A method for introducing microwave energy into a combustion chamber of a reciprocating internal combustion engine with at least one cylinder with a cylinder head in which the microwaves reach the combustion chamber through a microwave window, wherein the microwaves are run about a circumference of the combustion chamber and radially injected into the combustion chamber through at least a portion of a combustion chamber wall functioning as a microwave window. The method and the internal combustion engine facilitate a precise control of a beginning of a space ignition of a fuel air mix in the combustion chamber so that an optimum low emission combustion of a fuel is achieved with an efficiency that is higher compared to conventional reciprocating piston combustion engines. In general the invention provides safe ignition of lean fuel air mixtures.