How do Diesel Engines Run Without Spark Plugs?

Nowadays, the prices of fuel are rising day by day. Thanks to the incredible invention of diesel fuel that is not just efficient but cost-effective at the same time. The reason behind this engine being so affordable is that it does not need a spark plug (as needed in petrol engines). Now, one question must be popping in your mind “How on earth do diesel engines run without spark plugs?” Well, there is no rocket science behind it.

Just a little difference in the functioning leads this engine to ignite without needing a spark plug and run your car smoothly.

Do Diesel Engines Have Spark Plugs?

Unlike gasoline engines, diesel engines do not need spark plugs to create the ignition that ignites the intake air mixture. At the end of the compression stroke and the beginning of the expansion stroke, diesel fuel is injected into the combustion chamber of the diesel engine. Under the influence of high temperature and pressure of compressed air, self-combustion will take place. To put it simply, diesel engines have a high compression ratio, so they don’t need an ignition device (spark plug). However, in fact, self-detonation still occurs in both gasoline and diesel engines at the time of low compression ratio. 

In addition, some diesel engines will have an extra heating spark plug. In winter, the compressed air does not create enough temperature and pressure so when injecting diesel, self-combustion can occur. The reason is that the combustion chamber in the engine is too cold. At this time, the heating spark plug has the effect of heating the combustion chamber before starting the engine. 

As such, there is no ignition stage in diesel engines. In conclusion, diesel should have a high flash point and a low auto-ignition temperature. The definition of “flash-point” of a fuel is the lowest point at which it can mix with air to form a flammable mixture. The “high flash point” in diesel fuel means it doesn’t burn as easily as gasoline, which is a safe condition. A flash point that is too low will also damage the engine because fuel combustion continues and the fuel burns before the explosion – work.

As we know, the ignition system is an essential part of a vehicle (gasoline engine). If the ignition system doesn’t work, your car won’t start and that’s why the ignition system is so important. In contrast, in diesel engines, we do not see the presence of this system. Let’s find out why diesel engines do not have an ignition system (spark plugs).

Major Reason Behind Diesel Engines Run Without Spark Plugs

A spark plug is used in gasoline engines and vice versa in diesel engines that do not use them. Gasoline and diesel have different properties, one of which is automatic ignition (thanks to pressure and temperature). Diesel has a low self-ignition temperature. Autoignition temperature means that the fuel starts burning automatically at a specified temperature. Because of its low self-ignition temperature, diesel can easily ignite. To understand more about the reasons behind this question, Let’s read these reasons below to know why diesel engines are sparking without requiring a spark plug at all, right now!

1. Low volatility

In diesel engines, the infusion of air and fuel takes place at different stages. The ratio of compression may lead from 14:1 to 25:1. The idea behind designing the diesel engine is to have higher compression ratios. The less volatile nature of the diesel, as a result, tends to have a low self-ignition temperature.

However, another reason is their power cycle according to which the higher compression ratio (mentioned above) is not possible to achieve by petrol, as the chances for onset auto- ignition or engine knock are high.

But, to include such a high-pressure ratio, the cost of the engine will increase, so to keep it commercial, the manufacturers have come up with a solution to increase the temperature of the fuel as well as the combustion chamber thanks to spark plugs (drying spark plugs). It also provides convenience, as during low starting temperatures or in cold weather, the spark plug will also deliver some heat to the cylinder and combustion chamber. Diesel has low volatility so it cannot be like a mixture of gasoline and air to be ignited by a spark plug.

2. Self-ignition temperature

Due to the lower self-ignition temperature properties of diesel than those of petrol and gasoline, the temperature generated by the compression of air while compression stroke is on the action is enough to ignite the atomized diesel fuel. The temperature could be 1000F or higher, which is enough to ignite the fuel. This clearly shows that diesel engine is an improvement over petrol engines as in these engines only air is compressed during compression stroke making them capable of working even at the much higher temperature ratio as well.

Major reasons for diesel engines run without spark plugs
Why do diesel engines run without spark plugs? (Photo: CarTreatments)

On the other hand, petrol engines have an inability to work at higher temperature ratios even if they do work at some high temperature, air-fuel mixture may pre-ignite. This can result in knocking and detonation and for that, you might also require some maintenance tips. This is why the lesser self-ignition temperature capability makes the diesel engine an improved version of SI (petrol) engine.

So, this is how diesel engines run without spark plugs efficiently and quickly.

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3. Neglected usage of spark plug

The atmospheric air is stuck in the cylinder of the diesel engine, which then gets compressed to high pressure. This process eventually increases the temperature and as the engine came into action at the end of compression stroke, ignition happens, and fuel burns. This naturally eliminates the requirement of a spark plug to ignite the engine.

Diesel engines run without spark plugs to save fuel
Diesel engines run without spark plugs due to overuse of Spark Plug. (Photo:

So, this is why the diesel engine doesn’t require spark plug in addition these engines have overcome the disadvantages of earlier engines like noise and spark plug maintenance. Therefore, diesel engines run without spark plugs to save fuel, time, and money at the same time.