How is my vehicle powered – how does petrol make my engine work? Part 2 0

Remember from Part 1:  When fuel burns, the space taken up by the exhaust gases is much much greater than the space that the original petrol and oxygen took up to start with. This is due to the gases produced, and a lot of heat.

In an internal combustion engine, like the one in your car, a couple of special things happen to get something much more useful from the burning fuel.

Firstly, a spoonful of fuel is sprayed from your fuel tank, into the engine’s cylinder, and is mixed with air. A source of ignition is applied to the fine drops of fuel and vapour – from a spark plug in petrol engines, or just from getting hot from being squeezed in a diesel engine – and produces a flame which spreads rapidly through the air/ fuel mix. A miniature explosion occurs, and produces a huge amount of heat which enhances the expansion of the volume of exhaust gases (as discussed in Part 1).

Second, this little explosion is contained within one of the engine’s cylinders; cylindrical hollows, surrounded by thick aluminium or cast iron, that are a key part in the engine. The explosion pushes on the walls of the cylinder to no avail. Instead, a piston that plugs one end of the cylinder, is pushed violently out as the gases expand. Think champagne bottle corks and you would not be too far off!

A bit of mechanical engineering to do with crank shafts, valves and timing belts, turns the ‘out’ motion of the pistons (the popping cork) into a rotation, and preps up the next piston with fuel for its turn for an explosion. Fix some wheels on the rotating bits with a gear box and you’re away!

Sound complicated? Think of each piston as the leg of a cyclist on a bike. The cyclist’s leg goes down and pushes the pedals round. She lifts her other leg back up, bending it so she can push down hard on that pedal at the right moment. A motor vehicle engine is not so different….

Part 1, Part 3

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