Do Diesel Engines Have Spark Plugs?

why diesel engines don't have spark plugs
July 9, 2021

Diesel Versus Gas Engines

Ask many and they will mention that the primary advantage to a diesel engine is that it provides more torque. Based on the combustion of the engine and the different use in plugs between gasoline and diesel engines, diesel provides a considerable amount of work-power over horsepower. Hence the big-rigs on the road are diesels. The utility trucks pulling boats, trailers and heavy loads are primarily diesels. And most work-site trucks are diesel vehicles to help drag away or haul heavy loads in their truck beds. With the importance in need for utility based vehicles, one can imagine how important it is to take care of a vehicle’s power-plant and understand some of the differences in their vehicle’s parts and maintenance.

“It takes torque to move weight,” states Joe Duke Skelton from Speedy Duke’s Diesel (Odessa, TX). “When comparing gas to diesel, if a gas vehicle is doing the same ‘work’ as a diesel, it’s going to burn five times more fuel.” Skelton claims that for many work-duty truck owners, once they buy a diesel, they never go back to gas for a work truck. “For the everyday driver, gas is great,” he says. “But for pushing and/or pulling any kind of a load in a vehicle, the gas owner typically finds themselves renting a diesel vehicle to get the job done.”

How Many Spark Plugs are in a Diesel

Diesel engines do not require spark plugs. 

With a diesel, obviously the first difference that comes to mind is the fuel. Where gasoline can ignite in its current state, diesel must first become atomized for ignition. “Diesel is a cleaner burning fuel than gas,” says Skelton. “Diesels have all of the emissions but the fuel burns richer and cleaner with a regeneration system helping burn solids in the emissions of the exhaust.” The difference in ignition methods, also explains why diesel engines do not need to have any spark plugs, which we’ll explore in the following section.


Why Diesel Engines Don’t Need Spark Plugs

A gas engine requires spark plugs to create a spark to ignite the gasoline and create the combustion stroke of the piston. A diesel engine has no spark plugs, so instead, diesels have a compression ignition and glow plugs that heat the combustion chamber to aid ignition if a diesel engine is cold. According to Skelton, “The difference in diesel is that diesel fuel doesn’t ignite. A spark plug has no use with diesel fuel because there is no need to ‘light’ the diesel fuel. Instead, the glow plug only heats the combustion chamber.”


Glow Plugs in Diesel Engines

In combination with the piston design, and the heated chamber from the glow plugs, the diesel fuel becomes atomized into a mist. The mist becomes more volatile, therefore igniting in the combustion chamber with a much more efficient explosion, forcing the piston with a stronger force, through it’s stroke. “In the bigger picture,” says Joe, “yes, both engines require combustion, but with diesel, the use of a glow plug instead of spark, utilizes heat instead of an arc, making the fuel molecules move quicker and thereby producing more efficiency in power.” A well-tuned gas motor typically pushes in the range of 130 psi in compression. A well-tuned diesel motor typically pushes in the range of 425 psi.


Common Diesel Diagnosis & Repairs

With the enhanced compression, every diesel diagnosis must consider the pressure being created in a diesel engine. Skelton says, “Regardless of a fuel leak, a coolant leak, blow-by, or almost any service on a diesel, compression affects almost everything, in comparison to a gas motor.” Many of the gasoline repair shops may miss the proper analysis in the difference of diesels and therefore sometimes an owner may want to consider a specialty shop for diesel maintenance.

Blow-by for example is considered a crankcase pressure. Crankcase pressure is produced by weakened piston rings allowing the compression blowing past the rings, down into the crankcase. With added diesel compression comes more opportunity to force the combustion past the rings. The proper diesel pistons, rings and oil however, helps create a tighter seal, while still reducing the friction of a tighter tolerance piston ring, providing an added bond to reduce blow-by. “Pulling an oil fuel cap that appears puffy can be an indication of blow-by,” says Joe, “also indicating some internal engine repairs may be ahead. The condition is more noticeable on a diesel due to the higher compression. Where a gas engine can sometimes run longer under the adverse blow-by conditions, a diesel engine cannot run very well, because the higher compression blowing by the rings causes a lack of torque, thereby causing much harder starting conditions.”


Preventing Carbon Build Up in Diesel Engines

With added diesel compression creating more force against the moving parts, added friction is also built into each revolution, requiring more lubricity and improved flow-through from the engine’s motor oil. “The internal components in a diesel work way harder than a gasoline engine,” says Skelton. “Fuel and oil additives can help counteract some of the intensified diesel energy by reducing the friction, reducing the wear and tear, helping eliminate engine noise on components that may be wearing and enhancing the combustion of the diesel fuel.”

By substituting the spark plug with a glow plug, the mist conversely ignites against more surface area, eventually causing a carbon build up. When build-up reduces the efficiency of the ignition, stiction eliminator additives act as a detergent throughout the motor, cleaning away the carbon interference and helping restore a more efficient and cleaner burn.

“Additives don’t make the motor run differently,” says Joe, “but they do help counteract the additional stresses incurred with a more efficient motor, thereby aiding in the longevity of the motor and helping reduce total cost of ownership in fuel economy, maintenance and repairs.”



“I have a handful of customers that come in after every over-the-road haul they make, to replenish their Hot Shot’s Secret additives and oil.”

I recently had a customer that came in weekly, for about three months straight. He had a lot of miles on his engine, but the motor was in very good condition. The only issue he experienced was some top end noise. So we kept filling him up with the Hot Shot’s Secret FR3 and Everyday Diesel Treatment. He would come in religiously to add these products and after a few months, his engine noises have now gone away and his truck is running great.

I had another customer with engine noise, that after only two treatments of FR3, his engine noise went away.

On the 6.0s, 7.3s that come in, Hot Shot’s Secret Stiction Eliminator saves us about 75% of the injectors that seem to be on their way out.”

– Joe Duke Skelton of Speedy Duke’s Diesel (Odessa, TX)