The Four Strokes of an Internal Combustion Engine

The four strokes are intake, compression, power (combustion/expansion), and exhaust. These phases describe how a typical four-stroke engine breathes air and fuel, ignites the mixture (or compresses air until it ignites diesel), converts that energy into motion, and clears the cylinder for the next cycle.

How the Four-Stroke Cycle Works

In a four-stroke internal combustion engine, the piston travels up and down twice (four strokes) for each full cycle, while the crankshaft makes two revolutions and the camshaft one. The timing of valve openings and, in spark-ignition engines, the spark event, is synchronized to ensure efficient operation. The following list breaks down each stroke and what happens inside the cylinder.

1. Intake (induction): The intake valve opens, the piston moves from top dead center (TDC) to bottom dead center (BDC), drawing in an air-fuel mixture (gasoline engines) or fresh air (diesel engines). Throttle position and intake design influence airflow.
2. Compression: Both valves close. The piston moves from BDC back to TDC, compressing the charge. In gasoline engines, a spark plug will ignite the mixture near the end of this stroke; in diesels, fuel will be injected near TDC into hot, compressed air.
3. Power (combustion/expansion): The mixture ignites—spark-triggered in gasoline engines, compression-ignited in diesels—creating high-pressure gases that force the piston from TDC to BDC. This is the only stroke that delivers mechanical work to the crankshaft.
4. Exhaust: The exhaust valve opens, and the piston moves from BDC to TDC, pushing spent combustion gases out through the exhaust system, preparing the cylinder for the next intake stroke.

Together, these four strokes complete one thermodynamic cycle. While details like fuel type, injection timing, and valve control vary by engine design, the intake-compression-power-exhaust sequence is consistent across modern four-stroke engines.

Key Details and Variations

Valve Timing and Overlap

Engines often open the intake valve slightly before the piston reaches TDC on exhaust (and keep it open slightly after BDC on intake), and open the exhaust valve slightly before BDC on the power stroke. This “valve overlap” improves cylinder scavenging and volumetric efficiency. Variable valve timing (VVT) systems adjust these events dynamically to optimize power, fuel economy, and emissions.

Spark-Ignition vs. Compression-Ignition

Gasoline (spark-ignition) engines premix air and fuel (either via port or direct injection) and use a spark near the end of the compression stroke to begin combustion. Diesel (compression-ignition) engines compress only air, then inject fuel into the hot, high-pressure air near TDC, where it auto-ignites without a spark plug.

TDC, BDC, and Engine Timing

Top Dead Center (TDC) is the piston’s highest point; Bottom Dead Center (BDC) is the lowest. The four-stroke cycle spans two full crankshaft revolutions. The camshaft, geared to run at half crank speed, ensures valves open and close at the right moments relative to piston position. Precise timing is critical; even small deviations can reduce power, increase emissions, or cause mechanical damage.

Efficiency and Cycle Variants

While the mechanical strokes are the same, thermodynamic strategies vary. Atkinson and Miller cycle engines, common in hybrids and some turbocharged applications, alter effective compression and expansion (often via late intake valve closing) to improve efficiency. Direct injection, turbocharging, and advanced ignition control further tailor combustion without changing the fundamental four-stroke sequence.

Common Misconceptions

Only the power stroke produces useful work; the other strokes consume a portion of that energy. Two-stroke engines combine these functions differently (with one crank revolution per cycle), but modern automotive and most motorcycle engines use the four-stroke design for cleaner emissions and better fuel economy.

Why It Matters

Understanding the four strokes clarifies how engines convert chemical energy into motion, informs maintenance (e.g., timing belts/chains, valve adjustments), and helps explain performance and efficiency differences among engine types and technologies.

Summary

The four strokes of a four-stroke engine are intake, compression, power (combustion/expansion), and exhaust. This sequence—coordinated by piston motion, crankshaft rotation, and cam-driven valves—governs how engines draw in air (and fuel), compress it, release energy through combustion, and expel exhaust, forming the basis of modern engine design across gasoline and diesel applications.

What’s the difference between 2stroke and 4stroke?

The main difference is that a two-stroke engine completes its combustion cycle in two strokes of the piston (and one crankshaft revolution), while a four-stroke engine requires four strokes and two revolutions. Two-stroke engines are simpler, lighter, and have a higher power-to-weight ratio, making them ideal for portable tools but less fuel-efficient and more polluting. Four-stroke engines are more complex, heavier, and more fuel-efficient with better emissions, making them the standard for most vehicles and applications requiring sustained use.
 
