Theory question: IP timing advance

[73MB280SEL]

Howdy,

In Gasoline engines, the spark timing is advanced typically with RPM and engine load (vacuum), but what about the older MB Diesels? Do they have an advance mechanism? How does it work?

Just curious.

Sholin

[P.E.Haiges]

Sholin,

Yes, the MB Diesels have an advance mechanism. It it located between the vacuum pump and the injection pump (IP).

It works similar to the weights in an older gasoline engine distributor. As the engine speed increases the weights are moved outward by centrifugal force against the force of springs. This outward movement is changed to a forward rotation on the shaft that drives the IP and thus advances the timimg. When the engine speed decreases, the springs pull the weights back and the IP timing goes back to nominal timing.

The reason for the advance is that it takes a finite time for the fuel to ignite and if the timing was not advanced, the ignition would be later as the engine speed increased and power and efficiency would decrease.

P E H

[73MB280SEL]

PEH,

That's interesting to know. I kind of figured there would have to be some kind of advance mechanism, but if you limit the RPM, you may get by without one.

Here's a couple of followups: (1) Does anybody ever tune the initial advance till it "runs right"? That's how one would tune most gas engines (using a timing light, of course). For instance, if you set my M117 4.5 to book (5 deg ATDC), it runs like poo-poo.

(2) What happens if the diesel timing is too advanced. In gasoline engines, too much advance at speed knocks. Too much at idle and the car won't start (kickback).

Thanks for the info,

Sholin

[psfred]

Check the vac retard on the M117 -- if the diaphram is shot or the mechanism stuck, you will be running VERY late timing. I set mine to 7-10 BTDC with NO vac, works better even though my vac system is OK.

yes, extra advance on the diesel will result in higher noise and more smoke at idle, better power and milage, and maybe melted piston crowns. If you set it really fast it will sound like a Mack trunk and run crappy with lots of smoke.

No, I've never set a diesel by ear. Don't set gassers by ear, either, always use a timing light. The timing instructions for deisels are the equivalent of using a timing light -- the newer ones have a pickup you can use with the proper "tool" to set timing dynamically.

Peter

[gsxr]

Diesels have a very specific window where timing is best for power & efficiency. Go outside that and it will either run crappy or you risk engine damage (I think). On my 1987 OM603.960 engines, I found that setting the IP timing to the far side of spec (advanced), I gained measureable MPG, but no power. One car was slightly retarded before, the other was at spec (+/-0). Advancing to the limit of spec gave me about 10% gain in MPG on the freeway. Tried this on my OM617 but there was ZERO gain on that car! Oh well. I'm waiting for someone else to try this too, in case my cars were flukes...

[Gurkha]

Apart from the spring weight governor, the older OM 616 diesel pump also featured a limiter rod attached to the governor and the throttle linkage. This would prevent un-neccesary richening of mixture in case the throttle was opened up fully with the vehicle on high gear.

[73MB280SEL]

Ok, I reviewed my OM601 manual and got the poop.

So what does a Diesel do when it is overadvanced at running speed? At idle, I can see that overadvancing would cause it to idle like a mack truck. What about if the springs are too weak or something and it runs too much advance at speed?

Not that I am having any problems. My car is running fine/I'm just curious.

On a Gas engine, the engine will start to knock if you have too much advance. Just before the knock limit is also the point where the greatest power and efficiency are to be had for most gas engines not running on pump gas.

Also, there are basically two advance mechanisms on most gas cars: load and rpm. On a gas engine, vacuum advance (load) is used because the throttle plate lowers combustion chamber pressure which in turn slows down combustion. Advance is required to compensate for this. What about Diesel, RPM advance I can understand, but would there be any advantage to load (injection quantity) advance?

Sholin

[psfred]

The reason for injection advance is to compensate for ignition delay. The best power and economy is achieved when the fuel/air mixture starts to burn just as the piston goes over top dead center, so that all the pressure developed is applied to the downward stroke.

