OMC's 4-Rotor Wankel Racing Engine - The Real Story

[Instigator]

Agreed.

I followed IMSA car road racing in the ‘80’s and Mazda ruled GTU which is Grand Touring Under 3.0 liters.

Many competitors complained that Mazda was given an unfair, +50% displacement capacity due to the unfair way a rotary was classified.

Love the motors and tech but please…..

How do you quantify cubic-inches/liters for a Rotary engine?

A) One rotor has 3 combustion surfaces/heads. Therefore, (per each rotor) 3 x combustion chamber area = cubic-inches/liters.

B) One rotor/combustion chamber. Therefore, each rotor is only designated as only having 1 combustion head. 1 x combustion chamber area = cubic-inches/liters.

I am wrong to think the rotor engine should be classified as example A. Giving the rotor engine & the conventional combustion piston engine equal footing in engine size comparison.

Example B, seems to me, to give the rotary engine an unfair advantage in racing classification against piston engines.

[Rotary John]

At the time of the rotary race engine, they were competing against 2 strokes. A 2 stroke engine has one power pulse per crankshaft revolution; so does a rotary. A 2 stroke completes all phases, intake, compression, power and exhaust for each revolution of the crankshaft; so does a rotary. Thus if compared to a 2 stroke the displace should be measured for 1 revolution of the crank. It's the 4 stroke that's the oddball. It takes 2 complete revolution of the crank to complete all 4 phases. I don't recall anyone ever saying a 2 stroke should be measured for 2 crank revolutions as a 4 stroke. A 4 stroke displacement has always been measured displaced volume per crank revolution, same as a 2 stroke and a rotary. The argument only comes up when the rotary dominates. Kinda like Andy Granitily's turbine. Kinda like when the big boys banned the 2 stroke in thunder boat racing. The establishment hates being beat. Enough for tonight, I'm going to the shower and bed.

[2us70]

I count 3 ignition events per crank revolution. One for each rotor face. Thus the total swept volume of all 3 faces equals displacement. Seems like basic trig to me?

[Rotary John]

I count 3 ignition events per crank revolution. One for each rotor face. Thus the total swept volume of all 3 faces equals displacement. Seems like basic trig to me?

You don't count too well. The rotor turns at 1/3 the speed of the crank. There is ONLY ONE spark plug fire for each rotation of the crank.
Wankel animation - YouTube

[techteam]

I count 3 ignition events per crank revolution. One for each rotor face. Thus the total swept volume of all 3 faces equals displacement. Seems like basic trig to me?

Remind me not to ask you to balance my cheque book.

[techteam]

Next thing John somebody will mention the usual.

Ha, looks like you beat me to the punch. I'm getting slow.

[Lake X Kid]

The rotor turns at 1/3 the speed of the crank. There is ONLY ONE spark plug fire for each rotation of the crank.
Wankel animation - YouTube

That clarifies a lot, thanks RJ.

[Rotary John]

The real difference in the rotary is each of the 4 cycles consumes approx 270 degrees of crank rotation. A 2 stroke by contrast consumes approx. 90 degrees for each cycle and a 4 stroke approx 180 degrees. ( forget about overlap and exact port timing). This is the reason the rotary has such high power density. Displace ment is measured max volume - min. volume x the number of cylinders or rotors. I you want equality give a fixed amount of fuel and run what you brung. Audi did that with their turbo diesels.

[Rotary John]

Next thing John somebody will mention the usual.

Ha, looks like you beat me to the punch. I'm getting slow.

James: I'm surprised Jackie hasn't jumped in and claim it should be 8 times because he raced a V-8 four stroke.

[Instigator]

Not that I matter in this equation but…, I still don’t get it John.

4 stroke to two, I do.

Rotary to everything else, I still don’t.

One crankshaft revolution what do we have correct?

[Rotary John]

Not that I matter in this equation but…, I still don’t get it John.

4 stroke to two, I do.

Rotary to everything else, I still don’t.

One crankshaft revolution what do we have correct?

One crankshaft revolution: One spark plug fire, one power stroke. one intake, one compression, one exhaust; just like a 2 stroke. The difference is these events happed simultaneously in a rotary, while they happen sequentially in a 2 stroke. You have to also remember the rotary is a 4 cycle. It has a positive exhaust cycle where a 2 stroke exhaust is dependent on blow down and the incoming fresh charge to force the spent charge out the exhaust port. Same is true for the rotary intake, compression and power cycles.

[powerabout]

I think were the punters get confused is which part is counted as a revolution?
See above for crank as explained by John.

[2us70]

So the crankshaft in overdriven 3 to 1 from the rotor? Does this have an effect on torque at the crankshaft output end? Also what are the revs seen by both the rotor and crank in operation? My only experience with a rotary was driving my brother-inlaw's Mazda RX-3 many years ago.

[Rotary John]

You have said it correctly but should read the rotor turns at 1/3 crank RPM. The torque on the crank is determined by the eccentric of the shaft and the combustion pressure on the rotor and is independent of the rotor RPM. However, because the shaft has to turn approx. 270 Degrees before the exhaust port opens, the force on the eccentric lasts longer than a piston engine. Hope this makes sense.
The OMC race engine ran between 7000/8000 RPM crankshaft meaning the rotor turned 1/3 of that; 2333/2667 RPM. Before someone asks again, the final version of the race engine produced between 265/280 prop shaft HP at those RPM's. (1974)

[Lake X Kid]

The rotor engine’s rotational components seem to be more conducive to generating crankshaft rotation, then the action of converting piston’s linear motion into crankshaft rotation. High RPMs would seem not to be a major obstacle (in spite of an eccentric non-circular rotor motion, and the rotor’s larger circumference turning the smaller diameter crankshaft should supply ample torque.

However, being the rotor engine has a more favorable geometry of internal moving parts, why does it not dominate the boat & car market? Is the rotor engine’s weak link(s) due to … cooling issues, seal problems, combustion efficiency – in comparison to piston engines, or …?