cylinder deactivation is an approach that minimizes that makes it to the drive wheels is used to overcome vehicle pumping losses by varying the total working displacement inertia, aerodynamic drag, and rolling resistance. The rela - of the engine. The “smaller” engine operates closer to wide- tive ranking of these energy uses is highly dependent on the open throttle at lower loads, which reduces the pumping vehicle and the application to which it is being applied. work. As indicated in the discussion of the pumping work above, Cam phasers allow the valve timing to be changed with the magnitude of these energy partitions is highly dependent engine operating conditions. For example, the timing can be on the engine size, its application in the vehicle, and the duty shifted with engine speed to optimize the engine breathing cycle under which the vehicle is operating. The best way to with engine speed. VVT can also be used in place of the in- quantify the partitioning shown in Figure 4-5 would be to take throttle. Either opening the intake valve late or closing take the specific data for the application of interest or through it late can regulate the amount of air/fuel mixture captured the application of a verified system simulation program. in the cylinder. This can be done with lower throttling losses Opportunities to reduce the fuel consumption of gaso- than would occur with the conventional intake throttle. line engines include improving engine efficiency (trying Different engine designs will lend themselves more eas- to reduce the proportion of the fuel energy leaving as heat ily to different valve control technologies. For example, transfer, exhaust energy, and pumping work), reducing the overhead valve systems with the cam in the block versus a energy lost to friction, and reducing the power required for single overhead cam versus a double overhead cam design running accessories. Brief descriptions of different technolo- will have to invoke different VVA approaches, which may gies for reducing fuel consumption are given below. favor different valve manipulation strategies for the differ- ent engine configurations. So decisions as to which valve technology to invoke would be based on the engine con- Variable Valve Actuation and Cylinder Deactivation figuration, the application’s duty cycle, and the incremental There are many approaches to VVA. These include cam cost of implementation. Estimates for the fuel consumption phasers, variable lift mechanisms, fully flexible valve trains, reduction achievable through variable valve lift and/timing and cylinder deactivation. The primary loss that the VVA sys- range from 1 to 3.5 percent (TIAX, 2009, p. 4-33). tems are trying to reduce is the pumping, or throttling, loss. A variable compression ratio allows the engine to operate at Gasoline Direct Injection Engines different compression ratios for different loads in order to maximize the engine efficiency over the widest load range GDI engines refer to an embodiment in which the fuel possible. The combination of VVA and variable compres- injector is mounted so that the fuel is directly injected into the sion ratio keeps the engine operation closer to its maximum cylinder, as opposed to the more common port fuel injection,

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 POWER TRAIN TECHNOLOGIES FOR REDUCING LOAD-SPECIFIC FUEL CONSUMPTION where the fuel injector is mounted in the intake port. There tive modes of combustion into the engine’s operating map are two philosophies of engine operation that fall under the will require much higher levels of sensing and control than classification of GDI engines. are currently in use in today’s engines. For example, it is In one case the engine is still operated with a stoichio- likely that real-time cylinder pressure sensing would be metric mixture of air and fuel. This approach enables a necessary to activate and control transitions between conven- three-way catalyst to be used in the exhaust, so emissions tional combustion and LTC-type combustion during engine standards can be met. However, by mandating operation operation. If these combustion regimes can be incorporated with a stoichiometric mixture, the engine still needs to be into the operating map of gasoline engines, fuel consumption throttled. For a direct-injected stoichiometric engine the could be reduced via lean combustion at light loads, with efficiency improvements come from less fuel being used minimal pumping losses, without the need for lean exhaust during transient engine responses and from internal cooling aftertreatment. Then during higher load operation, where of the cylinder charge from the fuel