What Is the Combustion Stroke?

The combustion stroke—also called the power or expansion stroke—is the phase in an internal combustion engine when the air–fuel mixture ignites and rapidly burns, generating high pressure that pushes the piston from top dead center (TDC) to bottom dead center (BDC) and turns the crankshaft. This article explains where the combustion stroke fits in the engine cycle, what happens during it, how it differs between gasoline and diesel engines, and why it matters for performance, efficiency, and emissions.

Where It Fits in the Engine Cycle

In a conventional four-stroke engine, the combustion stroke is the third act in a repeating sequence that converts fuel energy into mechanical work. Understanding the sequence clarifies the role of combustion.

• Intake stroke: The intake valve opens; the piston moves down, drawing in fresh air (and fuel in port-injected engines).
• Compression stroke: Both valves close; the piston moves up, compressing the mixture to raise temperature and pressure.
• Combustion (power/expansion) stroke: The mixture ignites near TDC; expanding gases force the piston downward, delivering work.
• Exhaust stroke: The exhaust valve opens; the piston moves up, expelling spent gases.

Together, these four strokes complete one thermodynamic cycle over two crankshaft revolutions; only the combustion stroke produces net positive work on the crankshaft.

What Happens During the Combustion Stroke

From ignition to exhaust blowdown, the combustion stroke is a tightly timed sequence that maximizes pressure on the piston while minimizing energy losses.

• Ignition timing: In spark-ignition engines, the spark occurs slightly before TDC to ensure peak pressure shortly after TDC; in diesel engines, fuel injection begins near TDC, and combustion starts as fuel auto-ignites in hot, compressed air.
• Rapid pressure rise: A flame front (gasoline) or multiple ignition sites (diesel) create a fast pressure increase, typically peaking a few crank-angle degrees after TDC.
• Piston work: High cylinder pressure pushes the piston down toward BDC, turning the crankshaft via the connecting rod and producing torque.
• Valve status: Intake and exhaust valves remain closed during most of expansion to contain pressure; the exhaust valve opens before BDC for “blowdown,” releasing pressure and reducing pumping work on the next stroke.
• Thermal dynamics: As gases expand, temperature and pressure drop; the engine’s design aims to extract as much expansion work as practical before opening the exhaust valve.

The effectiveness of this stroke depends on combustion speed, ignition timing, mixture preparation, and how closely peak pressure aligns with the crank angle that maximizes torque.

Gasoline vs. Diesel: Key Differences

While the purpose is the same—turn chemical energy into mechanical work—the way combustion starts and spreads differs between engine types.

• Spark-ignition (gasoline): A spark plug ignites a premixed air–fuel charge. Combustion typically propagates as a flame front. Knock control (via sensors and timing) prevents damaging pressure spikes.
• Compression-ignition (diesel): Air is compressed to high temperature; fuel is injected and auto-ignites. Combustion occurs in mixing-controlled phases, with strategies to manage soot and NOx.
• Advanced modes: Homogeneous Charge Compression Ignition (HCCI) and Spark Controlled Compression Ignition (e.g., Mazda Skyactiv-X) blend features of both to achieve faster, cooler combustion for efficiency and low NOx.

These differences shape the pressure profile during the stroke, influencing torque delivery, fuel efficiency, and emissions control requirements.

Characteristics That Define the Combustion Stroke

The combustion stroke is governed by timing, geometry, and combustion quality. These factors determine how much useful work reaches the crankshaft.

• Synonyms: Also known as the power stroke or expansion stroke.
• Timing window: Nominally ~180 crank degrees from TDC to BDC, with exhaust opening typically before BDC to start blowdown.
• Peak pressure position: Optimum peak often occurs 8–16 degrees after TDC in many gasoline engines; diesels vary with load and injection strategy.
• Mean effective pressure (MEP): A key performance metric; higher MEP during the combustion stroke translates to more torque.
• Knock and pre-ignition: Abnormal combustion raises pressures too quickly, risking damage; modern engines use knock sensors, direct injection, cooled EGR, and precise timing to mitigate.
• Efficiency strategies: Atkinson/Miller cycles extend the effective expansion relative to compression; turbocharging, direct injection, and variable valve timing tailor pressure and temperature profiles.
• Emissions impact: High temperatures form NOx; rich zones in diesel produce soot. Combustion phasing and aftertreatment (TWC, SCR, DPF) address these.

Optimizing these elements ensures the combustion stroke delivers strong torque with minimal fuel and emissions penalties across the engine’s operating range.

Common Misconceptions

Several myths persist about this phase of the cycle; clarifying them helps avoid confusion.

• “Only gasoline engines have a combustion stroke.” In fact, all internal combustion engines do; the mechanism of ignition differs.
• “Combustion ends at BDC.” In practice, much of the useful combustion energy is extracted before BDC; exhaust opens early to manage flow and efficiency.
• “More ignition advance is always better.” Excessive advance can induce knock and reduce efficiency; optimal phasing depends on load, speed, and fuel.

