Stringent emissions regulations and the need for lower tailpipe emissions are pushing the development of low-carbon alternative fuels. H2 is a zero-carbon fuel that has the potential to lower CO2 emissions from internal combustion engines (ICEs) significantly. Moreover, this fuel can be readily implemented in ICEs with minor modifications. Batteries can be argued to be a good zero tailpipe emission solution for the light-duty sector; however, medium and heavy-duty sectors are also in need of rapid decarbonization. Current strategies for H2 ICEs include modification of the existing spark ignition (SI) engines to run on port fuel injection (PFI) systems with minimal changes from the current compressed natural gas (CNG) engines. This H2 ICE strategy is limited by knock and pre-ignition. One solution is to run very lean (lambda >2), but this results in excessive boosting requirements and may result in high NOx under transient conditions. The volumetric efficiency of the engine is also reduced in a port-fueled application due to the low volumetric energy density of H2 which displaces fresh air. A novel mixing-controlled combustion strategy is proposed that significantly reduces the propensity of abnormal combustion at stoichiometric air/fuel ratios while also alleviating the need for extreme boosting.
The study was conducted on a pent-roof spark-ignited single-cylinder engine modeled from a large-bore medium-duty engine. A direct injection (DI) system capable of injecting H2 at 170 bar was integrated into the cylinder head. Both, lean and stoichiometric operation of the engine was explored in conjunction with various injection strategies. At a constant load of 8 bar at 1000 rpm test condition, it was shown that a homogenous split-injection strategy, where 50% of the total fuel mass was injected a few degrees after spark timing, was beneficial in NOx reduction while a stratified single-injection strategy exhibited the best thermal efficiency. Further, the results indicated that a stratified combustion strategy was able to increase the knock-limited load of the engine from 3.7 to 8.4 bar gIMEP load at 1000 rpm. This strategy also demonstrated increased efficiency compared to a homogeneous combustion mode and produced lower NOx at comparable loads. The diffusion-like combustion enabled by post-spark injection successfully demonstrated further knock mitigation and NOx reduction but was limited in performance due to challenges associated with in-cylinder mixing and DI injector flow rate.