Diesel knock is the clanking, rattling sound emitted from a running diesel engine. This noise is caused from 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 diesel 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 from 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 diesel knock 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.

The 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. Diesel knock 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. Diesel knock is seen by many as a tolerable side-effect of better fuel economy.