Diesel engines do not use spark ignition. Rather, they rely on high compression to ignite the fuel mixture. Since there is no need for spark ignition, there is no point in spark plugs as well.
So do diesel engines have spark plugs? Diesel engines do NOT have spark plugs installed in them. But, they do have glow plugs installed to heat up the combustion chamber to help in starting the engine.
Diesel engines are ignited by high compression of the fuel mixture. The only problem is if the engine is in cold condition, the ignition might not take place.
So to heat up the engine, glow plugs are used in diesel engines.
Let’s dig into this topic further.
Do diesel engines have spark plugs?
Diesel engines do NOT have spark plugs.
Diesel engines do not need spark plugs to ignite the engine fuel mixture. Rather, the ignition is done through high compression of the fuel mixture in the combustion chamber.
This distinction is one of the major differences between a diesel engine and a gasoline (petrol) engine.
While gasoline engines need spark plugs to ignite the fuel, diesel engines rely on high compression.
This difference in the ignition methods has led to design differences in these engines as well. Gasoline engines have spark plugs installed in them. And the diesel engines have not.
So motorcycle engines do not have glow plugs, but have spark plugs. And diesel engines do not have spark plugs but have glowplugs.
Since gasoline engines rely on spark ignition, they have a lower compression ratio (ranging from 8:1 to 13:1).
Diesel engines rely on compression ignition and do not have spark plugs. The compression ratios of diesel engines range between 14:1 to 23:1.
A typical diesel engine will have a compression ratio of 22:1. Compare that to a gasoline engine – which usually has a compression ratio of 8:1. That’s almost a 3x increase in compression.
Bottomline. Diesel engines are dependent on compression ignition. Not spark ignition. And that’s why there is no need for spark plugs in diesel engines.
What do diesel engines have instead of spark plugs
Although diesel engines do not have spark plugs, they do have a plug installed in them which looks similar to a spark plug.
These are called glow plugs.
These glow plugs are fitted in diesel engines whereas spark plugs are installed in gasoline engines.
First things first. Glow plugs do not ignite the fuel mixture in diesel engines. They help in the ignition for sure. But they are not the ones igniting.
Glow plugs do not produce sparks. The ignition is still carried out by high compression of the diesel fuel mixture.
What are glowplugs?
A glow plug is an electrical heating element fitted in each diesel engine cylinder that guarantees dependable starts regardless of the weather.
Basically, the glow plug heats up the combustion chamber and ensures fuel ignition. This heating plug is helpful, especially in cold weather conditions.
A glow plug is only required during the ignition process instead of spark plugs, which constantly operate while driving. For a more in-depth discussion on differences, here is a post on spark plugs vs glow plugs.
Why do diesel engines need glowplugs?
Now, glow plugs help in heating the combustion chamber. How does that exactly help?
In diesel engines, there are no spark plugs generating an electric spark to ignite the fuel mixture. The diesel fuel mixture is ignited by the high compression of the gas mixture.
The compression ignition is fine. But the problem arises when in cold weather conditions.
When cold, the fuel mixture in the engine needs to be heated up so that the fuel ignites easily when compressed.
If not heated up, the fuel mixture might not ignite properly even with high compression.
And that’s exactly where a glow plug fits in.
The glow plug heats up the combustion chamber to facilitate fuel ignition. Without glow plugs, there would be engine starting problems in diesel engines. Especially in cold conditions.
Spark plugs vs glow plugs
Although they look similar, spark plugs and glow plugs are too different.
First, a spark plug generates an electric spark in gasoline engines to ignite the fuel mixture. Glow plugs do not generate any spark. They heat the engine to facilitate ignition.
Second, spark plugs are required in each combustion cycle. They need to generate sparks for every combustion cycle. Glow plugs, on the other hand, are useful only once before the engine starts.
Third, the application. Spark plugs are used in gasoline engines and have no place in diesel engines. Glow plugs are used in diesel engines and are not found in gasoline engines.
To highlight the differences between a spark plug and a glow plug, here is a table below:
|Features||Spark Plug||Glow Plug|
|Type of engine||Gasoline Engine||Diesel Engine|
|Function||Used for fuel ignition||Used for heating the engine|
|Source of ignition||Spark produced at the tip||Glow produced at the tip|
|Duration||Produce spark continuously to keep the engine running||Glows only once in the beginning to start the engine|
|Types||Copper, Iridium, and Platinum||In-cylinder variety and Thermostat variety|
Diesel engines do not have spark plugs. But they do have a plug called glow plugs installed in them.
But, as discussed above, spark plugs and glow plugs are not at all the same. It would be wrong to call glow plugs as spark plugs of diesel engines since glow plugs do not ignite the fuel.
Glow plugs should be changed once every 100,00 miles of distance traveled. Or every 5 years, whichever is lower.
You do not need to replace glow plugs as often as spark plugs. Spark plugs need to be replaced every 10,000 to 15,000 miles. Of course, for iridium spark plugs the changing period is much longer.
Diesel engines can run without glow plugs. Provided, the engine fuel mixture remains heated up sufficiently.
If the engine is under cold weather conditions, then problems might arise in starting the engine since glow plugs are not there to heat the combustion chamber.
The number of glow plugs in a diesel engine varies depending on the application.
For cars, the glow plug numbers in the diesel engine can range from 4 to 10 per cylinder. There can be as high as 16 glow plugs in a diesel engine.