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Pport air-blow by to exhaust possible?

Old 05-31-17, 02:15 PM
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Pport air-blow by to exhaust possible?

This may seem like a strange question:

If you were to boost test a peripheral port turbo engine. Like attach an air-compressor to the charge pipe, and "boost leak test" the engine. Would you expect that there would be air that would blow through the peripheral ports and directly back out of the exhaust port on one or more of the rotors at specific crank positions? Just thinking back on the fact that intake/exhaust port are open at the same time so therefore just as much as exhaust gas can be pulled through the intake shouldn't intake be able to push through to the exhaust?

Now, this makes me wonder, under boost on a mechanically high overlap engine, either piston or rotary, maybe we need to think of tuning a little differently, because some of the incoming air and fuel charge will be bypassing directly into the exhaust manifold, unburned by the spark / combustion. So we could wind up with a necessity of more fuel than anticipated being utilized and also potentially no 100% accurate with the wideband oxygen sensor. Potentially we could see slightly false lean mixture as the air skipping through due to overlap isn't burned, which isn't picked up by the 02.

I've just noticed that my PPort engine requires an UNGODLY amount of fuel when chasing typical air-fuel ratios underboost, both in injector duty cycle and in fuel pump capacity! I mean UNGODLY as compared to my 3 rotor. The car is making 800-900hp, but using nearly 75% Injector duty cycle with 4x ID2000/4x ID1000 on 93 octane with a base fuel pressure of 50psi. Where is the fuel going lol. The car runs great, aside from a little bit of slight misfire which will be solved with IGN-1a upgraded coils. However, I just wonder, if there's anyone out there who totally disregards AFR in terms of absolute values on Pport engines...EGT are very low in boost, under 1500F which almost makes be believe it could be leaned all the way out to say a 12:0+ rather than running 10.5-11. Its almost as if the AFR is just a relative value for comparison rather than an absolute number.

For example, at idle, the car will run all the way up to to a 15.0 without dying. We know this isn't remotely accurate as no rotary can truly run on a 15.0. I keep it around a 13.5 as it seems to pull the most vaccuum. But I would seriously doubt its an absolute 13.5. The overlap is so intense, if you were to crack the throttle on shutdown, without the charge pipe, you can physically see the exhaust bellow out of the intake ! Its gnarly, but its crazy.

Just curious anyone's opinions on this...I've yet to meet anyone in person who's ran a pport with a turbo

Last edited by Monsterbox; 05-31-17 at 02:25 PM.
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Old 05-31-17, 03:22 PM
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Yes, intake loss out the exhaust port at all engine speeds is the normal dynamic of a peripheral port intake and peripheral exhaust rotary engine. This is noted in the Norbye rotary engine book.

It is the closed throttle plate that interrupts this dynamic and causes the increasing volume of the rotor sweep in the intake cycle to pull up exhaust gas instead of air through the throttle body.

Both intake out the exhaust port and exhaust dilution of the intake charge require more fuel.

If you make did a semi-direct injection engine you could avoid the fuel loss out the exhaust with the intake under load, but with low load (throttle plate closed) you still need a bunch of fuel to overcome the exhaust dilution of the intake.

If you eliminate the throttle plate (such as in extreme lean burn gasoline or diesel application) and use semi-direct injection you solve both issues.

This is something Mazda was able to do to improve efficiency of the peripheral intake, side exhaust port rotary generator for the range extender EV.

This is something Mazda will be able to do on a Hydrogen-Boosted gasoline rotary as well (extreme lean burn with no NOx)
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Old 05-31-17, 11:20 PM
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Originally Posted by Monsterbox View Post
However, I just wonder, if there's anyone out there who totally disregards AFR in terms of absolute values on Pport engines...EGT are very low in boost, under 1500F which almost makes be believe it could be leaned all the way out to say a 12:0+ rather than running 10.5-11. Its almost as if the AFR is just a relative value for comparison rather than an absolute number.

For example, at idle, the car will run all the way up to to a 15.0 without dying. We know this isn't remotely accurate as no rotary can truly run on a 15.0. I keep it around a 13.5 as it seems to pull the most vaccuum. But I would seriously doubt its an absolute 13.5. The overlap is so intense, if you were to crack the throttle on shutdown, without the charge pipe, you can physically see the exhaust bellow out of the intake ! Its gnarly, but its crazy.

Just curious anyone's opinions on this...I've yet to meet anyone in person who's ran a pport with a turbo
this is how i tuned mine. the o2 sensor is obviously not right at idle, and then seems to become more correct at higher throttle openings. it does seem to be consistent, so i just use the W/B as a reference, but really i'm paying attention to how it runs.

i'm running 14.4 at idle, it cruises around 13.5, and wot its in the 11's (for some reason the next jet leaner is 12.8, and i street tuned it) the plugs come out looking great, and the engine seems happy.

actually to expand on this, at idle i tried all the jets i had, and 14.4 (give or take) is where its happy. at cruise i tried from 14.5:1 which it didn't like, to 10:1 which it also didn't like, and 13.5 is as lean as it would go without getting mad.

WOT it doesn't seem to care

so i found that the P port is a balance between what I want, what the engine wants and what my neighbors want. IMO it was a really good experience, it really taught me to pay attention to how the engine is running
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Old 06-01-17, 08:43 AM
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Great! Thanks for the feedback guys. This makes sense why its taking so much fuel
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Old 06-01-17, 10:50 AM
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Additional to the throttle closed exhaust dilution I would think in boost it depends on EBP vs Boost, if the backpressure is higher you'll have exhaust recirculation. If boost is higher then boost loss to the exhaust. Yes? No?

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Old 06-01-17, 12:38 PM
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It is not directly related to static measurements such as intake pressure and exhaust manifold pressure.

On an NA rotary with overlap there is intake loss to exhaust port at all rpm at WOT even though exhaust manifold pressure is always above ambient and intake pressure is (almost always) below ambient.

This is because there is a strong dynamic from the much greater volume, high velocity exhaust exiting the engine below and the low velocity intake above the apex seal as it crosses the exhaust port.

Basically, the exhaust port creates a siphon.

It is like a paint gun; the high pressure/velocity compressed air siphons the much denser paint up the tube and out the paint gun BECAUSE the air pressure is higher.

All the pictures/graphs that show rotary overlap are basically wrong because they are showing the overlap only between the exhaust and intake phases on the SAME rotor face. They show the peripheral exhaust port closing, which it doesn't; it only shifts which rotor face it is open to.
It is only when you look at the exhaust overlap to the intake phase on the PREVIOUS rotor face you see the strong advantage of rotary overlap.

When the exhaust port opens and peak exhaust velocity out of the motor occurs the previous rotor face is in the intake stroke and open to being entrained by the high velocity exhaust gas exiting the engine.

It is a phenomenon that does not have a direct equivalent in a piston engine and it is not counted as "overlap" in the traditional figuring of rotary overlap.

Closest a piston engine gets is in the header collector when one exhaust pulse entrains the exhaust from another runner that is in the exhaust/overlap phase and that exhaust entrains the intake.
But that is separated by many feet, much exhaust volume, intake and exhaust valves and the piston in the way. And it is still a strong enough effect in a piston engine that every race team and most OEMs will design it into the exhaust.
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