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170 The preceding appendices have focused on alternative fuels and emerging vehicle technologies in the light-duty fleet, including passenger cars, pickup trucks, vans, and sport utility vehicles. This appendix shifts focus to MHDVs, begin- ning by introducing the types and applications of MHDVs and then discussing the alternative fuels and vehicle technol- ogy improvements currently being examined in the context of MHDVs. The appendix closes with a summary of the most-promising alternative fuels and vehicle technologies for different classes of MHDVs based on their size and use characteristics. F.1 Classifying Medium- and Heavy-Duty Vehicles Trucks provide services, and there are almost as many dif- ferent types of trucks as there are services to be provided. For example, vans can be configured for package delivery, bev- erage delivery, and refrigerated food delivery, and there are likewise many possible configurations for trailers hauling varying types of freight over long distances. Different truck applications are also subject to very different usage patterns, ranging from the low speeds with frequent starts and stops of a waste-collection vehicle to the long periods at cruise speed for long-distance tractor-trailers. In the following are dis- cussed several possible ways of classifying MHDVs, with the aim of providing greater insight into the potential utility of alternative fuels and vehicle technologies across the spectrum of MHDV applications. F.1.1 Classifying MHDVs by Gross Vehicle Weight Vehicles in the United States are commonly categorized into eight classes based on gross vehicle weightâa term describing the maximum operating mass of a vehicle as speci- fied by the manufacturer, including the weight of the vehicle itself and fuel, cargo, passengers, and the like. Class 1 vehicles have a GVW of less than 8,500 pounds and are typically used for passenger travel. Class 2 vehicles, with a GVW of between 8,500 and 10,000 pounds, include large sport utility vehicles, pickup trucks, vans, and some multipurpose vehicles. While some Class 2 vehicles are used for passenger travel, others (often referred to as Class 2b) are employed as service vehi- cles. Classes 3 through 6 include various types of medium- duty vehicles with a GVW of from 10,000 to 26,000 pounds. Classes 7 and 8 are described as heavy-duty vehicles with a GVW of greater than 26,000 pounds. The maximum GVW of trucks in the United States is limited to 80,000 pounds in most instances. The forms and functions of MHDVs vary significantly both within and across these classes. For example, Classes 2b, 3, and 4 include large pickup trucks, step vans, smaller delivery trucks, and mini-buses. Class 5 and 6 vehicles include large walk-in delivery trucks, school buses, and bucket trucksâ trucks configured with telescoping booms and buckets used to perform utility maintenance and other functions. Class 7 and 8 vehicles include fire engines, urban and intercity buses, cement trucks, refuse trucks, tankers, and tractor-trailers. Tables F.1 and F.2, drawing on an NRC study, list typical applications, gross vehicle weight, payload capacity, approxi- mate share of the combined medium- and heavy-duty fleet, load-specific fuel economy, aggregate fuel consumption, and annual mileage for the different classes of MHDVs. As indicated by the data in the tables, there is significant crossover in the types of applications across classes, with the main differentiating factor being GVW. As of 2010, there were about 18 million MHDVs in the United States. As shown in Table F.2, Class 2b accounted for just under half of this total. Most of the MHDVs in Classes 3 through 8 are diesel powered; of the Class 2b vehicles, how- ever, only about a quarter rely on diesel (ORNL 2012). Because of their weight, number, and long-distance travel patterns, Class 8b vehicles are responsible for about 60% of the fuel A p p e n d i x F Medium- and Heavy-Duty Vehicles
171 consumption of all MHDVs, even though they only make up about 16% of the entire MHDV fleet. Class 6 and Class 2b vehicles are each responsible for another 12% to 13% of total MHDV fuel consumption. The fuel economy of MHDVs in miles per gallon, not surprising, generally declines with increased GVW. As shown in the table, however, fuel economy measured in ton-miles per gallon increases with greater weight; in other words, larger trucks are the most fuel-efficient transporters. F.1.2 Classifying MHDVs by Sector Another possibility for classifying MHDVs is by the sector in which they work. Until 2002, the U.S. Department of Com- merce periodically carried out the Vehicle Inventory and Use Survey (VIUS), which recorded the number, uses, and other characteristics of vehicles in the United States (ESA 2004). Drawing on the last VIUS survey from 2002, Table F.3 shows the total number of MHDVs in Classes 3 through 6 (combined medium-duty) and Classes 7 and 8 (combined heavy-duty) as well as the share in each of these groupings employed in differ- ent sectors. Although the total number of MHDVs has likely increased in the past decade, it is possible that the allocation of this fleet across sectors may still be similar. As shown in the table, the dominant sectors for medium- duty vehicles in Classes 3 through 6 are, in descending order, construction, agriculture, and for-hire transportation and warehousing. For heavy-duty vehicles in Classes 7 and 8, the Class Share of MHDV Fleet Typical Load- Specific Fuel Economy (ton- miles/gallon) Annual Fuel Consumption Total for Class (billion gallons) Average Annual Vehicle Miles (thousands) Annual Miles Total for Class (billions) 2b 48% 26 5.5 15â40 93 3 12% 30 1.5 20â50 12 4 4% 42 0.53 20â60 4 5 4% 39 0.26 20â60 2 6 6% 49 6.0 25â75 41 7 7% 55 1.9 75â200 9 8a 4% 115 3.5 25â75 12 8b 16% 155 28 75â200 140 Source: NRC (2010). Table F.2. Fleet share, fuel use, and annual miles for MHDV classes. Class Applications Gross Vehicle Weight (lbs.) Typical Payload Capacity (lbs.) 2b Large pick-up, utility van, multipurpose, mini-bus, step van 8501â10,000 3,700 3 Utility van, multipurpose, mini-bus, step van 10,001â14,000 5,250 4 City delivery, parcel delivery, large walk-in, bucket, landscaping 14,001â16,000 7,250 5 City delivery, parcel delivery, large walk-in, bucket 16,001â19,500 8,700 6 City delivery, school bus, large walk-in, bucket 19,501â26,000 11,500 7 City bus, furniture, refrigerated, refuse, fuel tanker, dump, tow, concrete, fire engine 26,001â33,000 18,500 8a Straight trucks: dump, refuse, concrete, furniture, city bus, tow, fire engine 33,001â80,000 20,000â50,000 8b Combination trucks: tractor-trailer, van, refrigerated, bulk tanker, flat bed 33,001â80,000 40,000â54,000 Source: NRC (2010). Table F.1. Applications, weight, and payload for MHDV classes.
