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In this study, systematical experiments were carried out to analyze the performance and emission characteristics of CNG using a single cylinder, variable compression ratio, spark ignition engine. Studies were also carried out to evaluate optimum spark timing at different equivalence ratios for the maximum brake torque. Effects of compression ratio on fuel consumption and exhaust emissions were also studied. Compressed natural gas showed 3-5% higher thermal efficiency and 15% lower fuel consumption as compared to gasoline. Also CO emissions were lower by 30-80% in rich zone and NOx emissions were lower by 12% at equivalence ratio of 1.0. At 50% throttle opening, MBT timings for natural gas were 4 to 6 degree more advanced depending on equivalence ratios as compared to gasoline. Increasing the compression ratio from 8 to 10 and 12 improved the thermal efficiency by 3 and 8 percent respectively. However, NOx emissions were increased by 15% and 40% respectively.

In this study, experiments were carried out to evaluate fuel consumption and exhaust emissions of carbon monoxide (CO), oxides of nitrogen (NOx) and hydrocarbons (HC) with compressed natural gas (CNG) and gasoline fueled SI engine powered vehicles. Studies were carried out to evaluate optimum spark timing to operate both on CNG as well as gasoline mode for fuel consumption and emissions. In this connection, mass emission tests were carried out at various spark timings. Conversion efficiency of a 3-way catalytic converter was also evaluated at different air-fuel ratio for CNG operation. The results showed that exhaust emissions of CO and NOx were significantly less with CNG as compared to gasoline. Optimum spark timing for better fuel economy and lower emissions on gasoline and CNG modes were 10° before TDC and 24° to 26° before TDC respectively.