Disclosed is a method of injecting natural gas into a diesel fuel engine for combustion with diesel fuel therein. One aspect includes injecting natural gas into an air-stream of an engine air intake or manifold, measuring the percentage of natural gas injected into the airstream or other efficiency gauge, varying the rate of injection of natural gas into the airstream in response to the measured percentage of natural gas therein and injecting the natural gas at a pre-determined rate so as to maintain an natural gas concentration in the air intake stream in the range of 0.6% to 3.0% by volume of natural gas.