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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.

Automotive engineers have recognized the fuel economy advantage of elevated coolant temperatures for many years. Evaporative engine cooling is a well established technology which maintains the engine coolant at its boiling temperature regardless of operating conditions. By assuring that the coolant entering the engine is at nearly the same temperature as that leaving the engine, overcooling is avoided. Excellent temperature uniformity is assured throughout the engine, permitting coolant temperatures to be raised without raising peak metal temperatures. Evaporative cooling allows engine coolant to boil freely at atmospheric pressure, taking advantage of the inherently high heat transfer rates obtained with boiling and condensing. The benefits derived from evaporative cooling are faster warm-up, reduced friction, (which contribute toward fuel economy) greater engine durability, and safe, low pressure operation.