The Four Strokes of Combustion in a Four-Stroke Engine

The four strokes of combustion in a typical internal combustion engine are: intake, compression, power (combustion/expansion), and exhaust. These steps occur in sequence over two full rotations of the crankshaft and define how air and fuel are drawn in, ignited to produce work, and expelled as exhaust.

What Each Stroke Does

The following list explains each stroke in the four-stroke cycle, including piston movement, valve positions, and ignition events. While the core sequence is the same for gasoline (spark-ignition) and diesel (compression-ignition) engines, the method of initiating combustion differs.

1. 
   

   Intake (Induction)

   Piston motion: from Top Dead Center (TDC) to Bottom Dead Center (BDC). The intake valve opens and the exhaust valve remains closed, allowing a fresh air-fuel mixture (gasoline engines) or air alone (diesel engines) to enter the cylinder due to the pressure drop created by the descending piston.

2. 
   

   Compression

   Piston motion: from BDC back to TDC. Both valves are closed. The trapped charge is compressed, raising temperature and pressure. In gasoline engines, a spark will ignite the mixture near TDC; in diesels, the air is compressed hot enough that fuel injected near TDC auto-ignites.

3. 
   

   Power (Combustion/Expansion)

   Piston motion: from TDC to BDC. Combustion rapidly increases pressure, forcing the piston downward to deliver work to the crankshaft. In gasoline engines, ignition is by spark just before TDC; in diesel engines, fuel injection begins near TDC and self-ignites in the hot compressed air.

4. 
   

   Exhaust

   Piston motion: from BDC to TDC. The exhaust valve opens and the intake valve is closed. The upward-moving piston expels combustion gases from the cylinder in preparation for the next cycle.

Together, these four strokes complete in 720 degrees of crankshaft rotation (two full turns). The cycle then repeats continuously while the engine is running.

Valve and Ignition/Injection Timing Essentials

The precise timing of valve events and ignition or injection is critical for efficiency, power, emissions, and drivability. The following points summarize typical timing strategies used in modern engines.

• Intake valve timing: Often opens slightly before the exhaust stroke ends (valve overlap) and closes after BDC to improve cylinder filling via inertia of the intake flow.
• Exhaust valve timing: Commonly opens before BDC on the power stroke to reduce pumping losses and closes after TDC to aid scavenging.
• Ignition timing (gasoline): Spark is advanced—fired several degrees before TDC—so peak cylinder pressure occurs shortly after TDC for optimal torque.
• Injection timing (diesel): Fuel is injected under high pressure near the end of the compression stroke; multiple pulses may be used (pilot, main, post) to control noise, emissions, and efficiency.
• Variable systems: Many engines use variable valve timing (VVT), variable valve lift, and direct injection strategies that adapt these events to load and speed.

These timing choices shape how effectively the engine breathes and combusts its charge, directly affecting performance, fuel economy, and emissions across the operating range.

Gasoline vs. Diesel: Same Strokes, Different Combustion

Both engine types use the same four strokes, but gasoline engines typically mix air and fuel before or during intake (port or direct injection) and rely on a spark plug for ignition. Diesel engines admit only air on intake, achieve higher compression ratios, and ignite fuel via compression heat when it is injected near TDC. This leads to different combustion characteristics, efficiency profiles, and emissions control requirements.

Common Misconceptions and Useful Notes

The points below clarify frequent misunderstandings and add practical context for the four-stroke cycle.

• “Combustion stroke” vs. “power stroke”: The combustion process initiates around TDC, but the useful mechanical work is primarily delivered during the expansion (power) stroke.
• Valve overlap is intentional: A brief period when both intake and exhaust valves are slightly open helps scavenging and improves cylinder filling at higher RPMs.
• Two-stroke engines are different: They complete intake, compression, power, and exhaust in one revolution using ports rather than discrete valve events, trading efficiency and emissions for simplicity and power density.
• Thermodynamic models: The idealized Otto (gasoline) and Diesel cycles approximate these processes but real engines deviate due to heat losses, finite combustion time, and pumping work.

