Glow Plugs

A glow plug is a heating element that assists in raising the temperature in the combustion chamber, which makes the engine easier to start in cold weather. The glow plugs are only used for cold, initial starting of the engine. They are needed because a diesel engine is a CI engine, which means the engine starts solely from the heat of compression.

If the engine is cold and the ambient temperature of the air intake to the diesel engine is too low, much of the heat generated by the initial rotations of the engine will be conducted away into the engine block and other components, and into the environment. If that happens, the air–fuel mixture temperature in the cylinder may not be raised sufficiently by engine compression for ignition to occur. Glow plugs preheat the incoming air so that ignition can occur more easily. This assists a cold engine to start more reliably. Once the engine is running, the glow plugs are no longer required.

A glow plug consists of a heating element and a resistor-type heating element attached to the end of a plug that penetrates the combustion chamber or precombustion chamber. When the glow plug is activated, current flows through the resistor causing it to heat up, which then raises the temperature of the incoming air. The glow plugs are activated by turning the key on before cranking of a typical modern diesel engine for somewhere between 10 and 20 seconds prior to the engine firing. Older and less efficient diesel engines, or worn engines, may require much longer preheating times to start the engine.

FIGURE 54-28
Heated air intake grid.

There are a variety of methods for altering the timing and style of the electronic fuel injection process in modern automotive diesel engines to ensure reliable cold starting. Even though glow plugs are fitted, they are rarely used for more than a few seconds. Glow plug resistor elements, also called filaments, must be made of materials that are resistant both to heat and to oxidation. Examples of such materials are platinum and iridium.

Some diesel engines use a heated air intake grid rather than glow plugs. The grid heater is usually placed in the intake manifold Figure 54-28. This component is activated prior to starting the engine, and the heated grid raises the temperature of the incoming air similar to the glow plug.

Applied Math
AM-34: Time: The technician can use different time measurement tools and techniques to determine if the system’s timed or sequenced operating parameters conform with the manufacturer’s specifications.

A medium-duty diesel-powered truck is in the shop with fuel injection pump problems. The owner had not maintained the fuel system properly and water entered the injection pump, causing its failure. A diesel technician is assigned to install a replacement fuel injection pump along with new fuel filters. The engine is a mechanically injected diesel and the fuel pump can be turned slightly right or left due to a mounting flange, which is slotted.

The technician carefully installs the pump and aligns the timing marks as accurately as possible at 2 degrees before top dead center (TDC). After starting the engine and checking for leaks, the technician asks his supervisor to listen to the sound of the engine. The supervisor and technician both agree that a diesel timing light is needed to assure that the timing is correct.

A diesel timing light differs from the timing light that is used on gasoline engines. There are no spark plugs on a diesel engine, so a spark-triggered light can’t be used. The diesel timing light uses a photoelectric sensor that installs in the glow plug opening to have access to the combustion area. In addition to this, an inductive sensor is placed very close to the harmonic balancer to process a signal from a mark on the balancer.

The timing light evaluates readings from these two sensors to determine a timing result. This method is much more accurate than merely aligning the timing marks that are found on the injector pump mounting flange. If a timing light is not used, the engine timing could be too far advanced or retarded. This could result in excessive emissions being produced or engine power output could be reduced.

When starting an engine with glow plugs or a heated air intake grid, do not use starting fluid or an explosion could occur. Be sure to follow starting instructions given in the manufacturer’s service information.