vaporization. This leads throttling requirements are low, the engine could revert back to a higher knock margin, which allows a higher compres- to stoichiometric operation where the three-way catalyst is sion ratio to be used. These engines are also more tolerant of effective. It has been estimated that incorporation of LTC EGR, so higher compression ratios can be used without an operation into the engine could reduce fuel consumption by NOx penalty. There will be an attendant efficiency improve- 10 to 12 percent (TIAX, 2009). See Table 4-2. ment with the higher compression ratio. The reduction in fuel consumption achieved via stiochiometric direct injection Turbocharging and Downsizing will be dependent on the extent to which this technology is combined with other technology packages. Based on esti- Turbocharging a gasoline engine is similar to turbocharg- mates from a light-duty study (NHTSA, 2009), referenced ing a diesel engine in that it is motivated by the desire to by TIAX, a fuel consumption reduction with stoichiometric redirect energy that was leaving the engine in the exhaust direct injection engines relative to a port-injected engine gases back into the engine. The turbocharger converts ex- with VVT described above would range from 2 to 3 percent haust energy into higher pressure and temperature intake (TIAX, 2009, p. 4-33). gases using a turbine in the exhaust gas stream and a com- The other approach to GDI is to attempt to replicate the pressor in the intake air stream. Because of the differences breathing characteristics of the diesel engine by minimizing in the fuels and the combustion processes between the two the throttling of the intake air and controlling the load by types of engines, there are different constraints that limit the varying the air/fuel ratio. Reduction in fuel consumption with application of turbocharging in a spark ignition engine rela - this approach to direct injection comes from reduced pump- tive to a compression ignition engine. Because the air and ing losses and higher efficiency from the lean-burning mix- fuel are premixed in a spark ignition engine, as the pressure ture, as well as the potential to increase the compression ratio and temperature of the mixture at the start of compression as with the stoichiometric direct injection. The drawback to is increased via turbocharging, the possibility of knocking this approach is that the three-way catalyst is no longer ef- combustion increases. Consequently it is common for the fective, so the engine has the same aftertreatment challenges compression ratio to be decreased when the spark ignition and expense as the diesel engine. The application of the lean engine is turbocharged. This tends to decrease the thermal burn direct injection technology will almost certainly be efficiency of the engine. It is also likely that more exhaust gas coupled with using turbocharging (described below) as well, will be recirculated when the engine is turbocharged. How- so one must consider the combination of technology pack- ever, if successfully implemented, the turbocharged engine ages. Referencing the same NHTSA report referred to above, will have a higher power density than the nonturbocharged TIAX reported a potential reduction in fuel consumption by engine so it can be smaller and lighter for the same power applying turbocharged lean burn GDI technology relative to output. This reduction of weight and potential reduction in the VVA nonturbocharged stoichiometric engine of 10 to 14 frontal area of the vehicle has an attendant fuel consumption percent (TIAX, 2009, p. 4-33). reduction benefit. Turbocharging can be combined with other technologies, like in-cylinder direct injection and/or VVA to compound the Different Combustion Modes benefits of the various technologies. The direct injection of VVA mechanisms and fuel injection systems open up the fuel results in in-cylinder evaporative cooling, which helps possibility of incorporating advanced combustion regimes counter the increased tendency toward knocking attributable into the engine operating map. For example LTC, a general to the turbocharging. Consequently the compression ratio classification for auto-igniting combustion modes such as may not need to be lowered to avoid knocking combustion HCCI, partially PCCI, and compression-aided ignition, of- to the same extent it would with turbocharging without direct fers potential for lean low-emissions combustion in which injection. This coupled with the increased EGR tolerance throttling losses are minimized and catalytic converters are of the direct injection and turbocharged engines helps keep not needed (Zhao et al., 2003). Incorporating these alterna- NOx emissions from rising. In addition, because the power

