Prechamber Mod "Flame" Front

GT2256V

109

01-24-2011, 07:47 PM #25

I figured I would chime in on this.

An indirect injection (IDI) engine has one clear advantage over a direct injection (DI) engine, rpm range. The key factor for an IDI engine to providing power at a higher rpm range is the resistance to flow provided by the prechamber at higher rpms. DI engines don't have this resistance to the flow which is why the have a narrower rpm range.

A DI engine is limited in rpm by the expansion rate of the combustion stroke. Past a certain rpm in a DI engine the fuel doesn't burn fast enough to maintain high enough pressure to burn the fuel completely. The higher the rpm goes past this point in a DI engine the less fuel it can actually burn and extract power from.

In an IDI engine at lower rpms the resistance to flow in and out of the prechamber is negligible and the IDI engine behaves much like a DI engine. As rpms increase so does the resistance to flow in and out of the prechamber. At the RPM where a similar DI would start to loose its ability to burn fuel because of the rate of expansion of during the power stroke, the pressure in the IDI engines prechamber is kept higher by the resistance to flow. This allows IDI engine to burn more fuel at higher rpm ranges than a DI engine and thus produce more power at those rpm ranges.

On the OM 617a the size of the prechamber holes effects the power produced at higher rpms. In general, with bigger holes the power peak will be moved to a lower rpm and the torque peak will be at a slightly higher rpm and slightly higher in value. As the holes get bigger your IDI engine becomes a DI engine. In general, with small holes the power peak will move to a higher rpm and the torque peak will drop in rpm and be lower in value. As hole size decreases from being oversized the power peak will grow and move to a higher range. At some point it will reach a maximum value and then decreasing the hole size will reduce the peak power while continuing to move the peak to a higher rpm.

<Flame War snipped>

(01-24-2011, 01:23 PM)OM616 Relevant portions of my response;

Any way, I have measured the pressures in the cylinder and prechamber during compression, (pressure differential), at different piston velocities, and found that above 2000RPM the 616/617a prechamber VE really starts to fall off. Boost increases the cylinder pressure, and in turn helps the prechamber VE, but also creates tremendous pre-injection cylinder pressures and temperatures.

Actually boost will drop the volumetric efficiency because it increases the density and thus the viscosity of the air. It does effectively increase the mass of air in the prechamber though.

Quote:With out increased fuel delivery at the bottom end, the torque will not vary much because the prechambers VE is adequate for the slower piston velocities and expansion volume from the quantity of fuel that is delivered over the length of time, by the stock elements. The benefits of increasing the high speed VE of the prechamber can be seen in the drop in EGTs at high speed operation.

The egt drop is probably due to less fuel being burned.

Quote:(After reading this again, I thought I would expand on the thought. The prechamber seems to be calibrated around a couple of variables, 1) being piston velocity and max cylinder volume, and 2) the pulse width of the injection, which for the most part sets the rate of expansion by slowly injecting fuel over a long time vrs, the same amount of fuel in ½ the time. With the slower injection, the pressure builds slower and can be bled through smaller holes over a longer period of time, allowing sufficient residual pressure to be sustained in the prechamber after the cylinder pressure has dropped. If larger elements were used, a given amount of fuel would be injected faster and the pressure curve would be steeper, (would riser quicker), there for, higher flowing holes would allow a faster release of the pressure, (more torque). Since the injection was shorter, the length of time the sustained pressure would need to be maintained to support continued combustion would be less. My goal with the 10mm elements is to be able to get the fuel injected and burnt before the cylinder pressure starts dropping, at that point the holes as calibrated orifices are not needed, and I can get as close to a direct injection with an external combustion chamber as I can get.

There is a rate limit to how fast you can burn the fuel dictated by the rate at which the fuel encounters oxygen. Dumping all the fuel at once can have the effect of using up all the O2 in the locality of the fuel. This can cause some of the fuel to hit the prechamber wall where it woun't burn very well. Spreading out the fuel delivery can promote better mixing by allowing the turbulence in the prechamber to replenish the O2 supply in the location of the fuel.

Quote:On the other side, if the boost is increased, the max cylinder volume is increased, which will require higher flowing holes to allow the additional air to make it into the prechamber at high piston velocities. Now one could argue that with out additional fuel, the increased holes would allow the prechamber pressure to drop too soon. Possible, I would point out that there is a higher volume of gasses in the prechamber creating a higher pressure at the start of injection. As long as the pressure stays above that which is needed to promote complete combustion for the length of time that is required to burn the injected fuel, all should be well and balanced)

The governor reduces the fueling at high speeds because the prechambers can't breath that well, and the fuel needs to be reduced to that which can burn in the lower O2 environment. In other words, the holes are sized for an engine operating speed of 55 mph at most, any faster and the increased volume of expansion gasses from the additional fuel needed, to run faster or make more power, simply can't get out of the prechamber before the next compression cycle starts. (At high speeds the prechamber never fully depressurizes and scavenging is reduced resulting in a dirty combustion environment, which generates smoke and high EGTs). (Determined from my pressure differential tests on 616/617a prechambers)”

The holes are sized to balance a high power peak and to give a range of rpm suitable for driving between the torque and power peak that matches the gearing (both the number of gears and the ratios). Also the peak power output of the OM617a is at 4250 RPM, much much faster than 55 mph.

Quote:Now that there are many examples of how to structure a technical response, there really is no excuse why we can not see a multi paragraph counter argument. Apposing thoughts are productive as long as they are well thought out and explained in context with supporting foundation.

I haven't done experiments but this is the knowledge I have gleaned from books on the the subject and several technical reports from NACA experiments where they tested several variables with prechambers.

If you increased the boost substantially I could see justification for enlarging the holes a little but not much.

ConnClark