The Four-Stroke Diesel Engine Cycle: Intake, Compression, Power, and Exhaust

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 The four-stroke cycle is the foundation of diesel engine operation in heavy equipment. Every movement of an excavator, bulldozer, dump truck, wheel loader, motor grader, and mining machine ultimately depends on combustion events that are repeatedly completed inside the engine cylinders.

During one complete combustion cycle, the piston completes four strokes: intake, compression, power, and exhaust. The crankshaft completes two revolutions or 720 degrees, while the camshaft completes one revolution or 360 degrees.

Understanding these four strokes is essential for troubleshooting. Hard starting, white smoke, black smoke, low power, high blow-by, misfiring, and uneven exhaust temperatures can often be connected to a problem occurring during one or more parts of the combustion cycle.

For a broader explanation of the complete engine system, read How Heavy Equipment Diesel Engines Work.

What Is the Four-Stroke Diesel Cycle?

The four-stroke cycle is a sequence in which air enters the cylinder, is compressed, receives injected fuel, produces combustion power, and is discharged as exhaust gas.

The four strokes are:

1. Intake stroke.
2. Compression stroke.
3. Power stroke.
4. Exhaust stroke.

A diesel engine does not normally use a spark plug to start combustion. Air is compressed until its pressure and temperature rise. Diesel fuel is then injected into the hot compressed air and ignites through compression ignition.

Important Basic Terms

Top Dead Center

Top dead center, or TDC, is the highest position reached by the piston inside the cylinder.

The piston reaches TDC at:

• The end of the compression stroke.
• The end of the exhaust stroke.

For this reason, piston position alone cannot confirm that a cylinder is at firing TDC. Camshaft position and valve condition must also be confirmed.

Bottom Dead Center

Bottom dead center, or BDC, is the lowest position reached by the piston.

The piston reaches BDC at:

• The end of the intake stroke.
• The end of the power stroke.

Crankshaft

The crankshaft converts the piston’s reciprocating movement into rotational movement. It completes two revolutions during one four-stroke cycle.

Camshaft

The camshaft controls the opening and closing of the intake and exhaust valves. In a four-stroke engine, it rotates at half the speed of the crankshaft.

Four-Stroke Cycle Summary

Stroke	Piston Movement	Intake Valve	Exhaust Valve	Main Process
Intake	TDC to BDC	Open	Closed	Air enters the cylinder
Compression	BDC to TDC	Closed	Closed	Air is compressed
Power	TDC to BDC	Closed	Closed	Combustion pushes the piston
Exhaust	BDC to TDC	Closed	Open	Exhaust gas leaves the cylinder

Important: This table represents the basic theoretical cycle. In a real engine, valves may begin opening before the piston reaches dead center and close after the piston has passed dead center. This is known as valve timing.

1. Intake Stroke

The intake stroke begins when the piston moves from top dead center toward bottom dead center.

During this stroke:

• The intake valve is open.
• The exhaust valve is closed.
• The piston moves downward.
• Cylinder volume increases.
• Cylinder pressure becomes lower than intake manifold pressure.
• Air enters the cylinder.

In a diesel engine, air is the primary material entering the cylinder during the intake stroke. Fuel is injected near the end of the compression stroke.

Airflow During the Intake Stroke

In a turbocharged heavy-duty engine, the air normally follows this path:

Outside air → precleaner → air filter → turbocharger compressor → aftercooler → intake manifold → intake valve → cylinder

The precleaner removes larger debris, while the air filter removes smaller particles. The turbocharger compresses the intake air, and the aftercooler reduces the air temperature after compression.

Symptoms of Insufficient Intake Air

• Black smoke.
• Low engine power.
• Slow engine response.
• Low boost pressure.
• High exhaust temperature.
• Increased fuel consumption.

Components to Inspect

• Precleaner.
• Primary and secondary air filters.
• Air restriction indicator.
• Intake hoses and clamps.
• Turbocharger compressor wheel.
• Charge-air piping.
• Aftercooler.
• Intake manifold.
• Intake valves and valve clearance.
• Boost pressure sensor.

2. Compression Stroke

After the piston reaches bottom dead center, the intake valve closes. The piston then moves upward toward top dead center.

During compression:

• The intake valve is closed.
• The exhaust valve is closed.
• Air is trapped inside the cylinder.
• The space above the piston becomes smaller.
• Air pressure rises.
• Air temperature rises.

The purpose of compression is to create enough heat for ignition when fuel is injected.

Causes of Low Compression

• Worn, broken, or stuck piston rings.
• Worn or damaged cylinder liner.
• Damaged piston.
• Leaking intake valve.
• Leaking exhaust valve.
• Damaged valve seat.
• Incorrect valve clearance.
• Leaking cylinder head gasket.
• Cracked cylinder head.
• Incorrect valve timing.
• Low cranking speed.

