OpenTopRotary

Why do high overlap engines (bridgeport/peripheral port) behave so poorly at part throttle/low RPM? I am assuming that they must exhibit cyclic misfiring similar to at idle, but the vibration is amplified because the difference in torque peaks is much greater than at idle, and also now connected to the driveline.



But does this happen just because they dont flow enough to overcome the exhaust pressure? Or does it have something to do with the throttle plate breaking up the pressure wave in the intake and not allowing it to gain the benefits of it traveling back into the intake port?



I ask this because I wonder if a PP engine could be run without a throttle in the intake path and behave much better at low speed and part throttle. I think this could be accomplished by regulating the intake flow by allowing the two cyclinders to communicate with each other? What I am thinking is connecting the two primary intake ports not to the intake manifold, but to each other through a separate manifold with a throttle valve between the two. This would then connect the opening port to the opposite port that will be closing very shortly. This should regulate the flow without having the throttle plate actually in the intake path. At full throttle the throttle plate connecting the two would be completely closed and the engine should then flow even better because it would have zero restriction from a throttle plate in the intake path.



Pat

Lynn E. Hanover

Too many questions in one package.



Bridgeported engines mostly have more overlap than Pported engines. Some of that difference is lost by the Pport having big flow just a few degrees after opening its ports.



The big difference is in opening point. For the "J" bridgeport opening is around 115 degrees BTDC while the Pport is around 86 degrees BTDC.



The bridgeport is a pain to drive around. It has no torque at all just off idle. We idle at 2,200 RPM, but just letting out the clutch get you a dead engine. You must rev the engine and dab the clutch to get rolling or it just quits again and again. It is better with a street transmission, where the first is areound 350:1 or so. The race trans has either a 2:1 or a 1.96:1 first gear.



The Pport engine is much less of a pain to drive. It to idles around 1,800 RPM, but can be pulled down to very low revs while letting out the clutch and get rolling. It does not spit and fart and bounce around like the J port. It also has next to no torque, but it is drivable. It would be streetable with a stock first gear.

This is a Pport with ports very much bigger than the factory castings. The stock Pport ports are a bit on the timid side. These were done by Mandville years ago.



Now as to the why of it all, I can only speculate as follows:



Usable power down low is usually connected to intake closing point, and less to the overlap involved. There are some piston race cars that can drive slowly with no fuss at all.



The Pport closes at 75 degrees ABDC and the "J" at 72 degrees ABDC. But the Pport is still flowing a ton just a few degrees before closing, and the "J" is just about closed and flowing little a few degrees before technical closing. So, the closing point in the rotary seems to have little affect on idle and off idle power.

Which is real nice because it has a big affect on high RPM power, and where the peak power is going to be.



So it must be a combination of total overlap and and the gas flow and direction during overlap. At just off idle there is so little gass flow that there should be little back pressure, some of course, but not much.



So total overlap and intake gas dilution from the exhaust, pulses between the intake port and the butterflys and the like.



Now for some real serious guessing.



I think the big short large volume runners of the Pport can null quickly and re-establish directional flow quickly, so that the forces in play have much less affect on its idle, and off idle operation. I think the smaller longer runners of the bridge port can get into an opposition bounce at a number of harmonic RPM and the idle will (and does) jump all over the place. There is just no low RPM where the bridgport is going to be happy. And when the clutch is let out a tiny amount it just stops the engine dead.



In racing all of this is a bigger (perhaps more obvious) problem because we have a clutch and flywheel weight of under 7 pounds. A stock or near stock flywheel would help out quite a bit.



The Pport just does not have the problems being attributed to it. If it has anything like some flywheel weight it can be OK on the street.



I can not follow your intake idea. I will have to see a picture.



Lynn E. Hanover

j9fd3s

well maybe just maybe you'd get exhaust gas dilution, cause if the intake and exhaust are both open, its easier to pull from the exhaust, than against the closed throttle plates? maybe?

heretic

Makes sense. Not enough velocity at low speed to keep things moving in one direction in the system, so when vacuum gets high enough, it pulls exhaust gases up instead of the intake charge's inertial energy (what inertial energy at low speed?) forcing things down the port. Then the rotor misfires from all the dilution, vacuum drops, and no major exhaust gas pressure, so the next time around, the rotor has a fighting chance of getting an easily ignitable mix.



Brap brap brap. Take a recording and slow it down so you can more easily hear it, and you will hear six strong exhaust pulses and six weak or nonexistent ones. Not a coincidence.

OpenTopRotary

Lynn, my intake idea is completely different from anything that I have ever seen or heard. Instead of regulating flow by creating a restriction, and a subsequent vacuum, it would work by decreasing the rate of expansion in the chamber.



This would be accomplished by opening the two cylinders together (one is in strong vacuum, one is nearing zero pressure[theroetically BDC at a certain RPM] or even positive pressure[ABDC]). When positive and negative pressure are combined the flow will be from positive to negative, pulling some of the intake charge out of the cylinder nearing BDC. This will regulate flow without actually creating any restriction up flow of any cylinders.



This could then be regulated by restricting the amount of volume allowed to flow between the two chambers. At low-to-part (throttle) the cylinders would be able to communicate a significant amount of flow between each other. At full (throttle) the valve would be completely closed and it would behave exactly like the whole thing did not exsist, except that the throttle would not be up stream creating any kind of restriction at all.



Just a concept at this moment.



If this is still not understood I will draw a picture, but i guess what it would look like would be a slide valve directly in a primary port. The primary port would not have any runner to the the carb, only a 3-4 inch runner between the two rotor chambers. This could be done by breaking the wall in the directly at the bottom of the primary port of the intermediate housing.



Pat

RONIN FC

Where are these cylinders again?



The automotive industry is well beyond just letting air in to the engine with least restriction. Its more involved than that.

OpenTopRotary

The "cylinders" I am referring to are the chambers near the intake ports. Seeing as there are 6 working chambers in a 2 rotor engine, I am trying to refer to the two that are in the intake phase. Obviously these move around and with this much overlap, there may actually be 3 or 4 of the working chambers in the intake phase, so what i am referring to is the working chambers in the intake phase where the primary intake ports are. I guess "cylinders" was definitely the incorrect term because the chambers change shape as they travel around the engine and at no point do they actually resemble a cylinder like in a piston engine.





I am sure that it is more involved, but obviously intake wave tuning is very important for rotary engines and has a greater effect than it does on piston engines. By reducing any restrictions that still exist, we may make a significant increase in torque and horsepower all across the operating range.



Pat