Why Diesel Engines Don’t Have Spark Plugs

While spark plugs are universally used in petrol-fuelled internal combustion engines and vehicles, and in gas-powered industrial stationary engines, diesel engines don’t have spark plugs. Instead, diesel engines are fitted with special components called glow plugs, which are similar to spark plugs in some ways but have some crucial differences.

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The reason why diesel engines don’t have spark plugs is due to the unique way that diesel engines operate. Let’s look at what happens in a diesel engine in more detail:

What Happens In A Diesel Engine? 

In a gas or petrol powered engine or generator, spark plugs are essential to initiate the combustion cycle that sets the application in motion. The spark plug delivers a small arc of electricity across an ignition gap, which ignites the fuel mix in the combustion chamber. This is the classic internal combustion process used by both vehicle engines and stationary industrial engines alike.

In a diesel engine, the ignition process is almost identical, except that it uses compression to generate the required heat to initiate the process, rather than an electric spark. When the diesel engine is switched on, air is compressed within an engine cylinder, generating heat as the pressure increases. When the correct temperature is reached, the air is quickly siphoned into the ignition chamber at the same time as the diesel is injected. When the fuel meets the influx of superheated and pressurised air, instant combustion occurs.

The compression process allows diesel engines to reach full operating capacity more quickly than gas and petrol-powered engines, and also accounts for their greater fuel efficiency.

What Are Glow Plugs?

The compression-ignition process means that diesel engines don’t need spark plugs, but they do sometimes have glow plugs, which are, in effect, mini heaters. The glow plugs aren’t used to ignite the fuel but help warm up the compressed air in the engine cylinder. This overcomes one of the main drawbacks of diesel engines, which is the time taken to reach ignition temperature, especially in cold ambient environments. (It’s the reason diesel cars take longer to start up on winter mornings, but are less likely to stall once they get going.) When you hear people or businesses talking about ‘diesel spark plugs’, what they’re actually referring to is glow plugs.

Next Steps

While we don’t stock diesel engine spark plugs – for reasons we have explained – we do have an extensive selection of spark plugs for every type of gas-powered industrial stationary engine. As your one-stop shop for gas ignition equipment, at RM Walsh we aim to supply the best components at competitive prices, and ship to businesses throughout the United Kingdom. Please browse our product pages to find out more or get in touch directly if you would like to speak with an adviser.


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