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Engines

What is Diesel Knock?

By Lori Kilchermann
Updated Feb 16, 2024
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Diesel knock is the clanking, rattling sound emitted from a running diesel engine. This noise is caused by the compression of air in the cylinders and the ignition of the fuel as it is injected into the cylinder. This is much the same as a gasoline engine suffering from pre-ignition or spark knocking. The timing of the fuel being injected into the diesel engine is critical to prevent parts breakage, which can result from severe knock.

A diesel engine functions differently than its gasoline counterpart. In a gasoline engine, fuel is mixed with air and then compressed before an electric spark ignites the mixture. In a diesel engine, only the air is compressed. The fuel is then injected into the cylinder filled with compressed air, and the heat from the compressed air ignites the fuel without the aid of an electric ignition.

The telltale sound of an operating diesel engine is due in part to the fuel injection process. By injecting raw fuel into extremely hot compressed air, the fuel ignites as the piston is still traveling up in the cylinder, causing a detonation and subsequent rattling sound to be heard. The process is compression driven, and the higher the compression ratio within the cylinder, the greater the power output of the engine.

While gasoline engines typically operate at 8:1 to 10:1 compression ratios on the street, the typical diesel engine operates at 14:1 to 25:1 compression ratios. This higher compression allows a diesel engine to operate much more efficiently than its gasoline cousin. Diesel knock is a by-product of the raised compression and fuel injection process and is an acceptable result of the ignition sequence.

A diesel engine is difficult to start in cold weather due to its lack of an electronic ignition system. Many manufacturers equip diesel engines with glow plugs to aid in starting the engine in cold climates. A glow plug uses the battery to heat a wire coil red hot in the combustion chambers. This causes more noticeable diesel knock in the engine until it reaches operating temperature. Knocking declines as the fuel begins to ignite more easily within the engine.

Some manufacturers have created special engine mounts that help muffle diesel knock from passenger compartments. As the cost of fuel rises, diesel engines are being fitted into an increasing amount of passenger vehicles due to superior fuel efficiency. Knock is seen by many as a tolerable side-effect of better fuel economy.

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Discussion Comments

By anon993957 — On Jan 02, 2016

The normal diesel engine knocking is like a drummer hitting solid blocks of metal in regular timing. It should be smooth and bearable, like a cat purring.

The knocking is caused by the combustion explosion. It is normal in diesel engines, although they are being made to run more and more quietly.

In a gasoline engine, the combustion knock is known as pinking (or pinging) and is not at all desirable. It can indicate overheating or that you are in too high a gear for the engine to cope with comfortably. Most likely because the extra load has increased the compression in the cylinder and pre-ignition is occurring.

Timing the ignition in a gasoline engine is a precision event and too early or too late can cause damage. This is even more so when pre-ignition occurs, because it is irregular and uncontrolled, so one cylinder may be igniting as it is trying to compress air/fuel mixture on it's way to the top of the cylinder.

Diesel engines have roughly twice the compression ratio of gasoline engines, and so are built to withstand compression ignition. The timing of the ignition is controlled by when the diesel is injected into the cylinder.

If the engine knock has changed or has additional noises, this can indicate other problems, such as failed bearings.

To isolate which cylinder, you can remove each spark plug wire one at a time or loosen each injector pipe one at a time. If the effect is very different for one or more cylinders, then the problem is likely specific to that cylinder.

In a diesel engine, it can be a single fuel injector that is injecting droplets rather than atomizing the fuel. This can cause serious damage as the fuel is not combusted evenly. The sudden appearance of white smoke and a smoky waxy smell may indicate this.

By anon993364 — On Nov 08, 2015

Bad injectors can cause knock as well as grey smoke and a stinking exhaust. The job of the injector is to shatter the fuel into microscopic droplets that will burn rapidly. Often an injector "pop-pressure" of 1800 psi will be most unsatisfactory as the droplets will be too big but a pop pressure between 2200 and 2400 psi will give a dramatic improvement. Some old industrial diesel engines often sound as if the pistons are being hit with sledge-hammers but allegedly on those types this is normal! Many years ago Rover-Perkins produced a reasonably quiet Direct Injection car engine by using two-stage injectors but Common Rail has now superseded mechanical injection.

Extremely high injection pressures are used to "atomize" the fuel which burns as soon as it meets the red hot air. With this type the noise is relatively low. It is all about air chasing fuel and fuel chasing air and big droplets are a definite no-no.

By anon345544 — On Aug 20, 2013

It's an economical process. Gasoline is cheaper in USA and Diesel is cheaper in Europe. Europe exports excess Gasoline into USA and USA exports Diesel into Europe.

That is also the reason why engine lubricants are more expensive in Europe: they are made of the same basic Petroleum oils as diesel increasing the demand even more.

By nextcorrea — On Jun 07, 2011

@Summing - In Europe t is fairly common for cars to run on diesel engines. For complicated reasons this feature has never caught on in the US. But you are right that it makes a lot of sense. I am hoping that we can begin incorporating easy ideas like this to start making a meaningful difference in the world today.

By summing — On Jun 04, 2011

If diesel compression leads to greater engine efficiency and fuel economy, why don't more cars use diesel engines. Wouldn't this make a lot of sense considering everyone's focus on saving energy and being green?

By chivebasil — On Jun 03, 2011

I used to have an old Mercedes that took diesel and it would make a knocking sound occasionally. I guess it must have been diesel knock. My mechanic always told me not to worry about it, but I never knew what was making the sound

By jonrss — On Jun 01, 2011

I used to work as a trucker and the sound of diesel knock always makes me kind of nostalgic. There is really no other sound like it in the world. When you hear that sound, you know you're about to drive a rig.

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