Spark plugs are used in gasoline engines to ignite the fuel. Diesel engines don't use spark plugs, as they ignite the fuel by compressing it. However, they do have a plug called a "glow plug" which is essential to using a diesel engine vehicle in cold weather. A glow plug has many important functions.
Warm the Engine
The most important function of the glow plug is to help warm the engine during cold periods. The diesel engine requires a lot of heat to operate consistently, as the diesel fuel will only fire if it is compressed heavily. This isn't a problem during warm periods or if the engine has been running awhile. However, if your diesel engine has been sitting around in cold weather for a long period of time, the engine block will get very cold. The cold engine block will suck up all the heat when you try to start the engine, causing it to struggle to start. A glow plug sticks just a bit out into the firing chamber. It will use battery power to heat itself up excessively, which will then cause the rest of the engine block to warm up. A glow plug can cause the engine block to heat to 800 degrees Celsius, greatly improving the engine's ability to start.
Diesel engines create a whitish blue smoke while they burn. This is caused by the burning diesel fuel creating fumes which are harmful to the environment. Glow plugs help reduce these emissions by up to 49 per cent by creating a more efficient burning environment. With a warmer engine, the fuel will be burnt more easily and will be "optimised," meaning it will be burnt more smoothly and fully, eliminating much of the smoke after-affect of running a diesel engine. This is because the glow plug will still be incredibly hot after starting the engine. It will slowly cool down as the engine drives, but the extra heat caused by the cooling plug as well as the remaining heat left from heating the engine block will help the fuel burn more efficiently than it would otherwise.
Early diesel engines were incredibly loud. The pistons ran less smoothly than in gasoline engine pistons, which created a din of poorly moving pistons. They were especially loud when cold, as the fuel would burn much less efficiently, increasing the noise. However, while diesel engines are still louder than gasoline engines, glow plugs have helped increase the efficiency of the fuel burning and thus reduced the noise created by the engine running.