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Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles
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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Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles
FIGURE 4-1 Energy audit for a typical diesel engine. SOURCE: Adapted from Vinod Duggal, Cummins, Inc., “Industrial Perspectives of the 21st Century Truck Partnership,” presentation to the 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) estimates the fuel savings of an improved-efficiency single-stage turbocharger at 1 percent. Another source projects that a high-pressure-ratio axial compressor will reduce fuel consumption by 1.1 to 3.6 percent.2
Almost all heavy-duty diesel engines sold in North America today use high-pressure loop EGR for control of engine-out NOx levels. To get EGR to flow from the exhaust manifold to the intake manifold, the pressure in the exhaust manifold must be higher than the pressure in the intake manifold. When the exhaust manifold pressure is higher than the intake manifold pressure, this is called having a negative ∆p, where ∆p refers to the difference in pressure between the intake and exhaust manifolds. High-efficiency turbochargers naturally produce a positive ∆p over much of their operating range, so turbocharger efficiency must be intentionally compromised in order to facilitate EGR flow. If it is possible to produce adequate EGR flow without reducing turbo efficiency, the overall engine efficiency will increase.
Dual-Stage Turbocharging with Intercooling
Modern engines use high-pressure ratios, which limit the efficiency of turbochargers. Using two turbochargers in series with intercooling would allow higher turbocharger efficiency, but this adds cost and packaging complexity and requires an EGR pump or other device such as a turbocompound system to facilitate EGR flow. Air-to-water intercooling is used after the first-stage compressor, in some applications, and air-to-air aftercooling is used after the second-stage compressor.
Conventional two-stage turbocharging employs two turbochargers working in series at all times. True sequential turbocharging switches turbochargers in and out of use as required, but they are normally connected in parallel. A modulated two-stage system brings some of the benefits of each of these two approaches. At low engine speeds it works as a two-stage system, delivering high-boost pressure despite the low engine speed. At high engine speeds it bypasses the small high-pressure turbocharger, allowing the bigger, low pressure turbocharger to work on its own and produce the higher flows at high engine speeds. Modulated two-stage systems offer the benefits of both high-boost pressure and wide-flow range, mainly due to the fact that two different-sized compressors are used. Using two compressors replicates the effect of a variable compressor without the need for a complex housing. The modulated two-stage system can have a high-pressure turbocharger far smaller than that of a conventional two-stage system, improving transient performance by reducing the turbo lag that affects both drivability and emissions.3
Dual-stage turbocharging is used in production by Navistar, Daimler Trucks, and Caterpillar in the United States and by MAN and Mercedes in Europe. Ford has announced that the 2011 diesel engine used in its Class 2b to 7 trucks will use a twin-compressor turbocharger (back-to-back compressors on the same shaft). Another source estimates a 2 to 5 percent reduction in fuel consumption.4 These benefits are only available if a way to provide the required EGR flow is available.
2
Personal communication between Steve Edmonds and David F. Merrion, committee member, September 2008.
3
See www.cummins.com/turbos.
4
Private communication from S.M. Shahed to David F. Merrion, October 1, 2009.
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Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles
Mechanical Turbocompound
The base turbocharged engine remains unchanged and a power turbine is added to the exhaust stream to extract additional energy from the exhaust. The power turbine is connected to the crankshaft to supply additional power (NESCCAF/ICCT, 2009, p. 81). Typically, the attachment includes a fluid coupling (to allow for speed variation and to protect the power turbine from engine torsional vibration) and a gear set to match power turbine speed to crankshaft speed. Published information on the fuel consumption reduction from mechanical turbocompounding varies, as evidenced by the following: 3 percent, according to the Detroit Diesel Corporation,5 which has a turbocompound engine in production; 2.5 to 3 percent (NESCCAF/ICCT, 2009, p. 54); 3 percent (K.G. Duleep, Energy and Environmental Analysis)6 and 4 to 5 percent (Kruiswyk, 2008, pp. 212-214); TIAX (2009, pp. 4-17) used 2.5 to 3 percent. Some of these differences may depend on the operating condition or duty cycle that was considered by the different researchers. The performance of a turbocompound system tends to be highest at full load and much less or even zero at light load.
Electric Turbocompound
This approach is similar in concept to mechanical turbocompound, except that the power turbine drives an electrical generator (NESCCAF/ICCT, 2009, p. 29). The electricity produced can be used to power an electrical motor supplementing the engine output, to power electrified accessories, or to charge a hybrid system battery. Electric turbocompound is a technology that fits particularly well with a hybrid electric power train for long-haul applications where regenerative braking opportunities are limited. The benefits of electric turbocompound and an electric hybrid power train can be additive. Energy and Environmenal Analyis7 has said that “electric turbo-compound is more efficient and possible as part of hybrid packages.” Fuel consumption reduction benefits as large as 10 percent are claimed. The NESCCAF/ICCT study (p. 54) modeled an electric turbocompound system and estimated benefits at 4.2 percent, including electrification of accessories. Caterpillar, Inc., as part of Department of Energy (DOE) funded work, modeled a system that showed 3 to 5 percent improvement, while John Deere investigated a system (off-highway) that offered 10 percent improvement (Vuk, 2006; TIAX, 2009, p. A-10). None of these systems have been demonstrated commercially. TIAX (2009, pp. 3-5) used a range of 4 to 5 percent for its estimates, which included the benefits of electric accessories. Achieving the full benefit of electric turbocompound requires the electrification of vehicle accessories, the addition of an electric motor to apply turbocompound energy to supplement engine output, and an electric storage system (battery) to store any energy from the power turbine that is not immediately required. Making all of these changes to the vehicle will pose significant development and cost challenges.
Variable Valve Actuation
Variable valve actuation (VVA), also called variable valve timing or discrete variable valve lift, allows the valve actuation to be adjusted independently from the crankshaft angle. There are many implementations of VVA. Some are hydromechanical, such as the system used on some BMW passenger car engines. Other designs use electromagnets or high-pressure hydraulic systems. Some versions offer “full authority,” or unlimited, control of valve timing and lift, while other implementations offer limited control, such as variable duration only, variable lift only, or even more limited control, such as with the system used on some Caterpillar engines to permit a Miller cycle to be used under some operating conditions. VVA technology can also be used for cylinder deactivation. One of the primary drivers for introducing VVA in diesel engines is to facilitate the use of nonconventional combustion modes. According to several sources, variable valve timing can improve fuel consumption by about 1 percent when standard diesel combustion is used (NESCCAF/ICCT, 2009, p. 55).
Low-Temperature Exhaust Gas Recirculation (Also Called Advanced EGR Cooling)
Most medium- and heavy-duty vehicle diesel engines sold in the U.S. market today use cooled EGR, in which part of the exhaust gas is routed through a cooler (rejecting energy to the engine coolant) before being returned to the engine intake manifold. EGR is a technology employed to reduce peak combustion temperatures and thus NOx. Low-temperature EGR uses a larger or secondary EGR cooler to achieve lower intake charge temperatures, which tend to further reduce NOx formation. If the NOx requirement is unchanged, low-temperature EGR can allow changes such as more advanced injection timing that will increase engine efficiency slightly more than 1 percent (NESCCAF/ICCT, 2009, p. 62). Because low-temperature EGR reduces the engine’s exhaust temperature, it may not be compatible with exhaust energy recovery systems such as turbocompound or a bottoming cycle.
