The combination of lean mixtures to provide oxygen in the exhaust and of exhaust heat conservation to enhance exhaust reactions yields significant exhaust reaction and lower hydrocarbon emissions. It requires adequately uniform mixtures, but does not require injection of additional air into the exhaust. A vehicle with this combination exhibits low emissions and good performance and fuel economy. Full evaluation of this approach must await general application. The results of this experimental exploration are presented as a contribution to the multi- industry effort to provide the public with acceptable atmospheric cleanliness at minimum vehicle and fuel cost.