There are no dumb questions and this one is included. This article is meant to save our new mechanic friend from embarrassment at the parts counter. After 18 years as a Diesel mechanic, I feel very qualified to answer this question.
Do diesel engines have spark plugs?
There are no spark plugs in diesel engines. Unlike gasoline engines, diesel engines combust by compression alone. The piston squeezes the diesel fuel and air mixture until it combusts at around 234 psi.
Now that the answer is out of the way. How does all that work? Let’s take a dive.
Gasolene is unpredictable and very volatile. You know this because of how careful you are when you handle it. Every smoke while trying to start a fire with gas? Nope. The fumes of gasolene are extremely flammable.
Diesel fuel is not this way. the flashpoint of gasoline -45°F. The flashpoint of diesel is 125.6°F. So manufacturers use spark plugs to create perfect firing timing. If you just compressed gas would not explode at a reliable time. It is not predictable.
Diesel is not this way. You know you can light a match and put it out in diesel. So diesel will reliably “blow up” when compressed the right amount. So, the piston just squeezes the diesel and air mixture until it heats to the amount needed to explode.
Diesel will reliably flash around 125 degrees. The compression of a diesel is also much higher than gasoline running from 14:1 to as high as 25:1. The gasoline engines with the highest compression only reach 12:1.
So, no spark plugs. What do glow plugs do?
If you are new in the diesel space you may be familiar with what a glow plug is, but may not be sure what it does.
It is the job of the glow plugs to heat up the combustion chamber before starting up your engine. That is why you will see a “Wait to start” on some diesel engine-equipped vehicles. When you turn the key over electrisity flows to either your intake heater, grid heater or glow plugs deending on the manufacture.
Diesel engines have an optimal temperature of the air inside the combustion chamber that makes it possible for a clean and efficient burn.
A Glow plug is essentially a heating element with a metal tube surrounding it. The tube has insulating ceramic material and is sealed on the end. When the glow plug is switched on, electricity flows, and its tip can exceed 2000 degrees F within just a few seconds. This warms the air in the combustion chamber to the ideal level for operation.
Do all Diesel engines have glow plugs?
There are many diesel engines that do not have glow plugs. Many larger diesel engines do not have them. Some manufacturers use an intake heater to accomplish the warming of the air being inducted.
Some diesel engines have them and others don’t.
Why do some diesel engines not have glow plugs?
As a general rule, Diesel Engines that do not have glow plugs have some other form of an air heater in the intake. Other forms of air heaters in diesels include intake heaters and grid heaters.
There are different solutions to solve the cold air in the combustion chamber problem.
Grid heaters are located inside the intake manifold. They are a simple heating element that heats the air as it passes over the fins of the heater. They are usually located on the top and are easily accessible.
Intake Heaters are similar in design as they are simple heating elements. They are unique because they are wrapped around an insulator and inserted into the intake around the connection to the ducting for the filter. They are very easy to remove and service.
Glow plugs are different because they only heat up on the tip. The screw-in to the cylinder head and the tip are placed directly in the combustion chamber. They are more difficult to service because of their placement.
A modern development
Some manufacturers are now using a “spark plug” of sorts to ignite the carbon during the after-treatment regen process. This uses the DEF fluid in the spray and then heats up and effectively cleans the diesel particulate filters.
There is a fuel spray that is released into the exhaust and the spark plug ignites the fuel spray inside the burner (ARD) to heat the DPF and burns the soot down into ash. It is then released out of the tailpipe.
This is not all manufacturers and it does NOT count as a spark plug 😉.
Diesel engines have no spark plugs at all. It is a common misconception. So now that you know this, You can bravely tell your buddies that you will not be going to the parts counter for them. Don’t be fooled. Glow plugs are not the same and are used for different purposes. Until next time, Keep wrenchin.