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Do Diesel Engines Have Spark Plugs?

202410 do diesel engines have spark plugs scaled jpg Diesel engine glow plug | Cars.com illustration by Paul Dolan

You own a diesel-powered vehicle and are thinking of giving it a tuneup. But does your diesel even have spark plugs to change? The short answer is no, diesel engines do not have spark plugs. Instead, they use the super-heated air created by their high compression ratios to ignite the diesel fuel as it’s injected into the cylinder.

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This concept, called compression-ignition, works because air gets warmer as it’s compressed. Diesel engines have a compression ratio that’s about twice as high as that of a gas engine, typically in the range of 14:1 to 25:1. In the latter case, this means 25 cubic inches of air are compressed into the space of just 1 cubic inch as the piston rises in the cylinder. This high compression heats the air in the cylinder to such a degree that it will ignite the diesel fuel as it’s squirted directly into the cylinder. Higher compression results in greater efficiency.

However, in winter, the incoming air could be so cold that the temperature increase created by compression alone may not be enough to ignite the diesel fuel. Thus, many diesel engines have what are known as glow plugs — essentially little electric heaters — that can heat the air in the cylinder to a higher temperature before the engine is started. Glow plugs are typically good for up to 100,000 miles; bad glow plugs can lead to poor fuel economy, misfiring, a rough idle, smoke or hard starting.

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Are There Compression-Ignition Gasoline Engines?

If you tried to use the compression-ignition principle on a gas engine, the gasoline — which is more volatile than diesel fuel — could ignite too early, potentially causing engine damage. While there is at least one commercially available gas engine that uses compression-ignition — Mazda’s SkyActiv-X — even that starts the combustion process with a spark plug because the spark can be accurately timed. After combustion is started, the resulting pressure and temperature increase prompts the rest of the air-fuel mixture to burn. The advantage in this case is that SkyActiv-X can run on very lean air-fuel ratios — about twice as lean as normal — which provides what Mazda says is a 20%-30% increase in fuel economy over its SkyActiv-G engine.

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