Understanding what actually occurs in-cylinder helps explain why modern control systems constantly adjust timing and fueling.

Related Terms

The combustion stroke connects to several other engine concepts commonly referenced in technical discussions.

• Top Dead Center (TDC) and Bottom Dead Center (BDC): Piston travel endpoints that bookend the stroke.
• Blowdown: Early exhaust opening near the end of expansion to release pressure before the exhaust stroke.
• Compression ratio and expansion ratio: Geometric factors dictating potential efficiency; Atkinson/Miller alter these effectively.
• Cycle types: Four-stroke vs. two-stroke; in two-strokes, the power event occurs every revolution but overlaps with scavenging.

These terms frame how engineers describe, measure, and optimize the power-producing phase of engine operation.

Summary

The combustion stroke is the power-producing phase of an internal combustion engine, where an ignited charge generates high pressure that drives the piston from TDC to BDC and turns the crankshaft. Also known as the power or expansion stroke, it is defined by precise combustion phasing, closed valves for most of expansion, and early exhaust opening for blowdown. Its dynamics differ between spark-ignition and compression-ignition engines, and modern strategies—direct injection, variable valve timing, and advanced combustion modes—aim to maximize efficiency and torque while curbing emissions.

Which stroke is also referred to as the combustion stroke?

Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C.

What are the 4 types of strokes in an engine?

What Are the Strokes of a 4-Cycle Engine?

• Intake stroke. The piston descends the cylinder bore from top dead center (TDC) to bottom dead center (BDC).
• Compression stroke. The piston moves up the cylinder bore from BDC to TDC.
• Power stroke.
• Exhaust stroke.

What does the compression stroke do?

The compression stroke compresses the air-fuel mixture within a four-stroke engine’s combustion chamber, significantly increasing its pressure and temperature to make ignition more efficient and powerful. During this stroke, the piston moves upward with both the intake and exhaust valves closed, squeezing the mixture into a smaller volume before a spark plug ignites it for the next power stroke. 
How the Compression Stroke Works

1. Closed Valves: Opens in new tabAfter the intake stroke, which draws the air-fuel mixture into the cylinder, the intake valve closes. The exhaust valve also remains closed to trap the mixture inside. 
2. Piston Movement: Opens in new tabThe piston then moves upward from the bottom of the cylinder towards the top. 
3. Compression: Opens in new tabAs the piston moves up, it decreases the volume of the combustion chamber, forcing the air-fuel mixture into a much smaller space. 
4. Increased Pressure and Temperature: Opens in new tabThe physical act of compression increases the pressure and temperature of the mixture. 
5. Preparation for Ignition: Opens in new tabThis compressed mixture is now highly pressurized, making it easier to ignite with a spark plug, which in turn generates a more powerful explosion to drive the piston down on the subsequent power stroke. 

Why Compression Is Important

• More Power: Opens in new tabA more compressed fuel-air mixture leads to a more efficient and powerful explosion, resulting in greater mechanical power for the engine. 
• Higher Pressure: Opens in new tabThe compressed mixture creates a much higher peak pressure than an uncompressed mixture would, according to the laws of thermodynamics. 
• Efficiency: Opens in new tabCompressing the mixture is a key step in the internal combustion process that makes four-stroke engines highly efficient and reliable. 

What is a combustion stroke?

A combustion stroke, also called the power stroke, is the third stroke in a four-stroke engine’s cycle, where the compressed air-fuel mixture is ignited, causing a powerful expansion of gases that pushes the piston down, generating mechanical work to turn the engine’s crankshaft. During this critical phase, the intake and exhaust valves remain closed, ensuring that the explosive force is fully utilized to create rotational power.
 
The Four Strokes of an Engine Cycle
The combustion stroke is part of a four-stroke internal combustion engine cycle, which consists of four distinct movements of the piston: 

1. Intake Stroke: Opens in new tabThe piston moves down, drawing a fresh air-fuel mixture into the cylinder. 
2. Compression Stroke: Opens in new tabThe piston moves up, compressing the air-fuel mixture to a high pressure and temperature, preparing it for ignition. 
3. Combustion (Power) Stroke: Opens in new tabThe compressed mixture is ignited by a spark plug (in petrol engines) or by self-ignition (in diesel engines), and the resulting expansion of hot gases pushes the piston downward, creating the engine’s power. 
4. Exhaust Stroke: Opens in new tabThe piston moves up again, pushing the burnt gases out of the cylinder through the open exhaust valve. 

Key Aspects of the Combustion Stroke

• Ignition: The stroke begins with the ignition of the high-pressure, high-temperature air-fuel mixture. 
• Power Generation: The rapid expansion of the burning gases creates an immense force that drives the piston down, converting chemical energy into mechanical work. 
• Closed Valves: For the force to be effective, both the intake and exhaust valves are closed during the combustion stroke. 
• Crankshaft Rotation: The linear motion of the piston is converted into the rotary motion of the crankshaft, which ultimately powers the vehicle’s transmission and wheels.