What are glow plugs?
To overcome this problem most diesel engine vehicle manufacturers provide an engine heating system and/or a provision to start the engine with ether or petrol and then change over to diesel. Electric heating systems can be auto or manual.
How do glow plugs work?
When the glow plug is turned on, the heating element at its tip gets heated. This heat is focused into the engine cylinders and heats the engine block that surrounds them. While generating heat, glow plugs also emit light causing them to glow. When the engine reaches a certain temperature, fuel mist is sprayed into the cylinder causing it to combust and power the engine.
Why are glow plugs needed?
The temperature in a diesel engine is raised by air compression where the piston moves upwards and causes air to compress and heat. Diesel engines are powered when the heat in the cylinder is high enough to cause fuel sprayed into the cylinder to combust and push down the piston.
Generally during hot weather or after the engine has already been running, the engine block is at a high enough temperature to cause fuel combustion to happen quickly when the ignition switch is turned on.
However during cold weather or when the engine is to be started in the morning, the engine temperature is not high enough. In this case a diesel engine heating system with glow plugs comes into play. Apart from glow plugs, a diesel engine system generally has a temperature sensor, a glow plug relay, a timer and an indicator lamp. When the ignition key is turned to the "on" position, the glow plug relay turns on the glow plugs in turn causing the indicator lamp to be switched on. The glow plugs are kept on for a few seconds to heat the engine block. When the temperature sensor detects that the engine has reached a certain temperature, the glow plug relay will switch off the glow plugs and the indicator lamp will go off indicating that the ignition key can now be turned to the "start" position. This cycle is the pre-heating cycle. The glow plugs can be kept on even after the pre-heating to cut down on emissions.
More info on Diesel Engine Heating