Differences Between Heavy-Duty and Light-Duty Hybrids

Requirements for heavy-duty hybrid vehicles are significantly different from those of light-duty (LD) hybrid vehicles, and they necessitate unique solutions. In this chapter, the term heavy-duty vehicles refers to both medium heavy-duty trucks and heavy-duty trucks, as defined in Figure 1-3 in Chapter 1 of this report. Many technologies that apply to light-duty vehicles do not apply to heavy-duty vehicles. The heavy-duty truck and light-duty vehicle hybrid technologies leverage each other only at the most basic level. Consequently, the 21CTP hybrid program is needed to address the unique technology needs of heavy-duty vehicles.

Unlike LD hybrid vehicles, the broad class of vehicles comprising the heavy-duty fleet is very diverse and includes tractor-trailer, refuse, dump and utility trucks, package delivery vehicles, buses, and large pickups. These vehicles have highly differentiated mission profiles, which make it difficult to establish architectures or performance metrics that are commonly applicable to this broad range of heavy-duty vehicles. Key differences between heavy-duty trucks and light-duty vehicles (LDVs) include the following:

•   Volume: Annual sales volume for heavy-duty trucks is about 5 percent of that for LDVs, and the former can be bought in a thousand times more configurations than the latter.

•   Buying criteria: Heavy-duty truck buyers prioritize reliability and cost of ownership, whereas LDV buyers prioritize a variety of attributes including cost, functionality, reliability, performance, and styling.

•   Weight: A heavy-duty truck can weigh up to a hundred times more than an LDV and has peak horsepower up to twice that of LDVs.

•   Life expectancy and driving cycles: Heavy-duty vehicles have longer life expectancy and more demanding duty cycles than those for LDVs. Heavy-duty vehicles have expected lifetime mileages nearly 10 times greater than those of LDVs.

Driven by these vehicle differences, factors differentiating hybrid systems for heavy-duty vehicles and those for LDVs are power rating, energy storage capacity, number of relevant driving cycles, and economics. Unlike cars and light trucks, which are available in only a relatively restricted range of sizes and weights and whose missions have been characterized by a few standardized driving cycles, heavy-duty trucks span a size range from 8,500 lb (Class 2b) to greater than 33,000 lb (Class 8), with gross vehicle weights (GVWs) of up to 200,000 lb (DOE, 2011). As a result, heavy-duty hybrid vehicles require high energy storage density, much like an EV, as well as high power density for acceleration and deceleration, like light-duty hybrids.

Functional differences between hybrid systems for heavy-duty trucks and those for LDVs result in substantial economic differences. Light-duty vehicles are fundamentally similar. Their weight range is relatively limited (up to Class 2a, under 8,500 lbs GVW), and their driving schedules are characterized by a small number of driving cycles. They are manufactured in high volumes, and their expected lifetime mileage is up to 150,000 miles. Hybrid systems for these vehicles can be manufactured in volumes large enough to benefit from the economies of scale.

Heavy-duty trucks by contrast vary widely in both tare (empty) and gross weights. Their missions vary from daily runs with frequent stops (e.g., the work of a delivery van) to 24-hour-a-day, multiple-day long hauls of tractor-trailers up to 200,000 lb GVW. The total fleet of heavy-duty vehicles is 10.99 million, and the average life of a vehicle is up to 1 million miles, resulting in a small market with low turnover and a challenge to making an economic argument for hybrid systems generally applicable to the heavy-duty fleet. A conventional hybrid design applied to vehicles whose missions incorporate a lot of stop-and-go driving, such as delivery vans, urban transit buses, or refuse trucks, has the potential to be economically sound (i.e., to result in a favorable payback period) by providing substantial fuel economy benefits of 20 to 40 percent (17 to 29 percent reduction in fuel consumption) (Greszler, 2009).1 A further benefit of hybridization in these vehicles is the reduced brake wear and maintenance resulting from regenerative braking.

In contrast to medium-duty delivery vans, urban transit buses, and refuse trucks, a heavy-duty, Class 8 long-haul truck makes few stops, maintains a relatively constant speed, and requires high power for long periods of grade climbing. According to the 21CTP, there are three primary reasons to consider hybridizing a Class 8 long-haul powertrain (Greszler, 2009):

1.   Reduced engine idle time, through the hybrid energy storage and use of electric auxiliaries;

2.   Reduced fuel use, through the electrification of components, thereby improving efficiency; and

3.   Reduced fuel usage during cruise, through energy management with traffic-induced speed variation and in rolling terrain.

An additional reason to consider hybridizing a long-haul truck is that, as mentioned above, the large-capacity batteries can be recharged during daytime operation to provide for hotel loads so that overnight idling of the main engine can be eliminated.

Hybrid Technology for the SuperTruck Program

In FY 2010, the DOE announced the establishment of the SuperTruck program, with an overall goal to develop and demonstrate a 50 percent improvement in freight efficiency


1 Answers provided by Ken Howden, DOE Office of Vehicle Technologies, to committee questions 9(a) and 42.

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