Electrification of Engine-Driven Accessories
Accessories that are traditionally gear or belt driven by a vehicle’s engine can be converted to electric power. Ex-
5
Detroit Diesel Corporation, DD15 Brochure, DDC-EMC-BRO-0003-0408, April 2008.
6
K.G. Duleep, Energy and Environmental Analysis, “Heavy Duty Trucks Fuel Economy Technology,” presentation to the committee, Washington, D.C., December 5, 2008, slide 17.
7
K.G. Duleep, Energy and Environmental Analysis, “Heavy Duty Trucks Fuel Economy Technology,” presentation to the committee, Washington, 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, the power-steering pump, cooling fans, and the vehicle’s air-conditioning system. In many cases this can result in a reduction in power demand, because electrically powered accessories (such as the air compressor or power steering) operate only when needed if they are electrically powered, but they impose a parasitic demand all the time if they are engine driven. In other cases, such as cooling fans or an engine’s water pump, electric power allows the accessory to run at speeds independent of engine speed, which can reduce power consumption. Electrification of accessories can individually improve fuel consumption, but as a package on a hybrid vehicle it is estimated that 3 to 5 percent fuel consumption reduction is possible.8 The TIAX (2009, pp. 3-5) study used 2 to 4 percent fuel consumption improvement for accessory electrification, with the understanding that electrification of accessories will have more effect in short-haul/urban applications and less benefit in line-haul applications.
Engine Friction Reduction
Reduced friction in bearings, valve trains, and the piston-to-liner interface will improve efficiency. Any friction reduction must be carefully developed to avoid issues with durability or performance capability. An example would be to develop heavy-duty diesel engines to run on 10W-30 oil instead of the current standard of 15W-40. The lower viscosity oil would reduce friction, at the expense of bearing capability. Fuel consumption improvement from one source9 was 2 percent, whereas another source10 claims 1 to 1.5 percent. The use of a thermatic oil cooler (thermostatically controlled oil cooler) in conjunction with lower viscosity lubricating oils could yield 1.5 percent improvement.11 The effect of friction reduction and oil temperature control will be greatest during cold starts and under light load operation, where friction accounts for a larger portion of total energy consumption.
Alternative Combustion Cycles
Alternatives to the standard diesel combustion cycle are available, such as low-temperature combustion (LTC), homogeneous charge compression ignition (HCCI), and premix charge compression ignition (PCCI). These combustion modes can be more efficient than standard diesel combustion under some conditions, particularly when very low NOx is a requirement. There are significant control requirements to make these alternative combustion modes work, and these modes cannot generally be used over the whole operating range of the engine, nor have they demonstrated inherent fuel consumption advantages (NRC, 2008, Finding 3-8, p. 42). The primary purpose of alternative combustion cycles is to lower engine-out emissions. This can lead to either lower overall emissions or lower cost for exhaust aftertreatment.
Effects of DPF and SCR on Engine Efficiency
The use of emissions control devices has an influence on engine efficiency. This is true whether the emissions are controlled on an in-cylinder basis or via the aftertreatment. In most cases, the effect of adding an emissions control device increases fuel consumption, either directly by reducing the efficiency of energy extraction from the combustion process or indirectly by requiring the use of additional fuel to maintain the performance of an aftertreatment system.
Improved SCR Conversion Efficiency
NOx is formed in a reaction that occurs naturally whenever nitrogen and oxygen are heated above a certain temperature. The higher the temperature, the more rapid the NOx-forming reaction occurs. In-cylinder technologies to control NOx formation in diesel engines are aimed at reducing the maximum temperature reached by the gases in the combustion chamber. The approaches used include retarded injection timing, multiple injection events and injection rate shaping, EGR, charge air cooling, and alternative combustion modes (such as HCCI, PCCI, LTC). Some of these approaches leads to a decrease in work output of the engine due to exhaust emissions control (NRC, 2008), except charge air cooling.
The DPF is used to eliminate PM on an aftertreatment basis. A DPF requires energy for regenerating the filter on a periodic basis. This energy most commonly comes from 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. A DOC or other device oxidizes the fuel in the exhaust stream, providing the heat required for DPF regeneration and increasing the fuel consumption of the vehicle. Another method to provide the heat required for DPF regeneration is to revise the air/fuel ratio of the engine to produce exhaust constituents and heat that are used to regenerate the DPF. This approach also increases the fuel consumption of the vehicle.
The SCR aftertreatment system for reducing NOx also requires a fluid, which is urea mixed with water (called Adblue in Europe and DEF [Diesel Exhaust Fluid] in the United States), to supply the reducing agent. The urea is made from natural gas. The energy use of this fluid and/or its cost must be accounted for in the calculation of energy consumption. The use of SCR can allow a higher engine-out NOx level, which in turn can be used to reduce fuel consumption, but
8
Anthony Greszler, Volvo Powertrain, “Reducing Emissions in Heavy Vehicles,” presentation to the committee. Washington, D.C., December 5, 2008, slide 23.
9
Anthony Greszler, Volvo Powertrain, “Reducing Emissions in Heavy Vehicles,” presentation to the committee. Washington, D.C., December 4, 2008, page 14.
10
Site visit to Daimler/Detroit Diesel, April 7, 2009.
11
Site visit to Daimler/Detroit Diesel, April 7, 2009.
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Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles
FIGURE 4-2 Historical trend of heavy-duty truck engine fuel consumption as a function of NOx requirement. SOURCE: Tony Greszler, Volvo, October 2009.
this improvement must be weighed against the urea consumption of the SCR aftertreatment system. The upcoming 2010 heavy-duty emissions standards reduce the allowable NOx level by a factor of 6 from 1.2 g/hp-hr to 0.2 g/hp-hr, which limits the ability of some manufacturers to use SCR to increase engine-out emissions.
There is a close relationship between emissions control requirements and fuel consumption. In particular, certain technologies that are used to control NOx emissions have the effect of increasing fuel consumption. See Figure 4-2 for an example of this trend. Figure 4-2 also compares truck engines with the most efficient large marine engines, which so far do not face any emissions constraints. The efficiency of large marine engines is due to several factors that cannot be reproduced in vehicle applications. Marine engines are very large and heavy, they run at very low speeds, they have a source of unlimited cooling capacity (seawater), and they face (for the time being) no emissions constraints. All four of these factors contribute to the high efficiency of marine engines. According to information provided by Volvo,12 the fuel consumption of truck diesel engines is about 10 percent higher than for marine engines due to the size, weight, and cooling factors that are limits faced in vehicle applications. The 2010 model truck engines will suffer an 11 percent fuel consumption penalty for NOx control (compared with an 8 g/hp-hr NOx engine, see Figure 4.2), which is less than the penalty of 2002 and 2007 engines. This penalty does not include the energy content of DEF (urea) for the SCR system.
