Lean burn and EGR concepts are being used increasingly in spark-ignition engines. Compared with a stoichiometric petrol engine, however, these dilution concepts are said to give a certain reduction in power output, in particular with higher EGR ratios or under lean mixtures near the minimum flammability limit. Some of the reduction can be recovered by correctly optimizing the MBT, and increasing the intake pressure which in addition reduces the pumping losses. In this work, experimental and theoretical studies of engine performance and emissions have been carried out with special emphasis on the combustion process. These have been analyzed from both combustion images in the early stages of the flame kernel development, and from the pressure-time history. Simultaneous measurements of engine operating conditions, pressure traces and sequences of combustion images have been made in a single-cylinder four-stroke engine. The early stages of the combustion have been analyzed using image analysis techniques. From the pressure time history the actual instantaneous flame speed has been derived for the main burn phase. The experimental part includes stoichiometric and lean burn operating conditions, while the simulation extends the scope into th studying of EGR. The detailed spark ignition engine simulation predicts the effects of EGR ratio, flame speeds, ignition timing and the lean equivalence ratio on the engine performance and on the emissions of CO, HC and NOx.