How Bike Engine Works With Diagram

The complete four-stroke cycle. It's hypnotic.

We now have piston at the top of its stroke again. But the crank still has momentum and is still turning, and it starts to pull the piston back down the cylinder – which is an ideal opportunity to open another pair of valves (controlled by another camshaft) and let the low pressure of the descending piston draw a fresh pulse of fuel/air mixture into the cylinder, a bit like drawing blood in a syringe.

Once again the piston reaches the bottom of its stroke, and the cylinder above it is full of swirling fuel/air mix. Still driven by crank momentum, the piston starts to rise again, for the second time, which compresses the mixture. When the piston reaches the top, the spark plug sparks again, igniting the mixture and pushing the piston back down the cylinder all over again.

So that's a complete cycle of an internal combustion engine. If you've been counting, you'll realise the piston actually went through two up strokes and two down strokes per cycle – that's why it's called a four-stroke engine (there are many other types of engine – two-strokes, wankels, diesels etc – but almost all modern bike engines are four-strokes).

The four strokes are often shortened into memorable verbs: in the order described above, it'd be bang, blow, suck, squeeze (but it's more often ordered suck, squeeze, bang, blow, because it trips off the tongue better).

So now all we've got is a piston flying up and down, and a spinning crank. How does that make a bike go forwards?

As you'll appreciate, the four-stroke cycle described above happens very, very quickly. Unbelievably quickly. When your bike is ticking over, the crank will be spinning at around 1200 revolutions per minute. That's 600 sucks, 600 squeezes, 600 bangs and 600 blows every minute (because each one is half a crank revolution). And that's per cylinder.

So the crank is spinning very quickly, but if you then just connected it with a chain to a back wheel, it would only have enough force to drive a motorbike very, very slowly, and it would be revving its nuts off to do it. If you imagine choosing first gear on a pushbike, then trying to pedal as you ride downhill; same thing.

What we need to do is somehow select a higher gear on our pushbike instead. We need a system of gears to slow down the rate at which the crank is spinning, and which then – because of a phenomenon called mechanical advantage – actually increases the strength of the force (also called torque) to the point where we can drive a motorbike forward at a decent rate but with the engine revving more sensibly. We want to swap high crank speed and low torque for low crank speed and high torque.

Ah, how about, a gearbox? So the crank has a cog, or gear, on the end of it, which turns a bunch of other gears of different sizes – and we can select which one we want, using a clever mechanism called a gear lever – before the drive appears, slower but much more forcefully, at an output gear – and from where we can hook a chain round it and drive the back wheel.

Now your bike has a running engine, and is moving. Hooray!

Source: https://www.bennetts.co.uk/bikesocial/news-and-views/features/bikes/how-does-a-motorcycle-internal-combustion-engine-work

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