A post in another forum brought me to wanting to ask the Tech guru's in here about the desing of the top of the pistons in the Mercedes Diesel engines. Esp. the DI's of the recent past and up to what is in production today.

Im wondering which (if any) have a design on the top of the piston made for enhancing the automization of the fuel... via a cup or some other form of indentation or pocket on the top of the piston. This seems to be the design in the VW diesels and the Ford powerstroke diesels.

Also if anyone knows..... which of any of MBZ (or any other diesel engine for that mater) engines have injectors aimed with the intent of keeping fuel from contactiing the cylinder walls. (As I understand it; this is the VW design being used in VW diesels.)

All MB diesels up to the TDI use pre-chamber injection rather than direct injection, so fuel reaching the cylinder walls isn't an issue, by and large.

There are several prechamber designs, with associated piston top shapes, not interchangeable. The appearance is basically an off-center hole with flow channels radiating from it. Prechamber has several holes, I don't remember how many at the moment.

The TDI pistons have a "chamber" in them, unlike older direct injection diesels, and I don't know just exactly what the nozzle lookes like -- the older DI nozzles had four holes at a right angle to the axis of the nozzle, while the MB injector for the prechamber diesels has a waisted pintle, injects directly out the axis of the nozzle.

The new nozzles also inject a very small amount fuel before the main injection -- this ignites, so that the main fuel charge is injected into a burning fuel mix. reduces knock and soot considerably, but makes the nozzles EXPENSIVE ($600 or so each).

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

In precombustion chamber Diesels engines, the fuel is injected into the pre-chamber and ignites there. The fuel that comes out of the nozzel into the combustion cylinder on the top of the piston is already burning. It is a rich mixture and completes its burning with the air in the cylinder. There is a small recess in the piston to clear the nozzle because the nozzle projects below the surface of the cylinder head.

P E H

The recess in the top of the direct injection piston is a swirl chamber. As the piston reaches TDC, the air is forced to move in a motion similar to the shape of the swirl chamber. This helps insure a complete air/fuel mixture so that the combustion process is as smooth and complete as possible. This also helps to equalize the pressures in the cylinder during combustion - avoiding excessive stress on internal components. Even prechamber diesels have some form of flow control on them. Look at an MB piston. The prechamber detent in the piston is nearly on center, as the prechamber protrudes on center. Combustion gases push down evenly on the top of the piston. VW IDI diesels have an off center prechamber. The top of the piston has two circular patterns connected at the bottom where the prechamber expends the burning fuel. It is meant to cause the burning fuel to swirl in the cylinder while pushing down on the top of the piston.

Now if I could only fix the damn things.

Thanx muchly.

Was trying to figure out why in reality VW DI's seem to not coke up when runnign WVO compared to what they "say" about other motors. A few of us got to thinking maybe it was the pistion swirling action of the VW piston.

I Dont know about Mercedes DI's coking up. Not much data on that. GM DI's seem to do ok too? It seems all DI's have swirling chamberish type pistons so maybe its not cause of that. Any ideas?

I think it depends a lot on compression -- "typical" american diesel engines tend to run very low compression by diesel standards, used to be 16:1 or 18:1, with direct injection and fixed injection timing. Not always true, please don't start a flame war!

European (MB, Peugout, VW/Audi/Volvo) practice has been much higher compression ratio (21:1 on the 61x engines, 22:1 on the 60x, I think, 23:1 (and actually as high as 26:1 in manufacture) on the VW/Audi). This results in much better combustion, along with the use of prechambers of some sort and high swirl heads.

American diesel design was essentially stagnant for many years -- the basic engine designs weren't upgraded significantly, at least for the automotive/truck applications, until the middle 90's when the EPA started applying pressure due to the carcongenic nature to the inky black soot from the low compression, fixed injection timing engines. We've all seen and old Mack or Detroit blowing huge clouds of smoke on each gear change.....

That said, fuel quality also has a lot to do with carbon accumulation, and American #2 diesel is both rather low grade by international standards, and highly variable.

Use of a diesel treatment like RedLine will help reduce soot accumulation, but the best thing is to find a fuel that doesn't smoke.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!