Thermodynamic cycle Most petrol engines use either the four-stroke
Otto cycle or the
two-stroke cycle. Petrol engines have also been produced using the
Miller cycle and
Atkinson cycle.
Layout Most petrol-powered piston engines are
straight engines or
V engines. However,
flat engines,
W engines and
other layouts are sometimes used.
Wankel engines are classified by the number of rotors used.
Compression ratio Cooling Petrol engines are either
air-cooled or
water-cooled.
Ignition Petrol engines use
spark ignition.
High voltage for the spark this may be provided by a
magneto or an
ignition coil. In modern car engines, the
ignition timing is managed by an electronic
Engine Control Unit. Ignition modules can also function as a rev limiter in some cases to prevent overrevving and the consequences of it, such as valve float and connecting rod failure.
Primer Primers may be used to help start the engine. They can draw fuel from fuel tanks and vaporize fuel directly into piston cylinders. Engines are difficult to start during cold weather, and the fuel primer helps because otherwise there will not be enough heat available to vaporize the fuel in the
carburetor. == Power output and efficiency ==