Reciprocating internal combustion engines can be divided into spark-ignition engines, which require spark plugs to initiate combustion, and compression-ignition engines (diesel engines), which compress the air and then inject diesel fuel into the heated compressed air mixture where it autoignites. Spark plugs may also be used in other applications such as furnaces where a combustible mixture should be ignited. In this case, they are sometimes referred to as flame igniters.