The Four Stages of Engine Operation Explained

The four stages of a four-stroke internal combustion engine are Intake, Compression, Power (combustion/expansion), and Exhaust. These strokes govern how most modern gasoline and diesel engines breathe air, burn fuel, and convert energy into motion, shaping performance, efficiency, and emissions across cars, motorcycles, generators, and more.

The Four Stages at a Glance

The following list outlines the core sequence of a four-stroke cycle, pairing each stage with what the piston, valves, and fuel/air charge are doing as the crankshaft turns.

1. Intake: The piston moves down; the intake valve opens to draw in fresh air (and fuel in port-injected gasoline engines); the exhaust valve is closed.
2. Compression: Both valves close; the piston moves up, compressing the charge to raise temperature and pressure in preparation for ignition.
3. Power (Combustion/Expansion): Near top dead center, the charge ignites—by spark in gasoline engines or auto-ignition in diesels—forcing the piston down and producing useful work.
4. Exhaust: The exhaust valve opens; the piston moves up to expel spent gases; the cycle then repeats.

Together, these four strokes span two full crankshaft revolutions (720 degrees). Only one of the four strokes—the power stroke—produces torque; the others position and prepare the charge or clear the cylinder.

How Each Stroke Works

1. Intake (0–180° crank rotation)

As the piston descends from top dead center (TDC) to bottom dead center (BDC), the intake valve opens, creating a pressure drop that draws in air. In port-injected gasoline engines, fuel mixes with air in the intake port; in direct-injection gasoline engines, fuel may be sprayed during late intake or early compression. Diesel engines typically admit air only during intake, with fuel injected later. Variable valve timing often opens the intake valve slightly before TDC and closes it after BDC to improve volumetric efficiency and take advantage of inertia in the incoming airflow.

2. Compression (180–360°)

With both valves closed, the piston rises, compressing the trapped charge. Gasoline engines (spark-ignition) generally run compression ratios around 9:1 to 14:1, while modern diesels (compression-ignition) range roughly 14:1 to 22:1. Near the end of compression, gasoline engines fire a spark plug—typically a few to dozens of crank degrees before TDC depending on load and speed—so peak pressure occurs shortly after TDC. Diesels inject fuel near TDC; the hot compressed air ignites the spray without a spark. Effective mixture motion (swirl/tumble) aids rapid, stable combustion, and knock control in gasoline engines manages premature auto-ignition.

3. Power (Combustion/Expansion) (360–540°)

Combustion raises cylinder pressure sharply, pushing the piston down to deliver work to the crankshaft. In gasoline engines, a flame front propagates from the spark plug; in diesels, the burning occurs as injected fuel mixes with hot air (a combination of premixed and diffusion-controlled combustion). Peak pressures typically occur a few degrees after TDC. Engine management balances spark timing or injection timing, air-fuel ratio, and exhaust gas recirculation (EGR) to optimize efficiency, emissions, and knock or soot formation. This stroke is the source of the engine’s output torque.

4. Exhaust (540–720°)

As the piston moves back up, the exhaust valve opens—often slightly before the piston reaches BDC on the power stroke to “blow down” pressure and reduce pumping losses. Hot combustion products exit through the exhaust manifold, spinning a turbocharger if equipped and warming catalytic converters for emissions control. Near the top of this stroke, the intake valve usually begins to open (valve overlap), improving cylinder scavenging and breathing at higher RPM. The cycle then restarts with the intake stroke.

Variations and Modern Enhancements

While the four-stroke sequence is foundational, modern engines employ technologies that reshape timing, mixture formation, and combustion to boost efficiency, power, and emissions compliance.

• Variable valve timing/lift (VVT/VVL): Adjusts when and how far valves open to trade off torque, power, efficiency, and emissions across the rev range.
• Direct injection (GDI/DI): Injects fuel directly into the cylinder for better mixture control, enabling higher compression and reduced knock in gasoline engines.
• Turbocharging and intercooling: Increase air mass per intake stroke and reduce charge temperature, raising power and efficiency.
• Exhaust gas recirculation (EGR): Lowers combustion temperatures to reduce NOx in both diesel and gasoline engines.
• Atkinson/Miller cycles: Alter effective compression/expansion via valve timing or boosting to improve thermal efficiency, common in hybrids.
• Cylinder deactivation: Shuts off some cylinders under light load to reduce pumping and heat losses.
• Start-stop systems: Cut idling fuel use by shutting the engine off at stops and restarting quickly.
• Advanced combustion (e.g., HCCI/PCI): Seeks diesel-like efficiency in gasoline engines via controlled auto-ignition for cleaner, leaner burn.
• Hybridization: Uses electric motors to supplement the power stroke and recapture braking energy, letting the engine run in its most efficient zones.
• Aftertreatment: Three-way catalysts, particulate filters, and SCR systems clean the exhaust products of the cycle.

