How gasoline burns must be controlled. If it burns too quickly, it can cause detonation, sometimes called ping or knock. So special additives are used to slow down the rate of combustion.
Octane rating is a measure of how well a gasoline resists detonation. The higher the rating, the less likely it is that detonation will occur.
Higher operating pressures in the combustion chamber can also cause abnormal combustion. A lead-based, anti-knock additive used in leaded gasoline can allow these higher pressures, without loss of performance. Lead additives are also lubricants. In high compression engines, lead compounds coat valve faces and seats and help prevent wear.
Leaded gasoline is used in older cars that have few or no emission controls. It should never be used in engines designed for unleaded fuel. Governments now limit levels of lead in gasoline and unleaded gasoline is replacing leaded gasoline. So-called “unleaded gasoline” may contain small amounts of lead but maximum levels are tightly controlled.
For gasoline to burn properly, it must be mixed with the right amount of air. For a gasoline engine, the air-fuel ratio by mass is about 15 to 1. By volume, it is about 11,000 to 1. Not much gasoline, but lots of air.
A lean air-fuel mixture has more air in proportion to the amount of fuel.
A slightly lean mixture gives good fuel economy and low exhaust emissions - suitable for cruising conditions. A mixture that’s too lean can make an engine run roughly and overheat.
A rich air-fuel mixture has less air in proportion to the amount of fuel.
A slightly rich mixture can produce more power at lower temperatures, but the extra fuel it uses steps up fuel consumption and emissions. A too-rich mixture fouls spark plugs and causes incomplete burning, and that reduces power.
In normal combustion, the spark plug ignites the mixture, and a small ball of flame forms around the tip of the plug. The piston compresses the mixture. The flame spreads faster and moves evenly to halfway through the mixture. The piston reaches top dead center. The flame picks up more speed, then shoots out to consume the rest of the mixture. Combustion ends with the piston a short way down the cylinder.
Detonation is a violent collision of flame fronts in the cylinder, caused by uncontrolled combustion. It occurs after the spark plug has fired. The sudden rise in pressure can cause a knocking sound. Sustained detonations can lift temperatures enough to cause pre-ignition, also called auto-ignition.
Pre-ignition occurs before normal combustion, when something in the combustion chamber heats up enough to ignite the mixture before the spark plug fires. A mixture may also ignite simply because it is unstable under the higher heat and pressure. Detonation and pre-ignition can cause severe damage and must be avoided. Engine management systems use knock sensors to detect detonation and they retard ignition to stop it.
An engine that keeps running after it is switched off is said to be running-on or dieseling. That’s because, as in a diesel engine, the fuel is igniting just from heat and pressure, with no spark from the plug. It may even cause an engine to run backwards for a short time.
Dieseling can be caused by a high idling speed, an overheated engine, too many carbon deposits in the chamber, or by using a gasoline with an octane rating that is too low. It can be prevented by a special valve in the carburetor idle circuit that cuts off fuel when the ignition is turned off. Or a solenoid can be used to return the throttle to a below-idle position.
