Somebody explained this to me before, but I am getting old

[KINGTUT]

My question which will beg my ignorance on diesel engines. Why are they so much louder than gassers. I understand compression combustion vs. fire combustion. And even though the newer models are still quiter they still have that rattle. I have a Cummins 6.7 in my motorcoach, it is quite, comparatively to my 97 7.3 but still has that unique diesel "rattle". Someone explain it to me in simple terms that I can understand.

[MarkEkberg]

I think most of it is the injector actually firing. I don't think the combustion process is louder. If you have a stick coast down a hill in a low gear. It gets quiet as a gasser.

I know there is less exhaust noise from a diesel v/s gas due to the longer stroke. Lots of gas engines open the exhaust valves while combustion is still taking place.

[action4478]

Originally Posted by KINGTUT

My question which will beg my ignorance on diesel engines. Why are they so much louder than gassers. I understand compression combustion vs. fire combustion. And even though the newer models are still quiter they still have that rattle. I have a Cummins 6.7 in my motorcoach, it is quite, comparatively to my 97 7.3 but still has that unique diesel "rattle". Someone explain it to me in simple terms that I can understand.

I don't know for sure ,, I thought it was from detonation of the fuel ...

[fireguywtc]

You pretty much have it nailed on the head as why diesels are louder is the compression combustion part. The much higher pressures created when they fire compared to gassers it why it makes more noise. I can't find any numbers for that but is atleast 2 times or more then gas motors.

As for your 6.7 still has the common diesel rattle noise vs your old 7.3 that probably sounds like a bus, it's just newer technology helping to make them quiter. We can't totally get rid of the noise but have suppresed it a bunch.
I like it though, a diesel running is music to my ears

[massbadass]

Originally Posted by fireguywtc

I like it though, a diesel running is music to my ears

Amen to that, and a turbo whistling too!!

[klhansen]

The difference in terms of thermodynamic cycles:

The gas engine uses the Otto cycle, which is a constant volume combustion process, meaning the firing happens nearly instantaneously with the piston at or near TDC, and the pressure drops nearly immediately as the piston starts the expansion stroke.

The diesel engine uses the Diesel cycle (of course), which is a constant PRESSURE combustion process, meaning that the injection and combustion of the fuel sustain the pressure over a longer duration, even as the piston begins it's expansion stroke. The whole thing occurs at higher pressure as well (gasser compression pressure about 120-140 psi versus about 400 psi for a diesel.) It's kind of like ringing a bell with a hammer. The combustion chamber surfaces take a much harder hit than with a gasser, which is why diesel are so much heavier.

[KINGTUT]

Thanks for the responses. I guess I sorta get it now. BTW, I was out running the coach a little this afternoon, 6.7 Cummins, and I got down by the tail pipe and I can not discern one typical odor of diesel exhaust, really no smell at all. I took a look at the filter underneath, the one Ford calls the DPF and it is HUGE, I mean about 4 feet long and as big around as Walter Payton's thigh. It does not have to be cleaned for hundreds of thousands miles, it must work. The Cummins still sounds nice, just quite. It sounds best before it gets warmed up, after that, well not much diesel sound to it, but clearly not a gasser. Up in the cabin on the road, you can not hear it, really, but it is 40 feet behind me underneath a bunch of insulation. The torque is unbelievable, 750 ft. lbs at 1600. Computer limited to 2600 RPM's, so far I could not imagine what it would do at 3500, probably come apart. Very, very strong engine. The Allison 6 speed is butter smooth, the shifts are barely perceptible. Seems to be a really nice drive train, but they all are when they are new.

[THEBUNDO]

Since diesels were operating without injection in years gone by, some explanations don't seem right. Or did they always have some form of injection?

[klhansen]

Originally Posted by THEBUNDO

Since diesels were operating without injection in years gone by, some explanations don't seem right. Or did they always have some form of injection?

There was always some sort of injection, either into a precombustion chamber as in IDI (indirect injection) or directly into the combustion chamber. If the fuel were introduced into the intake, the heat of compression would ignite it WAY before TDC and it'd throw rods all over the place. That's assuming you could get it to run in the first place.

[TWeatherford]

I thought they were designed that way to let the gassers know we're better than them.

[Texrob1]

Originally Posted by TWeatherford

I thought they were designed that way to let the gassers know we're better than them.

That's what I thought Too..

[DeepRoots]

Since diesels were operating without injection in years gone by, some explanations don't seem right. Or did they always have some form of injection?

diesels with no injection? how would that work?
the first ones used blast injection... compressed air blew the fuel in. It was the only way with the machining abilities of the time. Obviously air pressure via compressor trying to overcome cylinder pressure is an issue. Engines at the time also needed the associated air tanks etc.

The true potential for diesels was not realized until guys like Bosch was inventing pumps.
Jerk pumps run off camshafts are about the simplist form, they are still used on small diesels like for trash pumps and the like. There were even multiple jerk pumps that were used with multi cylinder engines. Rotary and Inline pumps are more complex versions of these simple acting pumps.

in the marine industry I've had to deal with a number of different forms of injection, rotart pumps, inline pumps, detroits with thier injectors acting as pumps, rockers or camshafts building the pressure.

[drcampbell]

The characteristic "marble in a tin can" sound is the result of a high rate of pressure rise during the onset of combustion. High pressure, by itself, doesn't make any noise.
(there's much higher pressure in a SCUBA tank than in a Diesel engine, but they're silent)




Compression pressure in a Diesel engine is a lot higher than in a gas engine, but if two engines have the same displacement and are doing the same job, the average pressure during the power stroke is essentially the same, regardless of what kind of engines they are - gas, Diesel, steam, fu-fu powder, whatever. Look up "BMEP".


Nobody designs an engine to "open the exhaust valves while combustion is still taking place", except maybe drag racers. Doing so would throw away a lot of fuel and send the HC emissions soaring. What does frequently happen is that the exhaust valve opens while there's still high pressure in the cylinder, but if an engine is correctly calibrated, combustion is essentially complete by the time the piston is just past Top Dead Center. (ignition advance, remember?) Longer stroke has nothing to do with it.


Gas & Diesel engines don't "use" the Otto and Diesel cycles; the cycles provide a theoretical mathematical model to aid understanding of what's happening inside the cylinder. An actual Diesel engine is anything but a constant-pressure machine; the pressure rises very rapidly when combustion begins and drops significantly as both the combustion event and the piston downstroke progress.
Actual Combustion Cycles

[CDX825]

We have a lister petter two cylinder diesel pump at the farm and that thing has got to be the loadest diesel I have ever heard for its size. I was starting it up for the first time thinking oh this thing should be fairly quiet and cool sounding but man it was loud. Was not so much the exhuast as it had a good muffler as it was just engine noise.

[roofeditor]

I think he asked why some diesels are louder than others? Many good answers in why diesels are louder generally.

As to why some are more quiet than others, they type of injection, the timing of the injection, placement of the injection all are major factors. Some injection is phased in and some is all or nothing instantly, which would you think makes more noise? Then there is the actual compression ratios. Some NA diesels have very high compression while some with big turbos and dual turbos have lesser compression making up the BMEP in the turbo rather than compression.

A good example. My 98.5 dodge/Cummins was not loud it was VERY loud but the same engine the little old 5.9 with different injection and turbo only a few years later was much more quiet. Combustion chamber design also factors in.

Now in Tut/Higs case the engine is way back there and I'm assuming there is some major sound deadening material back there.