Most 2010 engines will use SCR aftertreatment to reduce NOx emissions. Daimler’s Detroit Diesel estimates that, with the installation of SCR in 2010, fuel consumption will be reduced 3 to 4 percent by a combination of higher engine-out NOx, controls and fuel system improvements, reduced DPF regeneration frequency, and other efficiency gains, while Volvo estimates the potential improvement of its 2010 engines at 2 percent. As the SCR system conversion efficiency improves, it allows higher engine-out NOx emissions. Engine manufacturers can take advantage of this by making changes in fuel injection timing or by using less EGR, in order to reduce fuel consumption.
Thermal Insulation of Ports and Manifolds
Thermal insulation would reduce heat rejection to the engine coolant (from exhaust ports) or to the ambient air (from manifolds). The energy retained in the exhaust can
12
Tony Greszler, Volvo, private communication with Thomas Reinhart, 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 system or bottoming cycle. Caterpillar Inc. made components, such as air gap pistons and exhaust port liners as part of the 21st Century Truck Partnership, but reported results are not available (NRC, 2008, p. 30). The anticipated benefit is small.
Improved Work Extraction from Combustion Process
The compression ratio, expansion ratio, combustion 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 parameters that can be modified in an effort to reduce fuel consumption. Improved combustion chamber design allows for improved air management and mixing. Improved materials and structural design enable higher cylinder pressures. These enhancements allow more precise control of the timing and rate of heat release (combustion) as well as higher combustion temperatures, both of which can improve thermal efficiency. Unfortunately, higher combustion temperatures also lead to higher NOx. Combustion chamber design enhancements may require more advanced materials and a more complex machining process. More complex and expensive fuel systems allow greater control of injection pressure, timing, and rate shaping. In addition, because higher cylinder pressures must not result in higher NOx, measures must be taken to allow the improved fuel consumption without creating an increase in emissions. These measures may include improved NOx conversion efficiency by the aftertreatment, advanced fuel injection techniques (which enable more detailed control of combustion), and improved engine controls. The efficiency benefit of these improvements is estimated at 1 to 3 percent.13
Finely controlled, high-pressure fuel injection is a key enabler for more fuel efficient combustion and a cleaner, more consistent fuel burn. Current state-of-the-art systems planned for deployment in 2010 engines include very high pressure (2,000 to 2,400 bar) common rail injection systems with advanced nozzle designs that are capable of finely shaped and controlled spray, along with multiple injection events per cycle.
Potential future improvements will continue to improve control, allow more accurate timing and metering of injection with combustion events, and further increase fuel injection pressure. Improved material properties and controls could enable pressures of up to 3,000 bar in the 2015 time frame and perhaps 4,000 bar by 2020. Future systems will also utilize increasingly sophisticated injection techniques such as variable-spray nozzles, piezo-electric nozzles, or supercritical fuel injection (fuel changing instantaneously from liquid state to supercritical gaseous state at injection based on site visits). These advances may be possible in the 2013 to 2015 time frame (TIAX, 2009). Fuel injection systems were estimated on site visits to offer between 1 and 4 percent improvement in fuel consumption; Vyas et al. (2002) estimates fuel injection systems have the potential 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 2002. Real-time combustion control with start of combustion sensors can also yield a fuel consumption reduction of 1 percent to 4 percent.14
Engine Electronic Controller Calibration Management
Advanced engine controls will be enabled in part by the onboard diagnostic systems that are mandated for medium and heavy trucks beginning in 2010 on one family and across the board in 2013. Increasingly sophisticated engine controls, particularly a transition to closed-loop control approaches, will enable engine efficiency improvements. Closed-loop controls will feed information about the engine’s operating regime and emissions back to the system controls. This improved feedback will allow manufacturers to optimize emissions and fuel consumption within the constraints of emissions requirements across a variety of operating conditions.
Better use of calibration tools to improve control of EGR, injection rate shapes, multiple injection events, and increased injection pressure can yield 1 to 4 percent fuel consumption reduction. These benefits are redundant with those described above for improved work extraction from the combustion process. Another feature already in use on some long-haul trucks is adding 200 lb-ft of torque in the top two transmission gears, which manufacturers claim can give 2 percent reduction in fuel consumption by reducing the need for downshifting on modest grades. With the next generation electronic controller, using model-based controls, it is predicted that another 1 to 4 percent fuel consumption reduction will be achieved.15 Note that the reductions listed in here may be repeats of reductions from previous sections, and there may be some redundancy in the percentages quoted, but the concepts and percentages presented here came from committee site visits where engineers talked about these reductions. The overall approach of more sophisticated control of the combustion process tends to include several building blocks, such as upgraded fuel system capabilities, sophisticated control algorithms, additional sensor inputs for feedback control, and technologies such as model-based controls. The benefits of this approach are often claimed by each of the individual building blocks, leading to redundant claims on the same fuel consumption benefit.
13
Presentations by and discussions with Cummins, Detroit Diesel, and Volvo.
14
Committee site visit, Daimler/Detroit Diesel, April 8, 2009.
15
Committee site visit, Daimler/Detroit Diesel, April 8, 2009.
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Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles
Bottoming Cycle
A bottoming cycle is basically a secondary engine that uses exhaust energy or other heat sources from the primary engine to develop additional power without using additional fuel. The energy sources used by the bottoming cycle are sources that normally go to waste in a conventional engine. A typical bottoming cycle includes the following components: a feed pump to drive the working fluid from the condenser to the evaporator (or boiler); the evaporator, which transfers waste heat energy from the primary engine to the working fluid; an expander, which takes energy from the working fluid to make mechanical power; and a condenser that rejects unused heat energy from the bottoming cycle working fluid before starting a new cycle. The power generated by the expander can be used to make electricity, which in turn can power an electric motor supplementing the engine output, power electrified accessories, or charge a hybrid system battery. Sources of energy to power a bottoming cycle can include the EGR stream, exhaust stream, charge air stream, and engine coolant circuit (NESCCAF/ICCT, 2009, pp. 85-88). Cummins, Inc. has shown a projected increase of thermal efficiency from 49.1 to 52.9 percent (7.2 percent decrease in fuel consumption) using an organic Rankine cycle. Cummins also lists turbocompounding and a Brayton cycle as alternative methods of extracting work from unused energy in the exhaust stream. Cummins reports recovering 2.5 thermal efficiency points from the exhaust and 1.3 thermal efficiency points from the coolant and EGR stream.16 The NESCCAF/ICCT report (2009, pp. 55-56) showed the effect of a steam bottoming cycle to reduce fuel consumption by up to 10 percent.
Other Technologies
Other technologies for reducing the fuel consumption of diesel engines are discussed in the press almost every day (e.g., Automotive News, Transport Topics, Diesel Fuel News, DieselNet.com). Some are emerging technologies and may not become production feasible, including new diesel engines of two-stroke-cycle design, split-cycle design, free-piston design, rotary design and camless engines with digital valve control such as the Sturman Industries concept. The list of potential technologies also includes oxygen injection into the intake air, hydrogen injection into the intake air, air injection from the air compressor to overcome turbo lag, or the use of fuel-borne catalysts such as platinum and cerium.
