Effects of compression ratio on performance and emission characteristics of heavy-duty SI engine fuelled with HCNG

The wide range of hydrogen's flammable limits enables ultra-lean combustion. A lean burn reduces the combustion temperature, increases thermal efficiency, and reduces knock, which is a serious problem in a spark ignition (SI) engine. The anti-knock improvement from hydrogen addition makes it feasible to increase the compression ratio (CR) and further improve the thermal efficiency. Herein, the effects of the CR on performance and emission characteristics were investigated using an 11-L heavy-duty SI engine fuelled with HCNG30 (CNG 70 vol%, hydrogen 30 vol%) and CNG. These fuels were used to operate an engine with CRs of 10.5 and 11.5. The results showed that thermal efficiency improved with an increased CR, which significantly decreased CO2 emission. On the other hand, the NOx emission was largely increased. Nevertheless, for HCNG30, a CR of 11.5 improved thermal efficiency by 6.5% and decreased NOx emission by over 75%, as compared to a conventional CNG engine.

Figure optionsDownload as PowerPoint slideHighlights
► A higher compression ratio remarkably increases thermal efficiency.
► High thermal efficiency by a higher compression ratio reduces CO2 emission.
► However, NOx emission was largely increased at the same excess air ratio.
► Efficiency was 6.5% higher and NOx was over 75% lower compared to CNG engine.
► Reducing THC and CH4 emissions will be necessary for the emission regulation.