Homogeneous Charge Compression Ignition (HCCI), also known as Controlled Auto-Ignition (CAI), is a form of combustion which is not yet used in production cars. It is being developed by several manufacturers, including Mercedes-Benz, GM, Renault and Nissan.
HCCI engines use the principle of auto-ignition. A combustible air/fuel mixture is compressed in the cylinder so that it heats up, and when the temperature and pressure rise far enough the mixture begins to burn. In petrol engines that's generally bad news because the intention is to control combustion timing using a spark from the spark plug, but in HCCI it's positively encouraged. Combustion occurs throughout the chamber rather than spreading from a single point, which promotes clean, complete combustion and keeps temperatures low – all good for emissions. The clever bit is to control combustion timing by carefully managing the conditions inside the combustion chamber, closing the valve early to trap exhaust gas in the cylinder, where it helps to heat up the incoming charge to auto-ignition temperature. GM calls this 'recompression', because the exhaust gas is compressed by the piston as it continues to rise. Mercedes-Benz uses the same system and call it 'internal exhaust gas recirculation'.
Another way of ensuring the gas in the cylinder reaches auto-ignition temperature is to raise the compression ratio. Mercedes-Benz has opted to incorporate a variable compression system into its DiesOtto HCCI engine (above).