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Although an engine's lean limit fuel-air ratio may be easily found experimentally, it is difficult to predict accurately due to the influence of speed, load, ambient conditions, ignition characteristics, fuel chemistry, engine wear, etc. This paper will describe a new closed loop feedback control which adaptively tracks the engine's instantaneous lean limit using only one engine sensor. Test data is presented which indicates the control system response to changes in ignition timing, fuel chemistry, speed, and load. Results from laboratory and field tests indicate a significant improvement over programmed fuel-air scheduling systems, both in fuel economy and exhaust emissions.