Offroad-Express

I’m trying to learn a bit more about diesel engines - why do some diesel engines of the the exact same model make a lot of diesel rattle and some almost none?

The more rattle or knock they make, the better the condition of the engine

But you have to be able to distinguish between the the diesel rattle/knock and the heavy sound of a crook bottom end.

Diesels have a compression ratio of around 20:1 (most petrol engines are around 9:1).

The atomised fuel detonates by the heat and pressure created by compressing the charge air to the above ratio.

The knock is the sound of the fuel detonating.

The bigger the knock, the bigger the detonation.

So as a result, a good solid diesel knock or rattle indicates the engine is in good shape, the injector timing is where it should be, the rings are good and there is plenty of compression.

I'm always suspicious of a diesel that runs quiet, unless its a really late model engine with sound deadening built into it.

all newish engines all run pilot injection which reduces noise. it injects a small bit of fuel first then the main fuel is injected into the burning pilot fuel.
older engines in light vehicles are often IDI injection where it injects into a side chamber which also helps reduce noise.

diesel knock is the fuel igniting all at once rather than igniting as its injected and burning with a smooth flame front.
the lag time between injection and ignition depends on many factors. how well its injected, cetane rating of the fuel, compression, engine temp etc.

loud noise is not necessarily an indicator of how well an engine is going. if the diesel knock is louder than normal it can mean the injectors are not spraying as well as they should be, poor fuel, to cold, low compression.

also do not forget a lot of the noise can be from the injection pump itself.

so how come the new common rail like the tritons have a hua of a knock when under acceleration

mudlark wrote:so how come the new common rail like the tritons have a hua of a knock when under acceleration

It's a mitsi... I own a gen 3 pajero and it's a really noisy diesel. The next gens are common rail but one of the complaints was they were too noisy. They have the same engine as mine.

Pretty much every common rail I've heard is so much quieter than a mechanical pump setup. Obviously not all will be this quiet but in general they are.

CRD's rock, to me it's like the comparison between carb and fuel injection on a petrol engine. The new CRD's production diesel's are getting close to 100hp/litre blow a lot less smoke and seem to be very fuel efficient.

Alot of the common rail ones do knock, the hilux 1kd seems to be one of the worst. some are much worse than others yet the diesel specialists don't know why. My Ranger only does it when cold with a wee bit of load.

The 3.2 mitsi does make a good knock, much worse when they are cold.

hiluxs have a few injector issues. often due to the crap fuel we have. bit of 2 stroke oil in the diesel and it will purr

tweake wrote:hiluxs have a few injector issues. often due to the crap fuel we have. bit of 2 stroke oil in the diesel and it will purr

yep they need replacing from/around the 60k km mark. afaik toyota replace them under warranty. a guy i know has run two stroke since he bought it and at 130k mark still no issues

Have a read of this .

http://www.bankspower.com/techarticles/show/28-Diesel-Evolution