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How Do Glow Plugs Work in Diesel Engines

GLOW PLUG FUNCTION: There is one basic function of glow-plugs.

It is to provide diesel engines with extra heat energy during the start-up in cold weather.

In simple car terminology a glow plug is an electric heating device.

This principle is a necessity for most diesel engines. It is especially so for old or high speed diesels.

The reason is because they do not always start 'readily' on cold and frosty mornings. They need a little help from a heater plug.

The problem occurs most when an indirect injection diesel engine block is cold. Often, the normal arising heat is insufficient for starting a diesel by itself.

This means there is not enough heat of compression in the combustion chambers. By that, we refer to the cylinder block and the cylinder head.

Heat loss occurs through cold metal cylinders and during air intake. It impedes the engine's self-ignition process. Thus, we need to assist a diesel engine in reaching the necessary temperatures.

Some vehicles get fitted with heater plugs for diesel engine workings. They get used to provide enough additional heat - or replacement energy source - for the engine to fire up. This may mean using several glow plug cycles on very cold mornings.

Diesel Glow Plugs How they Work

The process needs to operate before the start of the engine. The vehicle temperature sensor detects a lack of warmth in the air. Next, the glow plug gets energized through the control circuit. The glow tube heats up to an incredible 800° Celsius.

The high temperature 'significantly' improves the engine's ability to cold-start. Once the engine has started, it warms up and generates enough of its own heat afterwards. It will then run as normal without needing the heater plugs.

Another benefit of this increased heat development is an optimization of raw combustion in the swirl chamber. It reduces the chance of having a smoky diesel engine and developing other vehicle emissions.

As most owners of 4x4 diesel cars know, you should wait until the light goes out on the dash (if there is one). It tells you that the glow plugs have finished the warming up process and you can attempt to start the vehicle.

Glow Plug Function: Diesel Engine Glow Plugs DiagramGlow Plug Installation and Engine Position

Diesel engine glow plugs get installed in a specific position of the cylinder head. The principle does not work unless the glow rod's extended inside the hot zone.

The crucial part is making sure that the glow rod gets positioned exactly at the edge of the fuel mixture.

This is the only way it can provide enough heat exactly where it needs it most. The technical automotive terms for the correct positioning is 'precombustion chambers'.

Note: Extending the glow rod too far inside the combustion chamber is not ideal. The result would be unlikely to guarantee an ignitable fuel-air mixture.

How Different Parts of a Glow Plug Work

Generally, diesel engine glow plugs are pencil-shaped devices made of metal. There is a heating element located at the tip of the rod. The element heats up when it gets electrified - due to its electrical resistance. It then begins to emit an orange-red light as electricity passes through it.

That is one reason why the electrical resistor gets called a 'glow' plug. It's because the light is visible. It is a similar effect to that seen in a toaster element or a light bulb filament when they heat up.

The fuel injector spray pattern impinges 'directly' on the red hot tip. The timing of this glow plug principle is crucial to work with efficiency. It should take place during the injection of fuel at top dead center (TDC).

As a rule, the fuel ignites even when the engine is 'insufficiently' hot for its normal operation. This reduces the cranking time needed to start an indirect injection diesel engine.

Direct Injection Engines

The fuel can also get injected directly into the cylinder (not a swirl chamber). Thus, direct injection diesel engines do not have glow plugs. Some may have an intake manifold heater instead. This heats up the air entering the cylinders before compression. Other diesel engine starting aids include:

Cold diesel fuel tends to gel too easy - much like engine oil sludge. This means it does not combust 'efficiently'. Even so, not all vehicles get fitted with heater plugs as standard by the manufacturer.