Diesel Engine Summary
In summary, to add up all these individual potential reductions to arrive at an overall potential fuel consumption reduction would not be correct because there would be double counting of some effects. The best recent attempts at packaging fuel-saving technologies for engines were in the DOE programs with a goal of demonstrating 50 percent thermal efficiency while meeting 2010 emissions. The National Research Council (2008) review of that program showed a baseline thermal efficiency of 42 percent with a goal of 50 percent, or a 19 percent improvement in thermal efficiency and a 16 percent fuel consumption reduction. Three engine manufacturers—Caterpillar, Cummins, and Detroit Diesel—were funded at a level exceeding $116 million over five years with the following result, according to the report: “These results show that none of the industry partners achieved the goal of measuring 50 percent thermal efficiency at 2010 emissions from a complete engine system.”
Cummins has supplied the committee with the following research roadmaps for achieving 49.1 percent thermal efficiency and 52.9 percent thermal efficiency. Figure 4-3 is a research roadmap for 49.1 percent thermal efficiency by 2016, which is an improvement of 17 percent from the 42 percent baseline (14.5 percent reduction in fuel consumption). Figure 4-4 is a research roadmap for 52.9 percent thermal efficiency by 2019, which is an improvement of 26 percent from the baseline 42 percent (20.6 percent reduction in fuel consumption). These roadmaps can be compared to the baseline shown in Figure 4-1. Note that these are plans and goals, not actual development results. Actual results that will be achieved in development may vary from the planned benefits.
In its report for the committee, TIAX (2009, Tables 5-8 and Table 5-9) summarized the diesel engine fuel consumption potential reductions by range of years and by application as shown in Table 4.1.
GASOLINE ENGINE TECHNOLOGIES
Gasoline engines operate with a premixed charge of fuel and air and use spark ignition to start the combustion process. They are used in many Class 2b applications as well as Class 3 to 6 applications. Within the medium-duty truck sector, all gasoline engines operate on the four-stroke-cycle principle and are of an in-line or “vee” configuration. Displacements of these engines typically range from 6 to 8 liters. These engines normally burn gasoline, but with slight changes they can burn natural gas (compressed [CNG] or liquefied [LNG]), propane, hydrogen, ethanol, methanol, and so forth.
The fundamental operating principle for gasoline engines used today relies on creating a well-mixed charge of gasoline and air at the time the spark plug fires. After the combustion process is over, a catalyst in the exhaust system is used to perform the final emissions cleanup. Emissions of NOx, carbon monoxide, and unburned hydrocarbons are the principal species being treated by the catalyst in the exhaust. The three-way catalyst treats all three of these emissions simultaneously; 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
FIGURE 4-3 Research roadmap for 49.1 percent thermal efficiency by 2016. SOURCE: Provided under license by Cummins Inc. Copyright 2009 Cummins Inc. All rights reserved.
properly only if the air/fuel ratio is carefully controlled to be chemically correct, or stoichiometric.17
Heavy-duty gasoline engines, and most heavy-duty engines using spark-ignited alternative fuels such as natural gas, use a relatively simple emissions control strategy. The engine operates at a stoichiometric air/fuel ratio across most of the operating range, with relatively high engine-out emissions levels. A three-way catalyst is used both to oxidize hydrocarbon and carbon monoxide emissions and to reduce NOx. The three-way catalyst can function properly only if the air/fuel ratio is carefully controlled in order to meet current and future emissions requirements. Additional considerations for gasoline engine emissions control include achieving rapid catalyst light-off on startup and controlling evaporative emissions, but the basic emissions control technology for spark-ignited engines is the relatively simple and inexpensive three-way catalyst.
There is a fuel consumption penalty that comes with the three-way catalyst used on gasoline engines. Because the air/fuel ratio must be maintained at stoichiometric all the way down to idle, the pumping losses from throttling are large. Lean operation could provide significant fuel savings but would not allow the NOx reduction function of the three-way catalyst to work. Many technologies that could be applied to gasoline engines to reduce fuel consumption are not used, primarily because of the need to maintain low NOx emissions. For example, lean gasoline direct injection (GDI) has the potential to provide double-digit percentage reductions in fuel consumption, but it is not used, because it would result in higher NOx emissions. The NOx emissions of a lean GDI engine could be much lower than those of an unregulated engine, but engine makers have not been able to make lean GDI reach the very stringent U.S. NOx standards applied to new cars and trucks today. GDI has been used in Europe, where NOx is less stringently regulated.
One consequence of requiring a stoichiometric mixture of air and fuel is that the intake airflow needs to be throttled for lighter load operation. Lighter loads necessitate a lower fuel flow rate into the engine, and since the air/fuel mixture is to be maintained in stoichiometric proportions, the airflow rate needs to be reduced in proportion to the fuel flow rate. The process of throttling the intake airflow results in significant pumping losses that are not present in diesel engines that operate using traditional diesel combustion. This pumping loss is one of the main reasons spark ignition engines are less efficient than diesel engines. The magnitude of the pumping loss depends on the operating duty cycle of the engine. If 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 work will depend on the engine size relative to the vehicle. A smaller engine size in a given vehicle application will spend a higher portion of its operation at a higher load, relative to a
17
Stoichiometric refers to the chemically balanced reaction of air and fuel. Under stoichiometric conditions there is a certain amount of oxidant (air) such that all of the carbon in the fuel could react to carbon dioxide and all the hydrogen in the fuel could react to water, with no oxidant or fuel left in the products.
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FIGURE 4-4 Research roadmap for 52.9 percent thermal efficiency by 2019. SOURCE: Provided under license by Cummins Inc. Copyright 2009 Cummins Inc. All rights reserved.
TABLE 4-1 Diesel Engine Fuel Consumption (percentage) by Years and Applications
Application
2013-2015
2015-2020
Tractor trailer
10.5
20
Class 6 box truck
9
14
Class 6 bucket truck
7.2
11.2
Refuse truck
10.5
14
Urban bus
9
14
Motor coach
10.5
20
Class 2b pickup and van
14
23
SOURCE: TIAX (2009).
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 2 to 5 percent of the fuel energy (Patton et al., 2002).
Compared to diesel engines, spark ignition engines are generally simpler and less expensive, they have more effective and lower cost exhaust emissions aftertreatment systems, and they have higher fuel consumption.
The current emphasis in the development of spark ignition engines is on reducing fuel consumption. Figure 4-5 gives a qualitative partitioning of the fuel energy for a typical gasoline-fueled vehicle. This is analogous to Figure 4-1, which gives an energy partitioning for diesel-powered vehicles. Figure 4-5 is illustrative in describing the technologies being considered to reduce gasoline engine fuel consumption.
The proportion of the fuel energy that gets converted into indicated work is a direct measure of the engine’s fuel conversion efficiency. Factors that affect an engine’s fuel conversion efficiency include irreversibilities18 in the combustion process, the amount of energy leaving the engine 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.
18
Irreversibility is a thermodynamic concept. It is used to describe and quantify the degree of imperfection in any real process. In the context used here it describes the degradation of energy during the combustion process 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 completely into work. Inherent in the chemical reaction of the actual combustion process are irreversibilities that render the resultant thermal energy of the combustion products not completely available to be converted into work, even though the quantity of energy is conserved.
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FIGURE 4-5 Partitioning of the fuel energy in a gasoline-fueled engine (proportions vary with vehicle design, type of engine, and operating conditions). SOURCE: NRC (1992).
