Volume = Total production of vehicles in the regulatory category
UL = Useful life for the regulatory category (miles)
VOCATIONAL VEHICLE AND TRACTOR HYBRIDS
For vocational vehicles and combination tractors incorporating hybrid powertrains, the agencies specify two methods for establishing credits. The first method uses chassis dynamometer evaluation of the vehicle, and the second method uses engine dynamometer evaluation with the powerpack in either a (1) pre-transmission format or a (2) post-transmission format. Each method requires a comparison test of the actual physical product, because the agencies are not aware of analytical models that can assess the technology.
Chassis Dynamometer Evaluation
Similar to heavy-duty pickup and van hybrids, to generate credits for hybrid vocational vehicles, full vehicle chassis dynamometer testing is a straightforward basis for comparing fuel consumption performance of hybrid vehicles to conventional vehicles. The agencies specify two sets of duty cycles for vocational trucks to evaluate the benefit depending on the vehicle application. The key difference between the two sets is that one does not include operation of a power take-off (PTO) unit while the other does. For example, delivery trucks do not operate a PTO while bucket and refuse trucks do.
The duty cycles that apply to hybrid powertrains without a PTO system are shown in Table F-1 .
The transient cycle, derived from the California Air Resources Board (CARB) Heavy-Duty Truck 5 Mode Cycle, is 668 seconds long and travels 2.84 miles. The cycle contains 5 stops and contains 112 seconds of idling. The maximum speed of the cycle is 47.5 mph with an average speed of 15.3 mph. The High Speed and Low Speed Cruise modes reflect constant speed cycles at 65 mph and 55 mph, respectively, which are representative of drivers using cruise control whenever possible. The final rules include a new optional PTO test cycle in addition to the standard set of test cycles in order for manufacturers of advanced PTO systems to demonstrate in the laboratory environment fuel consumption reductions that would be realized from their systems in the real world. The composite PTO test cycle for utility and refuse trucks is described in greater detail in EPA/NHTSA (2011b, see Table 3-23).
|Vehicle Category||Transient||55 mph||65 mph|
|Day cab tractor||19||17||64|
|Sleeper cab tractor||5||9||86|
Engine Dynamometer Evaluation
The engine test procedure involves exercising the conventional engine and the hybrid-engine system based on an engine testing strategy. Real-world functionality would need to be accurately represented. The testing would also need to recover vehicle kinetic energy. The agencies specify the use of the Heavy-Duty Engine FTP cycle for evaluation of hybrid vehicles, which is the same test cycle specified for engines used in vocational vehicles. Engine dynamometer evaluation may be undertaken in one of two ways:
1. Pre-transmission power-pack testing, which includes the engine and hybrid systems in a pre-transmission format, could utilize existing engine certification duty cycles. Changes to how the engine certification would be conducted to address energy capture and idle operation would need to be evaluated as a complete protocol is developed.
2. Post-transmission power-pack testing, which includes the transmission, would require a vehicle-like duty cycle, which provides the appropriate speeds and torques to match field operation.
Heavy-duty hybrid vehicles and hybrid powertrains can be certified using an A to B test method. This concept entails testing the conventional vehicle or powertrain, identified as “A,” and the hybrid version of the vehicle or powertrain, identified as “B.” The benefit associated with the hybrid system for fuel consumption would be determined from the weighted fuel-consumption results from the tests of each vehicle or hybrid powertrain, as described below:
1. Improvement Factor = (Fuel Cons_A - Fuel Cons_B)/(Fuel Cons_A)
2. Gallons/1,000 ton-mile benefit = Improvement Factor × GEM Fuel Cons Result_B
Note in the above equations that the GEM (Greenhouse Gas Emissions Model) result would be calculated for the base vehicle or powertrain without hybridization, and the Improvement Factor would account for hybridization of the vehicle or powertrain.
The following equation for the credits (debits) accounts for the amount that the family emission limit is below (above) the standard, the payload tons, the production volume, and the useful life: