Petrol engines have spark plugs, while diesel engines have glow plugs instead. These components are necessary for the diesel combustion process to run. But do all diesel engines have glow plugs?
No, not all diesel engines have glow plugs. Diesel engines produce the heat they need by compressing air very quickly, but that’s not enough heat for some engines to function, especially smaller diesel engines or if it's cold outside. Glow plugs make the process a lot easier.
In this guide, we’ll take a deep dive into what a glow plug does and why some engines don’t have them at all.
Keep on reading!
What Is a Glow Plug?
Naturally, the first question in your mind is, what is a glow plug? A glow plug is a heating element that you’ll find inside diesel engines.
Its name comes from how the plug looks when electrical energy flows through it. When it receives power, it heats up and glows.
What Does a Glow Plug Do?
The glow plug is there to raise the temperature on incoming fuel and air, allowing the mixture to combust and produce energy inside the diesel engine. There is one glow plug for every cylinder in the diesel engine.
So, let’s be clear on one thing: glow plugs do not ignite the fuel. The diesel/air mixture ignites by itself. Glow plugs just heat up the mixture to help things move along easier.
For the most part, glow plugs will only stay on for as long as it takes to start the internal combustion process.
However, in some engines, the glow plug will stay on for a brief period after the engine starts running. That period is called the ‘post-heating time’, and its duration depends on the engine’s type and temperature.
Whatever the case may be, the glow plug will shut off when the engine is running at the correct temperature. It will only start to glow again when you restart the engine.
Glow Plug Vs Spark Plug
At this point, you’re likely to notice several similarities between a glow plug for a diesel engine and spark plugs for petrol engines. Well, that’s because they have the same core purpose: they trigger the combustion process that these engines require to produce an energy output.
Naturally, there are some differences. The most notable is when each plug turns on and for how long.
Glow plugs turn on during the initial phases of starting a diesel engine. Once the cylinders are hot enough, they’ll turn off until you need them to restart the engine later. From that point on, the combustion process can continue without the help of any glow plugs.
Spark plugs, however, only turn on long enough to produce a spark. However, they’ll continue repeatedly firing for as long as the petrol engine is up and running. If the spark plugs stop sparking for whatever reason, the combustion process stops.
Glow plugs and spark plugs are siblings, each designed to handle diesel and petrol combustion, respectively.
Do All Diesel Engines Have Glow Plugs?
Interestingly enough, not all diesel engines have or need glow plugs. Most larger diesel engines do not need any glow plugs because their fuel/air mixture can ignite without any problem.
You’ll mostly find glow plugs in smaller diesel engines that struggle to maintain the correct temperature necessary for combustion.
Remember: The diesel/air mixture ignites by itself because it heats up when compressed quickly. That’s where the engine gets the heat necessary for the combustion process.
Unfortunately, smaller diesel engines have trouble maintaining the heat from compressing the air inside. That’s why glow plugs are necessary. They ensure everything runs as hot as they need to be.
Can a Diesel Engine Start Without a Glow Plug?
Yes, diesel engines can start without a glow plug. As mentioned, most large diesel engines do not have glow plugs inside them at all.
Glow plugs don’t trigger the combustion process in a diesel engine. Their purpose is to warm up the fuel/air mixture to combust by itself. The mix already has enough heat coming from the air, which gets hot as it becomes compressed.
As you might imagine, diesel engines will be more challenging to start in cold weather, like winter.
How Do I Get My Diesel to Start in the Cold?
Diesel engines need enough heat to start. Yes, modern diesel engines are much easier to start than older ones, especially those with glow plugs. Still, you’ll have to put more thought into starting your diesel engine safely and effectively during the winter months.
Here are a few things to keep in mind when it starts to get cold:
- Park somewhere relatively warm: When you own a diesel vehicle, and it starts to get cold, you'll want to park it somewhere fairly warm. A garage that's a few degrees warmer than the outside is good enough.
- Get a block heater: If the winter in your area gets excessively cold, you can also invest in a small block heater. This is a straightforward accessory that heats your engine block and makes it easier for the engine to start. Think of it as similar to a glow plug, except it's outside of the engine.
- Give the engine time to warm up: Don't drive immediately when your diesel engine starts running. And most certainly, don't put any excess load on the engine like towing something behind your diesel truck. A 5-minute warmup time is good enough.
Signs That a Glow Plug Needs Replacing
Eventually, you'll have to replace your glow plug like any other car component. Here are a few signs to look out for to know when your glow plug is wearing out and needs a replacement:
- Problematic starting even in warm temperatures
- Engine misfire or rough idling
- The engine starts burning more fuel than usual
- You’ll notice white or black smoke. White smoke means there isn't enough heat to burn the fuel correctly, while black means the diesel/air ratio is incorrect likely due to a faulty glow plug interfering with combustion.
Looking for new glow plugs or other diesel engine parts? Check out the Marketplace at CarPart AU!
By Ray Hasbollah