The Four Strokes of an Internal-Combustion Engine, Explained

The 4‑stroke cycle proceeds in this order: intake, compression, power (combustion/expansion), and exhaust. In modern gasoline engines the spark typically fires just before the piston reaches top dead center near the end of compression; in diesels, fuel is injected near the end of compression and ignites from heat. One complete cycle spans two crankshaft revolutions (720 degrees) as the piston travels between top dead center (TDC) and bottom dead center (BDC).

How the 4-stroke cycle works

At the heart of most road-going engines, the 4‑stroke cycle choreographs air, fuel, valves, and a moving piston to convert chemical energy into mechanical work. Each stroke corresponds to a half-turn of the crankshaft, and valves open and close in a tightly timed sequence to manage gas flow.

The following list breaks down the four strokes, describing piston movement, valve positions, and the combustion event that delivers power.

1. Intake (induction): The piston moves from TDC to BDC. The intake valve opens, the exhaust valve stays closed. Air (gasoline engines: air plus fuel from port injection, or air alone with later direct injection) is drawn into the cylinder by the descending piston. Many engines begin opening the intake valve slightly before TDC and close it after BDC to improve cylinder filling.
2. Compression: The piston rises from BDC to TDC with both valves closed, compressing the trapped charge. In spark‑ignition (gasoline) engines, the spark plug fires shortly before TDC to initiate combustion; in compression‑ignition (diesel) engines, finely atomized fuel is injected near the end of compression and self‑ignites from heat and pressure.
3. Power (combustion/expansion): Rapid combustion raises pressure, forcing the piston down from TDC to BDC with both valves closed. This is the only stroke that delivers work to the crankshaft. Peak cylinder pressure typically occurs slightly after TDC to maximize leverage on the crank.
4. Exhaust: The exhaust valve opens and the piston travels from BDC back to TDC, pushing out spent gases. The intake valve may begin to open before the exhaust valve fully closes at the end of this stroke (valve overlap) to aid scavenging and improve volumetric efficiency.

Together, these four strokes complete one thermodynamic cycle per cylinder every 720 degrees of crankshaft rotation. Camshafts and engine control systems fine‑tune the exact timing to balance efficiency, emissions, and performance.

Key timing and mechanics

Behind the clean four-part narrative is a set of precise timings and mechanical relationships that determine how smoothly and efficiently an engine runs.

• Two revolutions per cycle: A full cycle requires 720° of crank rotation; each stroke spans roughly 180°.
• TDC and BDC: Top and bottom piston positions anchor each stroke; combustion is timed so peak pressure acts just after TDC.
• Valve events: Intake typically opens slightly before TDC and closes after BDC; exhaust opens before BDC and closes after TDC, creating overlap that improves scavenging at higher RPM.
• Ignition/injection timing: Gasoline engines advance or retard spark with RPM and load; diesels shape fuel injection timing and duration to control combustion and emissions.
• Variable systems: Technologies like variable valve timing/lift and variable geometry turbochargers shift these events dynamically for torque, efficiency, and emissions gains.

The exact angles vary by engine design and purpose—for example, high‑revving performance engines often use more overlap, while efficiency‑focused designs reduce it for better idle and lower emissions.

Gasoline vs. diesel: what changes

Both gasoline (spark‑ignition) and diesel (compression‑ignition) engines follow the same four strokes, but the way combustion starts and is controlled differs significantly.

• Combustion initiation: Gasoline uses a spark near the end of compression; diesel injects fuel into hot, high‑pressure air, causing autoignition.
• Mixture formation: Gasoline may be port‑injected (premixed before entering the cylinder) or direct‑injected (mixed in‑cylinder); diesels are direct‑injection by design.
• Air control: Gasoline engines often use a throttle to meter air; diesels mainly regulate power via fuel quantity, with boost and EGR managing airflow and emissions.
• Compression ratio: Gasoline typically ~10:1–14:1 (varies with direct injection and knock control); diesels ~15:1–22:1 to achieve autoignition.

Despite these differences, both rely on the same intake–compression–power–exhaust sequence, coordinated by precise valve and injection/ignition timing.

Common misconceptions

It’s easy to conflate simplified diagrams with real-world engine behavior. These clarifications help align expectations with modern practice.

