I've been on this board for a while, and it occurred to me tha tnever knew how the prechamber and injector work together. Can anyone explain?

looking at a diagram of an OM617 that I have on my machine, I see that the injector sprays into a pre-chamber where the glow plug also sticks it head in. Since the injector doesn't spray directly into the cylinder, how does injector spray pattern / pre-chamber affect combustion, etc.. since it doesn't directly spray into the cumbustion chamber.

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#dieselFLEET
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'97 E300
'99 E300

You got the basic idea right. The purpose of the prechamber is to quieten the diesel combustion. By injecting the fuel into a prechamber as opposed to directly into a cylinder, the fuel burns more gradually. The combustion starts in the prechamber and then grows into the cylinder. Ironically, if a prechamber becomes very dirty it can actually make the combustion very loud, louder than it would be if the fuel were directly injected into the cylinder because a dirty prechamber doesn't facilitate a good swirl of the fuel, thus delaying the combustion and making it more sudden. Another disadvantage of the indirect injection design (IDI) is heat loss, as the prechamber effectively acts as a heat sink. This especially makes a difference when the engine is cold and that's why you need to let the glow plugs glow longer. Modern direct injection (DI) engines usually don't need any glow time unless it's extremely cold. However, one advantage of the IDI design is a higher redline, but with today's injection technology I don't think this is an issue anymore, plus today's DI engines are as quiet as equivalent IDI engines so it appears the OM606 will be the last of its kind.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

To add just a little to that explanation.
As the piston rises on the compression stroke, all the air in the cylinder is squeezed, just like in a gas engine--except that the piston comes all the way to the top of the cylinder forcing all the air into the precombustion chamber. It goes in thru small, calibrated orifices which promote great turbulence or swirl to the air. Thus, when the fuel is sprayed into the this hot, turbulent ball of air, it mixes very thoroughly and begins to burn. As it burns, the increasing pressure forces it back out into the cylinder area where the combustion process continues until all the available air is used forcing the piston down with great force.
Understanding that all the air has to go into, and then, out of the pre-chamber casues me to doubt that the efficiency of a diesel can be significantly raised by typical gas engine speed tricks--like porting, and polishing. They may help---some. But the limiting factor is the size of the holes into the pre chamber. I do not recommend enlargiung them, as they are specifically designed for the difficult task they accomplish. Changing them would affect the swirl and probably reduce the overall efficiency of the design.

Excellent information. Greatly appreciate the responses.

This should be a newbie sticky

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#dieselFLEET
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'97 E300
'99 E300

>Bump<



Great thread. I'm printing it out.

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Growth for the sake of growth is the ideology of the cancer cell. -- Edward Abbey

1984 300SD: Hilda the veggie vundercar

Part of a diesel's inherent efficiency is that fuel is injected throughout the power stroke, rather than just before as in a gasoline engine. Newly injected fuel follows an existing flame front into the expanding volume created by the downward stroke of the piston. This creates nearly constant pressure expansion of enclosed gases, which is more adiabatic, hence efficient.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

For the uninitiated, adiabatic means without heat transfer.

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Mahalo and Aloha,
S. L. "Red" Pike, Naval Architect/Marine Engineer, USCG Licensed Chief Engineer (Steam, Motor, and Gas Turbine)
Former nuc bubblehead (boomers)
"Sustained superior performance through continuous incremental improvement."

1987 300SDL White w/ Gray Leather interior, 160K (mine)
2009 Nissan Murano (leased - wife's)

I thought I saw in a diagram some kind of a flap inside the prechamber. What's the deal with that?

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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(='.'=) This is Bunny. Copy and paste bunny into your
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There is a semi-spherically ended protrusion in the space between the glow plug and the small holes at the bottom of the prechamber. I believe it's intended to increase swirl to improve atomization of the fuel. Pretty funny looking thing, sort of like a very miniature version of a common 110VAC incandescent light bulb in shape.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

when did MB introduce the direct injection to there diesels?

Actually I think the Injector Spray (Red Arrow) is directed at the Ball Pin (Green Arrow) and the spary hitting the Ball Pin further atomizes the Fue.

After the Engine is runing the Ball Pin must become extremely hot and when the Fuel Spray hits it; it is surely an aid in starting combustion. I think the Ball Pin is one of the reasons that the Injectors in a Mercedes seem to last longer than on other Enginges with prechambers. The Ball Pin aids in atomization and when hot ignition of the Fuel even when the Injectors Nozzles are worn and the Fuel Spray is poorly atomizing.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

Is Mercedes the only one who used the ball pin in the prechamber? Has anyone ceramic coated their prechambers to lessen heat loss?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

After my first pilgrimage to the holy city of Stuttgart, I got a DVD set that has LOTS of vignettes and a few animations on it. As soon as I figure out how to translate them to an upload-able form, I'll highlight the diesel info in "DD." The ball pin prechambers also induce a swirl turbulence pattern.

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1987 300D (230,000 mi on a #14 head-watching the temp gauge and keeping the ghost in the machine)
Raleigh NC - Home of deep fried sushi!

That will be very interesting to see. I look forward to it.

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1993 W124 300D 2.5L Turbo, OM602.962
2000 Chevrolet Cavalier, 2.4L DOHC
2002 Ford Explorer, 4.0L SOHC
2005 Toyota Prius, 1.5L

http://www.fuelly.com/sig-us/40601.png

I've heard of prechambers failing by burning up. If you look down the hole, it's gone! Never happened to me, but what would cause that? It seems to be related to timing (too advance?)..

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Cheers!
Scott McPhee

1987 300D