These advances refine how each stroke is timed and managed, but the underlying intake–compression–power–exhaust sequence remains the backbone of piston-engine operation.

Common Misconceptions

Several widespread myths can blur how the four-stroke cycle actually works. The points below clarify frequent misunderstandings.

• Two-stroke vs four-stroke: Two-stroke engines complete a power cycle in two piston strokes (one crank revolution), not four, using ports rather than conventional valves.
• Diesel differences: Diesels use the same four strokes but ignite fuel via compression heat, not a spark; they manage mixture by fuel quantity rather than throttling air.
• Rotary engines: Wankel rotaries don’t have pistons, but still execute intake, compression, combustion, and exhaust phases—just via rotating chambers.
• Combustion timing: Peak pressure ideally occurs shortly after TDC, not exactly at TDC; timing is optimized for torque and efficiency.
• Valve states: Both valves are closed during compression and most of the power stroke; controlled overlap occurs around the exhaust/intake transition.
• Jet engines: Turbofans and turbojets are continuous-flow machines, not “four-stroke” devices, though they share analogous thermodynamic phases.

Understanding these distinctions helps place the four-stroke cycle in context alongside other engine types and combustion strategies.

Why the Four Stages Matter

Mastering the sequence explains why engines deliver power only once every two revolutions per cylinder, why timing and compression are critical, and how diagnostics target specific strokes (e.g., intake leaks, compression losses, misfire on power, or exhaust restrictions). It also underpins design choices that affect fuel economy, drivability, and emissions in today’s vehicles.

Key Terms and Typical Numbers

The following quick-reference items define core concepts and typical ranges you’ll encounter when discussing the four-stroke cycle.

• TDC/BDC: Top/bottom of piston travel; strokes span TDC-to-BDC or BDC-to-TDC.
• Compression ratio: Roughly 9:1–14:1 (gasoline) and 14:1–22:1 (diesel), with higher ratios generally improving thermal efficiency.
• Ignition/injection timing: Spark often 5–40° before TDC; diesel injection typically near TDC with pilot/main events to control noise and emissions.
• Peak cylinder pressure: On the order of 40–80 bar in many modern gasoline engines and higher in diesels; peaks a few degrees after TDC.
• Valve overlap: A designed period when intake and exhaust valves are both slightly open to aid scavenging, tuned via VVT.
• Air–fuel ratio: Gasoline stoichiometric about 14.7:1 by mass; diesels run lean overall, controlling power with injected fuel.

These figures vary by engine design, application, and emissions strategy, but they frame how each stroke is optimized in practice.

Summary

The four stages of engine operation—Intake, Compression, Power (combustion/expansion), and Exhaust—define how most piston engines convert fuel into motion over two crankshaft revolutions. Modern technologies fine-tune valve events, mixture formation, and combustion, but the essential cycle remains unchanged, guiding performance, efficiency, and emissions in contemporary gasoline and diesel powertrains.

What are the 4 functions of the engine?

The intake function involves drawing a mixture of air and fuel into the combustion chamber. The compression function compresses the mixture. The power function involves igniting the mixture and harnessing the power of that reaction. The exhaust function expels the burned gases from the engine.

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 is a complete run through all four stages of engine operation?

Four-Stroke Cycle. The four-stroke cycle is defined as a sequence of four distinct piston movements in an engine, comprising the intake, compression, power, and exhaust strokes, which collectively complete one cycle of operation.

What are the 4 stages of the engine?

The four stages (strokes) of a 4-stroke internal combustion engine are: Intake, during which a fuel-air mixture enters the cylinder; Compression, where the mixture is squeezed into a smaller volume; Power (or Combustion), when the compressed mixture ignites, forcing the piston down; and Exhaust, in which the piston pushes the burnt gases out of the cylinder. This cycle repeats, with the piston’s up-and-down movement turning the crankshaft and generating power.
 
Here’s a breakdown of each stage:

1. Intake Stroke:
   • The intake valve opens, and 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 much smaller volume, which increases its temperature and pressure. 
3. Power (or Combustion) Stroke:
   • The compressed air-fuel mixture is ignited by a spark plug (in a gasoline engine). 
   • The resulting explosion or controlled burn creates a high-pressure force that drives the piston downward, generating power to turn the crankshaft. 
4. Exhaust Stroke:
   • The exhaust valve opens as the piston moves upward again. 
   • This upward movement pushes the burnt combustion products (exhaust gases) out of the cylinder through the open exhaust valve. 

This cycle then repeats, with the engine’s operation converting the linear motion of the pistons into the rotary motion of the crankshaft, which ultimately powers a vehicle.