I'm the guy who is 'supposed to know everything'. I like to maintain this fragile illusion but yesterday I was asked to explain why diesel engines make the 'clacking' sound they do at idle. I know it has to do with the way they run off of auto-ignition of the gas, but I honestly couldn't explain to a non mechanical person what causes it - which tells me that I really don't know. Anyone got a good explanation of this?

 

Okay, there are a few factors in the noise at idle. Combustion chamber temp., fuel pressue, and compression. The pump and injectors tend to make some noise as well. A lot of the noise is the cold fuel and hot air exploding.

 

Gas engines have all of this as well, excpet for such high compression but they don't sound like this - so there's something else causing it right?

 

Doesn't a diesel run in a perpetual state of detonation? I always thought that was why they sounded like that. All the pieces are bouncing off of each other.

 

I know I know

Diesels do work on spontaneous combustion. As the piston goes up, fuel is squirted into the combustion chamber. as the piston moves upwards and "squeezes" the fuel/air mixture it ignites. Some of it ignites prematurely as the piston is still on the move upwards, causing that characteristic diesel "clatter" at idle.

The newer diesels use a high-pressure common-rail system that shoots a small amount of fuel into the chamber as the piston is moving up, shoots the rest in (at very high pressure) when the pistons towards the top of it's "up" cycle, giving a more complete burn and reducing the noise at idle.

What'd I win?

 

So it's basically constant pre-ignition, like having plugs that are too hot in a gasser? Another thing that is weird, with at least my 6.0, is that there is a hill near my driveway. Every morning I go down this hill. I decided I don't want excessive break wear, so I recently started dropping it in 2nd (its an auto) to let the engine brake down the hill. So the engine runs at about 1600rpm down this hill and for the entire coast, the engine is so quiet I can't tell its running! Then when I get to the bottom and use the brakes and get below 1500rpm, it clatters again, as well as all the way through the rpm ranges as I get onto the highway. Weird.

Polarbear, you won.... let's see... hmmm - polarbears like fish right?

 

Exactly right- and it only happens at idle, which is why they quiet down so much once they get rolling. The common rail system utilizes a very high-pressure fuel delivery system and electronic controls to minimize that effect. The small amount of fuel injected acts as the "fire starter," then the majority gets injected when the pistons positoned ideally.

 

low rpm, and high compression, are the most importent ones

 

There is no air fuel mixture being compressed in a diesel. The fuel is not introduced until it is time for the cylinder to fire. There is no pre ignition or detination because the injector does not introduce fuel until it is time for the cylinder to fire. The fuel timing is comparable to ignition timing on a gasser. The Loud knock is caused by the introduction of high pressure cold fuel into very high cylinder pressures. The newer diesels are louder than older diesels because they are direct injected, where the older diesels had a pre combustion chamber to let the fuel start a flame front before it actually moved out into the combustion chamber. The latest trend on common rail fuel systems is to use pilot injection, a small burst of fuel before actual injection. I went to chrysler school when the common rail cummins first came out. I was told that before they released this new engine, a focus group said it was too quiet. The engineers reworked the software to put some of that knock back. The reason that the power stroke is quiet when it is coasting is that the timing is retarded under deceleration and very little fuel is introduced.

 

Since you're off the accelerator pedal and the engine is being driven by the rear wheels, the engine management system can basically shut the engine down by shutting off the fuel.

Diesels aren't the best at compression braking because they don't have throttle plates. The engine can pump as much air as required so the engine compression is the only resistance to being driven. That's why some big rigs have exhaust (jake) brakes. It's just a butterfly valve in the exhaust system to help it compression brake more effectively.

 

Jakes and exhaust brakes are 2 different things.

As for what causes the diesel clatter, idle or not, is the fact that fuel is detonated. The smack from the flame front on the top of the piston is what causes the noise.

 

True, jakes work by changing the valve timing around a bit, and exhaust brakes just use the butterfly valve. The exhaust brake will make a hissing sound, from the extra exhaust gasses passing through a pilot hole in the butterfly valve, whereas the "jake" brake will make a distinctive "blatting" sound, from the valve timing pretty much trying to force the engine to run "backwards".

 

hudsonmacbeth Hi and welcome to FTE! Be sure to read this to help you around this site: https://www.ford-trucks.com/forums/s...d.php?t=300691 There are also State, Provincial and International Chapters located near the bottom of the main forum page.

We are glad to have you. Hope you enjoy the site. Check out the various forums. Lots of great info for you.

See ya on the boards!

-Matt

P.S. We now have a General Diesel Discussion forum. This thread is moved there now.

 

Well now that's VERY interesting. I would prefer that they eliminate as much of the clatter as possible without sacrificing anything else.

 

i just thought it was high compression