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0 TECHNOLOGIES AND APPROACHES TO REDUCING THE FUEL CONSUMPTION OF MEDIUM- AND HEAVY-DUTY VEHICLES FIGURE 4-18 Fuel savings with respect to conventional cycles on standard drive cycles under (left) a 50 percent load and (right) a 100 percent load. SOURCE: ANL (2009). MDV Fig 4-18.eps bitmap FIGURE 4-19 Percentage of braking energy recovered at the wheels under (left) a 50 percent load and (right) a 100 percent load. SOURCE: ANL (2009). MDV Fig 4-19.eps bitmap FIGURE 4-20 Percentage average engine efficiency of conventional and hybrid trucks for (left) a 50 percent load and (right) a 100 percent load on standard cycles. SOURCE: ANL (2009). MDV Fig 4-20.eps bitmap cant improvement since start-stop is the only main feature Effect of Remoing Breaks from Highway Cycle in it to aid in engine efficiency. The full hybrid gains in the Since the HHDDT cycle is short and does not represent transient and urban cycles as the engine can be completely the real highway cycle, a new cycle was formulated with switched off in electric-only mode. original acceleration followed by a cruising part and finally a deceleration part. Figure 4-21 shows how the new drive cycle was obtained from the HHDDT cycle. The results of Effect of Drive Cycle on Hybrid Performance removing stops from the HHDDT cycle have been grouped The drive cycle or duty cycle plays an important role in in Figure 4-22. In every other bar the hybrids are compared determining the following: type of hybrid technology to be with the conventional vehicle, hence the values are greater. used, level of hybridization and sizing of components, and When the breaks are removed from the highway cycle, power management strategy. there is a significant drop in fuel savings in the hybrid con-

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 POWER TRAIN TECHNOLOGIES FOR REDUCING LOAD-SPECIFIC FUEL CONSUMPTION FIGURE 4-21 HHDDT 65 cycle repeated five times with stops (left) and without stops (right). SOURCE: ANL (2009). MDV Fig 4-21.eps bitmap FIGURE 4-22 Fuel consumption reduction due to stop removal, with respect to conventional vehicles without stops, and with respect to conventional vehicles with stops (50 percent load on theMDV Fig 4-22.eps right). SOURCE: ANL (2009). left, 100 percent load on the bitmap figuration. However, the conventional configuration benefits half-loaded and at or above 2.5 percent when fully loaded. the most when the stops are removed as the hybrids could The full hybrid hits its regenerative braking limit only when have recovered part of the kinetic energy while braking. In fully loaded at or above 3.5 percent grade. Thus, the full the case of the full hybrid, the savings are more than halved hybrid can capture more kinetic energy while braking, as (5.3 percent fuel saved on a cycle with stops, 2.4 percent fuel expected. saved on a cycle without stops). In general, the hybrids still The simulation results suggest that the mild hybrid has no outperform the conventional vehicles in all cases as there are advantage over the conventional vehicle when the grade is still some gains using the hybrid system even when the stops less than 2 percent, as there is not enough energy generated for accessories (see Figure 4-24).31 Charge balancing is hard are removed. to achieve, so the engine might be used to charge and hence may result in higher than expected fuel consumption. Fur- Fuel Saings in Grades for Hybrid Configurations Due to the lack of real-world drive cycles that include thermore, there is not much reduction in fuel consumption grades and to illustrate the potential benefits of hybridization with grades greater than 3 percent, as the electric machine in a “hilly” terrain, idealized sinusoidal road profiles were will reach its maximum potential. created. The elevation of such a road is a sinusoidal func- The results also indicate that the available fuel savings tion of the horizontal distance, with a “hill” period varying with the full-HEV configuration can increase by as much as between 1 and 3 km. Maximum grades also vary from 0 to 4 percent. All combinations of maximum grade and period 31 Note that a simulation of vehicle performance on bus routes in San were analyzed. Figure 4-23 shows the profile created. Francisco, where the grades can be demanding for conventional buses, For the mild-hybrid truck, the motor reaches its rated found that the hybrid bus performed the best on fuel savings as well as on power when braking for grades 3 percent and higher when emissions of NOx and particulate matter (SAE, 2004).

