Glow plugs are miniature heating elements enclosed in a metal housing with a probe at one end and an electrical contact at the other. One spark plug is screwed into each combustion chamber. When the spark plugs are turned on, they heat up quickly, increasing the temperature of the air entering each combustion chamber. The spark plug probe is located directly in the path of the fuel torch from the injectors. Therefore, the fuel passing through the spark plug also heats up, allowing the optimum combustion chamber temperature to be reached more quickly. Additionally, small particles of fuel passing through the spark plugs ignite, which helps to start the combustion process.
The duration of the warm-up period is determined by the warm-up control unit. This device monitors the air temperature in the engine compartment using an ambient air sensor (on earlier models) or coolant temperature (detected by a coolant temperature sensor screwed into the cylinder head - later models). The device changes the warm-up time period (the time during which the spark plugs are energized), to meet specific conditions.
A control lamp mounted on the instrument panel informs the driver that preheating is in progress. The lamp goes out when the cylinders are warm enough to start the engine. However, power is supplied to the glow plugs until the engine starts. If the engine does not start, power to the glow plugs is cut off to avoid discharging the battery and overheating the plugs. Note that on later models, the control lamp will remain lit while the engine is running if there is a problem with the preheating system.
Later models have a post-heating mode - after the engine starts, the spark plugs continue to function. This helps improve fuel combustion until the engine warms up. This mode makes the engine quieter, the ride smoother, and the emission of harmful components lower. The duration of the "post-heating" period is determined by the temperature of the coolant.
Original article is on the portal MercedesMan.ru