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What is a Four -Stroke Engine?

What is a Four -Stroke Engine?

There are two common types of power cycles for gasoline engines: Four-stroke and two-stroke. Let’s see how a four-stroke works. There’s also the rotary engine and Atkinson cycle engine, but those are uncommon.

First and foremost, an internal combustion engine needs to combust fuel and air. The force from this combustion is transferred to mechanical energy. Hence, an internal combustion engine needs to perform four functions to produce power i.e. intake, compression, combustion and exhaust. As for a four-stroke engine, those four functions take four distinct strokes (up and down movements of the piston) to produce power.

Therefore we start off with the four-stroke engine, as it is easier to understand. We will cover the two-stroke engine after this episode.

Here are the basics.


Also called suction, this stroke begins with the piston at top dead centre (TDC), as it travels downwards toward bottom dead centre (BDC). The intake valve opens and the piston travelling downwards causes a negative pressure (vacuum) in the cylinder, sucking in the fuel/air mixture and filling the cylinder.


As the piston travels back up from BDC to TDC, the intake valve closes and traps the fuel/air mixture inside the cylinder. The piston compresses the mixture in preparation for combustion.

The crankshaft will have travelled 1 full, 360o revolution (1 RPM) when the piston reaches TDC.


Also known as ignition or power stroke. The sparkplug (or sparkplugs) fire, igniting the compressed fuel/air mixture. The pressure raises by 3200 to 5000 kPa (32 to 50 bars), and the temperature to 600o Celcius.

The compression pressure forces the piston back downwards to BDC, creating the mechanical work (kinetic energy) to turn the crankshaft.


Also known as outlet stroke. The exhaust opens as the upward movement of the piston pushes the burned gasses out past the exhaust valve, into the exhaust port and exhaust pipe.

The crankshaft completes two full revolutions when the piston fully reaches TDC. A four-stroke engine needs two RPM to produce mechanical power. Thus if you hold 10,000 RPM for one minute on the tachometer, there are 5000 power strokes in that one minute. Yes, that’s how hard the engine is actually working.

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