Integrated optimization of high-efficiency natural gas engines and hybrid-electric powertrains for heavy-duty trucks

The integrated optimization of a high-efficiency engine that burns a low-carbon fuel and a hybrid-electric powertrain can reduce greenhouse gas (GHG) emissions and fuel consumption in heavy-duty commercial vehicles. This study combines a high-efficiency high-pressure direct injection (HPDI) of natural gas (NG) engine with hybrid-electric powertrains for long-haul commercial transport. A system-level simulation framework couples an experimentally validated 1-D GT-SUITE™ engine model with an experimentally verified Autonomie® model of a heavy-duty truck. The engine was sized to meet the demands of the hybrid propulsion system. Engine efficiency was optimized for use in the hybrid powertrain, featuring turbo-compounding, a high compression ratio, and advanced injection timing. Two hybrid configurations are compared: a pre-transmission (P 2 ) parallel hybrid with a 13 L HPDI NG engine and a series plug-in hybrid with a downsized 9 L HPDI NG engine. The model indicates that the P 2 hybrid NG truck with an optimized engine improves fuel economy by up to 8.7% on highways and 37% in urban conditions. The series plug-in hybrid delivers 25–34% fuel savings on highway cycles and 53–68% in urban cycles. Hybridization also reduced engine-out emissions of NO x and CH 4 , primarily by shifting engine operation to higher loads, reducing cycle-composite NO x by 50%–87% and CH 4 by up to 99%. Tailpipe GHG emissions decreased by 26%–50% in the NG P 2 hybrid and by 57%–75% in the NG plug-in hybrid relative to the baseline diesel. A well-to-wheel analysis further indicates that upstream emissions, especially the electricity grid carbon intensity for the plug-in hybrid, can substantially change the total GHG emissions. These findings underscore the strong potential of combining optimized HPDI NG engines with hybrid-electric powertrains to decarbonize freight transport.