Two-Stroke Engines

• Process: The entire process of intake, compression, combustion, and exhaust happens in just two piston strokes. 
• Lubrication: Oil is mixed with the fuel for lubrication, as there isn’t a separate oil reservoir. 
• Pros:
  • Simplicity & Lighter Weight: Fewer parts and a simpler design make them lightweight and easy to maintain. 
  • High Power-to-Weight Ratio: They deliver more power for their size and weight, making them excellent for small, portable equipment. 
• Cons:
  • Lower Fuel Efficiency: Less fuel-efficient than four-stroke engines. 
  • More Pollution: They produce more exhaust smoke and are less environmentally friendly. 
• Common Uses: Small engines, like those in leaf blowers, some chainsaws, and older or competition motorcycles. 

Four-Stroke Engines

• Process: The cycle takes four distinct piston strokes: intake, compression, combustion (power), and exhaust. 
• Lubrication: They have a separate oil reservoir and oil pump system, distinct from the fuel tank. 
• Pros:
  • Better Fuel Economy: They are more fuel-efficient and consume fuel more effectively. 
  • Lower Emissions: Produce less exhaust smoke and are more environmentally friendly. 
  • Durability & Less Vibration: Generally more durable, quieter, and produce less vibration. 
• Cons:
  • More Complex & Heavier: More moving parts and a more complex design result in increased weight. 
  • Higher Cost: The more complex design can also make them more expensive. 
• Common Uses: Cars, trucks, most modern motorcycles, and larger industrial equipment. 

What are the 4-strokes of an engine in order?

The four strokes of an engine, in order, are Intake, Compression, Power, and Exhaust. The piston moves down during the intake stroke to draw in an air-fuel mixture, then moves up during the compression stroke to compress it. A spark plug ignites the mixture, pushing the piston down during the power stroke, and finally, the piston moves up again to expel exhaust gases during the exhaust stroke. 
This video explains how a 4-stroke engine works: 52ssaVReeYouTube · Sep 2, 2023
Here’s a breakdown of each stroke:

1. Intake Stroke: Opens in new tabThe piston moves down, creating a vacuum that draws a mixture of air and fuel into the cylinder through an open intake valve. 
2. Compression Stroke: Opens in new tabThe intake valve closes, and the piston moves up, compressing the air-fuel mixture into a smaller space. 
3. Power Stroke: Opens in new tabA spark plug ignites the compressed air-fuel mixture, causing a rapid expansion of gases. This controlled explosion forces the piston down, generating the power that turns the crankshaft. 
4. Exhaust Stroke: Opens in new tabThe exhaust valve opens, and the piston moves up again, pushing the spent exhaust gases out of the cylinder. 

This cycle repeats, with the four strokes completing one operating cycle and generating continuous power.

What is the 4-stroke engine?

The 4-stroke engine cycle includes the intake stroke (air/fuel mixture enters the engine), compression stroke (mixture is compressed), power stroke (mixture is ignited to propel the vehicle) and exhaust stroke (expels spent gases), with the intake and exhaust valves being controlled by camshafts.

Is a V8 a 4-stroke engine?

Yes, an engine designated as a “V8” is a type of internal combustion engine with eight cylinders, and almost all modern V8s are 4-stroke engines that operate on the four-stroke cycle (intake, compression, power, and exhaust) to produce power.
 
Here’s why the terms are related:

• V8 refers to the engine’s design: Opens in new tabA V8 engine has eight cylinders arranged in two banks, forming a “V” shape. 
• “4-stroke” refers to its operating cycle: Opens in new tabThe “4-stroke” classification describes the four distinct movements (strokes) of the piston within each cylinder to complete one power cycle. 

In a 4-stroke V8 engine: 

1. Intake: The piston moves down, drawing a fuel-air mixture into the cylinder.
2. Compression: The piston moves up, compressing the fuel-air mixture.
3. Power (Combustion): A spark plug ignites the mixture, forcing the piston down and creating power.
4. Exhaust: The piston moves up again, pushing out the spent exhaust gases.

This cycle repeats in all eight cylinders, resulting in a smooth, continuous power delivery because a power stroke occurs every 90 degrees of crankshaft rotation.