> 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."
     >
    Doug,
    I wasn't going to say anything anymore.
    However, regarding your concern for the new people, above is also not
    correct.

    Both engine run at the same rpm. It is the number one aspect to have
    equal speed.
    Second we have to count two chamber not two rotations of the Rotary to
    be equal to the RPE.
    I know multiplying it out results in the same, but this would be
    incorrect as the rpm number would be double. It is the displacement of
    one rotation that needs to be doubled to make things equals with the
    RPE. That is the same as counting two chamber per rotor.

    There is difference between saying we we have to run the rotary at twice
    the speed and saying we have to calculate its displacement at two shaft
    rotations.

    Your sentence could read:
      " 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."

    Rolf



I haven't heard anybody talk about why RPE engines measure displacement
to start with..

I think "displacement" is a way to predict how much air (volume and
mass) are going to be moved at a given operating condition (output
RPM).  And this is useful because the amount of air moved in a given
amount of time is a way to predict engine output.

Thus, it makes sense to look at how much air is moved through a wankel
at a given RPM.  From this come up with a method of comparing chamber
volume to piston engine displacement.  Anything else is mental gyrations
for no good purpose.


Matt-

Hello Matt,

Why measure displacement? you ask.
You answered your own question.
Although, you compare engines from their airflow as that is an
indication of hp.

The common way is to provide the engine displacement which provides a
good idea of size and hp without any calculations. From here we
calculate up.

In simple terms, I would say, displacement is an indication how big an
engine is.
Some Governments use it to assign a tax value to the engine or vehicle.

Using it to calculate air flow and hp:
Remember hp is only affected by three things: displacement, mean piston
pressure (mep) and speed, nothing else.

Increasing mep is only possible within narrow limits. Speed is also very
much limited by inertial loads. The only real and easy control one has
is with engine displacement. (‘No replacement for displacement’, as one
saying goes.)

That is why small model aircraft engine have about 20 cc displacement
and large ship diesels as much as 1.8 cubic meters or 1,800,000 cc per
cylinder and as much as 14 cylinders. It is all about hp. With
displacement we can get an idea of hp quickly. That should give an
answer to your question in the outset.
See: http://www.bath.ac.uk/~ccsshb/12cyl/
<http://www.bath.ac.uk/%7Eccsshb/12cyl/>

We distinguish between “displacement” (in cc, litre, cuin, cuft), and
“air flow” in vol/time or in mass/time.
Air flow in l/s = displacement in litre * rpm / 120. (4-stroke engine)
Hp is the product of displacement, mep, rpm and a factor. But we talked
about that already.
Not being interested in ‘mental gyrations’ I stop right here. Most has
been said anyway.

Rolf



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