Besides the specific demands on mixture formation the main problem of lean-mixture SI engines is to assure the ignition of the air-fuel mixture even if very lean and at all operational conditions.In the scope of extensive research work an analysis was made of the operational behavior of a 4-stroke SI-engine with a newly developed plasma-jet ignition and a conventional transistorized ignition system, respectively. The investigations on the engine were completed by tests in a combustion chamber with methane-air mixture.With plasma-jet ignition the combustion delay and the combustion duration are clearly shorter. The engine tests also revealed a clearly extended lean misfire limit and a reduction of cyclic variations. The NOX-emissions are only slightly higher than with the transistorized ignition system although a considerable improvement of the brake efficiency can be obtained. In the case of plasma-jet ignition the analysis of the combustion process shows an essential reduction of the losses due to real combustion, while heat losses increase less distinctly.