peejay

Yep. (Overlap is *always* there, but I think I know what you're getting at)

It's not really RPM dependent as much as load dependent. At higher loads there is more pressure (less vacuum) able to draw exhaust gases up around the opened port. But at higher loads you're better off with the peripheral vs. the side port anyway...

It would seem to be important to me, but I haven't experimented in the field. But more volume between the port and the closing/switching valve would mean more of an air-spring effect. Or maybe that effect could be beneficial... as I said I haven't experimented in the field.

Snrub

iTrader: (1)

So if the valve was shut and the engine is at a lower load, could you give me an indication of how much exhaust dilution there would there be compared to when the valve was open vs. the same engine with just the side ports (no peripheral intake existing)?

peejay

Insufficient data. How long is a piece of string?

nkeehn

A foot

Snrub

iTrader: (1)

Would it be possible to pick a couple of "lengths" as examples and then give me an idea as to how they would effect the answer? Are there really that many different answers, a typical semi-PP will be fairly small and pretty much the same location, right? For instance wickedrx2 runs a 1" PP.

rotarygod

iTrader: (1)

Twice the length as from one end to the center.

nkeehn

Could using a rotary valve timed for the port opening help eliminate the exhaust dilution problem? I know that adding a cam to a rotary engine is paramont to blasphemy, but it wouldn't be a critical engine part and could even be electrically driven. Besides, it'd be a rotary cam, not a normal cam, the name says it all.

Kenku

I'd have to actually check some papers I have sitting around, but I'm somewhat skeptical about the supposed effects of overlap on peripheral motors. Short version of things, the rotor gets in the way and doesn't leave all that much area for actual gas flow. Plus, some numbers I have for late development MFR ports suggests they went to far less overlap than people might think. I'll have to check though.

Snrub

iTrader: (1)

Anyone know the answer to my question?

nkeehn

Not yet, but I'm trying to find out myself

Kenku

Well, roughly, if the valve to close off the peripheral ports is far enough away from the face of the housing that the cross sectional area of the port looking between the valve and tip of the rotor parallel to the direction of flow is greater or equal to half the cross sectional area of the port perpendicular to area of flow (like normal) then for purposes of exhaust gas reversion you can treat it like a normal peripheral port with zero airflow.

Let's do the math for a quickie example... looking at a square port because it'll place the valve farthest away from the housing face. Say, 30mm square. With the apex seal halfway in the middle of the port, which will be where both intake and exhausting chambers have the same area to them, there's 450mm^2 of area to either port. The port's 30mm wide, so if the valve to shut off the ports is more than 15mm away from the face of the rotor housing, the biggest restriction to flow is going to be at the port and it'll act like a straight pport motor. For a circular port of the same area, the diameter will be 33.85mm, so the valve has to be within 13.3mm of the face of the housing to have any restriction. Both of those would require the valve to be within the water jacket area.

That's a fairly rough approximation and ignores all sorts of things, but if you want a better approximation, I reccomend you dig up enough data to model it yourself.

How much exhaust gas dilution does a full out peripheral port actually have, should be the question. For that it's *really* worth looking at the geometry of the whole thing to figure out where the choke point is... which is... the rotor! So while in theory, you have around 20 degrees where both ports are fully open and 150ish degrees of overlap... the clearance between the rotor and housings means that for most of that, any possible flow is choked down to almost nothing. Thus I feel fairly confident in shifting it into the "crap that's not worth worrying about" column.

Someone want to put together a flowbench and measure the flow between intake and exhaust at various rotor positions to prove me wrong? I've been meaning to put one together myself, but... stuff to do, y'know?

couturemarc

one advantage to valving the semi pp is to control intake air velocity. the downside of course is the topic of this tread... overlap. It would be really interesting to find out exactly what effect this has at different rotor positions but the only way i can think of would be to modle it in simulation programs, i just don't think that a flowbench would be any help in this type of scenario.

We have a dynamic rotary flowbench that currently operates upto about 6000rpm and if anyone has any suggestions on how we could measure effects of overlap we would try it.

Intake opening around 65 BTDC and closing at 55 ABDC for a port size of roughly 0.55-0.70in^2 works very nicely. This setup reduces overlap slightly from a normal PP and makes it more like a BP.

rotarygod

iTrader: (1)

I agree with that entirely. The flowbench wouldn't be very useful because it doesn't allow for the scavenging effect of the exhaust. Just because a certain amount of air can flow between ports at overlap on a flowbench doesn't mean that the same amount will due to the velocity of the gasses in accordance with their direction. We wouldn't see it on the bench.

