Evaluation of 48V and High Voltage Parallel Hybrid Diesel Powertrain Architectures for Class 6-7 Medium Heavy-Duty Vehicles 2021-01-0720

Electrification of heavy-duty trucks has received significant attention in the past year as a result of future regulations in some states. For example, California will require a certain percentage of tractor trailers, delivery trucks and vans sold to be zero emission by 2035. However, the relatively low energy density of batteries in comparison to diesel fuel, as well as the operating profiles of heavy-duty trucks, make the application of electrified powertrain in these applications more challenging. Heavy-duty vehicles can be broadly classified into two main categories; long-haul tractors and vocational vehicles. Long-haul tractors offer limited benefit from electrification due to the majority of operation occurring at constant cruise speeds, long range requirements and the high efficiency provided by the diesel engine. However, vocational applications can realize a significant benefit from electrified powertrains due to their lower vehicle speeds, frequent start-stop driving and shorter operating range requirements.

As the heavy-duty industry deals with solving challenges around the application of electrified powertrains, there are multiple pathways that can be explored to meet future regulatory requirements. This paper is the first part of a two-paper series that focuses on evaluating electrified solutions for Class 6-7 medium heavy-duty vehicles in the 2027 and beyond time frame. In this paper the focus is on investigation of near-term hybrid solutions that provide reasonable fuel efficiency improvements within a two-year payback period.

To investigate the various hybrid electric architectures, FEV has developed a system level approach for the selection and sizing of heavy-duty diesel hybrid powertrain components using GT-SUITE. The approach has been applied for a Class 6-7 urban vocational truck, which typically experiences low speed driving with frequent start-stops. A dynamic model for the baseline diesel vehicle was developed and calibrated to test data. The baseline diesel vehicle was then updated with hybrid powertrain components to evaluate different parallel hybrid architectures (P1, P2, P3, P4) at two different voltage levels: ≤48V (mild hybrid) and >150V (full hybrids). The evaluation was conducted over multiple drive cycles, including the ARB Transient Cycle and a real-world drive cycle. In the evaluation, key trade-offs were identified between fuel consumption, initial investment cost, payback period and freight efficiency. The trade-off analysis demonstrated that for a two-year payback period, a P3 architecture provided the best fuel consumption value for full hybrid applications. In a P2 or P3 configuration, a 48V system also showed considerable fuel efficiency improvements compared to the baseline diesel vehicle. The final P3 hybrid powertrain configuration for Class 6-7 vocational truck shows a 27% fuel consumption reduction for a 350V system while a 48V system shows a 22% fuel consumption reduction when considering a payback period of two years.