Autoignition in Otto Engine – Engine Knocking
In an ordinary gasoline engines the compression ratio has its limits. The compression ratio in a gasoline-powered engine will usually not be much higher than 10:1 due to potential engine knocking (autoignition) and not lower than 6:1. Higher compression ratios will however make gasoline engines subject to engine knocking, caused by autoignition an unburned mixture, if lower octane-rated fuel is used. The unburned mixture may autoignite by detonating from pressure and heat alone, rather than ignite from the spark plug at exactly the right time. The engine knocking can be reduced by using high-octane fuel, which increases the gasoline’s resistance to autoignition. The higher the octane number, the more compression the fuel can withstand before detonating (igniting). Since the temperature attained by the fuel-air mixture during the compression increases as the compression ratio increases, the likelihood of autoignition increases with the compression ratio. The autoignition can reduce efficiency or damage the engine if knock sensors are not present to modify the ignition timing.
Higher compression ratios can be achieved in diesel engines (also referred to as compression-ignition engines), because they do not compress the fuel, but rather compress only air and then inject fuel into the air which was heated by compression. Compression ratios in the range of 12 to 20 are typical for diesel engines. The the greater expansion in diesel engines means they reject less heat in their cooler exhaust. The higher compression ratio (greater expansion) and the higher peak temperature causes that diesel engines reach higher thermal efficiency.
Examples of Compression Ratios – Gasoline vs. Diesel
- The compression ratio in a gasoline-powered engine will usually not be much higher than 10:1 due to potential engine knocking (autoignition) and not lower than 6:1.
- A turbocharged Subaru Impreza WRX has a compression ratio of 8.0:1. In general, a turbocharged or supercharged engines already have compressed air at air intake, therefore they are usually built with lower compression ratio.
- A stock Honda S2000 engine (F22C1) has a compression ratio of 11.1:1.
- Some atmospheric sportscar engines can have compression ratio up to 12.5 : 1 (e.g. Ferrari 458 Italia).
- In 2012, Mazda released new petrol engines under the brand name SkyActiv with a 14:1 compression ratio. To reduce the risk of engine knocking, residual gas is reduced by using 4-2-1 engine exhaust systems, implementing a piston cavity, and optimizing fuel injection.
- The Diesel engines have the compression ratio that normally exceed 14:1 and ratios over 22:1 are also common.