Which stroke is also referred to as the combustion stroke?

The power stroke—also known as the expansion or working stroke—is the phase referred to as the combustion stroke, because this is when the ignited air-fuel mixture rapidly burns and expands, forcing the piston downward and producing the engine’s useful work. In both gasoline and diesel four-stroke engines, combustion primarily occurs at the transition from the end of compression through the power stroke, converting chemical energy into mechanical motion.

How the four-stroke cycle works

The four-stroke internal combustion engine completes one power-producing cycle across four distinct piston movements. The following overview shows where combustion fits and why the power stroke carries that alternate name.

• Intake stroke: The intake valve opens as the piston moves down, drawing in an air-fuel mixture (gasoline engines) or air (diesel engines).
• Compression stroke: Both valves close and the piston moves up, compressing the charge to raise temperature and pressure.
• Power (combustion/expansion) stroke: Near the end of compression, the mixture is ignited—by a spark in gasoline engines or by autoignition in diesels—causing rapid combustion and expansion that drives the piston downward.
• Exhaust stroke: The exhaust valve opens and the piston moves up, expelling spent gases from the cylinder.

Together, these strokes transform intake and compression into a controlled burn that delivers power, followed by scavenging of exhaust. The combustion event defines the power stroke, hence its alternate name.

Why it’s called the combustion (power) stroke

During the power stroke, combustion pressure does the work that turns the crankshaft. In gasoline engines, the spark typically fires slightly before top dead center (TDC) at the end of the compression stroke to allow the flame front to develop so peak pressure arrives just after TDC. In diesel engines, precisely timed fuel injection into hot, compressed air initiates autoignition. The resulting rapid pressure rise pushes the piston down, converting thermal energy into mechanical work. Because this burn and expansion occur during this downward movement, the power stroke is also accurately described as the combustion or expansion stroke.

Terminology and variations

Different sources may use “power,” “combustion,” “expansion,” or “working” stroke interchangeably. While modern technologies—direct injection, variable valve timing, turbocharging, and advanced ignition strategies—optimize when and how combustion occurs, they do not change the fundamental definition: the combustion-driven expansion happens in the power stroke. Note that two-stroke engines combine functions (intake/exhaust and compression/power overlap), but in four-stroke designs the combustion event defining the work output is squarely the power stroke.

Common misconceptions

It’s sometimes assumed combustion happens entirely during compression because ignition is initiated near the end of that stroke. In reality, ignition begins around the end of compression, but the bulk of the energy-releasing burn and pressure-driven expansion occur during the power stroke. Likewise, exhaust valve timing and late combustion tails are carefully managed to avoid significant burning into the exhaust stroke in healthy, well-tuned engines.

The key takeaways below distill the relationship between combustion and the four-stroke cycle.

• The power stroke is when the piston is driven down by combustion-generated pressure.
• Combustion is initiated at or just before TDC but primarily unfolds during the power stroke.
• “Power,” “combustion,” and “expansion” stroke are synonymous in four-stroke terminology.

Understanding these points helps clarify why “combustion stroke” is a common alternate name for the power stroke across engine types and references.

Summary

The combustion stroke is the power stroke—the phase in which the ignited charge expands and drives the piston downward, delivering the engine’s useful output. While ignition begins at the end of compression, the majority of combustion energy is released and converted to work during the power (expansion) stroke.