Scuba

Why are they so noisy compared to petrol engines?

Dream Weaver

Only a guess, but I would think it is something to do with ignition temperatures and compression ratios. A diesel engine is usally twice as high comp ratio than a petrol engine.

Fuel is injected into the compressed air which ignites spontaneously, rather than being ignited by a spark plug.

misty

Perhaps this will shed some light on your question matey!!

Diesel engines use direct fuel injection (DI), that is to say the diesel fuel is injected directly into the cylinder. The diesel engine has no spark plugs. The air it takes in is compressed and the fuel is injected directly into the cylinder where the heat caused by the air compression ignites the fuel. In the old days this meant that it exploded and expanded very quickly, making a noisy engine. This is why most diesel cars were IDI (indirect injection); the rough behavior was fixed by injecting the fuel into a small pre-combustion chamber that is connected to the cylinder by a narrow passage.

This slows down the explosion, as the gasses have to escape from through the narrow passage into the cylinder. This gives a softer bang and a smoother engine, but the gasses have to work harder, which lowers the efficiency a little. However the newer breed of DI engines use other techniques to tame the behavior of the engine, such as two stage injection, electronic control, and acoustic shrouds and shock absorbing engine mounts to mask the rattle.
dave

red wrex

So are you saying that the engine runs by det on purpose

would this not have the same result as on a petrol engine

iminent death!!!

jjones

no cos diesels are built like brick ****houses. that's why they are so heavy!

misty

Well I suppose you could put it like that!!!
dave

Scuba

Sounds crazy "Yes thats right my car has no spark plugs"

red wrex

Yeah, Im sure its a little more technical than that but i get the basics

Dark Blue Mark

Desel does not ignite easily, and only combusts under high compression.

New Honda deisel is supposed to be silent as a mouse

MB

misty

yeh, a hippopotomouse!!!
dave

Mmmmm

They are noisy so you can hear them coming and shout

"TAXI"

MattOz

LOL

My 330d is as quiet as any 4 cylinder Scoob engine. Some diesels are rattly, some aren't. Don't know why mine is so quiet, but probably has something to do with counter rotating balancer shafts etc.

Matt

Duck_Pond

Matt - yours is quiet because it's always parked up on your drive, being pampered and polished!

rik1471

Ark at you - "counter rotating balancer shafts"

misty

And the 30kg of sound insulation!!!!
dave

MattOz

Dave,

Even bonnet open, side by side the 03 WRX was noiser

Matt

ALi-B

iTrader: (1)

Also another thing that is VERY noisy on diesels is the pumps....this applies to non-common rail diesels which rely on a mechanical (with electronic control in later variants) pump and injectors.

These pump units are responsible for timing the fuel delivery and quantity. And they make a right racket....In my previous life I've had these on their own (i.e no engine)on a calibration dyno. For a pump unit (containing 6 individual pumps) off a cummins truck engine running at 3000rpm the noise is so loud you are required to use ear protection, and the dyno is in it's own little sound insulated room.

Obviously the latest diesel use common rail and solonoid operated injectors which control the fuel timing and quatity rather than fixed pressure opening which mechanical injectors use. So the pump unit is no longer required to meter and time delivery to each cylinder, and only needs to maintain a constant pressure to the fuel rail so a smaller, less complicated single high pressure pump would be used instead of a unit of pumps that have to specfically vary the injector timing and fuel quantity.

Simpler pump = less components = less noise

Thought I'd add while the industry hoo hars on common rail....VW's PD engines are NOT common rail...which is why they are noisier. They use a a system known as pump duse (PD). which basically has the injector and pump in one single unit for EACH cylinder...4 pumps = 4 times as much noise as a common rail (well not quite...but you get the idea )

Why not use common rail? well, so far Common rail injection hasn't evolved enough to meet or exceed pressures that are capable in normal mechanical injection

MGJohn

Prior to Herr Diesel putting his Monika on them, diesels were known as compression-ignition engines. It's a very old concept. Yes, detonation is one way of explaining it. With badly adjusted,too hot or overheated petrol engines, these sometimes 'run on' after engine switch off. This is known as 'dieseling' and sounds exactly like a noisy diesel. Yes, diesels are built like the proverbial because the compression ignition places much higher stresses on their moving parts and that equals more metallic knocking noises ... there's no such thing as a quiet diesel..... noise reduction is mostly down to the sound reducing containment techniques and measures rather than improved engine design which does of course play some part.

Dark Blue Mark

Have you looked at the new Hinda Diesel engine?

MB

Phil M

i love stupid jokes hahaha

NotoriousREV

In fact, early compression ignition engines were designed to run on coal dust, fuel oil (or diesel as we know it now) wasn't introduced until later. They were often used as small pump engines at coal mines or to power narrowboats carrying coal (free fuel)