Do Diesel Engines Have Spark Plugs?

I recently discovered what glow plugs are after seeing a glow plug light on a car I drove. After learning what glow plugs are and that they look an aweful lot like spark plugs, I had a question. Do diesel engines have glow plugs in place of spark plugs? Do diesel engines have spark plugs?

Diesel engines are not equipped with spark plugs. Diesel fuel is ignited in the engine by intense pressure which increases the temperature to combustion. The glow plug warms up the diesel fuel which is susceptible to gelling unlike gasoline which easily remains liquid. There are only a few engines that are exceptions from this rule.

There are also a few important things to know about glow plugs for us beginners just learning about them.

I am interested in purchasing a diesel car myself and so this was something that I wanted to know! Here, I share with you what I have learned about diesel engines, spark plugs, and glow plugs.

Diesel Engines Do NOT have Spark Plugs

Diesel engines are not equipped with spark plugs. They are designed to have enough compression in the cylinder to ignite the diesel without a spark. This is beneficial to simplify the car’s ignition system. The largest downside of diesel is the fact that diesel fuel gels at a higher temperature and glow plugs are used to remedy that.

Diesel engines work on pressure to ignite the fuel while gas engines rely on ignition to light a spark that ignites the fuel. Diesel engines avoid spark plugs to simplify the engine because the spark plugs are not necessary.

Can a gas engine run without spark plugs?

Gas engines would operate like diesel engines (without spark plugs) if they could, but gasoline cannot handle the high pressures that diesel can and the energy expended to compress the gasoline sufficient to combustion would be too much to make the effort worthwhile.

This idea has been explored and is being researched. It is a very interesting topic and the best place to start your research is Wikipedia. Read all about homogeneous charge compression ignition.

Diesel engines, to give a ballpark estimate operate often around a compression ratio of 16:1 while gasoline engines often have a compression ratio around 8:1.

Diesel Engines Have Glow Plugs

Glow plugs serve to heat up the diesel fuel when the engine is cool before ignition. Glow plugs allow the potentially gelatinized diesel fuel (because it is cold) to flow and vaporize in the cylinder. Cars equipped with glow plugs have an indicator light that will turn off once the glow plugs are sufficiently warm and indicates the engine can be started.

Glow plugs are the spark plug-like things that diesel engines are equipped with and are not the same as spark plugs.

I obviously don’t cover much here on glow plugs but if you are interested in learning more, head over to my other blog post that is The 3 Most Common Questions about Glow Plugs

Most vehicles equipped with glow plugs have an indicator light that indicates the glow plug is heating up. Once the glow plug is sufficiently warm, the light will go off informing the operator that the engine can be started.

Diesel Engines With Spark Plugs

Are there any diesel engines equipped with spark plugs?

I am only familiar with one example of an engine that runs on diesel fuel so this will not be a comprehensive list but only serves to show that it IS possible for a diesel engine to have spark plugs although uncommon.

Old tractor engines have spark plugs along with diesel fuel systems. These engines are equipped with a carburetor, distributor and spark plugs on one side, with the diesel injection pump and injectors on the left side.

The engine will run on gasoline at low compression and once warmed up, is able to switch over to diesel fuel. I am aware that there are old international tractors that operate this way and others such as the 1941 Farmall MD Diesel truck that runs on gas then switches to diesel. Click here to head to the Diesel World site to read about this cool old tractor engine.

Farmall Tractor Runs on Gasoline to Start and then Diesel

A decompression valve lowers the compression of the engine from around 17:1 to around 8:1 to allow the engine to start on gas. Once the engine is warm, you open the diesel throttle and flip the decompression lever. Then it runs on diesel.

So, of course in this example the engine while running on diesel does not require the use of the spark plugs but it is one example of an engine that exists that runs on diesel while at the same time is equipped with spark plugs.

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