explain rotary motors to me

[rattlsnak]

I dont know much about rotary engines. I have a friend who cant get his engine running. He bought a used motor, rebuilt it, and cant get it to run and asked for my help. I understand how the engine cycles work but i think he is having a timing issue maybe. He had 60 PSI of compression and then took it back apart and went through it again, and now has 80psi. ( i know right?) But anyway, he says the rotors can only go onto the eccentric shaft one way. ( then why is his compression different now?) I have looked at a few websites and cant figure what turns what in a rotary engine.
TurboRX7.com > Rotary Engine Block pictures shows some good pics, but not all i need to see.

So i guess my question is, does this motor need to be 'timed' in any way? Do the rotors simply fit over the shaft in any position? What turns them? etc.. lol..

[Quicksilver]

Ahhhh, flopping lobe motors...
Yes the rotors will only go in in the correct position. If it assembled (with all the seals) then that part is ok.

This link has a decent animation part way down to help you visualize the way things go together:
http://www.rotaryaviation.com/

A rotary is a 2 stroke motor. It has no valves so that can't effect compression but the seals on the rotors don't seal real well when initially assembled. I can easily believe that you could get that kind of compression increase just by managing to get a better seal during assembly

To get an idea of how the seals "need some help" in a rotary: We would generally see the dyno numbers on a 13B increase during break in with it reaching its peak after an average of 13 hours of run time.


Basically you are up against the basics of combustion: Compression, fuel, and fire. It seems that either the issue is spark/timing or fuel delivery. That is what to chase down.

[Canada Kev]

All I know is the piston engine goes boing, boing, boing, but the Mazda goes mmmmm...

Mazda RX-3 Commercial - YouTube

[rattlsnak]

too funny, as this is an aviation engine! But i still dont see how the rotor connects to the eccentric shaft..

[winders]

Originally Posted by Quicksilver

A rotary is a 2 stroke motor.

Not true. The Wankel used in the RX-7 is a four-stroke engine.

Scott

[Flieger]

Originally Posted by rattlsnak

too funny, as this is an aviation engine! But i still dont see how the rotor connects to the eccentric shaft..

Gear teeth. Sort of like a planetary gearbox.

[Flieger]

Originally Posted by winders

Not true. The Wankel used in the RX-7 is a four-stroke engine.

Scott

Really? I had never heard of a 4 stroke rotary. The RX-8 is a two stroke from all I have heard, so why go from 4 to 2 with the ever tightening emissions restrictions. How does that differ from all the other 2 stroke ones?

[rattlsnak]

Everything ive read/seen videos on so far, says they are all 4 strokes.. They have intake, compression, power and exhaust. And the rotor attaches to a fixed gear on an intermediate plate, whereas is just slips over the eccentric shaft. I was wondering if there was a way to put the rotor on the e-shaft 'out of time' so to speak. Like slipping the rotor on where one of the apex is pointing at 12oclock as compared to 2 oclock. Does that make sense?

or that it doesnt matter how it goes on initially, as it will time itself as the e-shaft spins??

[TimT]

I have heard they are two stokes.... so who knows what is correct..

I do know they need an ignition set-up with "split-timing" and if you don't get the timing split correct the engine will not run, or will run and self destruct in short order..

We have a RX-7 in the shop that the owner abandoned, that we saw over 500hp on a two rotor turbocharged rotary.... The owner wanted Haltech so we worked with Oz snd got the timing split corrected and engine running very strong.... then the owner disappeared.....

[RSTarga]

If you stood next to one in the pits you would know they are 2 strokes.

[fanaudical]

Wankel (rotary) engines are indeed "4-stroke" combustion engines. They are commonly misunderstood as "2-strokes" as they appear to compress/ignite without a valve train and are often confused with 2-stroke reciprocating motors.

Wikipedia does a rather good job of describing how these work:

Wankel engine - Wikipedia, the free encyclopedia

[TimT]

If you stood next to one in the pits you would know they are 2 strokes.

+ ₶

At an IMSA race in the early eighties... Mazda had a few cars at Lime Rock... they were quite possibly the loudest racecars I have ever heard..

[hcoles]

Originally Posted by Flieger

Really? I had never heard of a 4 stroke rotary. The RX-8 is a two stroke from all I have heard, so why go from 4 to 2 with the ever tightening emissions restrictions. How does that differ from all the other 2 stroke ones?

sorry.. Scott is correct they are 4 stroke motors, as a general rule I don't think they win on efficiency, the cr is somewhat limited by the geometry but this was some years ago, some of the issues probably have been resolved. I like them myself, the power per pound is very good. Years ago a guy had one in a Super 7 and all but dominated the So. Cal. AX circuit. I think his name was Jim. Somewhere the pros/cons are likely listed, I think it would be a good motor for a e.g. 356. There are a few companies that specialize in hot rodding them.

[TimT]

OK curious George got me... I Googled Wankel and the Wiki explanation is that it is a four cycle engine...

[Flieger]

Well, all engines must have a way to have intake, compression, power, and exhaust.

A Wankel (read the Wiki before the link was posted) has one power stroke per combustion chamber per revolution, twice as many as a 4 stroke piston engine, so that sounds decidedly 2 stroke to me.

I think the differences in design between the engines make comparing them hard, but with a rotary being worth the power of a piston engine twice as big, together with the lack of valves I would consider it a 2 stroke. Especially with their appetite for oil and fuel.

[TimT]

In any event, whether it is a two, four or miller cycle engine...

The rotary requires special ignition timing...

Is the engine in question being run with the stock ECU? or aftermarket?

[rattlsnak]

Originally Posted by Flieger

I think the differences in design between the engines make comparing them hard, .

especially since they really dont have a 'stroke' at all.

[Nate2046]

I found this to be the best mod to keep my RX7 running

[Quicksilver]

Originally Posted by winders

Not true. The Wankel used in the RX-7 is a four-stroke engine.

Scott

Nope. 2 stroke. Every time the a rotor face passes the spark plugs the spark plug fires. There are no valves to supply a separate exhaust and compression stroke. How could you get an exhaust stroke when there is no intake valve to keep from lighting of the intake port?

If you look at the wikipedia page and its diagrams that show the "4 strokes" you will notice that the rotor completes all "4 strokes" on a single rotation. The crank rotates 3 times for every rotation of the rotor but that is a pretty flimsy excuse for a "stroke".

The wikipedia article does not say that the rotary is a 4 stroke. To quote the wikipedia page: "In the Wankel engine, the four strokes of a typical Otto cycle occur in the space between a three-sided symmetric rotor and the inside of a housing." This statement is factually incorrect. The "Otto Cycle" is a very specific term that refers to reciprocating piston engines.
The correct phrasing of the wikipedia quote should be: In the Wankel engine, the functional equivalent of the four strokes of a typical Otto cycle occur in the space between a three-sided symmetric rotor and the inside of a housing.

Everything about a rotary will tell you it is 2 stroke. No valves and all intake and exhaust timing is done by modifying the ports. Every time the combustion chamber comes by it is ignited. Every time a face of the rotor comes by the intake charge comes in. Every time the rotor face passes the exhaust port the burnt charge is expelled.. You have to run a straight through muffler on it. (reverse flow mufflers will balloon or explode!) Loud enough that your ears will ring even though you have ear plugs in and a 3-1/2 foot glass pack on it. Their emissions stink unless you cook them in a converter.

[winders]

Quicksilver,

No, you are incorrect. The RX-7 Wankel is a four cycle engine. No doubt about it. Just because it does all four cycles in one revolution does not make it a two cycle engine.

Scott