Experimental and numerical investigation of the effects of low-pressure, high-pressure and internal EGR configurations on the performance, combustion and emission characteristics in a hydrogen-enriched heavy-duty lean-burn natural gas SI engine

In this study, a detailed 1D simulation model of the hydrogen-enriched natural gas SI engine was built according to realistic engine layout in the lab with practical boundary conditions, and validated against the experimental data. Four different types of EGR systems were comprehensively investigated, including a low-pressure (LP) EGR, a high-pressure (HP) EGR, their combinations, and an internal EGR, using the validated 1D simulation model of the hydrogen-enriched natural gas SI engine for their combustion, performance, and emissions characteristics. The results indicated that the peak combustion pressure of the in-cylinder pressure decreased with increase in EGR ratio either using the HP, LP, or combined HP-LP systems. The NOx emissions with the HP EGR were observed to be lowest of all types of EGR systems. Combined the 10% HP EGR ratio with the 5% LP EGR ratio (total 15% EGR ratio) was predicted to achieve the highest indicated thermal efficiency compared with other EGR strategies. As for the internal EGR, the peak combustion pressure and the heat release rate slightly increased with decrease in the valve overlap. Moreover, the indicated thermal efficiency firstly increased with decrease in overlap and then slightly declined. The internal EGR was mainly determined by the timing of the exhaust valve closing; However, it was not the only influence factor. The positive and negative waves in the intake and exhaust systems also played a crucial role in the gas exchange process and volumetric efficiency.