One of the most important operating modes for SI engines is in the idle speed region. This is because SI engines spend a large part of their time operating in this mode. Moreover, a large measure of operator satisfaction is dependent on an engine operating smoothly and reliably in and around idle. In particular the operator expects that the idle speed will remain constant in spite of the engine loads due to power steering pumps and air conditioning compressors.In the idle speed region an SI engine is thought to be quite nonlinear because the engine loading can be quite significant, thus forcing the engine to be driven through a reasonably large portion of its lower operating range. Many of the earlier studies of idle speed control systems have dealt with linearized models which in principle have limited validity for the problem at hand. In order to improve this situation, it is necessary to deal with the more general nonlinear control problem.In this paper two nonlinear control strategies are compared with conventional PID and Linear Quadratic Regulator (LQR) regulator designs. The nonlinear controllers studied are of the Input-Output Linearization and Sliding Mode types and the design methodology is detailed in the paper. All of the regulators designed have been realized and tested on an experimental engine equipped with microprocessor driven, electronic throttle control.