An important element in nearly all engine control systems is the lambda control feedback system and its associated switching exhaust gas oxygen sensor (EGO). This feedback loop is necessary to keep the mean value of the normalized air/fuel ratio close to one. This is a necessary condition for proper operation of the three-way catalyst systems which are a part of nearly all production emissions control systems.Currently many systems are based on using classical proportional-integral (PI) controllers in lambda control feedback loops which are self-oscillating. Proper design of such systems is dependent on knowing the time delay between the injection time and the time when a corresponding signal appears at the engine exhaust EGO sensor. Recently a new method of designing the vital larnbda control loop has emerged which is claimed to be very robust with respect to the injection/exhaust time delay. The method is based on modem geometric control techniques and is called a sliding mode controller.In this paper the previous work on sliding mode controllers (SMC) for the exhaust air/fuel ratio is extended and a sliding mode controller with improved steady state performance is developed. The sliding mode controller is compared to a conventional PI based controller on the same engine at a number of different operating points during different aggressive driving scenarios. It is shown that a SMC has desirable speed and robustness characteristics compared to conventional PI lambda controllers.