Some gets used to overcome friction, some is used to pump air and fuel into the engine, and the exhaust gases out of the engine, and some is used to power accessories. The work that makes it to the drive wheels is used to overcome vehicle inertia, aerodynamic drag, and rolling resistance. The relative ranking of these energy uses is highly dependent on the vehicle and the application to which it is being applied.
As indicated in the discussion of the pumping work above, the magnitude of these energy partitions is highly dependent on the engine size, its application in the vehicle, and the duty cycle under which the vehicle is operating. The best way to quantify the partitioning shown in Figure 4-5 would be to take the specific data for the application of interest or through the application of a verified system simulation program.
Opportunities to reduce the fuel consumption of gasoline engines include improving engine efficiency (trying to reduce the proportion of the fuel energy leaving as heat transfer, exhaust energy, and pumping work), reducing the energy lost to friction, and reducing the power required for running accessories. Brief descriptions of different technologies for reducing fuel consumption are given below.
Variable Valve Actuation and Cylinder Deactivation
There are many approaches to VVA. These include cam phasers, variable lift mechanisms, fully flexible valve trains, and cylinder deactivation. The primary loss that the VVA systems are trying to reduce is the pumping, or throttling, loss. A variable compression ratio allows the engine to operate at different compression ratios for different loads in order to maximize the engine efficiency over the widest load range possible. The combination of VVA and variable compression ratio keeps the engine operation closer to its maximum efficiency point, with minimal pumping work, over larger portions of the duty cycle.
Cylinder deactivation is an approach that minimizes pumping losses by varying the total working displacement of the engine. The “smaller” engine operates closer to wide-open throttle at lower loads, which reduces the pumping work.
Cam phasers allow the valve timing to be changed with engine operating conditions. For example, the timing can be shifted with engine speed to optimize the engine breathing with engine speed. VVT can also be used in place of the intake throttle. Either opening the intake valve late or closing it late can regulate the amount of air/fuel mixture captured in the cylinder. This can be done with lower throttling losses than would occur with the conventional intake throttle.
Different engine designs will lend themselves more easily to different valve control technologies. For example, overhead valve systems with the cam in the block versus a single overhead cam versus a double overhead cam design will have to invoke different VVA approaches, which may favor different valve manipulation strategies for the different engine configurations. So decisions as to which valve technology to invoke would be based on the engine configuration, the application’s duty cycle, and the incremental cost of implementation. Estimates for the fuel consumption reduction achievable through variable valve lift and/timing range from 1 to 3.5 percent (TIAX, 2009, p. 4-33).
Gasoline Direct Injection Engines
GDI engines refer to an embodiment in which the fuel injector is mounted so that the fuel is directly injected into the cylinder, as opposed to the more common port fuel injection,
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where the fuel injector is mounted in the intake port. There are two philosophies of engine operation that fall under the classification of GDI engines.
In one case the engine is still operated with a stoichiometric mixture of air and fuel. This approach enables a three-way catalyst to be used in the exhaust, so emissions standards can be met. However, by mandating operation with a stoichiometric mixture, the engine still needs to be throttled. For a direct-injected stoichiometric engine the efficiency improvements come from less fuel being used during transient engine responses and from internal cooling of the cylinder charge from the fuel vaporization. This leads to a higher knock margin, which allows a higher compression ratio to be used. These engines are also more tolerant of EGR, so higher compression ratios can be used without an NOx penalty. There will be an attendant efficiency improvement with the higher compression ratio. The reduction in fuel consumption achieved via stiochiometric direct injection will be dependent on the extent to which this technology is combined with other technology packages. Based on estimates from a light-duty study (NHTSA, 2009), referenced by TIAX, a fuel consumption reduction with stoichiometric direct injection engines relative to a port-injected engine with VVT described above would range from 2 to 3 percent (TIAX, 2009, p. 4-33).
The other approach to GDI is to attempt to replicate the breathing characteristics of the diesel engine by minimizing the throttling of the intake air and controlling the load by varying the air/fuel ratio. Reduction in fuel consumption with this approach to direct injection comes from reduced pumping losses and higher efficiency from the lean-burning mixture, as well as the potential to increase the compression ratio as with the stoichiometric direct injection. The drawback to this approach is that the three-way catalyst is no longer effective, so the engine has the same aftertreatment challenges and expense as the diesel engine. The application of the lean burn direct injection technology will almost certainly be coupled with using turbocharging (described below) as well, so one must consider the combination of technology packages. Referencing the same NHTSA report referred to above, TIAX reported a potential reduction in fuel consumption by applying turbocharged lean burn GDI technology relative to the VVA nonturbocharged stoichiometric engine of 10 to 14 percent (TIAX, 2009, p. 4-33).
Different Combustion Modes
VVA mechanisms and fuel injection systems open up the possibility of incorporating advanced combustion regimes into the engine operating map. For example LTC, a general classification for auto-igniting combustion modes such as HCCI, partially PCCI, and compression-aided ignition, offers potential for lean low-emissions combustion in which throttling losses are minimized and catalytic converters are not needed (Zhao et al., 2003). Incorporating these alternative modes of combustion into the engine’s operating map will require much higher levels of sensing and control than are currently in use in today’s engines. For example, it is likely that real-time cylinder pressure sensing would be necessary to activate and control transitions between conventional combustion and LTC-type combustion during engine operation. If these combustion regimes can be incorporated into the operating map of gasoline engines, fuel consumption could be reduced via lean combustion at light loads, with minimal pumping losses, without the need for lean exhaust aftertreatment. Then during higher load operation, where throttling requirements are low, the engine could revert back to stoichiometric operation where the three-way catalyst is effective. It has been estimated that incorporation of LTC operation into the engine could reduce fuel consumption by 10 to 12 percent (TIAX, 2009). See Table 4-2.
Turbocharging and Downsizing
Turbocharging a gasoline engine is similar to turbocharging a diesel engine in that it is motivated by the desire to redirect energy that was leaving the engine in the exhaust gases back into the engine. The turbocharger converts exhaust energy into higher pressure and temperature intake gases using a turbine in the exhaust gas stream and a compressor in the intake air stream. Because of the differences in the fuels and the combustion processes between the two types of engines, there are different constraints that limit the application of turbocharging in a spark ignition engine relative to a compression ignition engine. Because the air and fuel are premixed in a spark ignition engine, as the pressure and temperature of the mixture at the start of compression is increased via turbocharging, the possibility of knocking combustion increases. Consequently it is common for the compression ratio to be decreased when the spark ignition engine is turbocharged. This tends to decrease the thermal efficiency of the engine. It is also likely that more exhaust gas will be recirculated when the engine is turbocharged. However, if successfully implemented, the turbocharged engine will have a higher power density than the nonturbocharged engine so it can be smaller and lighter for the same power output. This reduction of weight and potential reduction in frontal area of the vehicle has an attendant fuel consumption reduction benefit.