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 TECHNOLOGIES AND APPROACHES TO REDUCING THE FUEL CONSUMPTION OF MEDIUM- AND HEAVY-DUTY VEHICLES FIGURE 4-23 Representation of the grades considered. SOURCE: ANL (2009). MDV Fig 4-23.eps bitmap FIGURE 4-24 Fuel savings of hybrid trucks with respect to conventional trucks as a function of maximum grade for various hill periods; MDV Fig 4-24.eps (left) 50 percent load and (right) 100 percent load. SOURCE: ANL (2009). bitmap

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 POWER TRAIN TECHNOLOGIES FOR REDUCING LOAD-SPECIFIC FUEL CONSUMPTION 14 percent for a 4 percent maximum grade with a hill period Simulation-Based Assessments of Hydraulic Hybrids of 1 km. This suggests that the value of hybridization in trac- EPA predicted that the UPS prototype hydraulic hybrid tor-trailer trucks may be significant in hilly terrains. vehicles would be able to capture and reuse 70 to 80 percent of the otherwise wasted braking energy (EPA, 2009). Kim and Filipi (2007) from the University of Michigan Hydraulic Hybrid Vehicles conducted a simulation study on a series hydraulic hybrid light truck (mass = 5,112 kg). Approximately a 68 percent Hydraulic hybrids have demonstrated fuel savings for medium- and heavy-duty applications. The energy savings reduction in fuel consumption can be achieved in city can be attributed to optimization of engine operation and driving and about 12 percent in highway conditions. The regenerative braking energy absorption. energy savings can be attributed to regeneration and engine EPA has been actively involved in vehicle-level demon- shutdowns and a smaller fraction to optimization of engine strations of this technology by using hydraulic launch assist operation. Design optimization over the complete driving in retrofitted urban delivery trucks. A hydraulic package of cycle enabled right-sizing of all hydraulic pumps/motors, a reversible hydraulic pump/motor and accumulators was the accumulator volume, and the gear ratio of the two-step added to the vehicle to reduce fuel consumption, while keep- transmission. Downsizing the engine to roughly 75 percent ing the vehicle’s conventional engine and transmission. Fuel of the baseline matched the acceleration of the conventional savings of 25 and 45 percent were realized during city driv- vehicle. In addition, it was found that having two smaller ing. For instance, Ford Motor Company, working jointly with propulsion motors for each axle reduced fuel economy con- EPA, demonstrated hydrualic power assist on a full-size sport sumption by an additional 10 percent compared to a single utility vehicle platform fitted with a hydraulic pump/motor propulsion motor architecture. The advantage is that the rear and valve block provided by infield technologies and carbon electric machine could be used mainly for acceleration and fiber accumulators developed by EPA. The pump/motor was the front electric machine for regenerative braking, where connected to the vehicle driveshaft in parallel with the con- both electric machines operate on high loads and in turn yield ventional power train. The vehicle demonstrated the ability to greater efficiency. Wu et al. (2004), also from the University improve fuel economy by close to 24 percent on a start/stop, of Michigan, developed a power management algorithm for city-typical driving cycle (Kepner, 2002). Emissions were a parallel hybrid and predicted fuel savings ranging from 28 reduced by 20 to 30 percent. Better acceleration, reduced to 48 percent depending on the types of pumps used. brake maintenance, and reduced operating costs (consumer Anderson et al. (2005) conducted a simulation study com- payback of 4 to 5 years for city driving) were among other paring hydraulic hybrid and electric hybrid vehicles. Fuel benefits (EPA, 2004). If the transmission and transfer case savings over a variety of driving cycles were found to be 39 were replaced by a complete hydraulic drive train, larger fuel percent for parallel hydraulic hybrids, compared to 31 and 34 consumption benefits could be realized. In June 2006, EPA percent improvements for parallel and series electric hybrids, and United Parcel Service (UPS) demonstrated the world’s respectively. Gotting (2007) estimated the fuel consumption first full hydraulic hybrid delivery truck, which realized 60 benefit for the parallel HHV to range from 20 to 25 percent to 70 percent reductions in fuel consumption under urban for Class 3 to 6 box trucks. driving conditions and up to 40 percent reductions in green- While indicative of the range of potential benefits, it house gas emissions, with an estimated payback shorter than should be noted that simulations are carried out under ideal 3 years. conditions—hence results