Abstract: A reciprocating piston engine which is selectively operable in either a gas operation mode with a gaseous fuel or a diesel operation mode with a liquid fuel includes a main combustion chamber which has an inlet for receiving gas and/or air, an injection device and an outlet. At least one precombustion chamber is connected to the main combustion chamber by at least one precombustion chamber output that opens into said main combustion chamber. Each precombustion chamber has an applied-ignition device for igniting the contents of the precombustion chamber during gas operation. An optimization of exhaust gas values, particularly of the NO.sub.x concentrations, is attained by the applied-ignition device and a separate gas line for supplying the precombustion chamber with gas, thereby allowing a leaner fuel mixture to be used in the main combustion chamber.