With the implementation of a closed loop fuel control system, operation of lean-burn natural gas engines can be optimized in terms of reducing emissions while maximizing efficiency. Such a system would compensate for variations in fuel composition, but also would correct for variations in volumetric efficiency due to immediate engine history and long-term engine component wear. Present day engine controllers perform well when they are operated with the same gas composition for which they were calibrated, but because fuel composition varies geographically as well as seasonally, some method of compensation is required. A closed loop control system on a medium-duty lean-burn engine will enhance performance by maintaining the desired air-fuel ratio to eliminate any unwanted rich or lean excursions (relative to the desired air-fuel ratio) that produce excess engine-out emissions. Such a system can also guard against internal engine damage due to overheating and/or engine knock.In a project funded by the U.S. Department of Energy, West Virginia University has developed a closed loop control system using a Hercules turbocharged lean-burn natural gas engine fitted with a GFI Compuvalve and an Altronic spark ignition system as a test bed for the research. Closed loop fueling control is accomplished by means of feedback to the Compuvalve from an exhaust gas oxygen (EGO) sensor.Two types of EGO sensor, the NGK Universal Exhaust Gas Oxygen (UEGO) sensor and the Bosch LSM11 wide-range oxygen sensor, have been used in the feedback control. While the feedback control has proven successful, it is evident that gas composition has only a mild effect on engine performance provided that the Wobbe number of the fuel is controlled. This is found to be true even in open loop operation provided that the fueling strategy is based upon orifice discharge. In a speed-density system, however, variation in volumetric efficiency can cause significant unwanted (and potentially damaging) excursions in air-fuel ratio, performance, efficiency, and emissions. Data show that widely varying gas compositions of similar Wobbe number give rise to similar in-cylinder pressure histories.