Abstract: An apparatus and method is disclosed for minimizing the fuel usage in an internal combustion engine. The subject invention is particularly adapted for use with an engine installation subject to varying loads and which includes a governor for varying fuel flow as a function of load. In operation, the combustibles in the exhaust gas of the engine is continuously monitored. The measured level of combustibles is then compared with a predetermined level corresponding to optimum efficiency. A controller is provided for varying the air/fuel ratio supplied to the engine for maximizing efficiency in correspondence with the preset level. By this arrangement, energy output is increased permitting the governor to further reduce fuel flow, thereby minimizing energy costs.