The Four Stages of the Internal Combustion (Four‑Stroke) Cycle

The four stages are intake (induction), compression, power (combustion/expansion), and exhaust. In a typical four-stroke gasoline or diesel engine, these strokes occur in sequence over two full turns of the crankshaft (720°), coordinating piston movement with valve events to draw in air, compress it, extract energy from combustion, and expel the spent gases.

The Four Strokes at a Glance

Below is a concise overview of each stroke, describing piston direction, valve positions, and the main purpose of the phase within the engine’s operating cycle.

1. Intake (Induction): The piston moves downward with the intake valve open, drawing in fresh air (and fuel in port- or carbureted systems).
2. Compression: The piston moves upward with both valves closed, compressing the trapped mixture (gasoline) or air (diesel) to raise pressure and temperature.
3. Power (Combustion/Expansion): Near top dead center, the mixture ignites—by spark in gasoline, by heat of compression in diesel—rapidly increasing pressure and driving the piston downward.
4. Exhaust: The exhaust valve opens; the piston moves upward to push out combustion products, clearing the cylinder for the next cycle.

Together, these four strokes transform chemical energy into mechanical work, with one power stroke delivered for every two crankshaft revolutions in each cylinder.

Stroke-by-Stroke Detail

1. Intake (Induction)

As the crankshaft pulls the piston down from top dead center (TDC) to bottom dead center (BDC), the intake valve opens to admit charge. In gasoline engines, air mixes with fuel either in the intake tract (port injection/carburetor) or inside the cylinder (direct injection). Diesels typically take in only air, with fuel added later during compression. Modern variable valve timing (VVT) may hold the intake valve open slightly after BDC to improve volumetric efficiency and reduce pumping losses.

2. Compression

The intake valve closes and the piston rises, compressing the contents and sharply increasing temperature and pressure. Typical compression ratios are roughly 9:1 to 14:1 for modern gasoline engines (with knock control and direct injection allowing higher values) and about 14:1 to 22:1 for diesels. In spark-ignition engines, the spark plug fires just before TDC (spark advance) to ensure peak cylinder pressure occurs shortly after TDC for maximum torque. Diesels inject fuel near TDC; the hot, compressed air ignites the spray without a spark.

3. Power (Combustion/Expansion)

Rapid combustion elevates pressure, pushing the piston down to deliver the power stroke. In gasoline engines, a flame front propagates through a premixed charge; timing is tuned to avoid knock while maximizing work. In diesels, fuel burns as it mixes with air (diffusion combustion), producing the characteristic diesel torque and sound profile. The expanding gases transfer energy to the crankshaft via the connecting rod, turning chemical energy into usable mechanical output.

4. Exhaust

As the piston nears BDC, the exhaust valve often opens early (blowdown) to release pressure before the upward stroke pushes out remaining gases. Near TDC, brief valve overlap—when the exhaust remains slightly open as the intake begins to open—can aid scavenging, especially at higher rpm. Exhaust aftertreatment (catalytic converters, particulate filters, SCR) manages emissions downstream.

Gasoline vs. Diesel: Same Strokes, Different Ignition

Both gasoline and diesel four-stroke engines use the same four stages but differ in how and when fuel is added and ignited. Gasoline engines (spark-ignited) typically meter air with a throttle and ignite a homogeneous or stratified mixture via spark. Diesels (compression-ignited) draw in unthrottled air, compress it to high temperature, and inject fuel directly into the hot air near TDC, where it auto-ignites. These differences influence efficiency, torque characteristics, noise, and emissions control strategies.

Variations and Related Cycles

Engine designers adapt the four-stroke foundation to meet targets for efficiency, power, and emissions. The items below highlight common variations and what they change.

• Atkinson/Miller strategies: Alter effective compression/expansion via valve timing or supercharging to boost efficiency, common in hybrids.
• Two-stroke engines: Combine events so every downstroke is a power stroke, using ports and scavenging instead of distinct intake/exhaust strokes; lighter and simpler but typically higher emissions.
• Variable valve timing and lift: Shift valve events (and overlap) to broaden torque, improve fuel economy, and reduce emissions across the rev range.
• Forced induction (turbo/supercharging): Increases intake charge density to raise power; requires careful control of mixture, timing, and temperatures.

While these approaches modify timing, airflow, and mixture formation, the core sequence—intake, compression, power, and exhaust—remains the backbone of piston-engine operation.

Summary

The internal combustion four-stroke cycle proceeds through intake, compression, power, and exhaust over 720° of crank rotation. Gasoline and diesel engines share this structure but differ in fuel delivery and ignition. Modern technologies—VVT, direct injection, turbocharging, and Atkinson/Miller tactics—refine these strokes for better efficiency, performance, and emissions without changing the fundamental sequence.

What are the 4 stages of the internal combustion engine?

The four stages, or strokes, of a common internal combustion engine are Intake, Compression, Power, and Exhaust. During the intake stroke, the engine draws a mixture of fuel and air into the cylinder; the compression stroke compresses this mixture; the power stroke ignites the mixture, creating an explosion that forces the piston down; and the exhaust stroke pushes the spent gases out of the cylinder, completing the cycle.
 
Here is a breakdown of each stage:

1. Intake Stroke:
   • The intake valve opens. 
   • The piston moves downward. 
   • This 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. 
   • This action compresses the air-fuel mixture into a smaller volume, increasing its temperature and pressure. 
3. Power Stroke:
   • The spark plug ignites the compressed air-fuel mixture. 
   • The resulting explosion creates a powerful force that pushes the piston downward. 
   • This downward movement of the piston generates power, which turns the crankshaft. 
4. Exhaust Stroke:
   • The exhaust valve opens. 
   • The piston moves upward again. 
   • This action pushes the spent exhaust gases out of the cylinder through the open exhaust valve. 

This four-stroke cycle then repeats, allowing the engine to continuously generate power.

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 are the 4 strokes of an internal combustion engine?

The four strokes of an internal combustion engine are intake, compression, power (or combustion), and exhaust, which work together in a cycle to create energy. During these four distinct piston movements, a fuel-air mixture is drawn in, compressed, ignited to create power, and then the spent exhaust gases are expelled, repeating the process continuously. 
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 engine cylinder through the intake valve. 
2. Compression Stroke: Opens in new tabThe intake valve closes, and the piston moves back up, compressing the air-fuel mixture into a smaller volume. This increases the pressure and temperature of the mixture. 
3. Power Stroke: Opens in new tabThe spark plug ignites the highly compressed fuel-air mixture, causing a controlled explosion. The resulting expansion of gases forces the piston down, generating power that turns the crankshaft. 
4. Exhaust Stroke: Opens in new tabThe exhaust valve opens, and the piston moves back up, pushing the burnt exhaust gases out of the cylinder. 

After the exhaust stroke, the intake valve opens again, and the cycle repeats. This sequence of four strokes is also known as “suck, squeeze, bang, blow”.

What is the 4 stage combustion cycle?

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.