Lean H2 combustion strategies have shown promising gross thermal efficiency and ultra-low engine-out NOx emissions for H2-fuel based internal combustion engine (H2ICE) systems 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 possessed no external exhaust gas recirculation (EGR).
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 SET cycle: A75, A100, B75, B100, and C100. A wide range of lambda levels and 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. Later, implementing a low-pressure EGR system, the effects of air and external EGR dilution strategies were compared at A-speed conditions for both boost systems.
From the results, high lambda levels reflected the benefits of lean combustion operation. Applying Miller strategies 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 surge and the choke limits when nearing lambda 2.2 to 2.5 levels. A 2S boost system achieved higher lambda levels without risking compressors choke or surge limits. At similar lambda levels, an active HP-stage bypass control managed improved PMEP for the 2S boost system. For both boosting systems, large charge cooling capacity was observed that further increased with the increasing lambda levels. A detailed fuel-energy balance analysis was conducted to highlight trade-offs between the1S and the 2S boost-system based H2ICE concept.