Symptoms of Low Compression

• Hard starting.
• Long cranking time.
• White smoke during starting.
• Misfiring.
• Unstable idle.
• Low engine power.
• High blow-by.
• Increased oil consumption.

Recommended Tests

• Compression pressure test.
• Relative compression test.
• Cylinder leakage test.
• Cranking speed measurement.
• Blow-by measurement.
• Valve clearance inspection.
• Borescope inspection.

3. Power Stroke

The power stroke directly produces engine power.

Near the end of the compression stroke, the injector sprays high-pressure diesel fuel into the combustion chamber. The fuel must be atomized into small droplets so that it can rapidly mix with the hot compressed air.

Combustion Sequence

1. Air reaches the end of compression.
2. The injector begins delivering fuel.
3. The fuel is atomized.
4. Fuel mixes with hot air.
5. A short ignition delay occurs.
6. Combustion begins.
7. Cylinder pressure rises.
8. Combustion gases expand.
9. The piston moves toward BDC.
10. The connecting rod rotates the crankshaft.

The other three strokes require energy to move the piston. This energy is supplied by the flywheel and the power strokes occurring in the other cylinders.

Factors Affecting Combustion Quality

• Air quantity.
• Fuel quantity.
• Fuel injection pressure.
• Injection timing.
• Injector spray pattern.
• Compression pressure.
• Fuel quality.
• Engine operating temperature.
• Fuel atomization and distribution.

Pilot, Main, and Post Injection

Pilot injection delivers a small amount of fuel before the main injection. It may reduce combustion noise and control the rate of cylinder pressure rise.

Main injection supplies most of the fuel used to produce power.

Post injection occurs after the main injection under selected operating conditions. It may support exhaust temperature control or aftertreatment operation.

Power-Stroke Problems

• Restricted injector.
• Leaking injector.
• Incorrect spray pattern.
• Low fuel pressure.
• Incorrect injection timing.
• Contaminated fuel.
• Air entering the fuel system.
• Low compression.
• Injector wiring fault.
• Missing injector command.

4. Exhaust Stroke

After the power stroke, the exhaust valve opens and the piston moves upward from BDC toward TDC. The piston pushes the burned gases out of the cylinder.

A typical exhaust path is:

Exhaust valve → exhaust port → exhaust manifold → turbocharger turbine → aftertreatment or muffler → exhaust pipe

Effects of Exhaust Restriction

When the exhaust system is restricted, exhaust back pressure increases. The piston must use more energy to remove burned gases from the cylinder.

Possible symptoms include:

• Low engine power.
• Slow response.
• High exhaust temperature.
• Abnormal boost pressure.
• Increased fuel consumption.
• Engine derating.
• Frequent regeneration on selected engines.

Components to Inspect

• Exhaust valves.
• Exhaust manifold.
• Turbocharger turbine.
• Exhaust piping.
• Muffler.
• Diesel oxidation catalyst.
• Diesel particulate filter.
• Exhaust temperature sensors.
• Differential pressure sensor.

Crankshaft and Camshaft Relationship

During one four-stroke cycle:

• The piston moves downward during intake.
• The piston moves upward during compression.
• The piston moves downward during power.
• The piston moves upward during exhaust.
• The crankshaft completes two revolutions.
• The camshaft completes one revolution.

This relationship is maintained by timing gears, a timing chain, or another timing mechanism.

Incorrect timing can cause low compression, hard starting, low power, smoke, abnormal noise, and possible piston-to-valve contact on some engine designs.

What Is Valve Timing?

In a basic theoretical cycle, each valve opens and closes at dead center. In an actual engine, valves may open before dead center and close after the piston has passed dead center.

This timing improves cylinder filling and exhaust-gas removal across the operating speed range.

What Is Valve Overlap?

Valve overlap is the brief period near the end of the exhaust stroke and beginning of the intake stroke when both intake and exhaust valves are open.

Valve overlap helps remove remaining exhaust gas and begin the flow of fresh intake air. The correct amount varies by engine design.

Firing Order in Multi-Cylinder Engines

The cylinders do not produce power at the same time. Their power strokes follow a specified firing order.

The firing order helps:

• Produce smoother engine rotation.
• Distribute crankshaft loading.
• Reduce vibration.
• Maintain balanced combustion.
• Provide continuous torque.

Firing order depends on engine design and must be confirmed using the correct service information.

