Diesel engines run on compression ignition. Air in cylinder is compressed to ignition point, then fuel is injected into the now hot cylinder. The whole principle is that as air is compressed, it gets hot. Higher compression means hotter ignition. Typical diesel compression is 16 to 1 or higher with some going as high as 22 or 23 to 1.
Glow plugs are one of many devices used to help heat up the combustion chamber for cold engine starting. There are also grid heaters, intake manifold heaters, and other less direct methods of helping to get a cold engine started like block heaters and oil heaters.
Glow plugs operate on the principle of heating the inside of the combustion area to aid in reaching the required temperature to ignite the fuel. Glow plugs are normally only used with INDIRECT INJECTION. This is because they are used in conjunction with a pre-combustion chamber. Fuel is injected into the pre-combustion chamber and the hot glow plug along with the compressed air charge ignite the fuel.
With direct injection, there is usually a higher compression ratio used that requires less preheating.
Using starting fluids with glow plugs is a dangerous combination. Starting fluids will ignite explosively, and will generate much more pressure than normal compression and combustion. That higher pressure can result in something yielding to the pressure. The weakest link in the combustion chamber is the glow plug. When they fail, they can be turned into projectiles that have velocity and energy similar to a high powered rifle. Thus the warnings about not using starting fluids with glow plugs.
Glow plugs can also be used in the intake manifold of some engines as an intake air pre-heater. These are particularly dangerous if starting fluid is used. Manifolds are not able to withstand the pressure of rapidly expanding gases from combustion.
Normal usage of glow plugs is to preheat the combustion air for quicker ignition of the fuel. This is usually accomplished with a timer. When you turn on the ignition switch, the glow plugs are timed to heat for a time to help in starting. The timer can be keyed into the ambient temperature to determine the length of time that they will be heating. Most automotive applications will have an indicator light to tell you when the glow plugs are active, and how long to wait to engage the starter. A bad glow plug or two will make an engine difficult to start.
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