What Are the Four Cycles of an Engine?

The four cycles of a four-stroke internal combustion engine are intake, compression, power (combustion/expansion), and exhaust. These strokes occur in sequence over two full crankshaft revolutions (720 degrees) and underpin how most modern gasoline and diesel engines generate power. While the basic sequence is the same across engine types, details like ignition timing, fuel delivery, and valve control vary by design and technology.

The Four Strokes Explained

To understand engine operation, it helps to break the process into the four distinct piston strokes that occur in a repeating loop. Each stroke corresponds to a specific movement of the piston between top dead center (TDC) and bottom dead center (BDC), coordinated with valve timing and, in spark-ignition engines, a spark event.

1. Intake (suction): From TDC to BDC, the intake valve opens and the piston moves down, drawing in the charge. In gasoline engines this is typically an air–fuel mixture (formed via port or direct injection); in diesels it is air only. Throttled gasoline engines create intake vacuum during this stroke.
2. Compression: From BDC back to TDC, both valves are closed. The trapped charge is compressed, increasing temperature and pressure. Typical compression ratios are roughly 9–14:1 for modern gasoline engines (higher in Atkinson/DI variants) and 14–23:1 for diesels.
3. Power (combustion/expansion): Near TDC, combustion begins. In gasoline engines, a spark ignites the mixture slightly before TDC (spark advance); in diesels, fuel is injected into hot, compressed air and auto-ignites. The rapid pressure rise pushes the piston down to BDC. Both valves stay closed. Peak in-cylinder pressures are higher in diesels than in gasoline engines.
4. Exhaust: From BDC to TDC, the exhaust valve opens and the piston pushes spent gases out of the cylinder. Around the end of this stroke, brief valve overlap (both valves slightly open) can aid scavenging and cooling.

Taken together, these four strokes produce one power event for every two crankshaft revolutions. The timing and control of each stroke are carefully managed to balance performance, efficiency, emissions, and durability.

How the Cycle Is Coordinated

Valve timing and overlap

Camshafts (or cam phasers in variable systems) open and close intake and exhaust valves at precise crank angles. Designers often keep valves open slightly beyond the “obvious” points—known as overlap—to improve cylinder filling and scavenging, especially at higher engine speeds.

Ignition and fueling differences

In spark-ignition (gasoline) engines, ignition occurs via spark near the end of compression; mixture formation can be port-injected or direct-injected. In compression-ignition (diesel) engines, only air is compressed; fuel is injected late in compression, igniting spontaneously. This fundamental difference shapes combustion speed, noise, emissions, and efficiency.

Modern Variations and Technologies

Contemporary engines still follow the four strokes but use advanced hardware and strategies to alter how each stroke behaves, aiming to improve economy and reduce emissions without sacrificing drivability.

• Atkinson/Miller cycle strategies: Use valve timing (late intake valve closing) or boosting to change effective compression and expansion, improving efficiency in hybrids and some small turbo engines.
• Variable valve timing and lift (VVT/VVL): Adjust when and how far valves open to optimize breathing across the rev range.
• Direct injection and turbocharging: Increase charge density and control mixture formation; often paired with cooled exhaust gas recirculation (EGR) to cut knock and NOx.
• Cylinder deactivation and stop–start: Disable cylinders or shut the engine off at idle to reduce fuel use without changing the fundamental four-stroke sequence when active.
• Advanced combustion modes (HCCI/SPCCI): Promote auto-ignition in gasoline engines under certain loads for diesel-like efficiency, yet still within the four-stroke framework.

These innovations tweak the timing, pressure, and temperature conditions within each stroke, but the core sequence—intake, compression, power, exhaust—remains intact.

How Four-Stroke Differs from Two-Stroke

Two-stroke engines complete the same four events in just two piston movements (one crankshaft revolution) using ports rather than valves and overlapping processes. They are lighter and can produce more power per displacement but typically have higher emissions and different lubrication requirements. Most road vehicles use four-stroke engines for efficiency and emissions control.

Summary

The four cycles of a four-stroke engine are intake, compression, power, and exhaust, occurring over two crankshaft revolutions. Regardless of whether the engine is gasoline or diesel, and despite modern enhancements like variable valve timing, turbocharging, or hybridization, these strokes form the essential rhythm by which internal combustion engines breathe, burn, and expel gases to produce power.

How to tell if an engine is 4 cycle?

Look for stickers labeling the equipment (e.g., “Four Cycle” or “No Fuel Mixing”). Look for an engine oil fill cap. Only four-cycle engines have a separate engine oil reservoir on the back or side, with another cap (usually yellow) for checking & filling the oil reservoir.

What are the 4 processes of an engine?

The cycle includes four distinct processes: intake, compression, combustion and power stroke, and exhaust. Spark ignition gasoline and compression ignition diesel engines differ in how they supply and ignite the fuel.

What are the 4 cycles of a gas engine?

Four-stroke cycle used in gasoline/petrol engines: intake (1), compression (2), power (3), and exhaust (4).

What is the 4 stage engine cycle?

In an engine, a stroke refers to the movement of the piston in the cylinder as part of the combustion process. There are typically four strokes involved in the operation of a four-stroke engine: intake, compression, power and exhaust.