172 dominant sectors are for-hire transportation and warehous- ing, construction, and agriculture. Note that MHDVs in the leased vehicle category can be employed by the lessee for a broad range of applications, including transportation and construction (ORNL 2012). F.1.3 Classifying by Application Alternatively, MHDVs can be classified according to appli- cation, which combines features of weight-based and sector- based classes. The previously referenced NRC study classified MHDVs into seven general application categories: tractor- trailers, straight box trucks, straight bucket trucks, refuse trucks, transit buses, motor coaches, and pickup trucks and small vans (NRC 2010). The definitions for these application cate- gories, along with the classes of MHDVs included, are listed in Table F.4. F.1.4 Important Characteristics of MHDV Use As described, the types and configurations of MHDVs are quite diverse. For example, Class 8 vehicles include buses, tractor-trailers, and refuse trucks. These vehicles vary signifi- cantly in their purpose and use characteristics. The effect of these factors on the suitability of alternate fuel choices and vehicle technologies is significant. Key considerations are whether the vehicle is part of a centrally managed fleet, daily and annual mileage, typical travel speeds and duty cycle, and ownership cycles and resale requirements. Fleet use. MHDVs are often operated in fleets. Examples of fleets are groups of tractor-trailers used for interstate truck- ing, utility bucket trucks for line maintenance and emergency response, school buses, trash collection trucks, and cement trucks. Fleets, whether private or public, often benefit from shared vehicle maintenance and refueling stations. This can be quite helpful from the perspective of introducing alterna- tive fuels because the fleet can be supported with minimal investment in refueling infrastructure. To transition a fleet of municipal service vehicles to natural gas, for example, it may be sufficient to install a single natural gas dispenser at the facility where vehicles are stored between uses. The minimum fleet size for shared services to be economi- cal depends on the application and vehicle class. Drawing on data originally reported in the 2002 VIUS, Table F.5 shows the percentage of MHDVs in fleets of different sizes that rely on different categories of refueling stationsâretail gas sta- tions, retail truck stops, a refueling station operated by the fleet owner, or a refueling station operated by some other third party (such as the operator of another nearby fleet)â for their refueling operations. For fleets in the range of 11 to 50 vehicles, roughly a half of all vehicles are fueled at the fleet operatorâs own facility. In contrast, smaller-sized fleets rely to a much greater extent on retail gas stations. Therefore, in the context of deploying alternative fuels for MHDVs, fleets with more than 11 vehicles appear to be a promising target since any specialized refueling infrastructure can be central- ized at the ownerâs facility, where it will be well utilized by vehicles within the fleet. To introduce alternative fuels with Sector Class 3â6 Class 7â8 Number of vehicles (millions) 2.9 2.3 For-hire transportation or warehousing 9.6% 30.1% Construction 18.4% 15.9% Agriculture 16.2% 12.2% Retail trade 7.1% 5.4% Waste management, landscaping, or administrative support 5.4% 5.0% Manufacturing 3.3% 4.9% Wholesale trade 5.5% 4.8% Utilities 5.0% 1.1% Leasing 6.2% 4.6% Personal 4.8% 2.5% Mining 1.1% 2.4% Other services 6.6% 2.8% Not in use 6.4% 5.1% Unknown 4.4% 3.2% Source: ESA (2004), ORNL (2012). Table F.3. Use of MHDVs by sector.
