Poll : How much you know about diesels??? - Page 2 - PeachParts Mercedes-Benz Forum

leathermang · #16 08-25-2002, 11:09 AM

The work done inside an engine is not done by compression... that is only something which effects the amount of power/air/fuel that is in the cylinder able to be burned... all the work is done on the downward movement of the piston in the power stroke... the fact that the timing on non diesel engines may be before TDC is a reflection of both the time it takes the fuel/air mixture to ignite completely... and the shorter relative stroke... giving less "working" distance.

For several degrees of crankshaft rotation there is a very small movement of the piston...but the placement of the crank, so that the power is applied in the direction of rotation desired is accomplished during that time....

BlueBabyBenz · #17 08-25-2002, 12:02 PM

So, then I was right after all.

I did oversimplify.

But then again, the essence of my guess can still be true, that is, that the fuel must be in the chamber before TDC, and allowing for the time it takes the explosion to fully develop its power only reinforces that guess.

Damnit, I'm doing it again. Oversimplifying, that is.

I'll shut up and `listen' now.

the_good_fellow · #18 08-25-2002, 10:06 PM

Here is a link about Engines

kerry · #19 08-25-2002, 10:43 PM

More about running backwards. There is a WWII diesel tugboat based on Lake Ontario (Oswego I think) that is a National monument or some such thing. It has a huge diesel engine. The boat has no transmission. The only way to back it up is to stop the engine and restart it backwards. Needless to say, the Captain does not like manoevering in tight quarters where backing up is required. When I saw it docking in Rochester they were going thru all kinds of gyrations to assure they could get out of the harbor without having to back up.

P.E.Haiges · #20 08-26-2002, 10:05 AM

Mclarenf1,

You have not answered your question about Diesel ATDC or BTDC. If you ever removed the vacuum pump from an older MB diesel engine, you would see some weights and a mechanism to change to timing of the injection pump as the speed of the engine is increased. The newer CDI Diesels do the same thing electromically so it is not as evident. This is why I asked earlier at what engine speed were you asking about.

Both of these methods vary the timing of the furl injection BTDC for the same reason as in a non Diesel engine: it takes a finite time to ignite the fuel and the faster the engine is running, the ignition must be started earlier to increase the efficiency of the engine.

P E H

leathermang · #21 08-26-2002, 10:30 AM

PEH, you are having a Senior moment.... I did not post this question... MCLARENF1 did....and he did state that for this discussion we are talking about at idle...

P.E.Haiges · #22 08-26-2002, 10:57 AM

Leather,

Except for not answering the original question part, most of my post applies to your previous post. There isn't much difference between advancing the timing on Diesel and non diesel engines.

P E H

leathermang · #23 08-26-2002, 02:25 PM

PEH, I thought we had both guessed ATDC, and I thought that warden had pretty much answered the question....which I think meant we were correct...
Can you be more specific about which of my posts you are referring to ? What is "my (meaning me) question you are referring to ?
OK, I see now that you have changed the person your post was addressed to .... maybe he will tell us what he thinks the answer is to this question...

P.E.Haiges · #24 08-26-2002, 05:35 PM

Leather,

That's what I meant that Mclaren should answer the riddle he started and with some proof they his answer is correct.

P E H

psfred · #25 08-26-2002, 09:07 PM

Timing of actual injection is probably slightly before TDC for the same reason it is on gas engines -- takes a finite amount of time for the fuel to ignite and the pressure to come up.

As for setting the injection timing, there are a number of different procedures, and this is determined by the manufacturer. MB on 617 and older engines uses the point where the plunger in the IP covers the inlet and start to pressurize the fuel -- this is 24 degrees BTDC. This is NOT when injection starts, that is when the plunger has risen far enough to pressurize the nozzle, a relatively long way away, to 1600 psi or so.

Newer Ford (not Cummins or International engines) and some European diesels are dynamically timed dynamically via the pressure pulse expansion of the injector line and a sensor. Very slick, but I have no idea what the actual injection timing is, certainly not 24 degrees BTDC.

The 601/602/603 MB engines is set at 15 degrees ATDC by a hall effect sensor with a protrusion on an arm on the cam in the pump. This is an electronic version of "when the plunger covers the inlet on the sleeve", just using the position of the cam rather than the flow of fuel.

On a Stanadyne pump, the engine is set for TDC and the marks under a cover on the pump housing and cam are aligned. I don't have any idea what the exact injection timing is there, either, but would bet it is similar to any other diesel.

Hope I've helped rather than confused!

Peter

TimFreeh · #26 08-26-2002, 09:20 PM

I had the timing checked/set on my Dodge Cummins pickup checked by a shop that has a dynamic Diesel timing light. The factory spec was something like 14 before TDC. My shop set mine to 11 BTDC - they claimed it ran better with that setting. After driving it for about 20K miles I now agree. The shop manager also pointed out the the Cummins engine in the Dodge has NO timing advance capability. It is fixed at one timing position for all engine RPM's. Of course the engine does not spend much time over 2,100 RPM.

I was under the impression that ALL diesels injected fuel BEFORE TDC to allow for a time lag between injection and the start of combustion.

Tim

psfred · #27 08-26-2002, 11:23 PM

Tim:

Ah, the great drawback to American diesel engines, fixed injection timing, also direct injection. Why, oh why, does anyone want a constant speed engine in an automobile? Works fine for diesel-electric (trains), but with a mechanical transmission? No way.

Probably 11 BTDC is correct for 2100 rpm. I would think more like 5 BTDC at idle, more like 25 BTDC at 5000 rpm.

Note that unless you have a system for actual injection (like the sensor that goes on the injection line), you have to set the pump, and that may not be the same mechanical timing as the actual injection. I'm certain that my 603 doesn't actually inject fuel at 15 ATDC, but that is the mechanical set up for setting the pump timing. Ditto for 24 BTDC for the 615/617 -- that is the correct crank position for whatever the actual injection timing is.

Peter

#28 08-27-2002, 07:51 AM

kerry edwards,

My Dad was a motor machinist on a Destroyer Escort during WWII. I have heard him talk many times about "reversing the cams." This is how they make the engine run backwards. I think they had a clutch to disconnect the prop shaft so they could stop and restart the engine, then engage the prop to reverse thrust.

Have a great day,

dabenz · #29 08-27-2002, 08:10 AM

Here's the possibilities (he did say to think about it):
Start of delivery (at injector pump): BTDC
Injector crack: BTDC, but very close to TDC
Injector delivery: ATDC, but very close to TDC
Actual start of ignition: ATDC

Why: There is a fairly wide range of pressures surrounding TDC where engine compression will heat the air enough to ignite the fuel. Ignite too soon and the engine runs backward. Ignite close to TDC but BTDC and the flame stays too close to the nozzle tip (flame should begin near bottom of pre-chamber then get sucked out by the downstroke of the piston). Too late and the flame chases the piston instead of pushing it. Calculate average piston speed and the amount of time the piston is within 5% of TDC for your engine at idle and you'll see that timing is a very appropriate word. All this for a pre-chambered engine, in the spirit of this web site.

My vote: ATDC. The question (as amended) was "ignition" timing at idle.

Now my questions:
1. Has anybody seen an engine where the crank timing marks actually corresponded to the location of the piston? Or in Yen-speak: is TDC really TDC?
2. How much does the average pressure differ between prechamber and cylinder at TDC at idle for your engine?

kerry · #30 08-27-2002, 09:21 AM

Larry:
Can you say more about 'reversing the cams'. Is this just a slang expression for reversing the whole engine or is there a specific procedure that involves the camshaft? I had assumed the engine had a couple of different starters that rotated in opposite directions.