pre chamber?

[Ears]

what exactly is the pre chamber and what is it for?

[compu_85]

Indirect injection - Wikipedia, the free encyclopedia

Old MB diesels have prechambers. Similar vintage VW and GM diesels have swirl chambers.

VW Diesel Training Manual

-J

[BillGrissom]

I usually find wikipedia articles to be spot on, but the one linked counters my understanding on many levels. This statement is particularly strange:
" ... speed up the combustion process, in order to increase the power output by increasing engine speed. "
I would think the pre-chamber would slow combustion, if anything, and I don't know that engine speed is limited by the combustion process anyway. Usually, balance and reciprocating forces are the limiting factors.
I can't view the VW manual (at office).

My understanding, as a mechanical engineer who has read several paragraphs about it various places (and may be totally wrong), is that the pre-chamber was necessary in our diesels with "mechanical poppet injectors" because the engine would be much too loud without them. Large trucks often didn't have pre-chambers, but those engines dampened the sound better, plus people expected trucks to be noisy. The pre-chamber reduces efficiency from flow losses thru the passages and more quenching of the burning fuel drops, and the odd shape of the piston w/ recess to match the pre-chamber. The noise is due to the sudden opening of the injector pintle and abrupt pressure rise, similar to "pinging" (auto-ignition) in a gasoline engine.

In modern diesels, w/ electronically-controlled injectors, they are able to gradually start the fuel flow and thus able to avoid the clinking sound from the sudden pressure waves. These engines also have a constant fuel supply pressure (common-rail), all invented by Fiat I understand - why they had funds from M-B & others to buy Chrysler. This also allows much better efficiency.

One interesting question is why nobody ever invented a mechanical poppet that tapered the flow so they could eliminate the pre-chamber in smaller engines. That would be less complex and yield much of the benefits.

[compu_85]

Actually, 1.9 IDI vw engines have a dual stage pintle / poppet injector. You'll see people talking about them here from time to time, the engine code was AAZ.

Prechambers also helped reduce some emissions compared to early DI systems. Of course computer controls make modern DI diesels **much** cleaner than old prechamber systems.

-J

[Maxbumpo]

Originally Posted by BillGrissom

I usually find wikipedia articles to be spot on, but the one linked counters my understanding on many levels. This statement is particularly strange:
" ... speed up the combustion process, in order to increase the power output by increasing engine speed. "

I agree, that article has several problems. I also understand that MB went with the design in order to reduce engine noise, with the trade-off of a reduction in efficiency / power.

[mannys9130]

It does speed up the combustion process, because instead of squirting all the fuel into the piston dish like a DI engine, ours squirt the fuel into the prechamber. The stream hits the pintle ball and swirls, combustion first starts and begins to push air and fuel out through the prechamber holes, and the now mixed air fuel charge continues to burn better and quicker inside the main combustion chamber. The prechamber tip looks like a rocket engine under load, shooting the burning air fuel charge into the combustion chamber.

Prechamber diesels do have the ability to rev much higher than DI diesels. Horsepower is the way we rate owed output of engines (or KW). Since power is work over time, an engine that spins faster will have a bigger horsepower number. 200 lb/ft of torque at 4000rpm is more powerful than 200 lb/ft at 2000rpm.

So the quoted section from Wikipedia is correct.

[BillGrissom]

Interesting, though speed in diesel world is certainly relative, so I might change "pre-chamber diesels rev faster" to "DI diesels rev even slower", since I owned a Honda 350cc w/ 10,000 rpm redline and have a 1965 Dart w/ 4.5L V-8 and 7000 rpm.

I wouldn't have expected the combustion time to limit the rpm. It is partially controlled by how the injection pump meters the fuel in over the downward piston stroke. DI engine fueling is much different, so compromises there could also explain lower rpm capability, rather than the speed of combustion. Newer diesels have much finer sprays (due to >10,000 psi fuel rail), which speeds droplet combustion and gives more thorough burning.

20 years ago I worked in liquid spray research, which over-lapped with diesel researchers. At that time they were just beginning to learn what went on inside the engine. They were just starting to get photos inside the cylinder and measuring drop sizes. They did find that much of the spray crossed the cylinder and impinged on the far cylinder wall (in big DI engines). Finer sprays helped a lot with that. I don't know how much more they know today (or think they know). In the end, it all comes down to running an actual engine and optimizing it as a black box.

[Diesel911]

Mercedes Precombustion Chamber seem to be focused on the control of combustion. The tiny holes hat the bottom of the Precombustion Chamber cause the combustion gasses to squeeze through those holes. At the same time during compression squeezine the Air through those Holes must create a lot of Turbulance inside of the Prechamber.

Turbulance is supposed to be and aid to combustion.

Then the Injector spray is focused towards the Ball Pin. People have said it is there for the Injector Spray to hit and cause better atomization and it surely does that when thel Ball Pin is cold and if any droplets from the spray reach it.

Since the Ball Pin is inline with the combustion it must get extremely hot and that is going to be a big aid in vaporizing the Fuel if any Fuel makes it down there.

[Shortsguy1]

I would love to know how one of our engines would run if you simply cut the end off the prechamber. So instead of lots of little holes, you would just have one big hole. Perhaps fuel would get to the piston and burn it up. Perhaps particle emissions would go up due to less turbulence, resulting in lower efficiency. Perhaps efficiency would go up because of less expansion losses through those tiny holes.

Anyone have an engine on the way out willing to run an experiment?

[compu_85]

I can tell you what happened with GM's motors and their swirl chambers....

Early 6.2s had smaller holes in their sqirl chambers. This resulted in better fuel mix and better fuel economy, at the expense of power. Later on the hole size was increased which allowed more flow and more power, but mixing wasn't as good and fuel economy took a hit.

-J

[Diesel911]

Originally Posted by Shortsguy1

I would love to know how one of our engines would run if you simply cut the end off the prechamber. So instead of lots of little holes, you would just have one big hole. Perhaps fuel would get to the piston and burn it up. Perhaps particle emissions would go up due to less turbulence, resulting in lower efficiency. Perhaps efficiency would go up because of less expansion losses through those tiny holes.

Anyone have an engine on the way out willing to run an experiment?

Sounds like something someone over on the superturbodiesel forum would do. That or drilling out the holes to a larger size.

It is kind of nice to know a Glow Plug tip or fragments will not fall inside of the Engine.