An apparatus is provided for generation of combined heat and power comprising an engine configured in a free-piston arrangement comprising two opposed pistons arranged in an elongated cylinder having at least one inlet port and at least one outlet port with uniflow scavenging, an electricity generator, means for harnessing heat and means for transmitting load from the engine to the electricity generator. The engine is configured to operate at close to constant volume combustion, high compression ratio, a lean fuel to air ratio and uses homogeneous charge compression ignition. The engine further comprises a synchronizing mechanical or hydraulic linkage between said two opposed pistons. A method for generating combined heat and power and a method for configuring apparatus for generating combined heat and power are also provided.