typically represent best-case sce- Currently, EPA is focusing more on full-series hydraulic narios. Real-world savings in fuel consumption are likely hybrids, along with improved vehicle aerodynamics, tires to be lower, because of off-design duty cycles and practical and advanced ICEs, including HCCI gasoline engines, free production vehicle constraints. piston engines, completely variable displacement engines, alcohol engines, and exhaust heat recovery systems. Eaton Power Management in Hybrid Vehicles Corporation, in collaboration with the EPA, has developed a series hydraulic hybrid power system that combines a high- Power management is key in obtaining maximum perfor- efficiency diesel engine and a unique hydraulic propulsion mance and fuel savings from a hybrid vehicle. The objective system to replace the conventional drive train and transmis- of a power management algorithm in a hybrid vehicle is to sion (Eaton, 2009). The engine operates at its “sweet spot compute the optimum operating point of the overall system facilitated by the continuously variable transmission (CVT) for any amount of power demanded from the driver. The cost functionality of the series hybrid hydraulic system and by criteria are usually fuel economy and emissions. The way the regenerative braking.” Fuel savings of 50 to 70 percent have power is managed will depend a lot on the sizing and char- been achieved, corresponding to a 40 percent reduction in acteristics of each of the components and their instantaneous greenhouse gases, 50 percent reduction in unburned hydro- state of operation. Mechanical efficiencies of the components carbons, and 60 percent reduction in particulate matter (EPA, (i.e., transmission, torque converter, differential), rolling re- 2009). sistance, aero drag coefficients, and instantaneous operating

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 TECHNOLOGIES AND APPROACHES TO REDUCING THE FUEL CONSUMPTION OF MEDIUM- AND HEAVY-DUTY VEHICLES point of components, among others, are of great importance HEVs. The main aspects of this approach are concerned with respect to fuel consumption but are considered as given. with (1) the self-sustainability of the electrical path, which The hybrid power train architecture is also assumed to be must be guaranteed for the entire driving cycle since the given. There are three general power management algorithm storage system cannot be expected to be recharged by an types, as outlined below. external source (fuel converter primarily, brake regeneration secondarily) and (2) the fact that no, or only limited, a priori knowledge of future driving conditions is available. Heuristic Rule-Based Such algorithms consist of an instantaneous optimization This type of control is constituted by heuristic rules—for (Sciarretta et al., 2004; Rodatz et al., 2005; Pisu and Rizzoni, example, if/then statements to split the power demanded 2007); the objective function in this optimization is fuel from the driver into the electrical and mechanical subsys- consumption and emissions. Decisions on the energy flow tems. The majority of these rules are thermostatic—that is, path (engine path and electrical path) can be evaluated based actions are triggered when certain conditions are met. They on an Equivalence Consumption Minimization Strategy. The are simple and easy to implement and less computationally equivalence between electrical energy and fuel energy can expensive to develop, but they need a significant amount of be evaluated by comparing the cost of energy produced at tuning in order to achieve better results compared to a con- any instant. An instantaneous objective function combines ventional vehicle. Their basic simplicity means the system the weighted sum of electrical energy and fuel energy and will not operate at its best potential at all times and there is is evaluated with regard to selection of a proper equivalence significant room for improvement. A rule-based algorithm factor value at any instant. If the engine can produce the for a particular vehicle can never be readily used for another. required power more efficiently than the electrical system Nevertheless, this type of control is often used due to its ease (sweet-spot operating points), the engine energy path is of implementation and lower cost. favored, and when the motor can produce power more ef- Table 4-13 shows the difference in fuel economy figures ficiently than the engine (low speed/low load), the electrical obtained during simulation of a hybrid electric transit bus path is favored. using three different rule-based control algorithms. The key in a real-time power management system is Chu et al. (2003) developed a series HEV military bus simultaneous optimization of the operating points of the (15,000 kg) prototype model and formulated an energy entire system as a whole and not just the engine alone. It management strategy enabling the ICE to operate in its peak aims for the best overall efficiency from the engine to the efficiency zones for reduced fuel consumption. A parametric wheels or from the battery pack to the wheels as the case design methodology also was established. Simulation results may be. However, owing to greater interactions among the on four different driving cycles show that, on average, fuel involved subsystems (engine, motor, battery), the control savings improve by about 17 percent and, acceleration times complexity can rise rapidly with the number of agents or by 25 percent, with top speed and gradability being the same their behavioral sophistication. This increasing complexity compared to a conventional vehicle. has motivated continuing research on computational learning methods toward making autonomous intelligent systems that can learn how to improve their performance over time while Real Time interacting with the driver. These propulsion systems need Lately, there has been a growing research effort toward to be able to sense their environment and also integrate infor- developing real-time power management algorithms of mation from the environment into all decision making. the power split between the thermal and electrical paths of This challenge can be effectively addressed by applying principles of cognitive optimization techniques. The problem can be formulated as sequential decision making under un- certainty in which an intelligent system (e.g., hybrid-electric TABLE 4-12 Fuel Economy and Exhaust Emissions vehicle, power train system) learns how to select control of Hybrid Electric Transit Bus with Various Control actions so as to reduce fuel consumption over time for any Strategies, Taipei City Bus Cycle different driving cycle (Malikopoulos, 2009). Fuel Economy CO HC NOx PM Dynamic Programming Hybrid Electric Bus (km/L) (g/km) (g/km) (g/km) (g/km) Power management control algorithms employing dy- Speed control 1.82 1.14 0.31 28.52 0.29 Torque control 2.02 0.71 0.27 20.50 0.26 namic programming (Scordia et al., 2005; Lin et al., 2003; Power control 2.15 0.70 0.24 18.98 0.23 Perez et al., 2006; Ogawa et al., 2008; Karbowski et al., Diesel bus 2009) rely on computing off-line the optimal control policy Conventional control 1.29 4.12 0.59 55.56 0.61 with respect to the available power train variables—that is, SOURCE: Wu et al. (2008). the power split between the thermal (engine) and electrical

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 POWER TRAIN TECHNOLOGIES FOR REDUCING LOAD-SPECIFIC FUEL CONSUMPTION ics that affects the efficiency of the derived policy even for the given driving cycle derived. Despite the aforementioned shortcomings, power management results based on deter- ministic dynamic programming methods are useful to serve as the benchmark of possible performance. Table 4-14 compares the predicted fuel consumption of a conventional vehicle against a hybrid power management optimized using dynamic programming and rule-based algorithms. Crosscutting Issues and Future Outlook for Hybrids During the committee’s discussions with manufacturers and suppliers, a number of overarching themes emerged with FIGURE 4-25 Dynamic programming process and rule extraction MDV Fig 4-25.eps respect to hybrid technology discussed. from the result. SOURCE: Lin et al. (2003). bitmap Brake O&M Benefits paths (motor, generator, and battery), gear selection, etc., In addition to saving fuel, hybridization significantly re- over a given driving cycle or a family of given driving duces brake costs. Suppliers and OEMs expect that hybrids cycles. Even though dynamic programming is not directly will more than double brake life. For some applications these implementable, its results can be used to form an efficient savings can outweigh the fuel savings. rule-based control algorithm (as shown in Figure 4-25) or to develop vehicle-level control using neural networks (Delprat et al., 2001). The derived optimum policy is then approxi- mated with simple rules and shift logic functions in order to be implemented in real time. Figure 4-26 shows an example of a rule-based algorithm used in real time on an engine map to determine which of TABLE 4-14 Predicted Fuel Consumption Comparison: the power devices are being used depending on visitation Conventional (nonhybrid), Dynamic Programming (DP), points during the duty cycle. The main shortcoming of this and Rule-Based (RB) approach is that it is efficient only for the driving cycles used DP RB Conventional in deriving the optimal policy. In addition, due to the high computational cost of dynamic programming, only simpli- Mpg 13.85 12.65 10.39 Fuel (gallon) 0.5259 0.5757 0.7005 fied models of HEVs can be used. As a result, the extracted optimum policy omits a significant number of HEV dynam- SOURCE: Lin et al. (2003). FIGURE 4-26 Implementing dynamic programming as a rule-based algorithm in SIMULINK. SOURCE: Lin et al. (2003). MDV Fig 4-26.eps bitmap

