THE FOUR-STROKE CYCLE IN DETAIL

We hear a lot about an engine being four-stroke or two-stroke, but what is a stroke? Simply put, a stroke is one full movement of the piston in the cylinder, either up or down. In a four-stroke engine, this would mean that the piston moves up in the cylinder twice, and down twice before completing one cycle.

But why, and how, does the piston move?

There are a number of reasons for the piston to move, and just as many to explain why the piston moves.

Firstly, each piston has its corresponding counterweight on the crankshaft. This weight balances out the weight of the piston and keeps it moving. The pistons also move in a ‘firing order’ which means that in the standard four-stroke engine, there is always one piston in every part of the cycle. This ‘firing order’ also keeps the pistons moving, as combustion in one piston causes a downward pressure on the crankshaft, turning it. The turning of the crankshaft causes a different piston to move up. This all keeps the pistons moving, and the cycle continuing until you shut off your engine again. 

Video credit to: saVRee savree.com

The four strokes are as follows:

intake

During this stroke, the piston moves down in the cylinder (sleeve, chamber, etc.), with the intake valves open. In diesel engines, (as we’ll be discussing in this blog) cool air rushes in through the intake valve to fill the opening in the chamber. (On the other hand, in petrol engines, an air-fuel mixture enters the vacuum in the chamber, ready to be compressed and then combusted) 

Intake

compression

Compression

With all valves closed, the piston moves up in this stroke, tightly compressing the air in the chamber. The tight compression of the air causes its temperature to rise to almost 700°C. 

combustion

This is also called the “Power-Stroke”. (Please keep in mind that we’re talking about a direct-injection diesel engine here).  A fine mist of diesel is injected into the chamber, and into this compressed air, where it ignites instantly due to the high temperature of the air inside. This powerful combustion causes the piston to move down again for the second time in this cycle. This is the only stroke in the cycle where the piston exerts power on the crankshaft and not the other way around.

Combustion

exhaust

This is the final stroke of this cycle. With the exhaust valves open, the piston moves up for the second, and final time in the cycle. The gasses left over from the combustion get pressed out of the chamber, and out the exhaust valve by the upwards movement of the piston. 

This then completes one cycle, which repeats itself over and over as long as your engine is turned on. 

As you can see, this has to be one of the most important parts of your vehicle and must be well-maintained for optimal performance. Luckily, DeBoer Trekker Parte has all the parts you need to keep your vehicle’s engine running in tip-top shape. Just head over to our contact page, and give us a call for all your engine parts. 

In our next blog, we’ll discuss how the four-stroke cycle causes movement of your vehicle. Stay tuned to our FaceBook page to be updated when our next blog is up!

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