What Are the Four Cycles of a Gas Engine?

The four cycles of a typical gasoline (spark‑ignition) engine are intake, compression, power (combustion/expansion), and exhaust. These strokes occur in sequence over two crankshaft revolutions, forming the fundamental operating pattern known as the four‑stroke or Otto cycle that powers most modern cars and small machinery.

The Four-Stroke Sequence at a Glance

The following list outlines each stroke in order, the piston movement, and the valve states that enable the engine to draw in mixture, compress it, convert fuel energy into work, and expel exhaust gases.

1. Intake (Induction): The piston moves downward with the intake valve open and the exhaust valve closed, drawing in an air–fuel mixture (or air alone in direct‑injection systems).
2. Compression: Both valves close as the piston rises, compressing the charge; the spark plug typically fires just before top dead center (TDC).
3. Power (Combustion/Expansion): Ignited gases rapidly expand, forcing the piston downward with both valves closed, delivering useful work to the crankshaft.
4. Exhaust: The exhaust valve opens as the piston moves upward, expelling spent gases; the intake valve remains closed, with brief valve overlap possible at the end of the stroke.

Together, these four strokes complete one thermodynamic cycle and repeat: intake sets up the charge, compression prepares it, combustion releases energy into motion, and exhaust clears the cylinder for the next round.

How Each Cycle Works

While the sequence is simple, modern controls and hardware refine each stroke for efficiency, power, emissions, and durability.

1. Intake (Induction)

As the piston descends, the open intake valve and pressure differential draw in either a premixed air–fuel charge (port fuel injection, PFI) or fresh air (gasoline direct injection, GDI), with fuel injected directly into the cylinder later. Throttle position, intake runner design, turbo/supercharger boost, and variable valve timing (VVT) shape airflow. Mixture preparation targets an appropriate air–fuel ratio (often near stoichiometric, lambda ≈ 1) for catalytic converter efficiency in most conditions.

2. Compression

With both valves closed, the piston rises and compresses the trapped charge, raising temperature and pressure. Higher compression ratio improves thermal efficiency but is limited by knock (spontaneous end‑gas autoignition). Engine control units manage spark timing, fuel, and sometimes internal or external EGR to prevent knock. In GDI engines, stratified charge strategies at light load may concentrate fuel near the plug for lean operation.

3. Power (Combustion/Expansion)

The spark ignites the mixture near the end of the compression stroke. A propagating flame front rapidly releases heat, peaking cylinder pressure shortly after TDC to maximize torque. The piston is driven downward, converting chemical energy into mechanical work. Combustion timing is advanced or retarded based on load, speed, fuel quality, and knock sensors. Variants such as Atkinson/Miller timing reduce effective compression (via late intake valve closing) to boost efficiency while still using four strokes.

4. Exhaust

Near the bottom of the power stroke, the exhaust valve opens to begin blowdown, reducing cylinder pressure. As the piston rises, it expels remaining combustion products through the exhaust system where catalysts and particulate filters (in some GDI setups) clean emissions. Brief valve overlap—when intake opens as exhaust closes—can improve scavenging and help catalyst light‑off during cold starts, managed carefully to limit unburned hydrocarbon emissions.

Relation to the Otto Cycle and Variations

Thermodynamically, the classic gasoline four‑stroke engine approximates the Otto cycle: isentropic compression, heat addition at (nearly) constant volume, isentropic expansion, and heat rejection. Many modern engines employ Atkinson or Miller valve timing strategies, cylinder deactivation, and forced induction to improve efficiency and performance, yet the mechanical sequence remains the same four strokes. By contrast, two‑stroke engines perform the same processes in only two piston strokes using ports and crankcase compression but are less common in road cars due to emissions and efficiency constraints.

Common Misconceptions

Because “gas engine” can mean different things in everyday speech, it’s useful to clarify what this cycle does—and doesn’t—cover.

• Diesel engines can also be four‑stroke, but they use compression ignition, not spark; their mixture formation and combustion differ.
• Two‑stroke gasoline engines still complete intake, compression, power, and exhaust, but combine them into two strokes with port timing instead of valves.
• “Atkinson” or “Miller” gasoline engines still operate on four strokes; the difference is in valve timing and effective compression/expansion ratios.

