Instead of using spark plugs to ignite the fuel in a petrol engine, a diesel-powered engine actually relies on the high flash point of diesel itself to provide combustion. This means that once the diesel gets compressed to a certain point in a hot engine, it ignites of its own accord. The upside of this is that it doesn’t need another electrical part to make the engine work, but it can mean it’s harder for the engine to get going in cold conditions. This is where the glow plug comes in, which works essentially as a heat lamp to heat the combustion chamber while the rest of the engine warms up, meaning the engine can start working straight away no matter the temperature.