How is my vehicle powered – how does petrol make my engine work? Part 3 0

Remember from Part 2: Think of each piston in your engine as the leg of a cyclist on a bike.

Except you need to imagine the cyclist with four legs (a typical car has four cylinders) or maybe six, eight or more – so a fly on bike, or a spider on a bike.  To complicate matters, a bike is more akin to a ‘two-stroke engine’ – this is where the piston strikes every revolution. This is the type of (coarser) engine one finds in a lawn mower or moped. Cars have four-stroke engines where each piston is ‘pushed’ by an explosion once every second revolution. Yes, that is now starting to sound a bit harder to picture than a bike so we might stop there!

So in short, the engine works by lots of small, contained, petrol-powered explosions, one quickly after the other, pushing out pistons, which in turn drive the shaft round, and power the wheels.

And so there we have it, that’s how some dead animals under the sea bed, pipes across frozen wastes, giant ships, some school chemistry, popping champagne bottles and a four-legged cyclist means you can you can drive to the shops.

The down side of all this of course is the gases left after the spoonful of petrol has had its theatrical limelight in the cylinder. The piston has been pushed out, the work is done but there is no happy ending for the left over exhaust gas.

First of all, the gases are still pretty hot, as is the metal around the cylinders from the explosions. This energy is effectively lost/ wasted: only about a quarter of the energy stored in petrol actually drives the wheels round, the rest is wasted. In this way, combustion engines are inefficient and really quite wasteful. All this leftover heat also means the engine needs a cooling system, otherwise it gets too hot. We’ll cover the cooling system in an upcoming article.

Furthermore the exhaust gases neither enjoy the party the popped champagne bottle enjoys, nor give you a slim and fit cyclist… it finds itself unceremoniously cast out of your car’s rear end dirtying the atmosphere, becoming one of humanities big niggles.

So, that’s how your petrol makes your wheels go round – next time we’ll look at what your engine needs to run a whole lot sm-oooooooo-ther.


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