Turbocharging can be combined with other technologies, like in-cylinder direct injection and/or VVA to compound the benefits of the various technologies. The direct injection of fuel results in in-cylinder evaporative cooling, which helps counter the increased tendency toward knocking attributable to the turbocharging. Consequently the compression ratio may not need to be lowered to avoid knocking combustion to the same extent it would with turbocharging without direct injection. This coupled with the increased EGR tolerance of the direct injection and turbocharged engines helps keep NOx emissions from rising. In addition, because the power
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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).
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).
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).
cant improvement since start-stop is the only main feature in it to aid in engine efficiency. The full hybrid gains in the transient and urban cycles as the engine can be completely switched off in electric-only mode.
Effect of Drive Cycle on Hybrid Performance
The drive cycle or duty cycle plays an important role in determining the following: type of hybrid technology to be used, level of hybridization and sizing of components, and power management strategy.
Effect of Removing Breaks from Highway Cycle
Since the HHDDT cycle is short and does not represent the real highway cycle, a new cycle was formulated with 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 removing stops from the HHDDT cycle have been grouped in Figure 4-22. In every other bar the hybrids are compared with the conventional vehicle, hence the values are greater.
When the breaks are removed from the highway cycle, there is a significant drop in fuel savings in the hybrid con-
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FIGURE 4-21 HHDDT 65 cycle repeated five times with stops (left) and without stops (right). SOURCE: ANL (2009).
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 the left, 100 percent load on the right). SOURCE: ANL (2009).
figuration. However, the conventional configuration benefits the most when the stops are removed as the hybrids could have recovered part of the kinetic energy while braking. In the case of the full hybrid, the savings are more than halved (5.3 percent fuel saved on a cycle with stops, 2.4 percent fuel saved on a cycle without stops). In general, the hybrids still outperform the conventional vehicles in all cases as there are still some gains using the hybrid system even when the stops are removed.
Fuel Savings in Grades for Hybrid Configurations
Due to the lack of real-world drive cycles that include grades and to illustrate the potential benefits of hybridization in a “hilly” terrain, idealized sinusoidal road profiles were created. The elevation of such a road is a sinusoidal function of the horizontal distance, with a “hill” period varying between 1 and 3 km. Maximum grades also vary from 0 to 4 percent. All combinations of maximum grade and period were analyzed. Figure 4-23 shows the profile created.
For the mild-hybrid truck, the motor reaches its rated power when braking for grades 3 percent and higher when half-loaded and at or above 2.5 percent when fully loaded. The full hybrid hits its regenerative braking limit only when fully loaded at or above 3.5 percent grade. Thus, the full hybrid can capture more kinetic energy while braking, as expected.
The simulation results suggest that the mild hybrid has no advantage over the conventional vehicle when the grade is less than 2 percent, as there is not enough energy generated for accessories (see Figure 4-24).31 Charge balancing is hard to achieve, so the engine might be used to charge and hence may result in higher than expected fuel consumption. Furthermore, there is not much reduction in fuel consumption with grades greater than 3 percent, as the electric machine will reach its maximum potential.
The results also indicate that the available fuel savings with the full-HEV configuration can increase by as much as
31
Note that a simulation of vehicle performance on bus routes in San Francisco, where the grades can be demanding for conventional buses, found that the hybrid bus performed the best on fuel savings as well as on emissions of NOx and particulate matter (SAE, 2004).
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FIGURE 4-23 Representation of the grades considered. SOURCE: ANL (2009).
FIGURE 4-24 Fuel savings of hybrid trucks with respect to conventional trucks as a function of maximum grade for various hill periods; (left) 50 percent load and (right) 100 percent load. SOURCE: ANL (2009).
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14 percent for a 4 percent maximum grade with a hill period of 1 km. This suggests that the value of hybridization in tractor-trailer trucks may be significant in hilly terrains.
Hydraulic Hybrid Vehicles
Hydraulic hybrids have demonstrated fuel savings for medium- and heavy-duty applications. The energy savings can be attributed to optimization of engine operation and regenerative braking energy absorption.
EPA has been actively involved in vehicle-level demonstrations of this technology by using hydraulic launch assist in retrofitted urban delivery trucks. A hydraulic package of a reversible hydraulic pump/motor and accumulators was added to the vehicle to reduce fuel consumption, while keeping the vehicle’s conventional engine and transmission. Fuel savings of 25 and 45 percent were realized during city driving. For instance, Ford Motor Company, working jointly with EPA, demonstrated hydrualic power assist on a full-size sport utility vehicle platform fitted with a hydraulic pump/motor and valve block provided by infield technologies and carbon fiber accumulators developed by EPA. The pump/motor was connected to the vehicle driveshaft in parallel with the conventional power train. The vehicle demonstrated the ability to improve fuel economy by close to 24 percent on a start/stop, city-typical driving cycle (Kepner, 2002). Emissions were reduced by 20 to 30 percent. Better acceleration, reduced brake maintenance, and reduced operating costs (consumer payback of 4 to 5 years for city driving) were among other benefits (EPA, 2004). If the transmission and transfer case were replaced by a complete hydraulic drive train, larger fuel consumption benefits could be realized. In June 2006, EPA and United Parcel Service (UPS) demonstrated the world’s first full hydraulic hybrid delivery truck, which realized 60 to 70 percent reductions in fuel consumption under urban driving conditions and up to 40 percent reductions in greenhouse gas emissions, with an estimated payback shorter than 3 years.
Currently, EPA is focusing more on full-series hydraulic hybrids, along with improved vehicle aerodynamics, tires and advanced ICEs, including HCCI gasoline engines, free piston engines, completely variable displacement engines, alcohol engines, and exhaust heat recovery systems. Eaton Corporation, in collaboration with the EPA, has developed a series hydraulic hybrid power system that combines a high-efficiency diesel engine and a unique hydraulic propulsion system to replace the conventional drive train and transmission (Eaton, 2009). The engine operates at its “sweet spot facilitated by the continuously variable transmission (CVT) functionality of the series hybrid hydraulic system and by regenerative braking.” Fuel savings of 50 to 70 percent have been achieved, corresponding to a 40 percent reduction in greenhouse gases, 50 percent reduction in unburned hydrocarbons, and 60 percent reduction in particulate matter (EPA, 2009).
Simulation-Based Assessments of Hydraulic Hybrids
EPA predicted that the UPS prototype hydraulic hybrid 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 conducted a simulation study on a series hydraulic hybrid light truck (mass = 5,112 kg). Approximately a 68 percent reduction in fuel consumption can be achieved in city driving and about 12 percent in highway conditions. The energy savings can be attributed to regeneration and engine shutdowns and a smaller fraction to optimization of engine operation. Design optimization over the complete driving cycle enabled right-sizing of all hydraulic pumps/motors, the accumulator volume, and the gear ratio of the two-step transmission. Downsizing the engine to roughly 75 percent of the baseline matched the acceleration of the conventional vehicle. In addition, it was found that having two smaller propulsion motors for each axle reduced fuel economy consumption by an additional 10 percent compared to a single propulsion motor architecture. The advantage is that the rear electric machine could be used mainly for acceleration and the front electric machine for regenerative braking, where both electric machines operate on high loads and in turn yield greater efficiency. Wu et al. (2004), also from the University of Michigan, developed a power management algorithm for a parallel hybrid and predicted fuel savings ranging from 28 to 48 percent depending on the types of pumps used.
