Why do diesels shake?? - PeachParts Mercedes-Benz Forum

240diowa · #1 01-19-2006, 10:15 AM

Why do diesels shake when they idle?? Is it due to incomplete combustion, IP timing.

TX76513 · #2 01-19-2006, 10:20 AM

....bad motor mounts, rack dampener adj, low idle adjustment....

300SDog · #3 01-19-2006, 10:22 AM

20:1 compression ratio and 300+ lbs pressure in each cylinder seems the obvious answer to me. Thats roughly double the compression and combustion pressure of your typical gasoline engine.

stayalert · #4 01-19-2006, 10:30 AM

because they can

why do transformers hum?

becasue they don't know the words....yes I know....don't quit yer day job

#5 01-19-2006, 10:35 AM

Quote:

Originally Posted by stayalert

because they can

I think that's the answer to a different question.

BTW, a well tuned diesel does not shake very much. I believe they tend to shake more at idle because the IP timing is optimized for higher rpms and the timing is somewhat advanced for the low engine speed. In other words, too much combustion takes place before the piston reaches TDC.

Hatterasguy · #6 01-19-2006, 10:38 AM

They don't unless there is something wrong. My 617 shakes a little sometimes because the rack dampener bolt is probably getting weak. My 603 is like a turbine, their is zero shaking or vibration at idle.

Brian Carlton · #7 01-19-2006, 10:41 AM

The combination of the compression ratio and the fact that the 617 is a five cylinder is the reason that it can never be perfectly smooth. It's not possible to balance a five cylinder engine so that the internal forces are completely cancelled. A four cylinder engine is similar. A large four cylinder will shake your teeth out.......which is why there are so few of them.

A six cylinder in-line engine is perfectly balanced. Doesn't matter if it's gas or diesel, they run perfectly smooth due to the inherent design of the engine.

A V8 engine can also be perfectly balanced with the proper counterweights added to the crankshaft.......however.......it's not perfect by design.

Hatterasguy · #8 01-19-2006, 11:14 AM

Yes I have noticed that as well. Mine was older and had the 2.2 16 valve. Toyota seems to have gotten the vibration problem solved in the last couple of years. Although I have to give them credit even on mine at anything off idle it was pretty smooth.

Look at the 616 though it is 2.4L so a pretty good sized 4 cylinder. Did MB use engine shocks on them? I suspect a fresh set of mounts and engine shocks would help a bit. But remember this is 30 year old technoligy it isn't going to be as smooth as 2005 technoligy.

I'd right it off as the cars charm and live with it.

Jim H · #9 01-19-2006, 01:08 PM

Items that produce 'shake'

1. Reactions from combution forces. An excellent example is the 'lugging' felt when a manual transmission is driven at low engine rpm...

Each time a cylinder 'fires' pressure on the piston produces a linear motion which acts on a lever (crank arm) to produce a rotation. The instantaneous torque applied to the crankshaft is complex sum of pressure on the piston area at that moment and the effective length of the lever as the crankshaft rotates.

The result is that a crankshaft does not want to turn at a constant rpm. Rotating mass of the crankshaft itself, and belt-driven auxiliaries like alternator and pumps adds rotational inertia or 'flywheel effect' to smooth out the impulses.

2. Reactions from rotating masses. The net effect of all of the rotating parts.

Piston motion is linear (up and down) but the connecting rod motion is a lot more complicated. The 'big end' is orbiting the crankshaft, the other end is linear.

3. Reactions from linear unbalanced motion. The net effect of all of the 'up and down' parts.

1983/300CD · #10 01-19-2006, 05:30 PM

My 5... well, 5 sparkplug rotary RX-7 engine was smooth as silk.

Diesel Giant · #11 01-19-2006, 05:35 PM

Several factory. Odd number of cylinders, its a diesel, and a not very robust engine mounting system. After wrenching on these for some time, I really think the engine mounts are a joke. Go look at a Dodge Cummins powered truck, they have some stout mounts. I have NEVER had to replace one or even see one go bad.

Brian Carlton · #12 01-19-2006, 05:48 PM

Quote:

Originally Posted by Diesel Giant

............and a not very robust engine mounting system. After wrenching on these for some time, I really think the engine mounts are a joke.

The nature of the mounts dictates the problem. If you need a soft mount that isolates the engine from the frame, by definition it's going to take a beating. It's not easy to engineer a "robust" mounting system that is simultaneously "soft".

I'm not familiar with the Cummins, but, you can rigidly bolt a 6 cylinder engine to the frame and not feel much vibration from it.

#13 01-19-2006, 05:54 PM

At Idle, with the stereo on you cant tell mine apart from a well tuned gasoline engine. It is perfectly still and idles softly.

As a matter of fact, many people are shocked to learn that it is a diesel. Mine is a 6 cylinder though and has pretty good power. It will GET UP AND GO when I want it to and then you'll hear it. I hate to sell it because it is so perfect and one rarely gets lucky to find a car and the engine in such pristine condition but I would really like to have a 1999 S500.

stayalert · #14 01-20-2006, 08:39 AM

wankel. I built a cool model of one in ~1975?