A glow plug is basically a heater. Diesel being injected into the engine isn't ignited by a spark, but spontaneously by compression. This can't happen if the

A glow plug is basically a heater. Diesel being injected into the engine isn't ignited by a spark, but spontaneously by compression. This can't happen if the temperature in the cylinder is too low. Having your fuel being ignited by the engine running, as opposed to something like a spark plug, has interesting side effects like dieseling and runaway.