173 for-hire transportation and warehousing sector, in contrast, relies most frequently on truck stops. Still, in several of the sectors, including for-hire transportation and warehousing, agriculture, utilities, vehicle leasing, and mining, nearly 25% or more of all MHDVs refuel at their own facilities, creating greater potential for a transition to alternative fuels. Combining data from several of these tables, it is possible to develop some rough estimates of the percentages of trucks by an assumed reliance on retail gas stations or truck stops, the dispensers would need to be much more widely deployed, likely resulting in relatively low utilization at any given station. Table F.6 provides additional insight into the refueling characteristics of MHDVs, in this case listing the primary refueling facility by industrial sector. The most common type of refueling facility for most sectors is the gas station. The MHDV Fleet Size Share of MHDVs by Fleet Size That Refuel At: Gas Station Truck Stop Own Facility Other Facility 1â5 73.8% 6.1% 18.2% 1.9% 6â10 55.3% 5.7% 35.5% 3.4% 11â20 41.1% 5.1% 48.9% 4.9% 21â50 42.9% 3.7% 49.8% 3.6% 51 or more 48.3% 6.3% 44.4% 1.0% No fleet 96.4% 1.6% 1.7% 0.3% Source: ORNL (2012) using data from ESA (2004). Table F.5. Primary refueling facility by fleet size. Application Classes Notes Tractor-trailer 7, 8 Includes many use types: long haul, port service, full load, less than load, tankers, etc. These trucks may be part of a large fleet or may be independently owned and operated. Since most fuel is burned in long-haul applications, greater considerations are given to this use. Diesel engines are assumed. Straight truck, box 3â8 Includes all configurations of box trucks, including delivery vans, refrigerated trucks, ambulances, and shuttle vans. In general these trucks are part of a fleet that is managed and maintained centrally. Engines are both gasoline and diesel powered. (The share for gasoline engines was over 40% in 2008.) Generally used for regional hauling, covering approximately 150 miles per day at an average speed of 30 miles per hour. Straight truck, bucket 3â8 Includes bucket trucks in a range of sizes. Assumed to be centrally managed and maintained as part of a fleet. Typical work includes movement to a site followed by periods of use by auxiliary equipment, such as manipulating the bucket boom. Typically diesel powered. Refuse truck 7, 8 Large waste-collection vehicles. Assumed to be centrally managed and maintained. The load cycle includes several hundred stops per day. Diesel powered. Transit bus 8 Transit buses are part of a fleet that is centrally managed and maintained. Urban-use cycles are approximately 150 to 250 miles per day at low speed, with significant electrical load for air-conditioning and lighting. Primarily diesel powered, but increasingly fleets have been converting to natural gas in an effort to improve local air quality. Hybrid buses have been deployed to reduce fuel consumption. Motor coach 8 Intercity buses. Owned and maintained by service providers, but must travel long distances without refueling. Diesel powered. Pickup trucks and small vans 2b Large pickup trucks and small commercial vans. Owned in general by private parties. Typically gasoline powered. Source: NRC (2010). Table F.4. Generalized MHDV application categories.
174 Daily range. Operating range describes the distance from the central facility within which an MHDV must operate on a daily basis. Because some alternative fuels and technologiesâ most notably natural gas and battery electricâoffer more limited range than conventional fuels, quantifying the typical operational range provides insight into the potential applica- bility of such fuels and technologies for various MHDV classes and applications. As shown in Table F.7, over 70% of medium- duty vehicles (Classes 3 through 6) and 50% of heavy-duty class in each application area that refuel at a facility owned by the fleet operator. For heavy combination trucks in Classes 7 and 8, close to 8% (or approximately 200,000 vehicles) are refueled at a fleet facility. The data do not support the same level of disaggregation for other trucks in Classes 3 through 8. Given the larger number of such vehicles, however, it seems likely that there could be several hundred thousand vehicles each in for-hire transportation, construction, and agriculture that are fueled primarily at fleet operatorsâ facilities. Range and Refueling Facility Class 3â6 Class 7â8 Typical range of operation Under 50 miles 61.5% 40.7% 51â100 miles 11.7% 13.5% 101â200 miles 3.2% 6.7% 201â500 miles 1.8% 7.6% 501 miles or more 2.2% 10.4% Other, not reported, or vehicle not in use 19.6% 18.1% Primary refueling facility Gas station 62.4% 28.4% Truck stop 7.7% 31.9% Own facility 27.3% 36.2% Other nonpublic facility 2.6% 3.5% Source: ORNL (2012) using data from ESA (2004). Table F.7. Operating range and primary refueling by weight class. Application Share of MHDVs by Sector That Refuel At: Class 7 and 8 Combination Trucks by Sector That Refuel at Own Facility Gas Station Truck Stop Own Facility Other Facility For-hire or warehousing 33.3% 38.7% 25.8% 2.3% 7.8% Construction 84.7% 3.3% 9.8% 2.2% 4.1% Agriculture, forestry, fishingâ¦ 62.7% 6.7% 29.4% 1.1% 3.1% Retail trade 86.6% 3.5% 8.6% 1.2% 1.4% Waste mgmt., landscapingâ¦ 78.2% 3.0% 17.1% 1.6% 1.3% Manufacturing 81.5% 5.1% 11.9% 1.5% 1.3% Wholesale trade 76.2% 6.6% 12.0% 5.1% 1.2% Utilities 72.6% 1.8% 24.3% 1.3% 0.3% Vehicle leasing or rental 60.2% 1.3% 31.8% 6.8% 1.2% Personal 98.6% 0.6% 0.7% 0.2% 0.6% Mining 48.7% 8.5% 34.3% 8.5% 0.6% All 93.9% 1.8% 3.7% 0.5% 0.7% Source: ORNL (2012) using data from ESA (2004). Table F.6. Primary refueling facility by sector.