Understanding these nuances helps distinguish the textbook sequence from real-world engine behavior, which is optimized through advanced controls and hardware.

Summary

The four strokes of combustion are intake, compression, power (combustion/expansion), and exhaust. Across two crankshaft revolutions, these stages draw in the charge, compress it, convert chemical energy into mechanical work, and then clear the cylinder. Precise control of valve timing and ignition/injection defines how efficiently and cleanly an engine executes this cycle.

What is the correct order of 4 strokes in an internal combustion engine?

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

What is the 4-stroke cycle in aviation?

The 4-stroke cycle in an aircraft piston engine is a process of converting fuel energy into mechanical energy (thrust) through four stages: Intake, Compression, Power, and Exhaust. During this cycle, the piston moves down and up twice, and the crankshaft rotates twice to complete one engine cycle. This cycle is repeated rapidly to produce continuous engine power.
 
The Four Strokes Explained

1. Intake Stroke: Opens in new tabThe piston moves down, and the intake valve opens to draw a mixture of fuel and air into the cylinder. 
2. Compression Stroke: Opens in new tabThe intake and exhaust valves close, and the piston moves upward, compressing the fuel-air mixture into a smaller space. 
3. Power (Combustion) Stroke: Opens in new tabA spark ignites the compressed fuel-air mixture, causing a controlled explosion that forces the piston down with great force. 
4. Exhaust Stroke: Opens in new tabThe exhaust valve opens, and the piston moves upward to push the burnt gases out of the cylinder, clearing it for the next cycle. 

This video explains how a 4-stroke cycle engine works: 38sMechanics MixYouTube · Aug 29, 2014
Key Aspects

• Crankshaft Revolutions: Opens in new tabTwo complete revolutions (720 degrees) of the crankshaft are required to complete one four-stroke cycle. 
• Valves: Opens in new tabTwo crucial valves, the intake valve and the exhaust valve, are responsible for allowing the fuel-air mixture in and the exhaust gases out, respectively. 
• Ignition: Opens in new tabA spark plug initiates the combustion process during the power stroke, turning the mixture into rapidly expanding gases. 
• Mechanical Energy: Opens in new tabThe downward force of the piston on the crankshaft is converted into rotational motion, which is then used to produce thrust. 

How to tell a 4-stroke from a 2 stroke?

A simple way to differentiate is by examining the header. Two-stroke dirt bikes feature a large exhaust header, whereas four-stroke dirt bikes have a relatively straight tube header. Additionally, on a two-stroke dirt bike, the spark plug is visible.

What are the 4 strokes of a combustion engine?

The four strokes of the combustion cycle, often called suck, squeeze, bang, blow, are Intake, Compression, Power, and Exhaust. During the intake stroke, a fuel-air mixture is drawn into the cylinder, followed by the compression stroke, where the piston moves up to compress the mixture. The power stroke occurs when the mixture ignites, forcing the piston down, and finally, the exhaust stroke pushes the spent gases out of the cylinder.
 
1. Intake Stroke 

• The intake valve opens as the piston moves downward.
• This movement creates a vacuum that draws a mixture of air and fuel into the cylinder.

2. Compression Stroke 

• Both the intake and exhaust valves close.
• The piston moves upward, compressing the air-fuel mixture into a smaller volume.

3. Power Stroke (Combustion)

• With the valves still closed, the compressed air-fuel mixture is ignited by the spark plug. 
• The resulting explosion creates a powerful force that pushes the piston downward, generating power for the engine. 

4. Exhaust Stroke 

• The exhaust valve opens as the piston moves upward.
• The upward motion of the piston pushes the spent combustion gases out of the cylinder and into the exhaust system.

Once the exhaust stroke is complete, the intake valve begins to open again, and the cycle repeats. 
This video shows the four strokes of the engine cycle: 59sCARinfo3d (En)YouTube · Oct 28, 2022