Why are diesel car engines so much louder than petrol, they sound like tractors?



Why are diesel car engines so much louder than petrol, they sound like tractors?

In: Engineering

Diesel engines operate by causing the fuel to ignite by the rapid heating that occurs when the air and fuel is compressed. In a gasoline engine, the mixture is ignited by the spark plugs.

To get the fuel/air mix to combust without a spark requires *much* higher pressure than in an equivalent gasoline engine. The result is also a much more powerful explosion, making for more noise, but also more energy released for a given displacement size, which is why diesels tend to have higher horsepower and torque, and somewhat better efficiency than gasoline engines.

1 bar is atmospheric pressure. Remove the air from a steel oil drum – 1 bar is plenty enough to crush it.

In petrol cars the explosion in the cylinder reaches about 20 bar under low load, and about 70 bar full load.

In diesel engines the pressure explosion would be around 80 bar at light load, and potentially almost 200 bar at full load. This higher pressure makes the characteristic loud chugging sound.

Also the fuel injectors which work at a crazy 2000 bar (!) make a characteristic clicking sound each time they inject fuel. This makes the characteristic click.

Also tractor engines are generally diesel.

Incidentally lots of small, compact European diesels aren’t particularly loud nowadays since they are very effectively isolated and shielded.

In the diesel engines, the fuel is injected into the already compressed air inside the cylinder. These types of engines are much noisier than the petrol ones because their mechanics work under higher pressure.

A lot of the noise of a traditional diesel engine is actually from the injector pump.

Here is a mechanical fuel injection pump being driven by an electric motor on a test machine to measure the fuel output to each injector and make sure it is set up correctly.