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Why Do Diesels Rattle

Diesel motors are an absolutely awesome engineering feat and they have made a huge difference in the automotive world as well as the transport industry. So why do diesel motors always rattle?

Diesel motors were invented by a German mechanical engineer by the name of Rudolf Diesel in the 1890’s as a way to create a more efficient combustion engine. In 1897, Rudolf built the first running variant of his engine and since then, the motors have continued to improve.

One of the key differences between a petrol and a diesel motor is, in fact, the efficiency. The high efficiency of Rudolf’s design came, in part, by adding fuel directly to the cylinder and using heat and pressure to ignite instead of the normal spark plug.

By adding the fuel directly to the cylinder at the exact right time and relying on the heat and pressure to ignite the fuel instead of spark plugs, Diesel didn’t just create a more efficient motor, but also the cause of the rattling that is commonly related to diesel powered vehicles.

In a normal petrol motor, the cylinder pressures are fairly low, at around an 8-14:1 compression ratio. The fuel is mixed with the air and a spark used to ignite and burn the mixture. When the wrong fuel is used, the mixture is wrong or the spark is poorly timed, this mixture can detonate, meaning it burns before the spark ignites. This detonation causes a loud ticking/rattle type noise which is called pinging.

This pinging noise is the source of rattling in diesel motors as well. As a diesel motor relies on high compression and heat to ignite the fuel instead of a spark, they are essentially relying on the same detonation that causes a pinging noise in a petrol motor to run.

Because of this, and the higher compression ratio of 18-25:1, the noise caused by detonation is a lot louder, causing diesel motors to rattle. Though this is a common complaint about diesel motors, this high compression ratio (along with the efficiency of the fuel itself) is one of the many reasons diesels make more real world power in almost every application.

Check out this article we wrote recently about glow plugs and why diesel's take longer to start.

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