The depletion of fossil fuels is understood by most energy users to be an inevitability. Researchers all over the world focus their attention on the development of various alternative fuels. It is believed that the use of compressed natural gas (CNG), as an alternative to conventional fuels will result in low levels of emissions. However, the use of CNG as the main fuel in a diesel engine with the diesel fuel used as an igniter has always been associated with some problems. The main problem in such a dual-fuel engine is the increased tendency toward detonation because of the high compression ratio of the engine. The current work is an experimental investigation of such a problem. An extensive experimental program was carried out on a variable compression ratio Recardo E6 engine. The results of the current investigation indicate that the dual-fuel engine can operate detonation-free for a compression ratio of 16.5. The results also indicate that dual-fuel engines are lower than diesel engines in brake thermal efficiency, brake power, and brake mean effective pressure at all loads. On the other hand, CO and NO emissionsare higher at high loads. However, at low loads, dual-fuel engines give lower NO.