Kenku

Well... to the paper... hell, I'll have to scan it tomorrow. It's good stuff. Suffice to say they found that the actual area of the overlap is signigantly smaller than you'd think by looking at port timing irrespective of the rotors.

I brought up the flowbench as supporting the "there's not much port flow possible between the two" evidence, not as a break or bust argument or anything else. It would suppory my idea that the valve doesn't do much of anything to keep it from looking like a pport intake with respects to exhaust gas dilution... except the intake charge velocity could maybe counteract that if it weren't closed... ah, buggerit.

Specfics on dilution... nother paper maybe. Or it might be in here but I'm tired. Truth to tell, I really need to just sit down and make my own damned model. This kind of crap is irritating to demonstrate approximately.

FWIW, from the Mazda paper I've been mentioning, SAE# 900032, they come up with optimized timings of IO at 80btdc, IC of 80 abdc, and 25cm^2 of port... EO of 73bbdc, EC of 55atdc and 12.5cm^2 area. By the numbers they compare to the baseline, that's quite a bit less overlap too, but more area.

Tomorrow I'm going to sit down and start chundering out large scary formulas to come up with something that'll spit out **** about rotary engines. Tonight, I'm tired and I'm going to bed.

rotarygod

iTrader: (1)

That's 135 degrees of overlap. That's quite a bit.

Kenku

The normal MFR port timing (as seen everywhere else including Yaw's site) is 150-some degrees, so in comparison...

rotarygod

iTrader: (1)

151 degrees, but close enough. That extra 16 degrees of eccentric shaft rotation is only 5.3 degrees more of rotor rotation. Unlike bridgeports, the entire area of the intake and exhaust ports are also open at the same time for some of that. Even a bridgeport with the same timing doesn't have the same overlap characteristics as a peripheral port. That extra few degrees between the mfr housings and the timing specs from the SAE paper isn't enough to make a difference from an overlap or dilution standpoint.

couturemarc

back to "semi" pp....

you don't need as much timing when you have the side ports functional still. In fact the overall open area of a 6-port semi pp motor is much more than the 25-30cm^2 found on full pp motors.

Its hard to distinguish between the effects of overlap and intake velocity... both will harm your low and mid range power and this is what is common on pp motors.

A staged semi pp motor that opens the primaries , then secondaries , auxiliary, the finally pp gradually increases intake port area and maintains high intake velocity and volumetric efficiencies around or above 100% from 2500 rpm to redline.

This setup will be affected by overlap but how significant will this be???

What i would like to do is block off the side ports and run just the small pp and see how it behaves at low and mid range compared to the side ports. This might give a good indication as to the effects of overlap.

any thoughts?

Kenku

Well, the baseline they use in the paper opens the port at 100btdc, so 165 degrees... but the point is, flow during overlap is severely less than people think, because while the ports are fully open, the port isn't determining the point of greatest flow restriction.

Got the paper scanned at any rate... it's kinda big to attach for here unless I shrink the files to where it's illegible and I can't get at my webspace ATM, so PM me or something if you want me to email copies. It's about a meg, zipped.

Acutally, they do briefly look at combination port setups; I'd forgotten that. They were actually looking at the peripheral as the primary port and the side port as the secondary though.

peejay

Oddly enough the engine I am building has exhaust opening at 75BBDC, EC of 58ATDC, IO of 80BBDC, IC at 60ATDC.

Going for a wee bit more low end than you'd get from closing the intake port sooooo late.

And of course the port shapes will prefer to flow in only one direction - forwards.

The overlap period isn't all *that* bad. A lot of people forget that there is a rotor in the housing too, and in the period around TDC it's right up by the ports. Not much flow potential. As opposed to a boinger where the valves are right next to each other and it'd be a cinch for airflow to freely communicate.

z8cw

Resurecting...

Kenku any chance to get this paper you were talking about?

CW

ziig

iTrader: (1)

i did a peri bridge that seemed to work great..it was some years ago though....used a stock manifold and added tubes for the peri side....

Stews'7

I have recently been talking to my buddy about doing something similar in my FD. What I would really like yo do is a NA 20B but the cost of the motor it self is insane. I prefer NA motors honestly when it comes to RX-7s. Revo Tune makes a semi pp set up , housings and LIM to boot. http://revo-tune.jp/catalogue/detail.php?id=105 Rmagic has a set up that uses the primarys and huge pp. Through some 9.7 in there and could be fun.
-Stew

Judge Ito

i have a semi - p-port with a large bridgeport.. custom intake. works lovely..

crispeed

It sure does don't it.
It's even nicer when those ports are being filled with a lot of nitrogen and two parts of oxygen.