Smoke Color and the Combustion Cycle

Smoke Color	Combustion Condition	Possible Causes
Black	Fuel burns with insufficient air	Air restriction, low boost, excessive fuel, overload
White	Fuel does not burn or burns too late	Low compression, injector fault, timing, cold engine
Blue	Lubricating oil burns inside the cylinder	Piston rings, liner, valve seal, turbocharger
Gray	Unstable combustion or contamination	Injector, fuel quality, oil consumption, timing

Connecting Symptoms to the Four Strokes

Symptom	Related Stroke	Initial Inspection
Black smoke	Intake and power	Air filter, boost, injector, engine load
White smoke during starting	Compression and power	Compression, injector, timing, cranking speed
High blow-by	Compression and power	Piston rings, liner, piston
Low engine power	All strokes	Air, fuel, compression, exhaust restriction
High exhaust temperature	Power and exhaust	Fuel delivery, boost, overload, restriction
Single-cylinder misfire	Compression and power	Injector, compression, valve, wiring
Hard starting	Compression and power	Cranking speed, compression, fuel pressure, timing

How to Identify a Weak Cylinder

Check Diagnostic Codes

Check active and logged fault codes using the machine monitor or diagnostic tool. Codes may identify injector circuits, fuel pressure, crankshaft sensors, camshaft sensors, boost pressure, or temperature problems.

Compare Exhaust Temperatures

A cylinder with a much lower exhaust temperature may have a failed injector, low fuel delivery, low compression, or a misfire.

An excessively hot cylinder may have overfueling, a leaking injector, delayed combustion, or an uneven load contribution.

Perform a Cylinder Cut-Out Test

A cylinder cut-out test disables injectors one at a time and observes the change in engine speed or performance.

If disabling one cylinder produces almost no change, that cylinder may already be contributing very little power.

Perform a Relative Compression Test

A relative compression test compares cylinder compression by monitoring cranking-speed or starter-current variations.

An imbalance can be investigated further using a compression pressure test, cylinder leakage test, borescope inspection, or valve-train inspection.

Combustion-Cycle Inspection Sequence

1. Confirm the complaint and operating conditions.
2. Check active and logged diagnostic codes.
3. Measure cranking speed.
4. Inspect fuel level and quality.
5. Inspect the water separator and fuel filters.
6. Check the air filter and intake restriction.
7. Inspect intake and charge-air connections.
8. Check fuel supply and rail pressure.
9. Measure boost pressure.
10. Compare cylinder exhaust temperatures.
11. Perform a cylinder cut-out test.
12. Perform a relative compression test.
13. Measure blow-by.
14. Check valve clearance.
15. Check valve timing and timing components.
16. Disassemble the engine only when the test results support it.

Common Diagnostic Mistakes

Assuming All White Smoke Is Caused by Injectors

White smoke may also be caused by low compression, low cranking speed, a cold engine, delayed injection timing, or coolant entering the combustion chamber.

Removing the Cylinder Head Too Early

Fault codes, fuel pressure, compression balance, exhaust temperature, and valve clearance should be evaluated before major disassembly.

Ignoring Cranking Speed

Low cranking speed can prevent the air from reaching sufficient compression temperature, even when piston rings and cylinder liners remain serviceable.

Assuming Every Engine Speed Drop Is a Combustion Problem

On an excavator, the hydraulic pump may absorb excessive horsepower. Engine output and hydraulic pump demand should be evaluated together.

Frequently Asked Questions

How many crankshaft revolutions occur during one four-stroke cycle?

The crankshaft completes two revolutions or 720 degrees during one intake, compression, power, and exhaust cycle.

How many times does the camshaft rotate?

The camshaft completes one revolution or 360 degrees. It rotates at half the crankshaft speed.

When is diesel fuel injected?

Fuel injection begins near the end of the compression stroke before the piston reaches top dead center, according to the engine’s injection timing.

Which stroke directly produces power?

The power stroke directly produces power when combustion pressure pushes the piston from TDC toward BDC.

Is the intake valve open during the entire intake stroke?

Not necessarily. In an actual engine, the intake valve may open before TDC and close after BDC according to the valve timing design.

What happens when an intake valve leaks?

Compressed air may leak into the intake manifold, reducing compression pressure and causing hard starting, misfiring, or low power.

What happens when an exhaust valve leaks?

Compression and combustion pressure may escape into the exhaust port, causing hard starting, low power, misfiring, or abnormal exhaust temperature.

Why does a diesel engine not use a spark plug?

A diesel engine uses compression ignition. Air becomes hot during compression, and the injected fuel ignites in that hot air.

Does each cylinder produce a power stroke during every crankshaft revolution?

No. In a four-stroke engine, each cylinder produces one power stroke during every two crankshaft revolutions.

Conclusion

The four-stroke diesel cycle consists of intake, compression, power, and exhaust. These strokes are completed during two crankshaft revolutions and one camshaft revolution.

During intake, clean air enters the cylinder. During compression, the piston raises the air pressure and temperature. Near the end of compression, fuel is injected and combustion begins. Combustion pressure pushes the piston during the power stroke, and the burned gases are removed during the exhaust stroke.

Every stroke depends on the condition of the air system, valve train, pistons, cylinder liners, fuel system, turbocharger, exhaust system, and electronic controls.

Understanding what should happen during each stroke allows mechanics to connect symptoms with possible causes, test the correct systems, and avoid replacing components without sufficient evidence.