Effect of the variation of compression ratio on engine performance and exhaust emission on spark ignition engine

Studies on improving fuel economy and exhaust emissions in the internal combustion engines have been gained more importance. Currently, the improvements of the effects of operating variables on the engine performance is a subject of study. The designed geometrical compression ratio can be realized as an effective compression ratio at the wide open throttle for a certain engine speed since the effective compression ratio changes with the amount of mixture into the cylinder in spark ignition engines. So, this condition of the spark ignition engines force designers to change their geometrical compression ratio according to the amount of charging into the cylinder.In this thesis, the compression ratio of a four-stroke and single cylinder spark ignition engine with compression ratio of 8.5/1 has been converted to the engine with variable compression ratio and the effect of compression ratio variations on engine performance and exhaust emissions has been investigated. In order to convert the standart engine to the engine with variable copmression ratio, the cylinder cover was grinded five times with 0,5mm steps. By doing so, engine compression ratio was changed between 8,5/1 and 12,72/1. Especially at full load, improvement in specific fuel consumption was achieved by up to 14,5% with compression ratio changing from 8.5/1 to 11,28/1. Also, CO emissions were reduced by almost 27%, HC emissions were increased by up to 18% at the same compression ratio range.