Combustion engine

Internal combustion engine with at least one cylinder containing a piston movable therein in an engine block, wherein the microwaves enter a combustion chamber via a microwave window, the combustion chamber being formed by the piston crown and the cylinder head, wherein the combustion chamber has at least in some regions a combustion chamber wall acting as a microwave window, which consists of a wall layer made of a ceramic material, in which at least one circumferential waveguide annular space with at least one inlet opening for the microwave is arranged, and which has at least one outlet opening for the microwave guided in the waveguide annular space of the wall layer. The internal combustion engine allows precise control of the beginning of volumetric ignition of a fuel-air mixture in a combustion chamber, so that optimal low-emission combustion of the fuel is achieved with an efficiency increased compared to conventional reciprocating internal combustion engines. Generally, the invention enables safe ignition of lean fuel-air mixtures.