Used in starting a diesel engine in the cold. Since the fuel in a diesel engine is ignited by compression instead of by a spark as in a gasoline engine, the cylinder must be at a minimum temperature for ignition to take place. In warm weather this is not a problem, as temperatures are warm enough for the engine to run without any problem. When temperatures are very cold however, there can be a problem. If the temperature in the cylinder is not high enough, the compression of the piston will not raise the temperature of the air/fuel mixture to the flash point and it will not burn. The result is a diesel engine which will either start hard, or refuse to start at all. If you get the engine started, chances are that the engine is going to run quite rough until it can heat up.
To alleviate this problem, diesel engines have glow plugs in the cylinders where spark plugs would be in a gasoline engine. The purpose of the plug is to keep the cylinder at a temperature at which ignition will occur when the engine is off so that it will start easier in cold weather.
It may be time to check the glow plugs in your diesel engine if you have problems starting it during cold weather, or if it runs unusually rough after it is started. Take good care of the plugs and your diesel engine should not fail you in cold weather.