In the present study, a 17 kW, stationary, direct- injection diesel engine has been converted to operate it as a gas engine using producer-gas and compressed natural gas (CNG) as the fuels on two different operational modes called SIPGE (Spark Ignition Producer Gas Engine) and DCNGE (Dedicated Compressed Natural Gas Engine). The engine before conversion, was run on two other modes of operation, namely, diesel mode using only diesel and producer-gas-diesel-dual-fuel mode with diesel used for pilot ignition. The base data generated on diesel mode was used for performance comparison under other modes to ascertain the fuel flexibility. A technology development and optimisation followed by performance confirmation are the three features of this study. The exercise of conversion to SIPGE is a success since comparable power and efficiency could be developed. DCNGE operation also yielded comparable power with higher efficiency, which establishes the fuel flexibility of the converted machine. The spark advance needed for producer-gas operation is much higher at 35 °BTDC as compared to CNG operation where it was 22 °BTDC, the compression ratio being same, i.e., 11.5:1 in both the cases. Therefore, with ignition timing adjustment, almost full flexibility between these two extreme fuels is a reliable concept with the requisite gas induction fittings. Due to the lack of an adequate control system, part load operation of the engine is a problematic area with both the fuels. Compared to a diesel or dual-fuel engine, the SIPGE on producer-gas or on CNG gives zero smoke, almost comparable emissions of HC, which are very low in quantitative terms, and much lower CO. The nitrogen oxide emissions of SIPGE and DCNGE are lower than the normal diesel engines.