I would like to know, Does the prechamber cup itself get hot enough once running to help support combustion? Or does the glow plug stay hot enough once a few combustion cycles are achieved.

I used to fly glow plug engined model airplanes. Once in while they would stop unexpectedly and sure enough the glow plug wire loop was not in the present in the glow plug. I know that the glow plug engines were not strictly diesels but they seemed to have some similarities in that they they were not spark ignition. Their fuels were perhaps more active with methanol and nitromethane. I could see why they were two strokes to keep things simple lightweight and keep the glow plug hot.

__________________
85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.

Yes. The glow plug tip is just heated by the combustion.
The fuel is vaporized in the prechamber when the motor is running. The compression then causes the heat necessary to cause combustion. When running the compression achieved is much greater than when doing a compression test.
The glow plug motor comparison is without merit. You can get 2 stroke diesel model airplane motors. They are purely compression ignition.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

The Ball Pin certainly get hot enough to help with combustion; see red arrow.
I actually believe the location of the Ball Pin allows worn Injectors to still be functional once the Ball Pin heats up.

The other popular type of IDI precombustion Chamber is the Squish Chamber (Old VW Rabbit Engines). I focuses part of the Injector Spray against part of the Chamber Wall to do the same thing. However the Chamber Wall is against the Cylinder Head and it must remain a lot cooler than the Ball Pin does.

Anything that retains heat in the Precombustion Chamber must help with Comboustion. However, if the Injector spray gets too close to the Glow PLug it will burn through it.

Attached Thumbnails

   

__________________
84 300D, 82 Volvo 244Gl Diesel

I have an 86 Jetta TD which is probably just as you describe. I get 55 mpg (US) with it.

__________________
85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.

My Volvo Diesel Engine is made by VW and has many parts in common with the Rabbit.

When I rebuilt the block I used oversized Pistons for a VW Rabbit that were made in Brazil.

__________________
84 300D, 82 Volvo 244Gl Diesel

Think of the glow plugs as performing the task of a block heater - but instead of heating the water jacket around the cylinders they warm up the air in the combustion chamber.

As mentioned above by layback40 heat is produced during the compression stroke. (Quickly) Compress air and it gets hot. (Quickly) expand air and it gets cold => don't hold onto a nozzle of a CO2 fire extinguisher and release the gas!

Even though quickly compressed air gets hot; unfortunately on a cold engine this heat buggers off quite quickly into the surrounding parts of the engine. The glow plugs give a bit of a boost. They are only in use before the engine starts - they are not used like a spark plug (or the model aircraft example you gave) to ignite the fuel once the engine is running.

Here's a nice link telling more

Indirect injection - Wikipedia, the free encyclopedia

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

So the Ball Pin (sort of a bridle) in the pre chamber acts similar to a constantly running glow plug. I guess I didn't see them when I had the injectors out.

__________________
85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.

I don't believe the diffuser ball glows, as they're always black when you remove the injectors. Their purpose is to help atomize the fuel spray and reduce ignition delay. The reason Mercedes has such a complicated combustion system is to make the engine quiet enough for a luxury car.

__________________
'83 300D, 126K miles.

The Ball Pin should get nearly as hot as combustion. Between the combustion cycle there is some time for it to get cooler but there is a limited area for the heat to transfrer from it.

__________________
84 300D, 82 Volvo 244Gl Diesel

Next time you have the injectors out, turn the glow plugs on and you'll be able to see the ball pin. Very cool.

__________________
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