Assessing the Effect of Compression Ratio on the Performance, Combustion and Emission Characteristics of a Spark-Ignition Engine, and Optimum Spark Advance at Different Operating Conditions 2018-01-1668

Nowadays, emission regulations and the requirement to reduce greenhouse gas emissions have escalated engine development efforts. In the present work, the effect of compression ratio on the performance, combustion and emission characteristics of a spark-ignition engine is evaluated at different operating conditions. A single-cylinder, water-cooled, spark-ignition engine (modified from a compression-ignition version) was used, with combustion chamber geometry consisted of flat cylinder head and a hemispherical bowl in the piston. Results showed that the brake thermal efficiency was increased from 9.8% to 12.9% when compression ratio was increased from 6.7:1 to 9.4:1 at low operating load of 5 N-m. Carbon monoxide emission was decreased when compression ratio was increased at all operating loads. However, as expected, nitric oxide emission was increased with the increase in compression ratio, with lower difference at low loads compared to medium and high loads. At medium and high loads, optimum spark advance resulted in the location of peak cylinder pressure around 14^o ATDC. Moreover, at low loads and low compression ratios, the cycle-to-cycle combustion variations were found to be significant, and hence, coefficient of variation of IMEP was also considered to determine the optimum spark advance. Overall, utilizing the higher compression ratio of variable compression ratio technology at low loads is useful for the improvement of fuel economy along with the reduction of cycle-to-cycle combustion variations and carbon monoxide emission, with no significant increase in emissions of unburned hydrocarbon and nitric oxide.