The Four-Stroke Engine
The internal combustion engine cycle of intake, compression, power, and exhaust strokes, governed by the Otto cycle.
The four-stroke engine is the internal combustion engine that powers most cars, motorcycles, and small machines. It converts the chemical energy of fuel into mechanical work through four piston strokes per cycle: intake, compression, power, and exhaust — two crankshaft revolutions. The concept was patented by Nikolaus Otto in 1876, and the idealized thermodynamic description of its spark-ignition cycle is named the Otto cycle after him.
During the intake stroke the piston moves down, drawing in a fuel–air mixture through the open intake valve. The compression stroke moves the piston up, compressing the mixture to a small fraction of its original volume (typical compression ratios 8:1 to 12:1), which heats it. Near top dead center the spark plug ignites the mixture; the rapid combustion raises pressure and drives the piston down on the power stroke — the only stroke that produces work. The exhaust stroke then pushes the burned gases out through the open exhaust valve, and the cycle repeats.
The Otto cycle idealizes this as four processes: isentropic compression, constant-volume heat addition (combustion), isentropic expansion, and constant-volume heat rejection. Its thermal efficiency is η = 1 − 1/r^(γ−1), where r is the compression ratio and γ the specific heat ratio of the working gas — which is why higher compression ratios give better efficiency, and why diesel engines, which compress air so strongly that fuel auto-ignites without a spark, achieve higher efficiencies still. In practice engines fall far short of the ideal because of heat loss, friction, incomplete combustion, and pumping losses.
Real engines add valvetrain timing, cooling, lubrication, and emission control, and modern variants include variable valve timing and turbocharging. The fundamental efficiency ceiling is set by the second law of thermodynamics: no heat engine can convert all its heat input into work, and waste heat rejection is unavoidable. That is why roughly two-thirds of the fuel's energy leaves as heat and exhaust rather than motion — and why electric motors, which suffer no such combustion-cycle limit, are so much more efficient where batteries suffice.
Tags
engines internal combustion mechanical engineering thermodynamics
Related articles
Click here for easy-to-read helpful e-books for anyone, anywhere, and about anything