Anderson et al. (2005) conducted a simulation study comparing hydraulic hybrid and electric hybrid vehicles. Fuel savings over a variety of driving cycles were found to be 39 percent for parallel hydraulic hybrids, compared to 31 and 34 percent improvements for parallel and series electric hybrids, respectively. Gotting (2007) estimated the fuel consumption benefit for the parallel HHV to range from 20 to 25 percent for Class 3 to 6 box trucks.
While indicative of the range of potential benefits, it should be noted that simulations are carried out under ideal conditions—hence results typically represent best-case scenarios. Real-world savings in fuel consumption are likely to be lower, because of off-design duty cycles and practical production vehicle constraints.
Power Management in Hybrid Vehicles
Power management is key in obtaining maximum performance and fuel savings from a hybrid vehicle. The objective of a power management algorithm in a hybrid vehicle is to compute the optimum operating point of the overall system for any amount of power demanded from the driver. The cost criteria are usually fuel economy and emissions. The way the power is managed will depend a lot on the sizing and characteristics of each of the components and their instantaneous state of operation. Mechanical efficiencies of the components (i.e., transmission, torque converter, differential), rolling resistance, aero drag coefficients, and instantaneous operating
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point of components, among others, are of great importance with respect to fuel consumption but are considered as given. The hybrid power train architecture is also assumed to be given. There are three general power management algorithm types, as outlined below.
Heuristic Rule-Based
This type of control is constituted by heuristic rules—for example, if/then statements to split the power demanded from the driver into the electrical and mechanical subsystems. The majority of these rules are thermostatic—that is, actions are triggered when certain conditions are met. They are simple and easy to implement and less computationally expensive to develop, but they need a significant amount of tuning in order to achieve better results compared to a conventional vehicle. Their basic simplicity means the system will not operate at its best potential at all times and there is significant room for improvement. A rule-based algorithm for a particular vehicle can never be readily used for another. Nevertheless, this type of control is often used due to its ease of implementation and lower cost.
Table 4-13 shows the difference in fuel economy figures obtained during simulation of a hybrid electric transit bus using three different rule-based control algorithms.
Chu et al. (2003) developed a series HEV military bus (15,000 kg) prototype model and formulated an energy management strategy enabling the ICE to operate in its peak efficiency zones for reduced fuel consumption. A parametric design methodology also was established. Simulation results on four different driving cycles show that, on average, fuel savings improve by about 17 percent and, acceleration times by 25 percent, with top speed and gradability being the same compared to a conventional vehicle.
Real Time
Lately, there has been a growing research effort toward developing real-time power management algorithms of the power split between the thermal and electrical paths of HEVs. The main aspects of this approach are concerned with (1) the self-sustainability of the electrical path, which must be guaranteed for the entire driving cycle since the storage system cannot be expected to be recharged by an 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.
TABLE 4-12 Fuel Economy and Exhaust Emissions of Hybrid Electric Transit Bus with Various Control Strategies, Taipei City Bus Cycle
Hybrid Electric Bus
Fuel Economy (km/L)
CO (g/km)
HC (g/km)
NOx (g/km)
PM (g/km)
Speed control
1.82
1.14
0.31
28.52
0.29
Torque control
2.02
0.71
0.27
20.50
0.26
Power control
2.15
0.70
0.24
18.98
0.23
Diesel bus Conventional control
1.29
4.12
0.59
55.56
0.61
SOURCE: Wu et al. (2008).
Such algorithms consist of an instantaneous optimization (Sciarretta et al., 2004; Rodatz et al., 2005; Pisu and Rizzoni, 2007); the objective function in this optimization is fuel consumption and emissions. Decisions on the energy flow path (engine path and electrical path) can be evaluated based on an Equivalence Consumption Minimization Strategy. The equivalence between electrical energy and fuel energy can be evaluated by comparing the cost of energy produced at any instant. An instantaneous objective function combines the weighted sum of electrical energy and fuel energy and is evaluated with regard to selection of a proper equivalence factor value at any instant. If the engine can produce the required power more efficiently than the electrical system (sweet-spot operating points), the engine energy path is favored, and when the motor can produce power more efficiently than the engine (low speed/low load), the electrical path is favored.
The key in a real-time power management system is simultaneous optimization of the operating points of the entire system as a whole and not just the engine alone. It aims for the best overall efficiency from the engine to the wheels or from the battery pack to the wheels as the case may be. However, owing to greater interactions among the involved subsystems (engine, motor, battery), the control complexity can rise rapidly with the number of agents or their behavioral sophistication. This increasing complexity has motivated continuing research on computational learning methods toward making autonomous intelligent systems that can learn how to improve their performance over time while interacting with the driver. These propulsion systems need to be able to sense their environment and also integrate information from the environment into all decision making.
This challenge can be effectively addressed by applying principles of cognitive optimization techniques. The problem can be formulated as sequential decision making under uncertainty in which an intelligent system (e.g., hybrid-electric vehicle, power train system) learns how to select control actions so as to reduce fuel consumption over time for any different driving cycle (Malikopoulos, 2009).
Dynamic Programming
Power management control algorithms employing dynamic programming (Scordia et al., 2005; Lin et al., 2003; Perez et al., 2006; Ogawa et al., 2008; Karbowski et al., 2009) rely on computing off-line the optimal control policy with respect to the available power train variables—that is, the power split between the thermal (engine) and electrical
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FIGURE 4-25 Dynamic programming process and rule extraction from the result. SOURCE: Lin et al. (2003).
paths (motor, generator, and battery), gear selection, etc., over a given driving cycle or a family of given driving cycles. Even though dynamic programming is not directly implementable, its results can be used to form an efficient 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 approximated 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 the power devices are being used depending on visitation points during the duty cycle. The main shortcoming of this approach is that it is efficient only for the driving cycles used in deriving the optimal policy. In addition, due to the high computational cost of dynamic programming, only simplified models of HEVs can be used. As a result, the extracted optimum policy omits a significant number of HEV dynamics that affects the efficiency of the derived policy even for the given driving cycle derived. Despite the aforementioned shortcomings, power management results based on deterministic 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 respect to hybrid technology discussed.
Brake O&M Benefits
In addition to saving fuel, hybridization significantly reduces brake costs. Suppliers and OEMs expect that hybrids will more than double brake life. For some applications these savings can outweigh the fuel savings.
TABLE 4-14 Predicted Fuel Consumption Comparison: Conventional (nonhybrid), Dynamic Programming (DP), and Rule-Based (RB)
DP
RB
Conventional
Mpg
13.85
12.65
10.39
Fuel (gallon)
0.5259
0.5757
0.7005
SOURCE: Lin et al. (2003).
FIGURE 4-26 Implementing dynamic programming as a rule-based algorithm in SIMULINK. SOURCE: Lin et al. (2003).
