Diesel engined buses are the major means of transportation in many urban and suburban areas. Compared with other transportation systems, bus fleets are flexible, effective and low in capital cost. However, existing buses contribute to a serious air pollution problem in many cities. They also consume large amounts of diesel fuel, which is a concern for national economies where locally available natural gas could displace the more expensive petroleum-based fuel. New engine designs significantly reduce pollutants and some use alternative fuels. However, there is a huge infrastructure of existing diesel buses. Expensive new buses or bus engines will only gradually displace them, particularly in countries with weaker economies. The urgently required fuel replacement and pollution reduction benefits must be deferred into the future. These factors lead to the requirement for an economically viable, clean-burning conversion system to convert existing diesel engines to natural gas fuel.The system utilized is a dual fuel conversion using multipoint, timed natural gas injection at each intake port and uses the existing diesel injection system to inject a small quantity of diesel as an ignition source. At idle and low loads, the engine runs on diesel fuel only. At high loads, the diesel injection is reduced and replaced by natural gas under control of an engine computer. The natural gas injection system is installed in the intake manifold and the electronic controls added to the fuel pump with minimal modification of the original diesel engine. This keeps the conversion cost low and retains the efficiency advantages of the high diesel compression ratio. For maximum flexibility, it also allows normal diesel operation for extended range or interruptions in the gas supply.The paper discusses test results at maximum torque and over the Japanese 6-mode test schedule. In maximum torque testing, it was possible to increase peak torque and power significantly using the same exhaust temperature limitation. This was accompanied by a 91% reduction in soot, 17% reduction in reactive hydrocarbons, and 29 % reduction in CO, emissions over the 6-mode test schedule. In original tests with standard diesel injection timing, the NOx emissions were up by 23% due to the faster combustion of the pre-mixed natural gas. Further work with adjustable injection timing showed that retarding the injection timing could provide better torque and substantially reduced NOx emissions for the dual fuel conversion.