What is a glow plug all about?

By January 4, 2017Truck Blog
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We have discovered a lot of curiosity about the differences between glow plugs and spark plugs.


A common question exists among people considering a diesel engine, “What in the world is a glow plug”. A good place to start is explaining the two types of internal combustion. Diesel engines use glow plugs and gas engines use spark plugs.

The main difference is that diesel engines rely solely on compression to raise the temperature of the air to a level that allows the diesel to combust automatically. As the piston rises in the cylinder, pressure is increased. This causes the temperature in the cylinder to increase sustainably. By the time the piston reaches the top of the cylinder, the pressure needed to cause combustion is reached and fuel is introduced into the cylinder. Combustion happens at that point forcing the piston downwards. These demands on the engine require a large and robust engine block.

When the engine is cold compression alone is not adequate for combustion. This is where the glow plug comes into play. The glow plug solves the problem by adding the heat needed for combustion.

During cold weather diesel engines lack the heat necessary to start and require the help of additional heat. The cold cylinder and air intake reduces the ability for combustion to occur. Extra heat is necessary for combustion to ignite. The glow plugs main function is to supply additional energy for the start. Before the diesel engine starts, the glow plug is energized and heated to 1400 degrees. This heat development also optimizes combustion allowing smoke and emissions to be reduced.


Two Basic Types of Glow Plugs:

Glow plugs are basically categorized into two types: metal rod, and ceramic. In the metal rod a glow rod of metal produces the necessary heat. The ceramic type comes without a metal glow tube. Their heating element is encased in a special type of ceramic. Ceramic glow plugs are more compact and reach the necessary temperature very quickly. This characteristic is advantageous in modern engines. Testing and replacing glow plugs should always be performed by experts that are trained and knowledgeable in this field. Incorrect installation and testing can cause damage in the glow plug itself and the engine.

Removing a glow plug and connecting it to a power source can cause serious injury and burns.

Possible damage to a glow plug can be observed simply by its appearance. Some visible damage can be ascertained such as overheating.
Overheating can arise by:

  1. The injection time or spray pattern not being set correctly.
  2. The wrong fuel quantity is injected or oil infiltrates the combustion chamber


Overvoltage damage can result by:

  1. Using the wrong glow plug (12 volts instead of 24 volts)
  2. The control unit is defective
  3. The alternator generates too much voltage.


Other visible damage can be a broken glow plug. A broken glow plug can be the result of incorrect point of injection timing, or poorly set flow rate. Breaks in the case or connection can be caused by using the wrong tightening torque or the wrench was not used correctly.

In warm and dry weather diesel engines can start even if one glow plug is defective and only the other plugs are functional. In this case, there will usually be increased pollutant emission and the possibility of knocking during start. More than likely the driver will not consciously notice these signs, or will not know how to interpret them. There will be a unpleasant surprise once the weather becomes cold and the necessary heat to the diesel engine fails to function, and the engine will at best start with difficultly and produce smoke– most probably, however, nothing will work at all. Below is a list of typical damage and the related causes.

Finally, there have been discussions on removing glow plugs and connecting them to an auxiliary power source. These testing methods are dangerous and can cause injury and burns. In addition, incorrectly installing glow plugs can damage not only the glow plug but the engine. Testing and replacing glow plugs should always be left to trained technicians. They have the knowledge and the proper equipment to get the job accomplished.

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