This paper presents the engine performance, combustion process, and exhaust emissions from of a turbocharged spark ignition (SI) WP-10 off-road engine developed to operate on gaseous fuels applicable to a wide range of the higher heating value (HHV) (900 to 1400 BTU). The HHV of the fuels was varied by blending of propane or carbon dioxide (CO2) into natural gas (NG). The developed engine was designed to operate at 1800 rpm and 175 kW.

A new method of calculating the specific heat ratio of the bulk gases with the calculated bulk gas temperature and composition was proposed. The specific heat ratio calculated using this method was lower than the value derived from the conventional Log P-Log V method. The application of the specific heat ratio calculated in calculating the heat release process increased the heat release rate (HRR) and the total heat released during combustion. In addition, it also resulted in retarded phasing of CA50 and CA95 defined as the crank angle location when 50% and 95% of total energy was released.

The effects of the fuel composition on engine performance, combustion process, and exhaust emissions were experimentally investigated. It achieved a brake thermal efficiency of about 32.8%. The exhaust emissions are in compliance with both EPA and CARB regulations. The addition of propane to NG increased the HRR, accelerated the combustion process, and shortened the combustion duration. This was the result of the quicker flame propagation property of propane. The HRR observed with propane blending was featured with two heat release peaks. The peak HRR observed with 1400 BTU fuel was about 10% higher than that observed with NG only operation. As expected, the blending of CO2 to NG was shown to slow down the combustion process, and retarded the combustion phasing, especially during the completion of combustion.

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