The engine is in lean-burn mode about 80% of running time. The spark plugs ignite a small pulse of lean mixture injected into the combustion chamber. When fired a fireball is created acting like an air piston, increasing the pressure and temperature in the combustion chamber. Compression ignition of the very lean mixture occurs with a rapid and even and complete burn leading to a more powerful cycle. The combustion timing is controlled by the flame from the spark plug. This enables SPCCI to combine the advantages of both petrol and diesel engines.