175 another maintenance site at moderate speed, repeating the cycle several times over the course of the day. A transit bus may have several different modes of operation, depending on the type of route. If it is serving a business district, it may travel in relatively congested conditions and make frequent stops. If it is assigned to a commuter route, in contrast, it may make relatively few stops separated by relatively long periods of travel at cruising speed (NRC 2010, 2012). Although the weights of all these vehicles may be similar, the benefits of alternative fuels and other advanced-efficiency technologies are significantly different depending on the duty cycle. The long-haul tractor-trailer spends a large portion of its time at highway cruise speed and will thus benefit more from improved aerodynamics than, say, from hybrid technol- ogies, the principal benefit of which is the recovery of energy during braking. In contrast, hybrid technologies may provide significant benefits to refuse trucks and transit buses, which must stop and start frequently, or to bucket trucks, enabling the conversion of auxiliary equipment to electrical actuation, which is both lighter and easier to maintain. The duty cycle can also have a major effect on the emis- sions profile for internal combustion engines. The emissions of diesel engines are highest during periods of high load, as when accelerating the vehicle from a stop. This makes it chal- lenging to develop effective emission control equipment for vehicles that must make many stops and starts, such as transit buses and refuse trucks. Applying cleaner-burning fuels or advanced engine technologies may help reduce emissions in such cases. Actual duty cycles have been abstracted into short test cycles so that vehicle performance may be evaluated on a common basis in a laboratory environment. These test cycles are the standards by which emissions regulations are bench- marked. The EPA developed the Urban Dynamometer Driv- ing Schedule (UDDS) for light-duty vehicles, for example, which includes both highway and city driving over 23 min- utes of activity (EPA 2012a). The UDDS concept has also been applied to heavy-duty vehicles (NRC 2010). A set of schedules has been developed for MHDVs spanning several modes of operation, including a âcreep mode,â a âtransient mode,â and a âcruise mode,â corresponding to increased average speeds (Clark et al. 2007, NRC 2010). Relevant aspects of the duty cycle are the proportion of time at highway cruise speed, the proportion of time at idle, the proportion of time in a stopâ start driving cycle, and the need to power auxiliary loads. Ownership characteristics and life-cycle cost. Finally, life-cycle ownership costs, including capital, financing, fuel, maintenance, and resale, are an extremely important factor in evaluating alternative fuels and vehicle technologies. Alterna- tive fuels often involve trade-offs in which certain cost com- ponents increase but others are reduced. For example, hybrid drivetrains and natural gas vehicles increase the purchase price vehicles (Classes 7 and 8) have typical ranges of less than a hundred miles. MHDV applications for which the operating range is relatively modest, and in which vehicles can refuel at a facility owned by the fleet operator, may present some of the most promising MHDV market applications for alterna- tive fuels. Operational range has additional implications beyond refueling requirements. Consider the potential application of heavy-duty natural gas engines for long-haul transport in tractor-trailers. Should such a vehicle require maintenance en route, the existing service infrastructure in place for diesel- fueled trucks may be of no help. Understanding this to be an issue, Cummins Westport, a maker of both heavy-duty diesel and natural gas engines, provides parts and service for both fuel technologies from its existing network (Cummins Westport, undated b). Access to proper maintenance is also important to maintain the performance of advanced emis- sions control systems (NRC 2010). Annual mileage. MHDVs are used heavily and as a result tend to travel more miles per year than passenger vehicles. As indicated earlier, the typical mileage by class ranges from 20,000 miles per year for local utility vehicles to over 200,000 miles per year for tractor-trailers used in long-haul applica- tions. In comparison to passenger cars, then, this amplifies the relative benefit of alternative fuels and vehicle technologies that reduce fuel costs for MHDVs. With more miles traveled each year, fuel-cost savings can accrue more rapidly to help offset increased capital costs associated with the vehicle pur- chase or conversion. It should be noted, though, that even with the potential for fuel-cost savings, there is still a delicate balance between capital and operating costs for MHDVs since newer technologies may increase system complexity and result in increased maintenance costs. Duty cycle. The term âduty cycleâ refers to the pattern of use of a vehicle over a fixed time period. Duty cycles are as varied as the applications of MHDVs. For example, the duty cycle for a long-haul tractor-trailer may be as simple as driv- ing 6 hours at an average speed of 60 mph. A waste-collection truck, in contrast, might drive