What is the component or thing which makes a loud roaring sound in some diesel engines, like my school bus?

In a diesel engine the most noticeable thing to be heard are the valves "slamming" back into there respective seats when the detonation occurs. The next noticeable noise would be the "whistling/whirring/whining" sound of the turbocharger that spins up to 15000 RPM's.


https://en.wikipedia.org/wiki/Diesel_engine

https://en.wikipedia.org/wiki/Diesel_engine

Diesel engines are inherently noisy because of the auto-ignition of the initially formed mixture of fuel-vapour and air, which causes rapid rate of pressure rise producing the characteristic noise. This abnormality is due to the inborn feature of a diesel engine, which uses a high compression ratio to obtain high fuel efficiency and a high compression-temperature for ignition of the fuel injected into the cylinder at high pressure. In a diesel engine, unlike a petrol engine, the air and fuel do not mix outside the engine.

https://www.scholarshipsinindia.com/answer/diesel_engines.html

Thanks for the answers but the sound is not caused by the diesel engine or pistons themselves. I don't know how to explain the sound. Not many diesel engines have the distinctive sound. I've only seen 3 different trucks in my life with the sound. On my school bus, the belt slips a lot when it's wet or dusty. When the belt slips it causes a squealing sound but more importantly, the "sound"-producing component slows down (the tone decreases) leading me to believe that it is driven by the belt. The only possible thing I can think of is a supercharger. The only thing I can think of to compare the sound to is that of a fan or radiator fans.

If the bus is doing a lot of stop-start work without switching off dropping students, the engine temperature will be fairly high. Modern engines have electric fans rather than the old permanently belt driven ones, and these only come in when engine temperature is high. Most trucks will not do the stop-start work a school bus probably does, and so you would only hear their cooling fans when the truck was labouring up an incline on a hot day. My diesel engined car rarely switches its fan on, and it only does so when I'm starting to get alarmed about the temperature shown on the gauge. Possible reason for the change in noise when the belt squeals is that the belt is driving the alternator producing the electric for charging the battery. When there is more load on the battery (and thus on the alternator) the alternator is harder to turn and there is more liklihood of belt slip. When the belt slips, less power is generated and the voltage may drop slightly, which can affect the speed of the fan. Superchargers are usually driven by exhaust gasses nowadays rather than belts and hence are called turbochargers. For diesels, turbo-charging is the norm now rather than belt or gear driven supercharging, and this wouldn't produce the noise you refer to. The turbocharger produces more of a whine in use, and there would be no moise difference on tickover, where a fan would sound more on tickover.

I think Baloo is on to the answer! Yes the fan! It will make a very ominous sound when the engine is hot. The fan has a temperature sensitive clutch within it that will engage as the RPM's of the motor increase. The fan will literally ROAR!

Electric fans ROAR also! I still see Thermo-Clutches employed on Diesel trucks and the like here in the USofA.

I'm the person who asked the question. The thing making the noise sounds exactly like the radiator fans when they turn on in my car, except on the bus it's always on, it's about ten times louder, and the "fan" changes rpm with the engine, like a turbo or supercharger, leading me to believe it is somehow connected to the engine. (or it might be the alternator that's connected, and producing electricity for the fan). Thanks for all your answers so far.

Thanks for the answers but the sound is not caused by the diesel engine or pistons themselves. I don't know how to explain the sound. Not many diesel engines have the distinctive sound. I've only seen 3 different trucks in my life with the sound. On my school bus, the belt slips a lot when it's wet or dusty. When the belt slips it causes a squealing sound but more importantly, the "sound"-producing component slows down (the tone decreases) leading me to believe that it is driven by the belt. The only possible thing I can think of is a supercharger. The only thing I can think of to compare the sound to is that of a fan or radiator fans.