Thread: Noisy Diesel engines,, and why?

I guess a start would be compression ratio which in a diesel is 18 up to 20 to 1 whereas a petrol is 8 up to say 10 to one depending on vintage.
This drives higher pressures in the engine through the power cycle hence they have to be built stronger/beefier and not quite as elegant.
My guess
Regards
Barryp

EGR quietens idle rattle. It works because ignition is delayed with less oxygen content. Disconnecting EGR on a TD5 makes it rattle considerably more which I used to my advantage to alert pedestrians to my approach when idling through shopping carparks.

Diesels rattle mainly because compression and combustion pressures rise a lot faster than petrol engines, and it is particularly noticeable at low engine speeds and power settings because there is literally no throttle - there is no restriction on the amount of air going into the cylinder, so even at idle it gets a full feed of air, which is then compressed.

The change with small changes in engine speed is almost certainly as various parts of the engine and vehicle resonate at different frequencies.

Modern diesels are impressively quiet compared to older ones, mainly because of multi stage injection made possible by common rail systems, and attention paid to soundproofing as diesels have become mainstream in passenger cars (especially in Europe). A good comparison is my Isuzu and my son's I30 -driving the latter, you cannot hear or feel the engine once under way, road noise is louder.

John

The noise is created by the rapid rise in pressure when ignition occurs. BTW sound is transmitted as a pressure wave, but the direction of the vibration is longitudinal, not transverse like waves you see on the surface of water. The greater the change in pressure and the shorter the time, the greater the amplitude of vibration (sound level). Frequency is the number of cycle per second of the sound wave.

It is worse with older diesel fuel injection technology as a consequence of how diesel is ignited.

Diesel does not burn. Without getting too technical with the chemistry, before it can ignite the large, complex molecule has to be 'broken' into simpler molecules by the heat and pressure in the combustion chamber that was created during the compression stroke. The time for this to occur is called the ignition delay. Because this time doesn't get any shorter as rpm increase, is one reason why direct injection diesels can't run at high rpm's.

However with the old fuel injection technology fuel is still being injected during the ignition delay period. Computer controlled injection systems inject a small initial quantity to commence ignition. The 2 stage injectors, as used in the 300Tdi, were developed for the same goal.

Once ignition starts, temperature rises quickly, and the remaining diesel molecules break down quicker. So with the old injection systems, the large quantity of fuel injected during the ignition delay, now ignites quickly and the pressure rise is very high.

All of the fuel is injected and combusted during a small angle of crankshaft rotation around top dead centre. As more fuel has to be injected for higher loads, the injection period is extended to obtain a higher fuel rate. At maximum load it will normally be over before 10 to 15 degrees after top dead centre. Injection will normally start 10 to 15 degrees before TDC.

Edit: To achieve high torque, high combustion pressure is required near TDC.

Petrol is vapourised before it enters the cylinder and starts to burn as soon as the spark causes ignition, but the way it burns the pressure starts low and increases towards the end of combustion. Diesels produce very high pressure early and it reduces toward the end of combustion.

Most NOx (oxides of nitrogen) is created by high temperature early in the combustion, which makes it a problem for diesel engines to lower the NOx emission and produce high torque.

Drive a Td6 3.0ltre L322 RR and you wouldn't know it was a diesel,no knocking,rattling or smoke,the Europeans make excellent diesels. Pat

advance your pump timing a degree,
or get timing checked , will give smoother idle

Thanks guys, that clears a few things up.

John your comprehensive explanation is good without being too technical, thanks.
Oztrax, I'm not sure there's anything wrong with mine, but next time I'm up home I might get Ivan to have a look at it just in case. It's been running the same pretty much since I bought it, and it's had the engine management system and injectors replaced at 10000k service too, so I would think if there was an issue, they'd have picked it up then.

With a bit of luck, anyone who is similar to myself with this query will find this and then be in the know too

Originally Posted by oztrax

advance your pump timing a degree,
or get timing checked , will give smoother idle

Advanced timing makes an engine noisier. Retarding injection timing makes it smoother and quieter.