Better fluid mixing in automobile engines can contribute to fuel economy, smoothness of engine operation, and reduction of emissions. It is possible to improve the uniformity of mixture of fuel with air in the induction system, of trapped burned gas with the charge in the cylinders, and of air with the exhaust gas in a reactor.Criteria of uniformity of mixing are discussed and a more stringent criterion suggested. The mixing length concept is developed. Fluid flow mixing models for different passage geometries are presented and evaluated for mixing length. Mixing performance of the induction system and cylinders of a typical engine are estimated.Improved mixing means are proposed and described. Maximum uniformity of distribution of the charge involves effective vaporization of fuel and mixing with the air ahead of the intake manifold. Maximum uniformity of mixture and turbulence prior to combustion involves the mixture of the trapped burned gas with the charge in the cylinders.