to a neighborhood at a mod- erate speed, and then creep along as workers gather trash in that neighborhood. The crew may repeat this cycle several times throughout a workday, after which the truck delivers its load and returns to its depot. In an NRC study that examined the prospects for alternative fuels and advanced efficiency technologies in different MHDV applications, the assumed typical use cycle for a waste-collection truck was 700 load stops per day over 25 miles, including two trips to the dump, adding an additional 50 miles (NRC 2010). Alternatively, a utility-service bucket truck could travel from a service facility to a location for maintenance. On-site, the vehicle may idle and provide power for the hydraulic equipment supporting the workmen in the bucket. Then the vehicle may move to
176 appointed a special committee to assess fuel economy tech- nologies for medium- and heavy-duty vehicles (NRC 2010). The EPA has several programs, including the Clean Diesel pro- gram and the SmartWay freight program (EPA 2012b, Smart- Truck 2009). These programs are focused on advanced engine and vehicle technologies and other methods to reduce fuel consumption and improve the environmental performance of MHDVs. For example, the Clean Diesel program focuses on reducing air emissions from diesel engines by working with manufacturers, fleet operators, local and regional officials, and others to encourage the adoption of new emissions control technologies to reduce the contribution of diesel emissions to local and regional air pollution. Alternative fuels provide one means of improving emissions profiles. In addition to voluntary and collaborative publicâprivate partnerships, the United States is poised for the first time to regulate fuel economy improvements in the medium- and heavy-duty vehicle fleets. The federal government has man- dated CAFE standards for light-duty vehicles since the late 1970s, but MHDVs were not previously regulated in the same manner. In May of 2009, however, President Barack Obama directed NHTSA and the EPA to develop the Heavy-Duty National Program, which would specify fuel economy stan- dards for combination tractors, heavy-duty pickup trucks and vans, and vocational vehicles (buses, refuse trucks, utility trucks, etc.). The proposed rulemaking for this program was issued by NHTSA and EPA in November 2010 and was finalized in September 2011. Under the new standards, set to take effect in 2014 and escalate through 2018, combination tractors will ultimately be required to achieve a 20% reduction in fuel consumption and greenhouse gas emissions, heavy- duty pickup trucks and vans will be required to achieve a 15% reduction, and vocational vehicles will be required to achieve a 10% reduction (EPA and NHTSA 2011). The remainder of this section briefly reviews current die- sel and gasoline engines for MHDV applications, discusses of the vehicle significantly, adversely affecting both capital and financing costs. However, a hybrid drivetrain reduces fuel con- sumption, and natural gas vehicles can benefit from a lower- cost fuel. When purchasing new vehicles, owners of MHDVs typically assume that the vehicle will be sold for a resale value at the end of its term of service. Given the small installed base of support for both natural gas and hybrid vehicles, the ability of the purchaser to sell the vehicle could be compromised; diesel MHDVs operated using untraditional fuels such as biodiesel may likewise fetch less on the salvage market. By implication, given the importance of resale in most MHDV life-cycle cost computations, developing alternative fuels that substitute or may be blended with existing diesel is important. Many tax incentives and other programs at the federal and state levels attempt to change the mathematics behind ownership costs by offsetting the incremental life-cycle costs of natural gas and hybrid vehicles (Krupnick 2011). F.2 Fuels and Vehicle Technologies Most MHDVs in the United States today operate on diesel, which accounts for almost 90% of the 3 million barrels of fuel consumed by MHDVs each day. Gasoline is the second most common fuel for MHDVs, at around 10%, followed by much smaller shares for LPG, natural gas, and electricity (ORNL 2012). Table F.8 breaks down the amount of different types of fuels used for different categories of MHDVs. Motivated by concerns over energy security, climate change, and economic efficiency, there are several initiatives in the United States aimed at improving the performance of MHDVs and reducing fuel consumption. The 21st Century Truck Partnership is a cooperative research and development partnership among the DOE, the DOT, the DoD, the EPA, and 15 industry partners with the goal of increasing the abil- ity of trucks to move freight while reducing emissions and fuel consumption (NRC 2012). The National Research Council Vehicle Type 1000 Barrels per Day of Gasoline, Diesel, or Gasoline Equivalent Gasoline Diesel LPG Natural Gas Electricity Light vehicles 8,264 190 25 â â Transit buses 0.52 31 â 10 0.37 Intercity buses 14 â â â School buses 3.5 31 â â â Class 3-6 trucks 297 363 11 â â Class 7-8 trucks 26 2,229 0.10 â â Total MHDV 327 2,669 11.1 10 0.37 Source: ORNL (2012). Table F.8. Daily U.S. fuel consumption for highway vehicles, 2010.
