Computational study of hydrogen engine combustion strategies: Dual-Fuel compression ignition with Port- and Direct-Injection, Pre-Chamber Combustion, and Spark-Ignition

Abstract

This work attempted to identify the best strategy for hydrogen internal combustion (IC) engines. Computational simulations were conducted to evaluate the combustion and emission characteristics of four combustion strategies: H2/diesel dual-fuel combustion with port-(DF-PI) and direct-injection (DF-DI) of H2, pre-chamber combustion (PCC), and port-injection spark-ignition (SI). Two ultra-lean conditions with an overall lambda (λ) of 2.5 and 3.0 were considered. A pilot energy fraction of 3% was applied in both DF modes to limit carbon oxide emissions. Note that since the DF-DI mode is free of end-gas autoignition, a high compression ratio at 17:1 was applied in contrast to the other three cases (13:1). Of the four strategies, the DF-DI and SI operations tended to yield the higher optimal indicated thermal efficiency (ITE) than the DF-PI and PCC modes, due to the highest expansion ratio and usable earliest spark timing, respectively. However, the diffusion combustion-dominant DF-DI operation generated the highest nitric oxides (NOx) emission, owing to the wide stoichiometric high-temperature flame periphery. Because of the pilot-intensified combustion process, the DF-PI operation was easy to induce end-gas autoignition, which considerably limited its load extension and improvement of ITE. In addtition, for both the DF-PI and DF-DI operations, their NOx emissions were challenging to be eliminated even with a modeled exhaust gas recirculation rate at 50% using water injection, indicating that a DI water injection method or an after-treatment system must be implemented for the further reduction of NOx emission. In contrast, both the PCC and SI modes were able to fulfill the EU VI regulation limit of NOx emission with a relatively high ITE. Considering that these two modes only need a single fuel supply system, they are considered more promising for practical applications than the DF-PI and DF-DI modes.