Four-stroke engine, operation and principle

Four-stroke engine, operation and principle

In the past, every motorist was half a mechanic. Today, there are quite a few classic enthusiasts who are genuine enthusiasts, but who actually have no idea what is going on under the hood. Of course we know everything. But perhaps there is someone for whom this article about the four-stroke engine is enlightening.

The name four-stroke – or 'four-stroke' – engine is derived from the operation of the engine. It is based on four 'strokes' or strokes. And the only correct sequence is intake stroke, compression stroke, power stroke and exhaust stroke.

For the sake of convenience, let's take a four-cylinder engine for this article. Each cylinder has (usually with classics) an inlet valve and an outlet valve. More valves per cylinder were already known in the past, but quite exotic. When one of the four inlet valves opens, the piston in that cylinder moves from the highest point to its lowest point. The piston moves from its TDC (top dead center) to its ODP (bottom dead center). The downward movement increases the volume and creates an underpressure above the piston. Because the inlet valve is open, a fuel mixture is drawn into the cylinder through the negative pressure via the intake manifold and the carburettor. When the piston has reached its ODP, the inlet valve closes. The piston is attached to the crankshaft via the connecting rod. During the journey from the BDP to the ODP, the crankshaft is rotated 180 °.

The compression stroke

That is the second 'stroke'. During the compression stroke, the intake and exhaust valves of the four-stroke engine are closed. The piston now moves up to its TDC. The mixture is compressed in the combustion chamber. In order to compress the mixture optimally, it must of course not leak between the piston and the cylinder. If the piston rings or cylinders are worn, it does and this results in power loss. During the compression stroke, the crankshaft is also turned 180 ° again. He has now made a complete revolution.

The labor stroke

At the end of the compression stroke, the ignition causes the affected spark plug to spark. This spark ignites the mixture in the combustion chamber of the four-stroke engine. Because the mixture is strongly compressed, the ignition stroke causes a very strong rise in pressure in the combustion chamber. This expansion ensures that the piston is pressed down forcefully from its TDC because the inlet and outlet valves are closed. The power stroke ensures that the engine 'works'. During that working stroke, the crankshaft rotates another 180 °.

The exhaust stroke

After the power stroke, the piston is on its ODP. Above the piston is now a quantity of 'exploded' mixture. During the exhaust stroke of the four-stroke engine, the piston moves from its ODP to its TDC. Because the exhaust valve opens at the beginning of the exhaust stroke, the residual gases are forced out through the exhaust manifold and the exhaust system. During the exhaust stroke, the crankshaft makes another 180 ° travel and thus the entire trajectory has taken 720 °.

If the piston is then back in TDC, a complete cycle has been completed.

Rotate smoothly

Our example is a four-in-line engine. Because that is such a deliciously common example. The engine is designed in such a way that the four pistons together deliver four power strokes at two 'rpm'. This allows the engine to run smoothly. Of every four 'strokes' of each piston, only one provides new propulsion force to the crankshaft. The other three strokes are doing something different. The engine is constructed in such a way that for each of those non-productive strokes of a piston, a power stroke is made on one of the other pistons. This gives the crankshaft a nice force impulse every 180 °.


Such a four-cylinder four-stroke engine produces four power strokes per two revolutions (720 °). Since one power stroke lasts 180 °, the end of the power stroke of one piston coincides with that of the other. Characteristic of the conventional set-up is that the first and the fourth piston go up and down at the same time. That also applies to numbers two and three.

The order of the working layers is 1,3,4,2. . And that immediately explains the order of the spark plug wires on the distributor cap. The firing sequence is also the cause of the fixed sequence of valve adjustment. Always start at number 4, then 2, 1 and 3.




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Leave a Reply
  1. Hey Dolf, You always write funny and interesting texts! A few mistakes have crept into this story - you refer under 'The power stroke' in the first line to the power stroke where this compression stroke should be. And further on under 'The exhaust stroke' it goes wrong again.
    Furthermore, only compliments!


  2. 1, 3, 4, 2 is the normal and most commonly used sequence.
    Ford had to make something different again. And so the ancient Fiestas were equipped with that Kent engine, crankshaft mounted three times, with the pop order of 1, 2, 4, 3.
    Strange? Well, that's just how you look at it. With about 1100 you gave a 2L BMW at the traffic light the first meters of unexpected spanking. Pure pleasure !!

  3. Is it perhaps meant that at the end of the compression stroke in the Upper Dead Point the spark plug sparks and ignites the mixture and then the downward power stroke follows?….

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