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 TECHNOLOGIES AND APPROACHES TO REDUCING THE FUEL CONSUMPTION OF MEDIUM- AND HEAVY-DUTY VEHICLES System Integration TABLE 4-15 Hybrid Fuel Consumption Reduction Potential (percentage) Compared to a Baseline Vehicle As hybrid systems become more fully integrated into Without a Hybrid Power Train, by Range of Years and overall vehicle architecture, there are a number of opportuni- Application ties to further optimize the system: 2013-2015 2015-2020 • Depending on the application, hybridizing a vehicle Tractor trailer NA 10 can enable engine downsizing. Currently, this is not Class 6 box truck 22 30 widely done. Part of the reason is that industry is still Class 6 bucket truck 35 40 Refuse truck 20 25 figuring out which applications can use a downsized Urban bus 30 35 engine without sacrificing mission performance. An- Motor coach NA NA other important factor is that smaller engines that meet Class 2b pickup and van NA 18 medium-/heavy-duty warranty requirements may be NOTE: NA, not applicable. unavailable. One supplier suggested that they would SOURCE: TIAX (2009). be able to offer optimized, right-sized power trains at volumes of 5,000 to 10,000 vehicles per year. • There is opportunity for substantial integration be- TABLE 4-16 Estimated Fuel Consumption Reduction tween hybrid systems and exhaust aftertreatment Potential (percentage) for Hybrid Power Trains systems. For example, a system designer could vary e ngine load to manage exhaust gas temperature, 5-10a Tractor trailer thereby offering emissions benefits. However, current Class 6 box truck 20-35 Class 6 bucket truck 30-45 standards measure engine-out emissions. To get credit Refuse truck 20-35 for any benefit, EPA would need to switch to measur- Urban bus 12-50 ing emissions on a vehicle basis or develop a method Motor coach 5-40 to credit the lower vehicle emissions. Class 2b pickup and van 18-30 • Improved system power density, component efficiency, aIncludes some reduction in hotel load, some idle reduction, and some and design integration are expected to offer additional electrification of accessories. fuel-saving opportunities. • Electrification is viewed as an enabler for more ef- ficient waste-heat recovery systems, such as electric turbo-compounding or electric bottoming cycles. In a FINDINGS AND RECOMMENDATIONS hybrid vehicle, electric waste-heat systems can offer an additional 1 to 2 percent efficiency benefit at neutral Diesel Engine Technologies cost compared to an equivalent mechanical waste-heat Finding 4-1. Many individual technologies for reducing system. load-specific fuel consumption of diesel engines were identi- • By narrowing the design window, hybrid systems fied. Some technologies are being used in 2010 by nearly all enable the engine to be optimized to deliver peak manufacturers (common rail fuel injection and selective cata- fuel economy within a narrow operating band. This lytic reduction, SCR), and some are being used by a limited opportunity applies primarily to dual-mode or series number of manufacturers (turbocompounding and multiple systems. turbochargers). One manufacturer, Cummins, has shown a roadmap for 49.1 percent thermal efficiency by 2016 and Taken as a whole, improved systems integration can offer 52.9 percent by 2019, which are 14.5 and 20.6 percent reduc- an additional 5 to 10 percent improvement in fuel efficiency tions in fuel consumption, respectively, from a 2008 baseline, in future systems in the years 2015-2020. In tandem, higher compared to current diesel fuel consumption. Significant sales volume can reduce costs by a factor of 2. technical challenges remain to be overcome before many of the fuel-saving technologies described in this section can be Hybrid Power Train Summary successfully implemented in production. In its report for the committee, TIAX (2009) summarized the hybrid fuel consumption potential reductions by range of Gasoline Engine Technologies years and by application, as shown in Table 4-15. Finding 4-2. Technologies exist today, or are under develop- Based on work discussed in this chapter as well as on ment, that offer the potential to reduce the fuel consumption the TIAX summary for hybrid power trains, the committee of gasoline-powered vehicles operating in the medium-duty estimated potential fuel consumption reduction as shown in vehicle sector. The most beneficial technologies and the Table 4-16.

