Why do diesels make the noise they do? - PeachParts Mercedes-Benz Forum

jbach36 · #1 10-29-2007, 10:48 PM

I was just wondering about something, as my diesel now sounds louder than it did just a few days ago. I'm thinking it's because the weather got cooler, maybe, but I don't know.

But that beckoned the questions, why do diesels make the sounds they do anyways? A gas engine compresses fuel and uses spark. The diesel engine compresses the fuel a lot more, with no spark, making it ignite due to the pressure. Either way, the pistons go up and down to provide power for the engine.

Maybe it's just the extra compression and exploding of the fuel, rather than sparking it?

Jeff 1991 300d, 112k

viggen9 · #2 10-29-2007, 11:45 PM

great question. I've been wondering the same thing myself and took a trip over to wikipedia where I read the following:

"The two main factors that held diesel engine back in private vehicles until quite recently were their low power outputs and high noise levels, characterised by knock or clatter, especially at low speeds and when cold. This noise was caused by the sudden ignition of the diesel fuel when injected into the combustion chamber. This noise was a product of the sudden temperature change, hence it was more pronounced at low engine temperatures. A combination of improved mechanical technology (such as two-stage injectors which fire a short "pilot charge" of fuel into the cylinder to warm the combustion chamber before delivering the main fuel charge) and electronic control (which can adjust the timing and length of the injection process to optimise it for all speeds and temperatures) have partially mitigated these problems in the latest generation of common-rail designs. Poor power and narrow torque bands have been helped by the use of turbochargers and intercoolers."

psfred · #3 10-30-2007, 12:44 AM

Several sources, but the big one is a supersonic shock wave that appears in front of the burning fuel during injection. When cold, the fuel also ignites late, so instead of burning nicely, it more explodes (commonly known as injector knock). If it's only for a very short time after starting, it's not a problem, but if it goes on all the time (and isn't a rod or bad lobe on the cam) you probably need to fix the injector or the low compression that's actually causing the noise.

Low compression will make knocking much worse (adjust the valves on 617.x engines -- the exhaust valves get tight with wear and you have low compression), so does overly advanced timing (stuck injection timer).

Peter

#4 10-30-2007, 01:47 AM

When the weather gets cold the stations switch to winterized D1. It's got a lower cetane rating so, as you can see by the graph, lower cetane results in later ignition and a higher pressure spike.

Deni · #5 10-30-2007, 04:08 AM

Why does a diesel make much less noise when it's running on vegetable oil, or some oil that is thicker than diesel?

IMO one of the noises is from injectors "opening" and "closing". Thicker fuel, IMO, dampens the impact.

#6 10-30-2007, 04:13 AM

It's the lower energy content and much slower burn rate, not the injectors.

Deni · #7 10-30-2007, 07:12 AM

The engine becomes quieter even by using engine oil. Has the engine oil a lower energy content and slower burning rate?

Mark DiSilvestro · #8 10-30-2007, 08:37 AM

On a related note, I have a pair of 240Ds - an '81 and an '82. While idling, sometimes the '82 has a much louder diesel 'knock' than the 81. I've heard of a phenomenon called 'nailing'. Is this something I should worry about?

Happy Motoring, Mark

rrgrassi · #9 10-30-2007, 12:39 PM

Nailing is like a ping on a gasser.

ConnClark · #10 10-30-2007, 03:32 PM

Quote:

Originally Posted by ForcedInduction

When the weather gets cold the stations switch to winterized D1. It's got a lower cetane rating so, as you can see by the graph, lower cetane results in later ignition and a higher pressure spike.

D1 has a higher cetane rating but a lower BTU value. If D1 had a lower cetane rating then it would be more difficult to start in the winter.

TheDon · #11 10-30-2007, 03:40 PM

I was wondering why my car was getting louder.. I.E. more turbo whine (its gotten colder) and slightly more clatter

DieselAddict · #12 10-30-2007, 05:55 PM

No, D1 actually has a lower cetane rating than D2 given the same amount of additives. The reason it's used in the winter is its much lower gel point.

Diesel noise is a funny thing and it can be made quieter in 2 ways: reduced ignition delay from higher cetane (good) or slow/incomplete combustion from low volatility (resulting in bad emissions and engine deposits). Veggie/engine oils fall in the 2nd category and that's why they run quieter. A better way to quieten a diesel engine is by using a high-cetane fuel and keeping up on maintenance.

DieselAddict · #13 10-30-2007, 11:38 PM

I noticed no one answered why diesels are usually louder than gassers. To my understanding this is mainly because gasoline is made to burn slower by limiting the amount of oxygen in the combustion chamber. Diesel engines are open to the atmosphere and take in as much air as they want. If gassers were allowed to do that they would be much louder and they would tear themselves apart from the violent combustion.

jbach36 · #14 10-31-2007, 12:42 AM

Quote:

Originally Posted by DieselAddict

No, D1 actually has a lower cetane rating than D2 given the same amount of additives. The reason it's used in the winter is its much lower gel point.

Diesel noise is a funny thing and it can be made quieter in 2 ways: reduced ignition delay from higher cetane (good) or slow/incomplete combustion from low volatility (resulting in bad emissions and engine deposits). Veggie/engine oils fall in the 2nd category and that's why they run quieter. A better way to quieten a diesel engine is by using a high-cetane fuel and keeping up on maintenance.

I put a post up a while back that Diesel Kleen makes my car run quieter. I think a few people thought I bought into some kind of snake oil, but the post above, about reducing ignition delay with a higher cetaine proves I'm not crazy. Diesel Kleen does make my car run quieter. It takes a few tankfuls, but it definitely does work.

Jeff

jbach36 · #15 10-31-2007, 12:42 AM

Quote:

Originally Posted by DieselAddict

No, D1 actually has a lower cetane rating than D2 given the same amount of additives. The reason it's used in the winter is its much lower gel point.

Diesel noise is a funny thing and it can be made quieter in 2 ways: reduced ignition delay from higher cetane (good) or slow/incomplete combustion from low volatility (resulting in bad emissions and engine deposits). Veggie/engine oils fall in the 2nd category and that's why they run quieter. A better way to quieten a diesel engine is by using a high-cetane fuel and keeping up on maintenance.

I put a post up a while back that Diesel Kleen makes my car run quieter. I think a few people thought I bought into some kind of snake oil, but the post above, about reducing ignition delay with a higher cetaine proves I'm not crazy. Diesel Kleen does make my car run quieter. It takes a few tankfuls, but it definitely does work.

Jeff 1991 300d, 112k