Design and performance analysis of hybrid electric class 8 heavy-duty regional-haul trucks with a micro-pilot natural gas engine in real-world highway driving conditions

Abstract

Diesel-fueled heavy-duty (HD) trucks are a key component of global freight transport, but consume 25% of the transportation industry's energy and emitting substantial greenhouse gas (GHG) emissions. Utilizing low-carbon fuels like natural gas (NG), combined with hybridization, presents an excellent opportunity for reducing GHG emissions and fuel consumption. Micro-pilot dual fuel diesel/NG engines offer high engine brake thermal efficiency (BTE) with an efficient three-way catalyst (TWC) to limit pollutant emissions like CO2 without the need for costly exhaust aftertreatment systems. This engine is particularly well-suited for hybridization, as it features high-efficiency under stable, mid-, and high-load conditions, but with reduced performance at low loads. This paper validates two conventional baseline models (X15 Cummins diesel engine and micro-pilot NG engine) based on a tested class 8 regional-haul truck on highway. The study investigates the combined effect of low-carbon fuels and hybridization on improving the fuel economy and GHG emissions of class 8 regional-haul trucks. This involves designing pre-transmission parallel and plug-in hybrid configurations with downsized 6.7-liter micro-pilot NG engines. The cargo load and performance requirements are maintained equivalent to the conventional diesel baseline. Results show comparable fuel economy and engine BTE between conventional diesel baseline and NG models on highways with full cargo load. The parallel and series plug-in hybrid NG designs exhibit fuel economy improvements of 2.9% and 26%, respectively, compared to the micro-pilot NG over highway with full cargo load. In the same conditions, the CO2 emissions reductions for parallel and series plug-in hybrid NG designs are 15% and 51%, respectively, relative to the conventional diesel baseline. In addition, this article provides a detailed comparison and analysis of non-plug-in and plug-in hybrids based on component energy losses and internal combustion engine (ICE) operating points.