At the beginning ?  there was the reciprocating engine.
2-strokes and 4-strokes.

Nowadays, 2-strokes are hardly used anymore.
Any reputable engine is a four-stroke. This is our standard, our base
engine we compare another engine with.

Then, the Mazda Rotary engine arrived on the scene. What is this? How
does it work? How much power per its displacement? Well, what is its
displacement?

All these questions come up, naturally.

So what do we do? We simply compare it with the known 4-stroke RPE. Not
least, as the Rotary also works on the 4-stroke principle.

A [SINGLE CYLINDER] reciprocating piston four-stroke requires "two"
crankshaft
revolutions to complete its four-cycle operation. Accordingly, its
displacement is
that achieved at two shaft rotations. [I don't think so Rolf. Check Taylor]

In comparing power and displacement, we must obviously use the same rpm
for all to be equal and one standard for all engines regardless if RPE
or Rotary.

The Mazda RX7/8 displaces 650 cc per rotor and per rotation. At two
rotors and two rotations the displacement is 2.6 litres.

Up to just recently we were all in agreement with that.
Then, Gordon arrived at the scene?.

Gordon makes no mistakes with his calculation; however he does with his
designations.

Gordon uses now two terms: A) the "equivalent displacement" of 2.6 litre
and B) the "real displacement" of 1.3 litre. For the term 'real' he uses
at times 'actual' or 'effective'.

We all know the Rotary is equivalent to a 2.6 litre RPE. By doing so we
are able to use the same formulae, the same rpm, the same everything. No
problem.

Saying the Rotary is a 1.3 litre does confuse things. The formulae must
be changed; both engines can no longer be compared on the same basis.
All is just 'wrong'.

Why coin a new term for the Rotary such as "real displacement" when it
is not usable with the same existing formulae? Doing this we are unable
to compare it with the RPE, which is what we wanted to do at the first
place. We want to know how it 'compares' to the RPE we are all familiar
with. That is what it is all about.

Coining a new term for the Rotary and calling it by only half its
displacement is simply deceit. Mazda used it for tax reasons. As I
understand it, lately they are not using it anymore, but just calling it
the Rotary.

To make the four-stroke Rotary 'equal' and 'comparable' to a four-stroke
RPE, we must call it a 2.6 litre engine. Otherwise we are cheating. We
are trying to say it can make twice as much power per displacement as an
RPE, which it cannot.

Regards
Rolf Pfeiffer

I agree with almost everything except were noted above in brackets.

Paul Lamar

Rolf,

I agree with everything you said except as Paul noted but as I am 100% sure
you know how these engine work so I think this was the accuracy of words and
not your intent.

"Accordingly, its displacement is that achieved at two shaft rotations."

It's displacement is the volume of air that cylinder displaces. In the RPE
that is roughly the volume within the cylinder from the bottom of the piston
stroke to the top of the stroke.

The rotary engine  which is closer in analogy to a rotary vane pump, has to
use its three  varying chambers in a different manner to physically rotate
the gases  along to displace  a stated quantity of exhaust at each rpm of
the e-shaft.  That volume per rotor housing is 650cc. Multiply times the
quantity of rotors and that is how the 13B came to be defined as 1300cc.
That is the amount it actually displaces on one rpm. Sorry but the RPE
engine makers and racing associations of the world don't have a monopoly on
the definition of the English word 'displacement'.  That is the ridiculous
point. This discussion is mostly about semantics and it is easy to change
the understanding with the wrong word.

HOWEVER: you, Paul and racing associations are correct in that when we want
to fairly  compare the RE to a RPE, then we have to measure the notary's
displacement at twice the number of e-shaft revolutions. It is then RATED at
a 2600cc displacement. Racing authorities will rightly define 'how to
measure' displacement in their rules and say that we should measure all
engines at twice the rpm of the RPE engines. The RPE is their base line data
source as you say.  Fine no problem, This is a correct and logical statement
because of way the RPE displaces air. Gordon is also correct and that is why
he used the word  'equivalent' displacement as in the word's  Latin root
prefix 'is equal to'.