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System Integration
As hybrid systems become more fully integrated into overall vehicle architecture, there are a number of opportunities to further optimize the system:
Depending on the application, hybridizing a vehicle can enable engine downsizing. Currently, this is not widely done. Part of the reason is that industry is still figuring out which applications can use a downsized engine without sacrificing mission performance. Another important factor is that smaller engines that meet medium-/heavy-duty warranty requirements may be unavailable. One supplier suggested that they would 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 between hybrid systems and exhaust aftertreatment systems. For example, a system designer could vary engine load to manage exhaust gas temperature, thereby offering emissions benefits. However, current standards measure engine-out emissions. To get credit for any benefit, EPA would need to switch to measuring emissions on a vehicle basis or develop a method to credit the lower vehicle emissions.
Improved system power density, component efficiency, and design integration are expected to offer additional fuel-saving opportunities.
Electrification is viewed as an enabler for more efficient waste-heat recovery systems, such as electric turbo-compounding or electric bottoming cycles. In a hybrid vehicle, electric waste-heat systems can offer an additional 1 to 2 percent efficiency benefit at neutral cost compared to an equivalent mechanical waste-heat system.
By narrowing the design window, hybrid systems enable the engine to be optimized to deliver peak fuel economy within a narrow operating band. This opportunity applies primarily to dual-mode or series systems.
Taken as a whole, improved systems integration can offer an additional 5 to 10 percent improvement in fuel efficiency in future systems in the years 2015-2020. In tandem, higher sales volume can reduce costs by a factor of 2.
Hybrid Power Train Summary
In its report for the committee, TIAX (2009) summarized the hybrid fuel consumption potential reductions by range of years and by application, as shown in Table 4-15.
Based on work discussed in this chapter as well as on the TIAX summary for hybrid power trains, the committee estimated potential fuel consumption reduction as shown in Table 4-16.
TABLE 4-15 Hybrid Fuel Consumption Reduction Potential (percentage) Compared to a Baseline Vehicle Without a Hybrid Power Train, by Range of Years and Application
2013-2015
2015-2020
Tractor trailer
NA
10
Class 6 box truck
22
30
Class 6 bucket truck
35
40
Refuse truck
20
25
Urban bus
30
35
Motor coach
NA
NA
Class 2b pickup and van
NA
18
NOTE: NA, not applicable.
SOURCE: TIAX (2009).
TABLE 4-16 Estimated Fuel Consumption Reduction Potential (percentage) for Hybrid Power Trains
Tractor trailer
5-10a
Class 6 box truck
20-35
Class 6 bucket truck
30-45
Refuse truck
20-35
Urban bus
12-50
Motor coach
5-40
Class 2b pickup and van
18-30
aIncludes some reduction in hotel load, some idle reduction, and some electrification of accessories.
FINDINGS AND RECOMMENDATIONS
Diesel Engine Technologies
Finding 4-1. Many individual technologies for reducing load-specific fuel consumption of diesel engines were identified. Some technologies are being used in 2010 by nearly all manufacturers (common rail fuel injection and selective catalytic reduction, SCR), and some are being used by a limited number of manufacturers (turbocompounding and multiple turbochargers). One manufacturer, Cummins, has shown a roadmap for 49.1 percent thermal efficiency by 2016 and 52.9 percent by 2019, which are 14.5 and 20.6 percent reductions in fuel consumption, respectively, from a 2008 baseline, compared to current diesel fuel consumption. Significant technical challenges remain to be overcome before many of the fuel-saving technologies described in this section can be successfully implemented in production.
Gasoline Engine Technologies
Finding 4-2. Technologies exist today, or are under development, that offer the potential to reduce the fuel consumption of gasoline-powered vehicles operating in the medium-duty vehicle sector. The most beneficial technologies and the
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magnitude of fuel savings will be dependent on the configuration of the engine and the duty cycle of its application. Under optimal matching of technology and duty cycle, fuel consumption reductions of up to 20 percent appear to be possible compared to 2008 gasoline engines in the 2015 to 2020 time frame. The economic merit of integrating different fuel-saving technologies will be an important consideration for operators and owners in choosing whether to implement these technologies.
Recommendation 4-1. Development of fuel-saving engine technologies and their effective integration into the engine/power train is critical for reducing fuel use by medium- and heavy-duty vehicles and helping the nation to meet national goals related to energy security and the environment. The federal government should continue to support such programs in industries, national labs, private consulting companies, and universities.
Diesel Engines Versus Gasoline Engines
Finding 4-3. Diesel engines can provide fuel consumption advantages, compared to gasoline engines, of 6 to 24 percent depending on application, duty cycle, and baseline gasoline engines.
Finding 4-4. Diesel engines are increasing in cost primarily due to emissions aftertreatment equipment (DPF and SCR), which can cost over $17,000. Because of this cost increase (and diesel fuel prices), dieselization of Class 6 trucks in the new sales fleet went from 75.8 percent in 2004 to 58.0 percent in 2008. The effect of 2010 emission regulations has yet to be felt, but it is expected to accelerate the trend toward gasoline engines in medium-duty trucks.
Recommendation 4-2. Because the potential for fuel consumption reduction through dieselization of Class 2b to 7 vehicles is high, the U.S. Department of Transportation/National Highway Traffic Safety Administration (NHTSA) should conduct a study of Class 2b to 7 vehicles regarding gasoline versus diesel engines considering the incremental fuel consumption reduction of diesels, the price of diesel versus gasoline engines in 2010-2011, especially considering the high cost of diesel emission control systems, and the diesel advantage in durability, with a focus on the costs and benefits of the dieselization of this fleet of vehicles.
Transmission and Driveline Technologies
Finding 4-5. The transmission ratio and axle ratio affect fuel consumption by determining the engine speed versus road speed of the vehicle. A properly specified transmission and axle will allow the engine to run at its best fuel consumption operating range for a given road speed.
Finding 4-6. Manual transmissions have the least mechanical losses. An automated manual transmission can reduce fuel consumption by reducing driver variability (4 to 8 percent benefit). The fully automatic transmission can improve productivity by reducing the shift time (power shift) and by avoiding engine transient response delays and can reduce fuel consumption (up to 5 percent) by reducing driver variability, but the AT has higher parasitic losses.
Recommendation 4-3. The industry should continue its practice of training dealers and provide training materials for truck specifications affecting fuel consumption, such as transmission ratios, axle ratios, and tire size.
Hybrid Power Trains
Finding 4-7. Fuel consumption reductions on hybrid vehicles 5 to 50 percent have been reported by enabling optimum engine operation, downsizing in certain cases, braking energy recovery, accessory electrification, and engine shutdown at idle.
Finding 4-8. A wide range of hybrid electric and hydraulic architectures have been demonstrated. The selection of a particular system architecture depends mainly on application, duty cycle, and cost-benefit trade-offs.
Finding 4-9. The realized fuel consumption benefits of a particular hybrid technology and architecture implementation are strongly dependent on application and duty cycle. Optimization of component sizing and power management are keys to maximizing the potential for fuel consumption reductions while satisfying performance and emission constraints.
Finding 4-10. Computer simulation of medium- and heavy-duty vehicles is an effective way to predict fuel consumption reductions considering the additional variables in a hybrid vehicle system, but such systems are not standardized, leading to a wide variety of results and unpredictability.
Recommendation 4-4. NHSTA should support the formation of an expert working group charged with evaluating available computer simulation tools for predicting fuel consumption reduction in medium- and heavy-duty vehicles and developing standards for further use and integration of these simulation tools.
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