Prechamber Mod "Flame" Front

OM616

10mm MW

572

01-27-2011, 07:45 PM #75

OK, I added to #2s list of goodness. (See post #59 for theories #1 and #2.)

Given that #1 is getting the most votes, I have decided to post my thoughts regarding #1 to see if I am missing something that may sway me from my #2 position.

My apologies if I don’t make sense, I am multitasking and don’t have the time to have it proof read by others, so I thought everyone here could proof read it for me.

Although I am an advocate of the #2 theory, I will do my best to impartially lay out my thoughts regarding how theory #1 would work, remember this is from my personal perspective and I am open to counter arguments of any sort. Here we go.

I have not measured the volume of the prechamber, either the 61X or 60X, but I do have a good idea of the size and shape of the chamber, the ID of the burn tube, and the distance from the nozzle to the tip of the burn tube. This is important to me because of what has to happen in order for the fuel that is injected into the prechamber, to be expelled from the burn holes into the cylinder, to ignite.

Another important volume to me is the amount of fuel that is injected, and how long does the injection last. For example, if at idle, the IP puts out 1cc every 1000 revolutions of the IP, that would equal .001 CCs of fuel injected per injection cycle at idle. Now that .001cc of fuel is injected into the prechamber in a fine mist creating a fog of fuel as apposed to a stream, the point being that a fog disperses and a stream has a trajectory.

Lets say for simplicity, that the distance from the injector nozzle to the bottom of the burn holes is 2 inches, (it is a little more than, but 2 is a nice number). The fog of fuel that is injected into the prechamber needs to, per the #1 theory, be heated to further atomize the fuel, and propelled through the oval chamber it self, down the burn tube, and out the small holes with enough velocity to disperse the fuel throughout the cylinder where it then ignites.

It has been suggested that a very small portion of the fuel that is injected ignites in the prechamber, creating heat that will preheat the remaining injected fuel, that is not ignited, and pressurize the prechamber causing the heated fuel fog to be expelled from the burn holes.

This is where I have a problem visualizing how this would actually be able to happen. For example, if only a very small portion of air is allowed to enter the prechamber, then the prechamber pressure will be lower than the cylinder pressure, and the less air that can get into the prechamber, the grater the pressure differential.

Sticking with pressure portion of the #1 optional theory, in order for anything to be expelled from the prechamber burn holes, there needs to be a higher inside the prechamber, than the pressure in the cylinder. In other words, pressure naturally wants to be equal or balanced, if you have a flat tire and a portable air tank, the air will leave the higher pressure tank, and go into the lower pressure tire. This will continue until the pressure in the tank and the pressure in the tire are equal, and at that point, the flow of air will stop. The same thing would take place in respect to the prechamber and cylinder pressures, only the prechamber starts off at a lower pressure than the cylinder, quite a bit lower because of the small holes.

Now considering the order of operations that would result in warmed, non-ignited fuel, to be blown into the cylinder, wouldn’t all the fuel need to be injected into the prechamber before a small portion of it is ignited? The heat from the combustion would then warm the rest of the fuel, and increases the pressure inside the prechamber high enough to cause the prechamber and cylinder pressures to try to balance each other, resulting in gasses and fuel being expelled from the prechamber into the cylinder, where it can then burn.

A couple of concerns come to my mind given that sequence of events. First, what ignites the fuel, or more correctly stated, what prevents the fuel from igniting in the prechamber, until after all the fuel is injected into the prechamber? If the initial portion of fuel injected into the prechamber ignited and then went out, the remaining fuel that is injected would be on the wrong side of the pressure wave wouldn’t it?

Also, what keeps all the fuel from burning in the prechamber? Is it a lack of oxygen because very little air is allowed to enter the prechamber during compression? That would be very dirty as the flame would smother out causing lots of CO. I also have an issue with the idea that the main quantity of fuel is blown into the cylinder to ignite and burn, would that not tend to nail like mad, (like a DI engine), as there would be a large amount of fuel ignited, vrs the fuel igniting as it is injected?

And another thing, if the combustion chamber is in the cylinder, and not the prechamber, how does the prechamber have any affect the ability of the engine to rev faster than a DI engine?

Statement (A) of theory #1

The cylinder pressure is always higher than the prechamber pressure, (pre-injection), because of the small holes restricting air flow into the prechamber. Fuel is dispersed from the prechamber and throughout the cylinder to be burnt, by a pressure differential created by a small amount of fuel that is burnt in the prechamber, which raises the prechamber pressure above the cylinder pressure, causing gasses to flow from the prechamber to the cylinder.

Thought pertaining to Statement (A) of theory #1;

If, per theory #1, the majority of fuel is burnt in the cylinder, then the pressure in the cylinder will quickly become higher than the pressure in the prechamber when the fuel ignites. This pressure differential would cause flow from the cylinder into the prechamber.

If there is only enough air in the prechamber to support the initial combustion of the small amount of fuel that propelled the fuel into the cylinder to begin with, then unless all the fuel has made it into the cylinder before it ignites, any fuel that is flowing out of the prechamber will be pushed back into the prechamber in an effort to balance the higher cylinder and lower prechamber pressures, and any flame would be smothered out once it entered the prechamber, then expelled back into the cylinder once the cylinder pressure drops bellow the prechamber pressure. This would produce lots of partially brunt fuel, CO, not a good emission.

Even if the fuel gradually ignited as it leaves the prehamber, the moment the cylinder pressure is equal to, or higher than, the prehamber pressure, the fuel will stop flowing from the prechamber to the cylinder, and the fire will go out because there is not enough air in the prechamber to sustain combustion.

Because the air in the prechamber was used up during the initial push, another burst of pressure from combustion in the prehchamber can not be possible unless additional air from the cylinder is forced into the prechamber, allowing another burst that will push a second wave of fuel into the cylinder until the pressures switch again. This would cause multiple ignitions in the cylinder as the main portion of the fuel would be expelled from the prechamber in pulses, as the cylinder and prechamber trade high and low pressure states

Keeping in mind that the start of injection takes place before TDC, and therefore the cylinder pressure is continually rising from compression, before and during, the duration of the injection. With out a continuously higher pressure in the prechamber than the cylinder, a sustained dispersion of fuel into the constantly increasing pressures of the cylinder is not possible, as flow is from high pressure to low pressure.

Unless the prechamber pressure is constantly increasing, maintaining a higher pressure than the cylinder pressure throughout the entire injection and combustion cycle, then I do not see how a continuous flame front can be maintained in the cylinder.

Additionally, there seems to be a consensus that the prechambers ability to maintain a high pressure longer than the cylinder, allows the IDI engine to rev higher than a DI engine. If that is the case, and the majority of the fuel is burned in the cylinder and not the prechamber, then how does the pressure in the prechamber, (that does not contain any burning fuel because the little amount of air that was allowed to enter the prechamber was used up during the pulse sequence of the injection), have anything to do with the combustion environment in the cylinder? Once the fuel is in the cylinder, the combustion environment in the cylinder is the same as in a DI engine isn’t it?