Hi

I'm still learning so was hoping you guys could help me understand this properly.

I understand that overlap occurs in larger ported (eg bridgeport) engines. When the intake and exhaust ports are open at the same time, which causes exhaust gas to get into the intake/fuel and gives them the pulsing idle we all love.

Searching around I read something along the lines of "exhaust gas getting into the intake causes a near stall effect", which to me reads as "pulse, near stall, pulse, near stall, pulse".
With that info what I don't understand is why the pulse isn't much faster. I'm not just talking about idle RPM.
In a two rotor, both rotors are facing in opposite directions to each other so when one rotor is on the power stroke, the other is on the compression stroke just about to enter the power stroke, so you have a lot of power strokes going when the engine is running with rotors having three sides. That with the speed of an engine at idle (at around say 1k RPM) you can hopefully see why I'm confused to why the pulse isn't much faster. The first RX2 in the video below has a mild pulse and the second has a higher pulse so I will use these as an example.

Could some people explain it like I am 10? Maybe I misinterpreted what I quoted above?

One more question. In a two rotor, the rotors face in opposite directions (if one is pointing upwards, then other is pointing downwards).
Is this the same for a three and four rotor? So in a three rotor - up down up. A four rotor - up down up down??

Videos.

Basically what the problem is that with both videos, in my head the rotors are moving very slowly with one pulse per power stroke but that obviously isn't the case and is what I'm confused about.

Here is the video. Another is below with a quicker idle.


With a higher and faster idle like the series 1 in the video below I still don't understand why the pulse isn't even faster with so many power strokes.

 

I have asked myself the same thing a few times and am quite interested to hear what others have to say! I'm not entirely sure myself, but have come to a similar conclusion that it seems odd lol.

The rotary engine IS a bit unusual as the rotor spins 1/3 the speed of the eccentric shaft. That said, since each rotor has 3 combustion faces you get one combustion attempt for one rotation of the eccentric shaft (per rotor).

If every combustion event was successful you should hear 2 "thumps" per engine revolution. At an idle of ~1200 rpm, this would be 2400 thumps per minute or 40 per second. I don't have a finely tuned ear, nor am I a musician but I would estimate a brappy idle to have around 200 braps per minute.
There is no way the engine is skipping 12 combustion events before firing again so it has to be some kind of harmonic. I've never bothered to work out the possible oscillations / overlap / amplitude, but in my mind the brap has to be multiple combustion events close together separated by multiple "misfires" close together. I speculate we are also getting a few normal combustion events along with braps but since they are spaced out you don't notice them as much.

2 rotors fire 180* apart
3 rotors fire 120* apart
4 rotors realistically can fire 90* apart or 180* apart

 

Years ago I accidentally adjusted the fuel map on my stock-ported engine too high by at least 20-30%, it blew black smoke out of the tailpipe and it made a similar 'brap brap' stumbling sound at idle. I've always wondered if high-overlap rotary engines sound like that because they are tuned too rich at idle for some reason.

 

i can't explain it, but i can support your experience with a somewhat similar one. i built a streetport 12A many years ago, my first one, and it would "brap" when i used the choke, but idle normal when the choke went off.

 

The 'brap' that was so prevalent and got so popular on vintage and drag race cars is just as much a function of high overlap as well as either poorly tuned (for the port configuration) vacuum advance and/or fixed advance at 20+ degrees causing unstable idle and brappiness (this is a whole tuning conversation unto itself). Yes a high overlap port config needs to idle higher but the 'near stall' effect and brappiness basically boils down to poor tuning either due to old and un-adjustable tech or because someone is hotboi-ing their standalone tunes.

 

I think you are asking what the mechanism of the rotary overlap induced misfiring idle and what dictates the speed of the missfires.

So with the throttle plates closed a vacuum is created as the intake stroke expands in volume. This draws in air and fuel but also draws exhaust up from the exhaust stroke.

This exhaust dillutes the air/fuel so that it cannot ignite.

Misfire (no new exhaust produced), misfire (no new exhaust produced so intake dillution is reduced), misfire (no new exhaust produced and further reduced intake dillution) BRAP it finally fires and exhaust is created again so the cycle restarts.

Its typically 2-3 misfires per fire event on the brap.

 

Thank you everyone for replying. Good to learn things which have been in the back of my mind for years

Just a couple more questions.

When accelerating, would the engine still be misfiring between combustions, resulting in some loss of power?

Another question. My friend had a 6 port 12A RX-4 which wouldn't brap on idle but WOULD brap on deceleration, not as strong as a bridgeport but it was definitely there. Another friend with a stock port 12A wouldn't get brap on deceleration. Could someone please explain why?

 

when you open the throttle intake vacuum drops, and so there isn't as much exhaust gas getting into the intake stroke. misfiring goes away with this, and some rpm.
also as the rpms rise things are moving faster, and any intake or exhaust tuning starts to work, which helps too.

 

My lil 12a Braps like crazy, if I don't open the hoop most think it's a 13b. In the winter I can set her at low idle (1600rpm) but during hot days like summer time I have to set her at 1900rpm because of the AC.

If I set her at around 1800rpm the car starts rocking with the braps, looks like she dancing

this video was sent to me, it's my car at Rotary Show Case 3 in Florida

 

Mazda called this "unstable combustion" in their SAE papers.

What is interesting is that all rotaries do it to some degree. Even stock port 12As do it, under deceleration when the intake vacuum is high enough to cause it, which made the cars buck-buck-buck when slowing down in gear at low RPM. Mazda came up with the shutter valve as a way to compensate, under those conditions it would block off half of the engine from the carburetor and pump air in to that rotor from the air pump, which reduced vacuum and made am unburnable mixture in that rotor.

With EFI, Mazda would do selective deceleration fuel cut.

As you increase overlap, the threshold of unsealed combustion drops until it gets into idle and low load.