What Is the Correct Order of a 4-Stroke Cycle?

The correct order of a 4-stroke internal combustion engine cycle is Intake → Compression → Power (Combustion/Expansion) → Exhaust. This sequence governs how most modern gasoline and diesel engines convert fuel into motion, with each stroke corresponding to one full movement of the piston either up or down the cylinder.

How the Four Strokes Unfold

The following list outlines each stroke in the cycle, describing what the piston, valves, and charge (air or air-fuel mixture) are doing at each stage.

• Intake: The piston moves down from top dead center (TDC) to bottom dead center (BDC); the intake valve opens, drawing in air (and fuel in port-injected gasoline engines). The exhaust valve remains closed.
• Compression: Both valves close as the piston moves up from BDC to TDC, compressing the charge to raise pressure and temperature. Near the end of this stroke, gasoline engines ignite via a spark; diesel engines continue compressing air until fuel injection triggers auto-ignition.
• Power (Combustion/Expansion): Combustion rapidly increases cylinder pressure, forcing the piston down from TDC to BDC. Both valves stay closed, and this is where useful work is extracted at the crankshaft.
• Exhaust: The exhaust valve opens as the piston moves up from BDC to TDC, expelling burned gases into the exhaust system. The intake valve remains closed until valve overlap near the cycle transition.

Together, these four strokes complete one thermodynamic cycle, coordinated by the camshaft(s) and crankshaft so that valve timing and ignition align with piston position for efficiency and power.

Piston Positions and Valve Actions

To clarify the mechanical choreography, here’s how piston motion, valve state, and ignition typically align across the cycle.

1. Intake: Piston down (TDC → BDC); intake valve open; exhaust valve closed; no ignition event.
2. Compression: Piston up (BDC → TDC); both valves closed; spark fires just before TDC in gasoline engines; in diesels, fuel is injected near TDC for auto-ignition.
3. Power: Piston down (TDC → BDC); both valves closed; expanding gases drive the piston.
4. Exhaust: Piston up (BDC → TDC); exhaust valve open; intake valve typically remains closed until overlap near TDC.

Although engineers may use valve overlap (a brief period when both valves are slightly open) to improve breathing, the fundamental order of the four strokes does not change.

Gasoline vs. Diesel—Same Order, Different Ignition

Both gasoline and diesel engines follow Intake → Compression → Power → Exhaust. The key difference is ignition: gasoline engines use a spark near the end of the compression stroke, while diesel engines inject fuel into hot, highly compressed air to achieve auto-ignition. Despite this distinction, the stroke sequence is identical.

Variants You Might Hear About

Some engine strategies adjust timing and duration of valves or expansion/compression ratios, but they retain the same four-stroke order.

• Atkinson and Miller cycles: Alter the effective compression/expansion ratio (often via late intake valve closing) for efficiency, common in hybrids.
• Variable Valve Timing (VVT): Shifts valve opening/closing to broaden torque and improve economy.
• Turbocharging/Supercharging: Increase intake charge density during the intake stroke, boosting power.

These approaches refine how the four strokes operate but do not change their sequence.

Why the Order Matters

Correct sequencing ensures proper cylinder filling, efficient combustion, and reliable exhaust scavenging. Deviations in timing or ignition within this order can cause misfires, power loss, higher emissions, and engine damage.

Summary

The 4-stroke cycle proceeds in this order: Intake → Compression → Power (Combustion/Expansion) → Exhaust. While valve timing and ignition strategies may vary across engine types and technologies, the fundamental sequence remains the same in modern gasoline and diesel engines.

What are the steps of a 4-stroke engine?

• Step 1: Intake Stroke. Air and fuel enter the small engine through the carburetor.
• Step 2: Compression Stroke. Just after the piston moves to the bottom of its travel (bottom dead center), the cylinder bore contains the maximum air-fuel mixture possible.
• Step 3: Power Stroke.
• Step 4: Exhaust Stroke.

What is the correct working cycle of a 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.

What is the order of the 4-stroke cycle?

The stroke of a piston is its movement in the cylinder from one end of its travel to another. Four-stroke cycle engines need four strokes to complete the operating cycle: intake, compression, power, and exhaust.

What is the correct sequence of a 4-stroke engine?

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.