• “The spark happens at TDC.” In reality, it usually occurs slightly before TDC; the flame needs time to develop so peak pressure arrives just after TDC.
• “Valves only move at stroke boundaries.” Valve events intentionally overlap and extend beyond TDC/BDC to improve breathing.
• “Every stroke makes power.” Only the power stroke does work; the others consume energy, which is recovered via flywheel inertia and other cylinders firing in sequence.
• “All 4‑stroke engines sound and behave the same.” Cam profiles, intake/exhaust tuning, turbocharging, and control strategies produce wide variation in torque curves and acoustics.

Understanding these nuances explains why the same basic cycle underpins everything from quiet commuter cars to high‑revving race engines and efficient long‑haul diesels.

Why the 4-stroke cycle matters

This cycle is the foundation of most contemporary automotive and small-engine design, balancing efficiency, emissions control, and power density. Advances like direct injection, turbocharging, variable valve timing, and sophisticated engine control units have refined—but not replaced—the core sequence of intake, compression, power, and exhaust.

Summary

The 4‑stroke cycle consists of intake, compression, power, and exhaust, completed over two crankshaft revolutions with carefully timed valve and ignition/injection events. Gasoline and diesel engines share the same strokes but differ in how they ignite the charge and control mixture and airflow. Precise timing and modern variable systems shape performance, efficiency, and emissions while preserving the fundamental four-step rhythm that drives internal‑combustion engines.

What is the order of operation of a 4-stroke engine?

A four-stroke engine has four piston movements in one cycle: intake, compression, power, and exhaust. Engines have cylinders, pistons, camshafts, valves, spark plugs, and a crankshaft. Four-stroke engines are more fuel-efficient, cleaner, and durable than two-stroke engines.

What are the steps of the four stroke process in the correct order?

The correct order of the four strokes is: Intake, Compression, Power, and Exhaust. In this cycle, the piston first draws the fuel-air mixture into the cylinder (Intake), then compresses it (Compression), followed by ignition of the compressed mixture to generate power (Power), and finally, the burnt gases are expelled (Exhaust).
 
Here is a breakdown of each stroke:

1. Intake Stroke: Opens in new tabThe intake valve opens, and the piston moves down, drawing the fuel-air mixture into the cylinder. 
2. Compression Stroke: Opens in new tabBoth the intake and exhaust valves close, and the piston moves up, compressing the air-fuel mixture. 
3. Power Stroke: Opens in new tabThe compressed mixture is ignited by a spark plug, creating an explosion that forces the piston downward, producing power. 
4. Exhaust Stroke: Opens in new tabThe exhaust valve opens, and the piston moves up again, pushing the spent combustion gases out of the cylinder. 

What are the stages of the 4-stroke cycle?

The four stages, or strokes, of a four-stroke engine are Intake, Compression, Power, and Exhaust. During the intake stroke, the piston moves down, drawing an air-fuel mixture into the cylinder. The compression stroke then compresses this mixture. Next, the power stroke ignites the mixture, creating pressure that pushes the piston down, generating power. Finally, the exhaust stroke pushes the spent gases out of the cylinder, and the cycle begins again.
 
This video demonstrates the four-stroke engine cycle: 38sMechanics MixYouTube · Aug 29, 2014
Here’s a breakdown of each stage:

1. Intake Stroke:
   • The intake valve opens. 
   • The piston moves downward, creating a vacuum that draws the air-fuel mixture into the cylinder. 
   • The piston reaches Bottom Dead Center (BDC) at the end of this stroke. 
2. Compression Stroke:
   • Both the intake and exhaust valves are closed. 
   • The piston moves upward, compressing the air-fuel mixture. 
   • This compression increases the mixture’s temperature, preparing it for ignition. 
3. Power (or Combustion) Stroke:
   • Near Top Dead Center (TDC), the spark plug ignites the compressed fuel-air mixture. 
   • The resulting explosion creates immense pressure, forcing the piston downward. 
   • This is the only stroke that produces work, converting the chemical energy into mechanical motion to turn the crankshaft. 
4. Exhaust Stroke:
   • The exhaust valve opens. 
   • The piston moves upward again, pushing the spent combustion gases out of the cylinder through the exhaust valve. 
   • Once the piston returns to the top, the exhaust valve closes, and the cycle repeats. 

What are the steps of the 4-stroke process?

Four Stroke Cycle Engines. A four-stroke cycle engine is an internal combustion engine that utilizes four distinct piston strokes (intake, compression, power, and exhaust) to complete one operating cycle. The piston make two complete passes in the cylinder to complete one operating cycle.