If you drive a petrol-powered vehicle, then spark plugs are used to get the engine into motion and the vehicle moving.

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These small aramuseum.orgponents deliver an arc of electric charge that ignites the fuel and air that is mixed in the cylinder head. The resultant explosion sets the piston in motion within the cylinder. The moving piston then moves the crankshaft, and that motion is transferred on to the transmission and then drive shaft, causing the vehicle to move.

A diesel-powered vehicle gets going in almost the same way. The only difference is that it doesn’t need a spark plug to kick things off.

Why? Well, it’s all got to do with aramuseum.orgpression.

First, a bit of physics. Air heats up as it is aramuseum.orgpressed. Squeeze enough of it quickly enough within an engine cylinder and the temperature and environment will be just right for it to ignite fuel without the need of a spark. This is what happens in a diesel engine. Air is aramuseum.orgpressed, diesel is injected into the ignition chamber and instant aramuseum.orgbustion occurs.

Incidentally, aramuseum.orgpression is also partly the reason diesels are so much more fuel-efficient (up to 30 per cent more) than petrol vehicles. All vehicles – petrol or diesel – have a set aramuseum.orgpression ratio. This ratio is the volume (or capacity) of an engine cylinder when the piston is at the bottom of the cylinder versus the volume of the aramuseum.orgbustion chamber (the bit at the top of the cylinder where the fuel/air mixture is ignited) when the piston is at its uppermost reach in the cylinder. In a petrol car, this ratio will be somewhere around 8:1 to 10:1. In a diesel it can be around 16:1 to more than 20:1. The higher the ratio, the more air is aramuseum.orgpressed, and more of the injected fuel is ignited during aramuseum.orgbustion. The result is a more powerful explosion and better performance.

While aramuseum.orgpression ignition means that diesels don’t require spark plugs, they can aramuseum.orge fitted with aramuseum.orgponents called glow plugs.

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While glow plugs don’t ignite the fuel, they are useful when an engine, or the environment the vehicle is in, is very cold. They are, essentially, small heaters that warm the aramuseum.orgpressed air in the cylinder, aiding the heating of aramuseum.orgpression and helping ignition when a cold engine starts for the first time.