Lean H2 combustion strategies have shown promising gross thermal efficiency and ultra-low engine-out NOx emissions for H2-fuel based internal combustion engines (H2ICE) in heavy-duty (HD) transport. Implementing lean combustion strategies require excessive air flow demand that further increases with the engine load increase. To meet such air flow demands efficiently across a wide engine operating region, a detailed system optimization is warranted including next generation turbocharging systems.
In this 1D system analysis campaign, a detailed study of various air-system configurations was conducted for a modified HD, direct-injection (DI), H2ICE concept based-off a Cummins heavy-duty 15L engine. The concept engine configuration had a geometric compression ratio of 10.4 and no external exhaust gas recirculation (EGR) was implemented.
First, a calibrated 1D engine model representing the H2ICE concept was developed. Using the 1D model, a detailed system-level analysis was conducted at five operating conditions from the heavy-duty SET cycle: A75, A100, B75, B100, and C100. A wide range of lambda levels, valve phasing, miller strategies were characterized by the gross engine performance improvements. Subsequently, different air-system configurations were evaluated for closed-cycle efficiency vs pumping losses trade-offs, while meeting the air flow targets. For next-generation turbocharging, both single stage (1S) and two-stage (2S) boost systems were simulated. Air versus external EGR dilution strategies were also studied at boost-limited engine operating conditions.
From the results, high lambda levels reflected the benefits of lean combustion operation. Implementing millerization and cam phasing further elevated these benefits, at the expense of high boost pressure demands. The 1S boost system, with advantages of low-complexity and post-turbine thermal performance, incurred rapidly deteriorating turbocharger performance from the choke and the surge limits for lambda levels beyond 2.2. A 2S boost system achieved higher lambda levels without risking compressors choke or surge limits. Irrespective of turbocharging, the required intake charge cooling was noted ~2-3x times of the conventional diesel engine levels, depending on the targeted lambda levels. A detailed fuel-energy balance analysis was conducted to highlight system trade-offs between the 1S and the 2S based H2ICE configurations.