177 than gasoline (Hileman et al. 2009) such that more energy can be carried in similarly sized tanks. Diesel engines are also more efficient than gasoline engines, for two reasons: first, as noted, diesel engines have higher compression ratios than gas- oline engines, allowing more work to be extracted during the expansion stroke of the engine; second, diesel engines do not throttle the air intake, which can lower efficiency, especially at light loads. The key advantage to diesels, however, is their dura- bility. The lifetime of a properly maintained diesel engine is several decades, whereas a gasoline engine has a much shorter lifetime. This is one of the reasons why the median lifetime of MHDVsâthe majority of which are diesel poweredâis so long. The estimated median lifetime of 2002 model year trucks, for example, is estimated to be 28 years (ORNL 2012). Gasoline engines are simpler, less expensive, and require less-expensive emissions control systems. The simplicity of gasoline engines derives from the relatively less-complex fuel-injection system. Diesel engines must withstand greater pressures than gasoline engines, and as such the engines must be heavier and more robust. These factors increase the cost of diesel engines over gasoline engines, which has led to an increased market share in recent years for gasoline-fueled MHDVs for Class 3 through 7 applications (NRC 2010). Advances in gasoline engines have narrowed the gap in fuel economy and durability for many vehicle configurations and uses. This helps to explain the relatively similar volumes of gasoline and diesel consumption for Class 3 through 6 trucks shown earlier. F.2.2 Advances in Conventional Fuels and Vehicle Technologies Significant efforts are being made to improve conven- tional fuels and engine technologies. As noted previously, the goals of the 21st Century Truck Partnership (NRC 2012) and other programs are to improve engine efficiency, improve performance, and reduce emissions. Some potential areas of improvement are detailed in the following. Ultra-low-sulfur diesel (ULSD). In December 2000, the EPA promulgated rules that on-road diesel fuel in the United States must have a sulfur content of no greater than 15 ppm [Office of Transportation and Air Quality (OTAQ) 2000]. The regulations, phased in from 2006 to 2010, were paired with more-stringent emissions regulations for new trucks beginning with the 2007 model year. While reducing the sulfur content of diesel yields significant emissions benefits on its own, it also enables the use of catalytic converters in diesel trucks, which otherwise would be damaged by higher- sulfur fuels. As the ultra-low-sulfur diesel was phased in, new trucks were then required to include catalytic converters. The implementation of the ULSD and clean diesel truck rules was completed in 2010. technologies for improving fuel economy and reducing emis- sions for diesel- and gasoline-fueled MHDVs, and considers alternative fuels that could substitute for diesel or gasoline in certain applications. It is likely that many of the technical options discussed could be implemented in the near term in order to comply with the new federal standards on MHDV emissions and fuel economy as they phase in between 2014 and 2018. F.2.1 Conventional Diesel and Gasoline Engines The diesel engine is a compression-ignition engine in which the fuel is injected into the cylinder late in the compression stroke. The pressure and temperature in the cylinder cause the mixture to ignite. The combustion process moves the cyl- inder, which extracts work. The volume of air a diesel engine consumes is generally constant at a given engine speed, with power output governed by the amount of fuel injected into the engine. This relates to the reason why older diesel engines emit sootâwhich is unburned fuelâunder high load condi- tions: more fuel is injected into the engine than can be burned during the expansion stroke of the engine. To increase power, diesel engines are often turbocharged, increasing the mass of air in the cylinder. Modern diesel engines are electronically controlled and use exhaust gas recirculation (EGR) to reduce in-cylinder formation of NOx. Filters and catalytic convert- ers are used to reduce the emissions of particulates. To meet more recent requirements for NOx emissions, new engines employ selective catalytic reduction units, mobile versions of the systems used to reduce emissions from power plants. Diesel engines have high compression ratios as compared to gasoline engines, increasing their efficiency. Gasoline engines, found in many medium-duty vehicles, rely on spark ignition. In a spark-ignition engine, the air and fuel are premixed in the intake manifold and then introduced into the cylinder, compressed, and ignited. Because the air and fuel are premixed, combustion occurs more rapidly than in a diesel engine. Compression ratios are lower than those in diesel engines to avoid the phenomenon of pre-ignition, also known as knocking. A catalytic converter is used to clean unburned fuel, CO, and NOx from the exhaust stream. In general, the fuel-air mixture must be stoichiometricâthat is, containing just enough air to combust the fuelâwhich requires that the intake air be throttled at light loads, in turn reducing efficiency. As with diesel engines, turbocharging is a technique that can be used to increase power. Additionally, engine manufacturers vary valve and ignition timing as a func- tion of load and engine speed to optimize efficiency, power, and the emission profiles of the engines. There are several reasons for the prevalence of diesels for MHDVs. Diesel fuel has higher volumetric energy content
178 and recycle energy that would otherwise be lost during brak- ing. The net effect is to improve fuel economy, especially when the use of the MHDV requires a significant amount of starting and stopping. In a hybrid-electric vehicle, an electric motor is used for traction, and a battery is used for tempo- rary storage of energy. In a hybrid-hydraulic vehicle, hydrau- lic motors are used for traction, and accumulators are used for the temporary storage of energy. There are many different configurations of hybrid drivetrains, which must be tailored to the specific application of the vehicle. Hybrid drivetrains are more costly, more complex, and potentially less reli- able than standard drivetrains, but may offer a number of benefits. These benefits include the ability to reduce engine size; the ability to operate the engine in its optimal zone for improved fuel efficiency and emissions; and, in the case of hybrid-electric, the ability to run auxiliary loads, such as air- conditioning and lighting, from the electric subsystem rather than directly from the engine (NRC 2010, 2012). F.2.3 Alternative Fuels and Vehicle Technologies Although accounting for only a small part of the MHDV market today, alternative fuels and engine technologies are in some cases able to offer significant improvements over conventional fuels and engine systems. A few examples are detailed in the following. Natural gas. Both gasoline and diesel engines have been modified to accept natural gas as a fuel. The natural gas may be carried on the vehicle in either compressed (CNG) or liq- uefied (LNG) form; the latter, while more costly, also supports improved vehicle range. Engines typically rely on spark igni- tion and apply many of the same technologies to improve power and reduce emissions, such as turbocharging and EGR. Cummins Westport is now marketing a heavy-duty natural gas engine for the Class 8 tractor-trailer market, and the firm already produces natural gas engines for buses and other mar- kets (Cummins Westport, undated a). The emissions char- acteristics