Gordon is a wordsmith and is rightly trying to establish the proper phrases
to accurately describe the situation. No deceit here. And I believe unless
we are accurate and nit picking and have a common rotary language then the
acceptance and understanding of the rotary into the aviation world will be
tough and this discussion will surface again.

Help me out here Paul. This is a point that could assist newcomers.  Perhaps
a rough explanation proof of this 'equivalent displacement' would be a
statement akin to:

" and so if at similar comparative VE the Wankel rotary engine will have
to run at about twice the rpm compared to an RPE to pump (displace) the same
amount of air."

Now as to the calculation sheet. Comparing it to a piston engine was not the
original intent but because of the enormous amount of performance data
available with the RPE it would prove valuable as a proof of the accuracy of
Gordon's calculations.  He has done an enormous about of work on this chart
to date and perhaps with some way to input the thermal efficiencies of the
two types of engines it would get even better.


Doug Fir

The rotary does not work like a vane pump at all. A vane pump does indeed
finish its cycle in one rotation of the output shaft. In fact some finish
two or more cycles in one rotation of the output shaft. There is no analogy
here.

Here is a graphic I did for the web site a long time ago. It has been on the
web site
for years.  Note in one rotation of the e-shaft the rotor rotates only 60 degrees.

******[My mistake. Not 60 degrees. 120 degrees as shown correctly in the graphic.]

A single 650 cc chamber cannot do didily squat on its own in one rotation of
the e-shaft.
About all chamber A can do is get partially through the expansion cycle
starting
with the intake cycle. This diagram is consistent with Ansdale and Bill Marvin
diagrams.

The bottom line is how much air can a given "displacement" engine pump in one
revolution?
The answer is in this chart. At 6000 RPM a 2.6 L (159 ci) engine can pump 215
cubic feet
of air in one minute at 80% VE. At 100% VE it can pump 258 cubic feet.
There is 1728 cubic inches in one cubic foot. So 1728 times 258 cubic feet is
445,824 cubic inches in 6000 RPM. So in one revolution the engine can
pump 74.3 cubic inches of air which is about half of the "displacement".

If you consider the rotary a 1300 cc (79.5 ci) engine it can only pump
39.75 cubic inches of air in one revolution. At 6000 RPM that is 239,500
cubic inches per minute or 138 cubic feet per minute. Air density at
sea level is .077 pounds per cubic foot so 138 cubic feet of air weighs
10.626 pounds of air a minute. That is 638 pounds per hour. At an air fuel
mixture
of 12:1 that is 53 pounds of fuel an hour. At a BSFC of .5 that is 106 HP
which is about half of what a "1300" rotary can really generate.

No way in heck is it going to generate 200 HP on 638 pounds of air per hour
at 6000 RPM.

The 13B came to be defined as 1300cc because Mazda incorrectly defined it as
1300 cc.
Mazda made a lot of decisions and some of them were incorrect. There was and
is a world
wide standard based on four cycle piston engines technology for the term
displacement
as applied to engines. We are not talking about a quart of milk here.

I suspect Gordon will agree with me on this analysis.

Check my arithmetic.

Paul, Matt and Rolf.

Thanks for your efforts. I think we are all on the same page here and I
understand how the Wankel rotary works. I don't understand the reference to
religion but I'll let that one pass.  Matt and Rolf both summed it up simply
and elegantly and now we have some usable definition to frame:)


" and so if at similar comparative VE the Wankel rotary engine will have
to run at the same rpm compared to an RPE and we have to count two
chambers per rotor to pump (displace) the same
amount of air."

Whew!!

Doug Fir



The Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site. http://www.rotaryeng.net
Copyright 1998-2006 All world wide rights reserved.