Motivated by investigations into catalytic converter modelling, this paper considers the problem of using an engine to generate a catalyst feedgas which has a predefined periodic air-to-fuel ratio. A simple feedback control scheme is proposed. The scheme is intended for fixed air flow and fixed engine speed operating conditions. Its main advantage is that it does not require a fuel puddling model, yet it can achieve high tracking performance that is usually associated only with feedforward schemes. Both theoretical and experimental results are presented.