Prechamber Mod "Flame" Front

OM616

10mm MW

572

01-24-2011, 09:26 PM #26

Now that is a perfect example of a technical counter argument! It should be noted that ConnClark was one of the members who contributed to my other thread.

I think we actually agree on more that first appears. I need to be more specific as to terminology.

I think we can agree that the purpose of the prechamber is to sustain a pressurized environment that supports continued combustion after the cylinder pressure starts to drop. (as the engine speed increases, the time that the cylinder pressure is at or above the pressure that is required to support combustion is less and less, there for, the prechamber maintains pressure longer, to aid in the combustion at high engine speeds).

When I spoke of power, I should have said torque to be specific. I agree that reducing the hole size will allow the engine to run faster, but due to the reduced flow capacity of the smaller holes, less air will be able to get into the prechamber and there for, the amount of fuel that can be burnt is reduced. This will allow the engine to spin up faster, but the torque will be very low. If I recall correctly, the OM617a engines peek torque is some where around 2K rpm, after that the torque starts dropping off, but because the RPM is increasing, the HP number increases.

One way to tune a prechamber is to increase the size of the holes until the max speed of the engine at full power is just above where you want it. This would allow the prechamber to breath very well at lower speeds, allowing an increased air density in the prechamber to burn lots of fuel, and the ability for the expanding gasses to get to the cylinder quick enough to perform work at high speeds.

The stock holes are sized to work with the stock amount of fuel. If the pump is turned up, increasing the amount of fuel that is injected, then there needs to be more air in the prechamber to burn it. One way to increase the air in the prechamber is to increase the boost, which creates incredible pressures and temperatures as not all the extra air can get into the prechamber. This pressure robs the engine of power as power is used to compress the air. Also the cylinder’s compressed air temps are tremendously high, I measured over 900 degrees F with only 15 lbs of boost, during some of the tests. This is before the fuel is injected.

By increasing the size of the holes (proportionately to the amount of boost and fuel increase), the pressure differential will be lower as more air will be able to get into the prechamber. This will lower the amount of power that is used up compressing the cylinder gasses, lower the cylinder compressed gas temperature, and put more air into the prechamber so more fuel can be burnt generating more torque at higher speeds.

The Stage III prechambers have the three 3mm hole area increased 15% each. On my NA 616 engine, this resulted in a stronger mid range and top end, and the EGTs dropped approximately 150 degrees at cruising speed of 75 mph, and I am able to give it more fuel than I could before to maintain that speed up hills, the EGTS climb slower with the larger holes. It pulls stronger as far as I dare spool it up to, than it did before.

Regarding your thought that my drop in EGTs is the result of less fuel being burnt due to the holes being too big, I am injecting the same amount of fuel with the enlarged holes as I was with the small holes. If less fuel was actually burned being burnt, then the EGTs would go to critical mass in a very short order. Unburnt fuel = high EGT, completely burnt fuel = lower EGT, the heat was converted into work instead of being pumped out the exhaust.

With the 617a, increasing the size of the holes resulted in a more dramatic reduction of EGTs, the turbo spooled sooner, the engine temp was lower as well. I was able to turn up the pump beyond the point where it was smoking with the stock sized holes. It ran very strong, but until I get an engine for my dyno, it is speculation as to any power increases or decreases.

Now, I bet my emission numbers are terrible, especially NOX as I am generating more heat because I have more air in my prechamber so that the additional fuel I am injecting can burn generating more heat.

I think a key to really being able to open up the prechamber is reducing the delivery time. Yes there is a point where you can deliver too much too fast, but we are not there yet.

Keep in mind how pitifully under powered these engines are, a 2.4l that puts out only 60-80HP at full steam, or a 3.0l turbo motor @ 120.

With everything, the prechamber is an exercise in compromise as you can easily have a high torque stump puller, or a low torque screamer with ease, the trick is getting the torque where you want it.

I am interested in your counter thoughts.

I hope more members will feel comfortable enough to add to this discussion.