How Internal Combustion Engines Work: Pistons, Fuel, and Power

What Is an Internal Combustion Engine?

An internal combustion engine (ICE) is a heat engine in which the combustion of fuel occurs within the engine itself — inside cylinders — rather than in an external furnace (as in a steam engine). The rapid expansion of hot combustion gases drives pistons, converting chemical energy stored in fuel into mechanical work. Internal combustion engines have powered automobiles, motorcycles, trucks, aircraft, ships, and countless machines since the late 19th century. Despite growing competition from electric motors, approximately 1.4 billion internal combustion engine vehicles were in operation globally as of the mid-2020s, and ICEs remain the dominant power source for road transport, aviation, marine shipping, and many industrial applications.

The two dominant ICE types are the spark-ignition (SI) gasoline engine — where fuel-air mixture is ignited by an electric spark — and the compression-ignition (CI) diesel engine — where fuel ignites spontaneously when compressed to high temperature. Both typically use the four-stroke cycle, though two-stroke engines are used in smaller applications.

The Four-Stroke Cycle

The four-stroke cycle (also called the Otto cycle for gasoline engines, after Nikolaus Otto who first demonstrated a practical version in 1876) describes how a gasoline engine converts fuel into motion in four distinct piston strokes — two up and two down:

• Stroke 1 — Intake: The piston moves downward, creating a low-pressure region that draws a fresh charge of air-fuel mixture (gasoline engines) or air alone (diesel engines) into the cylinder through the open intake valve. The exhaust valve is closed.
• Stroke 2 — Compression: Both intake and exhaust valves close. The piston moves upward, compressing the charge into a small volume — the combustion chamber. This raises the temperature and pressure of the mixture, increasing combustion efficiency. Typical compression ratios: 8:1–13:1 for gasoline engines; 14:1–25:1 for diesel engines.
• Stroke 3 — Power (Combustion): In a gasoline engine, a spark plug fires near the top of the compression stroke, igniting the compressed air-fuel mixture. Combustion releases energy rapidly, raising cylinder pressure and temperature dramatically. The expanding gases push the piston downward with great force, rotating the crankshaft and producing useful mechanical work. In diesel engines, the high compression alone heats the air sufficiently to ignite injected fuel.
• Stroke 4 — Exhaust: The exhaust valve opens; the piston moves upward again, pushing burned combustion gases out of the cylinder. At the top of the stroke, the exhaust valve closes, the intake valve opens, and the cycle repeats.

The crankshaft converts the piston's reciprocating (back-and-forth) motion into rotational motion. A flywheel attached to the crankshaft stores rotational energy to smooth out the single power pulse per two crankshaft revolutions in each cylinder. In multi-cylinder engines, the cylinders fire in sequence, producing more even power delivery.

Engine Components

Combustion Chemistry

Gasoline is a mixture of hydrocarbons (primarily C₄–C₁₂ compounds). Complete combustion of a representative hydrocarbon (octane, C₈H₁₈) follows:

2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O + heat

In practice, combustion is never perfectly complete or perfectly stoichiometric. The air-fuel ratio (AFR) critically influences emissions and efficiency. The stoichiometric AFR for gasoline is approximately 14.7:1 (14.7 kg of air per 1 kg of fuel). Modern engine management systems maintain AFR close to stoichiometric using feedback from oxygen sensors (lambda sensors) in the exhaust. A three-way catalytic converter in the exhaust system simultaneously oxidizes CO and unburned hydrocarbons (HC) and reduces nitrogen oxides (NOₓ) — but only functions efficiently at stoichiometric AFR.

Gasoline vs. Diesel Engines

Efficiency and Thermodynamic Limits

The maximum theoretical efficiency of a heat engine operating between a high temperature T_H and a low temperature T_L is given by the Carnot efficiency: η = 1 – T_L/T_H (temperatures in Kelvin). In a gasoline engine, peak combustion temperatures reach approximately 2,000–2,500 K; exhaust temperatures leaving the cylinder are 800–1,000 K. The Carnot limit for these temperatures is approximately 50–60%, but real engines achieve only 25–35% thermal efficiency. The gap arises from heat transfer losses to cylinder walls, incomplete combustion, pumping losses (gas flow resistance), friction, and the non-ideal nature of the combustion process compared with the ideal Otto cycle.

Engine developers use several strategies to improve efficiency:

• Variable valve timing (VVT): Adjusts intake and exhaust valve timing to optimize breathing across the RPM range, improving efficiency and power
• Direct fuel injection: Injects fuel directly into the cylinder (rather than the intake port), allowing higher compression ratios and more precise metering
• Turbocharging and supercharging: Force more air into the cylinder, allowing more fuel and greater power from a given engine displacement (downsizing)
• Cylinder deactivation: Shuts off fuel and closes valves on half the cylinders at low loads, reducing pumping losses
• Atkinson/Miller cycle: Uses a late-closing intake valve to effectively reduce compression ratio relative to expansion ratio, improving thermal efficiency at the cost of peak power

Context: The Transition to Electric Vehicles

The internal combustion engine faces increasing competition and regulatory pressure from battery electric vehicles (BEVs), which convert electrical energy to motion at efficiencies of 85–95% — 2–3 times that of the ICE. Many nations have announced ICE bans for new passenger car sales (UK by 2035, EU by 2035, California by 2035). However, ICEs are likely to remain dominant in heavy-duty transport, aviation, marine shipping, and many developing markets for decades, given the energy density of liquid fuels, existing infrastructure, and the challenge of electrifying high-power, long-range applications.