Characterization of Exhaust Gas Recirculation Effects in a Hydrogen-Fueled 4-Cylinder Engine with Direct Injection

The increasing importance of hydrogen as alternative energy source to reduce CO₂ emissions in the transport sector makes its adoption in spark-ignited engines an attractive and cost-efficient alternative to fuel cell-powered vehicles. Lean combustion is the preferred operating strategy for H₂-engines in order to achieve performance targets, enhance efficiency and at the same time avoid critical knocking and pre-ignition phenomena. Additionally, an effective approach to lower cylinder temperatures, relevant engine-out NOx emissions and boost pressure requirements at the same time, is an external exhaust gas recirculation (EGR) system. The aim of this work is to analyze and compare the effects of exhaust gas recirculation on the combustion of a lean hydrogen mixture in a turbocharged 4-cylinder H₂-ICE with direct injection. For this investigation a load point at 18 bar BMEP and 4000 rpm is selected with and without the utilization of additional external EGR. In this case, a BTE of 38 % is achieved at lambda 1.8 with an 11 % EGR rate. Under these conditions, the NOx emissions are also reduced by 80 % while the efficiency rises by 0.7 %pt., compared to the case without exhaust gas recirculation. Several experiments at the test bench were carried out and used to calibrate a 3D-CFD engine model, in which the complete 4-cylinder engine is virtually reproduced, including high-pressure injection and detailed chemistry for combustion and knock modeling in a single simulation. By investigating the impact of a varying residual gas concentration and air-to-fuel ratio on the mixture formation and the combustion properties, consequences for NOx emissions and abnormal combustion events are derived. A sensitivity study regarding the EGR temperature was additionally performed by means of 3D-CFD simulation to analyze the effects of 80 °C, 160 °C and 240 °C hot external EGR on the combustion process.