Challenges and Opportunities with Direct-Injection Hydrogen Engines

Abstract

<div class="section abstract"><div class="htmlview paragraph">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 &gt;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.</div><div class="htmlview paragraph">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.</div></div>