Glow plugs are heating elements that are used in diesel engines to keep the engine warm during cold starts. Diesel engines start much more quickly when they are warm than when they are cold because the fuel ignites immediately and does not require a flame from an external source (such as an electric spark). If you don’t have glow plugs and let the engine idle before driving, it will sputter and jerk at idle with black smoke coming out of the exhaust.
The number of glow plugs varies depending on the engine’s manufacturer. The most common is 6, 4 for starting, and 2 for running. Manufacturers also vary in how long it takes for them to heat up – anything from 3 minutes to 30 minutes after power is applied. The more glow plugs the engine has, the less the starting time will be.
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Glow plug number
The number of glow plugs in a diesel engine will differ based on the engine type. This can be four, five, or six for a standard diesel engine. They are used to get the combustion process going, and also have a heat sensor in them that functions as an early warning system if there is any kind of coolant leak that would otherwise go unnoticed without these sensors.
In many newer diesel engines, the glow plug is combined with other parts such as the fuel injector into what is known as a “glow plug injection” system. The main difference between this and a non-combined system is that in order for it to work properly you need to pre-heat your combustion chamber prior to injection with either an external flame or by using electrical heating coils.
Ford – The Ford Motor Company has experimented with glow plug ignition in the Triton V8 engine. There are 4 glow plugs per cylinder.
GM – General Motors offers 4 glow plugs per cylinder in medium- and heavy-duty versions of the Duramax diesel engine, with six cylinders total. Light duty versions use 5 glow plugs per cylinder for a total of 30 plugs for a 6.6 liter engine, and 18 per cylinder for a 3.0 liter version, for a total of 54 plugs for that engine size, as well as four pre-heating coils.
Mercedes – Mercedes’s OM646 engine has 16 glow-plug injectors and four individual pre-heat coils in the fuel system.
Nissan – The Nissan M62T engine has four glow plugs per cylinder, which are preheated with heat lamps below instead of with an electric coil. This technology was used in the earlier TA/TA engine design, which was previously used in some of Nissan’s previous engines (e.g. Cavalier, Serena).
Renault – Renault Trucks has 14 glow-plug injectors for their 8.9 liter V8 diesel engine.
Scania – Scania’s DSI 3.7 liter common rail direct injection engine has four glow plugs per piston, and 6 cylinders total. Early versions of this engine were said to have 24 glow plugs all together, but the number of glow plugs has subsequently been increased with more advanced modeling software, to avoid pre-ignition issues at colder temperatures.
Some engines have an extra glow plug or two for an auxiliary heater circuit which provides heat to the transmission and differential as well as the engine. There is usually a switch on your dash to turn off this feature if you don’t want it.
Throttle body fuel injection (commonly known as TBI) allows you to turn off one or more glow plugs so that you can restart on cold days without waiting for all of them to warm up again. This is also useful if you’re trying to save fuel by keeping your foot out of it until your engine warms up, but you need an immediate throttle response for passing maneuvers or emergencies.
Early models of the glow plug system were usually made up of a mechanism integrated into a motor which was called “glow starter”. The glow starter had its own battery and electronics which were able to control the glow plugs correctly.
On these types of systems, each glow plug has its own heating element and due to their electric nature, they will usually have a lighting indicator as well. There is also a timer that is used by the engine computer to time out when the glow plugs are supposed to shut off after being activated.
This timer prevents overheating of the spark plug or valve due to constant use of the ignition system for too long and it also doesn’t allow fuel to ignite before it should.
Can the engine run with a failing glow plug?
Your diesel engine will still run even if one of the glow plugs fails. The way the diesel engine works is that each cylinder’s exhaust is connected to a glow plug. Before the piston is released, fuel from the injectors mixes with air and goes into the cylinder where it ignites an explosion.
This creates heat which breaks down water to create steam and ultimately power for your engine. In order to work properly, all of your cylinders’ glow plugs must fire properly without failure so that they can ignite when needed while creating enough pressure at near-temperature levels in order to make power for your vehicle without damaging any other components of your engine.
Final thoughts
The importance of glow plugs in a diesel engine is well-documented. Some may argue that diesel engines cannot operate without the glow plug and can therefore be called “fossil fuel” engines. Others point to the importance of cold starts and say that a diesel engine can be used for years without the glow plugs.
The more glow plugs do a diesel has the more chances for starting this engine in cold weather and the fewer chances for a diesel engine to fail when using glow plugs.