Early ignition causes the pressure to go high on the UP stroke, greatly increasing the pressure and heat load in the cylinder without producing any usable power.

As the piston speeds up with increasing rpm, the injection or spark has to come sooner to get the start if ignition at the correct time.

Remember, there are LOTS of diesel engines out there with fixed injection timing -- almost all the pickup truck diesels are fixed injection (except the latest engines and possibly the old International engine with VE pumps) -- they clank terribly at idle and smoke on takeoff because the injection timing is too fast. Ditto for older big trucks -- lots of noise and smoke, exhaust clears as the rpm comes up, then another blast of inky smoke as the driver upshifts.

Fast timing at idle will give excessive smoke and noise. Fast timing at speed will also give excessive smoke, and cause overheating and possiblly piston damage. The amount of fuel injected at idle isn't enough to hurt anything but your ears.

Peter

[gsxr]

Good explanation, Peter. This is why I recommend setting IP timing to the advanced side of spec, or maybe an extra 0.5 degree if you're feeling lucky (that will also allow for a hair of chain stretch as you put miles on the motor). But otherwise I wouldn't screw around with extra advancement.

My personal opinion is that a LOT of Mercedes diesels on the road today are running around with retarded IP timing, because as the timing chain stretches (which is normal!), the IP timing will be retarded. Replacing the chain will cure this but you don't NEED to replace the chain if stretch is below spec. MB says OM60x engines are OK up to 4 degrees stretched, and OM61x engines to 5 degrees. But that means 2.0-2.5 degrees of IP timing retard at the stretch limit! See my point...?

[Mordecai]

Originally posted by psfred
The timing instructions for deisels are the equivalent of using a timing light -- the newer ones have a pickup you can use with the proper "tool" to set timing dynamically.

I've seen piezo-electric pickups that allow you to use an inductive type timing light with any diesel. The theroy is at the start of delivery the injector tube expands slighty and the piezo-electric sensor picks up that expansion and translates it to a magnetic pulse. The pulse is sensed by an inductive timing light and it flashes its light.

Has anyone ever used one of these? Would you use the standard start of delivery timing for the idle timing?

M.

[gsxr]

This has been asked before. Problem is, you can't use the specs in the manual, because the point of reference is different. We need SOMEONE to set the timing exactly with the Mercedes lock tool, then connect the piezo thingy, and see what that number is. Once someone does this, and posts the piezo number, THAT number will work for pretty much anyone who buys the same piezo pickup tool. I know I'm not explaining this clearly but I hope you kinda get what I mean...

[Mordecai]

Originally posted by gsxr
We need SOMEONE to set the timing exactly with the Mercedes lock tool, then connect the piezo thingy, and see what that number is.

There is no lock tool for the 123 injection pumps. Or is there, if so it's not in the manual. I set mine with a drip tube.

M.

[gsxr]

There is a lock tool for the turbo OM617.95x engines, well for most of them anyway - some of the early ones, or CA models may not have the RIV hole. My 1984 Federal 300D can use the lock tool. It's messy though, as a good pint of oil runs out. The later OM60x pumps have the plug higher up so there's no mess. BT, DT. Also, the 617 pump is a real b*tch to adjust because there is no turnbuckle like the 60x has! You have to loosen all the injection lines, then muscle it around - really NOT fun. The 603 is a breeze by comparison.... :p

[Mordecai]

Originally posted by gsxr
There is a lock tool for the turbo OM617.95x engines, well for most of them anyway - some of the early ones, or CA models may not have the RIV hole.

Mine is a 240D (616 motor?) grey market. I've never seen mention of a lock hole in any manual.

M.

[gsxr]

There is no provision for the 240D (2.4L, OM616) engine. ONLY the turbocharged 3.0L, OM617.95x engines, and then not all of them! It's easy enough to check, look for a 17mm hex plug directly above the engine mount shock top mounting nut... no plug and you can't use the RIV lock pin tool, you have to use the "wet" methods. All the newer OM60x engines with aluminum heads use the tool, which is very handy...