Understanding these distinctions helps avoid conflating the mechanical stroke count with the combustion method or valve‑timing strategy.

Why It Matters for Maintenance and Diagnostics

Knowing the four strokes aids troubleshooting: misfires and poor fuel economy often trace back to mixture issues on intake, compression losses point to valve sealing or piston rings, timing problems disrupt spark near the end of compression, and clogged exhaust paths affect the exhaust stroke. Tests like compression and leak‑down specifically evaluate the compression and sealing phases, while scan‑tool data on spark advance and fuel trims illuminate combustion quality.

Summary

The four cycles of a gasoline four‑stroke engine are intake, compression, power, and exhaust, executed in that order over two crankshaft revolutions. Intake draws in the charge, compression prepares it, combustion converts fuel energy to torque, and exhaust clears the cylinder. Modern controls refine each stroke, but the sequence remains the core of how most gas engines work.

What is a 4-cycle gasoline engine?

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.

What are the 4 cycles of an engine?

The four cycles (or strokes) of a four-stroke engine are Intake, Compression, Power, and Exhaust. In the intake stroke, a mixture of fuel and air is drawn into the cylinder as the piston moves down. The compression stroke involves the piston moving up to compress this mixture. During the power stroke, the compressed mixture is ignited, forcing the piston down and generating power. Finally, in the exhaust stroke, the piston moves up to push the spent gases out of the cylinder.
 
You can watch this video to learn how four stroke gasoline engines work: 52ssaVReeYouTube · Sep 2, 2023
Here’s a more detailed breakdown of each stroke: 

1. Intake (or Induction) Stroke
   • The intake valve opens, and the piston moves down the cylinder.
   • This movement creates a vacuum that draws a fuel-air mixture into the cylinder.
2. Compression Stroke
   • Both the intake and exhaust valves close.
   • The piston moves up the cylinder, compressing the fuel-air mixture into a smaller volume.
3. Power (or Combustion) Stroke
   • The compressed fuel-air mixture is ignited by a spark plug.
   • The resulting explosion rapidly expands the gases, forcefully pushing the piston down and producing power.
4. Exhaust Stroke
   • The exhaust valve opens.
   • The piston moves back up, pushing the burned gases (exhaust) out of the cylinder through the exhaust valve.

This four-stroke cycle repeats continuously, allowing the engine to operate and generate power.
 
You can watch this video to see the four stroke engine cycle in action: 56sYash VermaYouTube · Mar 26, 2015

What’s the difference between 2 cycle and 4-cycle gas?

The main difference is that 2-stroke engines complete the combustion cycle in two piston strokes (one revolution of the crankshaft) and require oil to be mixed directly with the gasoline, resulting in lighter weight, higher RPMs, and greater simplicity but also more emissions. In contrast, 4-stroke engines require four piston strokes (two crankshaft revolutions) per cycle, use separate oil and fuel systems, and offer more torque, better fuel efficiency, and lower emissions, though they are heavier and more complex.
 
Here’s a breakdown of the key differences:
2-Stroke Engines

• Cycle: Combines the intake, compression, power, and exhaust strokes into just two piston strokes. 
• Lubrication: Requires oil to be pre-mixed with the fuel, as it also lubricates the engine. 
• Design: Simpler design with fewer moving parts (no valves, cams, or lifters in small engines), making them lighter and easier to maintain. 
• Power: Produces twice as many combustion events per revolution, leading to higher horsepower and RPMs for their size. 
• Disadvantages: Less fuel-efficient, significantly higher emissions due to unburnt fuel and burnt oil, and typically louder. 
• Typical Uses: Small, portable equipment like chainsaws, leaf blowers, and older dirt bikes. 

4-Stroke Engines

• Cycle: Completes the four distinct strokes (intake, compression, power, exhaust) over two crankshaft revolutions. 
• Lubrication: Has a separate oil reservoir and requires periodic oil changes, similar to a car engine. 
• Design: More complex with more moving parts, including a crankshaft, connecting rod, valves, and camshaft. 
• Power: Produces less power per revolution but offers more torque, making them better for heavier loads. 
• Advantages: More fuel-efficient, much cleaner emissions, quieter operation, and longer lifespan. 
• Typical Uses: Cars, trucks, motorcycles, lawnmowers, and larger outboard engines. 

What is the order of a 4-cycle engine?

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