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 POWER TRAIN TECHNOLOGIES FOR REDUCING LOAD-SPECIFIC FUEL CONSUMPTION Finding 4-6. Manual transmissions have the least mechani- magnitude of fuel savings will be dependent on the configu- ration of the engine and the duty cycle of its application. cal losses. An automated manual transmission can reduce Under optimal matching of technology and duty cycle, fuel fuel consumption by reducing driver variability (4 to 8 per- consumption reductions of up to 20 percent appear to be cent benefit). The fully automatic transmission can improve possible compared to 2008 gasoline engines in the 2015 to productivity by reducing the shift time (power shift) and by 2020 time frame. The economic merit of integrating different avoiding engine transient response delays and can reduce fuel-saving technologies will be an important consideration fuel consumption (up to 5 percent) by reducing driver vari- for operators and owners in choosing whether to implement ability, but the AT has higher parasitic losses. these technologies. Recommendation 4-3. The industry should continue its Recommendation 4-1. Development of fuel-saving engine practice of training dealers and provide training materials technologies and their effective integration into the engine/ for truck specifications affecting fuel consumption, such as power train is critical for reducing fuel use by medium- and transmission ratios, axle ratios, and tire size. heavy-duty vehicles and helping the nation to meet national goals related to energy security and the environment. The Hybrid Power Trains federal government should continue to support such pro- Finding 4-7. F uel consumption reductions on hybrid grams in industries, national labs, private consulting com- panies, and universities. vehicles 5 to 50 percent have been reported by enabling optimum engine operation, downsizing in certain cases, braking energy recovery, accessory electrification, and en- Diesel Engines Versus Gasoline Engines gine shutdown at idle. Finding 4-3. Diesel engines can provide fuel consumption Finding 4-8. A wide range of hybrid electric and hydraulic advantages, compared to gasoline engines, of 6 to 24 percent depending on application, duty cycle, and baseline gasoline architectures have been demonstrated. The selection of a engines. particular system architecture depends mainly on applica- tion, duty cycle, and cost-benefit trade-offs. Finding 4-4. Diesel engines are increasing in cost primarily Finding 4-9. The realized fuel consumption benefits of a due to emissions aftertreatment equipment (DPF and SCR), which can cost over $17,000. Because of this cost increase particular hybrid technology and architecture implementa- (and diesel fuel prices), dieselization of Class 6 trucks in tion are strongly dependent on application and duty cycle. the new sales fleet went from 75.8 percent in 2004 to 58.0 Optimization of component sizing and power management percent in 2008. The effect of 2010 emission regulations has are keys to maximizing the potential for fuel consump- yet to be felt, but it is expected to accelerate the trend toward tion reductions while satisfying performance and emission gasoline engines in medium-duty trucks. constraints. Recommendation 4-2. B ecause the potential for fuel Finding 4-10. Computer simulation of medium- and heavy- consumption reduction through dieselization of Class 2b to duty vehicles is an effective way to predict fuel consumption 7 vehicles is high, the U.S. Department of Transportation/ reductions considering the additional variables in a hybrid National Highway Traffic Safety Administration (NHTSA) vehicle system, but such systems are not standardized, lead- should conduct a study of Class 2b to 7 vehicles regarding ing to a wide variety of results and unpredictability. gasoline versus diesel engines considering the incremental Recommendation 4-4. NHSTA should support the forma- fuel consumption reduction of diesels, the price of diesel versus gasoline engines in 2010-2011, especially consider- tion of an expert working group charged with evaluating ing the high cost of diesel emission control systems, and the available computer simulation tools for predicting fuel diesel advantage in durability, with a focus on the costs and consumption reduction in medium- and heavy-duty vehicles benefits of the dieselization of this fleet of vehicles. and developing standards for further use and integration of these simulation tools. Transmission and Driveline Technologies BIBLIOGRAPHY Finding 4-5. The transmission ratio and axle ratio affect fuel Alamgir, M., and A.M. Sastry. 2008. Efficient batteries for transportation consumption by determining the engine speed versus road applications. Paper No. 08CNVG-0036. Presented at SAE International, speed of the vehicle. 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