of natural gas engines are similar to those of new diesel engines, but natural gas, when used in MHDVs, offers simpler emissions compliance. Current market prices for nat- ural gas are well below those for diesel fuel on an energy equiv- alent basis. The principal barriers to the use of natural gas are increased vehicle costs, the relatively limited range afforded by currently available onboard storage tanks, increased weight due to specialized tanks, the limited availability of natural gas refueling infrastructure, and the limited availability of appro- priately trained mechanics [American Trucking Association (ATA) 2009]. Ensuring safety when handling compressed or liquefied natural gas is also a significant concern. Biofuels. Alternative liquid fuels, discussed in earlier appen- dices for the light-duty fleet, are also applicable to MHDVs. In Lighter-weight materials. Weight is a key consideration for MHDVs. Fuel consumption is directly related to the mass of the vehicle because a lighter vehicle requires less energy to accelerate, decelerate, and push uphill. Significant research is underway in developing new structural materials, including carbon fiber, to allow for the construction of lighter-weight trucks while also maintaining safety characteristics. Reduced weight would be most helpful for trucks not used for goods transportâbucket trucks and buses for exampleâbecause the saved weight in these cases would not likely be offset with increased load (NRC 2010). Even with freight trucks, though, the net effect of lighter vehicles with heavier loads would be beneficial, reducing the amount of fuel required to transport a ton-mile of goods. Aerodynamics and rolling resistance. At higher speeds, most of the power of the engine is used to overcome air resis- tance. For vehicles that routinely travel at high speeds, such as long-haul tractor-trailers and motor coaches, improved aerodynamics can result in significant fuel savings (NRC 2010). For example, many tractor-trailers are now outfit- ted with fairings to reduce turbulence in the undercarriage of the trailer. Similarly, new single-tire configurations have been developed that reduce rolling resistance, another major consumer of energy at high speeds. Gasoline direct injection (GDI). GDI is an engine tech- nology in which the gasoline is directly injected into the cyl- inder prior to ignition by a spark. GDI comes in two types: stoichiometric GDI (S-GDI) and lean-burn GDI. In both cases, the compression ratio can be increased by directly injecting the fuel into the cylinder, improving the efficiency of the engine. In S-GDI, a stoichiometric amount of fuel is added to the air, a requirement for three-way catalytic con- verters. The S-GDI system results in pumping losses at low loads. Alternatively, pumping losses may be reduced by allow- ing more air into the cylinder and burning less fuel, but then different and more expensive catalytic converters must be employed (NRC 2010). Homogeneous charge compression ignition (HCCI). The HCCI combustion process combines aspects of diesel and gasoline engines. A charge of fuel and air is introduced into the engine and is compressed until it auto-ignites (Ravi et al. 2012). In HCCI engines, combustion occurs much faster than in conventional gasoline or diesel engines, reducing the for- mation of NOx significantly. Unlike spark-ignition engines, HCCI engines can run lean, reducing throttling losses that compromise efficiency at low load. Compression ratios are higher than in a typical spark-ignition engine, improving efficiency. HCCI engines are under development and require precise control of the fuel mixture and engine characteristics to ensure proper operation. Hybrid-electric and hybrid-hydraulic drivetrains. The principal benefit of a hybrid drivetrain is the ability to recover
179 buses or small bus fleets operating in cities across the country [National Renewable Energy Laboratory (NREL) 2010]. Electric. Battery electric replacements for MHDVs are an option in applications where travel distances are short and there is ready access to charging systems. Although costly, bat- tery electric vehicles emit no pollutants locally, a compelling advantage in areas facing significant air quality challenges. One potential application is in port drayage systems, where containers must be moved and stacked within a confined area. F.3 Market Prospects Due to the longevity of most MHDVs, the year-over- year market for new vehicles is not especially large. As such, achieving high levels of penetration for alternative fuels and advanced engines and drivetrain technologies in the absence of environmental or policy motivators could take many years. More likely, the mix of different engine, drivetrain, and fuel types in use will continue to diversify, with traditional die- sel engines being supplanted in favorable applications with gasoline, natural gas, and other technologies. While a quan- titative assessment of the potential for alternative fuel and vehicle technologies is not presented, the key to understand- ing their applicability lies in the application, fleet, and usage characteristics, as discussed in earlier sections of this appen- dix. Table F.9 summarizes these characteristics for common MHDV applications. areas where it is mandated by law, MHDVs receive the same blends of gasoline and ethanol or diesel and biodiesel as do light-duty vehicles. Refined vegetable oils and synthetic die- sel fuels are also generally compatible as blends. Biodiesel, however, has slightly less energy content than petroleum die- sel, and it is thermally unstable and cannot be used in colder weather (Hileman et al. 2009). Hydrogen. The primary focus of research, development, and demonstration efforts for hydrogen is on fuel-cellâ powered vehicles, which convert the hydrogen into electricity that in turn powers an electric drivetrain. Hydrogen fuel cells emit only water vapor, thus offeringâdepending on how the hydrogen is producedâpotentially significant environment benefits. Hydrogen fuel-cell vehicles are not yet available for mass market adoption, and there are a number of technical and cost issues still to be resolved. In the absence of readily available hydrogen, natural gas may be substituted as a fuel for fuel-cellâpowered vehicles: an auto-thermal reformer can con- vert the natural gas to streams of carbon dioxide and hydrogen. This process could occur either on the vehicle or at the fueling site. In comparison to electric-only vehicles storing energy in a battery, fuel cells promise much greater vehicle range. The U.S. Department of Energy, partnering with the Federal Transit Administration (FTA), is evaluating the technical and economic viability of fuel-cellâpowered transit vehicles through the National Fuel Cell Bus Program. The program sponsors pilot tests of fuel-cell technologies and infrastructure for single Application Classes Fleet Typical Duty Cycle Typical Fuel Typical Refueling Location Tractor-trailer 7, 8 Local, regional, and national Extended periods at high speed Diesel Truck stop, own facility Straight truck, box 3â8 Local and regional Urban delivery, moderate average speed Diesel Own facility Straight truck, bucket 3â8 Local and regional Urban and rural use, significant auxiliary load Diesel Own facility Refuse truck 7, 8 Local Urban use with frequent stops and creeping at low speed Diesel Own facility Transit bus 7, 8 Local and regional Urban use with frequent stops at low speed Diesel but increasingly natural gas Own facility Motor coach 8 National Extended periods at high speed Diesel Truck stop Pickup trucks and small vans 2b Local Varied uses Gasoline or diesel Gas station Table F.9. Summary characteristics for MHDV applications.
