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Automotive engine emissions have been shown to be sensitive to changes in engine control variables. One of the most important variables is the fuel-air ratio (f/a) which is initially maintained by today's carburetors to about 3-5% but degrades with time. This research investigated several different sensors to be used in the exhaust for measurements of the f/a and investigated closed-loop control techniques for maintaining better accuracies of the f/a. Of the three sensors tested, a zirconia device which measures oxygen in the exhaust was found to have the most desirable properties from a control standpoint and was used in the f/a controllers. A laboratory engine was equipped with a fuel injector and used to perform studies on the closed-loop controller. It was found that f/a accuracies of 1% or less were readily achievable in the presence of conceivable disturbances.