RXBeetle

So as I understand it the whole pulse reversion thing in an intake relies on a port closing to cause a pressure reflection up the intake runner that is then bounced back into the intake port as it opens. So here's my question, how does this occur with a P-port motor? The intake never actualy closes, the next chambers just opens while the previous closes and the air flow is never interupted. If this is true then how does the 787b variable lengthe intake work? I thought the whole idea was to vary the length of the runner to match the time interval between intake closing (wave generated) and intake opening (wave reflected back into port). I guess it comes down to what generates the pressure wave in a P-port motor?

BuckyFD3

It will work in the same way, there will be a pressure drop as the motor sucks air in, then as the apex seal passes the port the pressure drop becomes less as it doesn't suck as much. then as the next face of the rotor comes round it starts again.

This causes the pulse. not as strong as a piston engine though as it wouldnt bonce back on valves.

speedturn

iTrader: (1)

Bucky is right. I have been racing P-port motors for 10 years now. They definitely have pressure waves; they wave does vary from strong to weak depending on where the rotor is. They do not have a fully closed off time, but they do have a time when the flow will essentially stop at low rpm or even back flow (reversion) at low rpm. When the rpms get up high enough and the intake tract length gets in tune, then the torque boost will push you back in the seat. The 787 variable length intake was made to widen the useful rpm band of the torque.

RXBeetle

Great replys, thanks. Yeah I see now how the air velocity is more of a sin wave than a square wave like in a piston engine or side port. I was also reading that the left over exhaust gasses still in the chamber will cause some of the pulse as the rotor moves from exhaust cycle to intake cycle. The 787b is what got me thinking on all of this. I was up till 2am making excel spreadsheets calculating the runner lengths based on rpm and temp.

crispeed

You'll never be able to calculate what's the best combination on paper. The only proven method is through trial and error. Also the exhaust length, collector id etc. on a PP is more important in many ways. Exhaust tuning will make or break a PP combination.

BuckyFD3

Also when the rotor comes round and pushes the flame out of the exhaust, because of the overlap on the intake, the intake charge gets sucked in as there is a pressure drop inside the rotor housing and some of the inlet (i.e air and fuel) gets sucked into the exhaust, its very small but enough.
Thats how you get bad emmisions at idle, this happens on large ports and also big degree cams on piston engines.
I suppose you can use this to your advantage aswell, it'll start the sucking action earlier to get the intake gasses moving, the quicker the revs the quicker it shuts off the exhaust port and the less time for overlap.

But as crispeed says, its all theory and best done in development.

Renisis engine helped get round this using side ports as we know.

ikari899

if you wouldnt mind i would like to see that excel sheet. there are so many different ways people calcuate exhaust and intake runner lengths it would be interesting to see. i think a problem with the calcuations most of the time is that with P port set ups there is never a time when air is not flowing though the ports (sorta). so the only real way to calcuate Pport runner lengths would be to use the wave created when the port opens and time it so that the high pressure wave related to it ends up ariving just when the port closes. right? so you really cant use helmholtz stuff on Pports right?

im trying to learn this stuff perfectly so the more information the better.

ikari899

and you want to time the exhaust pulse to end up at the other rotor just as it is closing correct?

and what you were talking about with the intake charge getting sucked out/though the housing would be caused by a properly designed exhaust savaging well.