180 EPA and NHTSA. 2011. âGreenhouse Gas Emissions Standards and Fuel Efficiency Standards for Medium- and Heavy-Duty Engines and Vehicles; Final Rule.â Federal Register, 76 (179). ESA. 2004. 2002 Economic Census: Vehicle Inventory and Use Survey, Geographic Area Series. Hileman, J., D. S. Ortiz, J. T. Bartis, H. M. Wong, P. E. Donohoo, M. A. Weiss, and I. A. Waitz. 2009. Near-Term Feasibility of Alternative Jet Fuels. RAND Corporation, Santa Monica. Krupnick, Alan J. 2011. Will Natural Gas Vehicles Be in Our Future? Resources for the Future, Washington, D.C. NRC. 2010. Technologies and Approaches to Reducing the Fuel Consump- tion of Medium- and Heavy-Duty Vehicles. The National Academies Press, Washington, D.C. NRC. 2012. Review of the 21st Century Truck Partnership, Second Report. The National Academies Press, Washington, D.C. NREL. 2010. Fuel Cell Transit Bus Evaluations Joint Evaluation Plan for the U.S. Department of Energy and the Federal Transit Admin- istration. ORNL. 2012. Transportation Energy Data Book, Edition 31. OTAQ. 2000. Regulatory Announcement: Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements. Ravi, N., H.-H. Liao, A. F. Jungkunz, H. H. Song, and J. C. Gerdes. 2012. âModeling and Control of Exhaust Recompression HCCI: Split Fuel Injection for Cylinder-individual Combustion Control.â IEEE Control Systems Magazine. SmartTruck. 2009. EPA Smart Truck Partnership. http://smarttruck systems.com/epasmartway.php (accessed September 24, 2012). While all MHDVs will benefit from general advances in engines and drivetrains, certain technologies and systems are likely to provide more benefits for some applications than for others. Table F.10 summarizes the technologies and fuels that appear to offer the most promise from different MHDV applications based on the considerations discussed in this appendix. References ATA. 2009. White Paper: Is Natural Gas a Viable Alternative to Diesel in the Trucking Industry. Arlington, Virginia. Clark, N. N., M. Gautam, W. S. Wayne, D. W. Lyons, and G. J. Thomp- son. 2007. Heavy-Duty Vehicle Chassis Dynamometer Testing for Emissions Inventory, Air Quality Modeling, Source Apportionment and Air Toxics Emissions Inventory. Coordinating Research Council, Inc. Cummins Westport, Inc. Undated a. Engines: Models. http://www. cumminswestport.com/models (accessed September 30, 2012). Cummins Westport, Inc. Undated b. Parts & Service. http://www. cumminswestport.com/parts-service (accessed September 17, 2012). EPA. 2012a. EPA Urban Dynamometer Driving Schedule. http://www. epa.gov/otaq/standards/light-duty/udds.htm (accessed September 17, 2012). EPA. 2012b. National Clean Diesel Campaign (NCDC). http://www. epa.gov/diesel/ (accessed September 24, 2012). Application Hybrid- Electric, Hydraulic Natural Gas Biodiesel, Ethanol Battery Electric Hydrogen Tractor-trailer Straight truck, box Straight truck, bucket Refuse truck Transit bus Motor coach Pickup trucks and small vans Notes: For pickup trucks and small vans, hybrid-electric technology is applicable but hybrid-hydraulic is not generally feasible. As shown by the absence of bullets in the battery electric column, electric vehicle technology is not viewed as promising for most medium- and heavy-duty vehicle applications. A possible exception, as suggested previously, is for certain contextsâsuch as at portsâwhere the ability of electric vehicles to mitigate severe local air-quality challenges may be highly prized. Table F.10. Promising fuels and technologies for MHDV applications.