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A four-stroke gasoline engine converts thermal energy from burning air-fuel mixture into mechanical energy through four distinct strokes. During intake, the piston draws the air-fuel mixture in. Compression stroke compresses it adiabatically. The spark plug ignites the gas, causing expansion that pushes the piston down in the power stroke, producing work. Finally, the exhaust stroke expels burned gases.

During the compression stroke, the intake valve closes and the rising piston compresses the air-fuel mixture adiabatically. This compression increases the pressure and temperature of the gas, preparing it for ignition. The compressed mixture is then ready for the spark plug to ignite it in the subsequent power stroke.

The compression ratio is the ratio of maximum to minimum cylinder volume and directly affects engine efficiency. For automobile engines, it typically ranges from 8 to 10. A higher compression ratio increases the pressure and temperature of the gas before ignition, allowing greater expansion during the power stroke and improving overall engine efficiency.

The intake valve opens during the intake stroke to allow the air-fuel mixture to enter the cylinder as the piston descends. The exhaust valve remains closed during intake and compression. During the exhaust stroke, the exhaust valve opens while the piston rises, expelling burned gases and preparing the cylinder for the next intake stroke.

An internal combustion engine uses the byproducts of combustion as the working fluid directly, rather than using a heat transfer medium to transfer heat. The combustion produces high-pressure products that expand through a piston to create work. This direct combustion approach relates to statements of the second law of thermodynamics governing energy conversion efficiency.

Internal combustion engines are categorized into three types: spark ignition gasoline engines, most commonly used in automobiles; compression ignition diesel engines used in heavy vehicles and industrial machinery; and gas turbines used in aircraft. Each type uses combustion to produce high-pressure products that generate mechanical work through different expansion mechanisms.

In the power stroke, the spark plug ignites the compressed air-fuel mixture, causing rapid heating. The heated gas expands almost adiabatically to its maximum volume, pushing the piston downward with significant force. This downward motion of the piston, transmitted through the crankshaft, produces the mechanical work that powers the engine and drives the vehicle.