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Date: Mon, 17 Oct 2005 16:38:01 -0600
From: Bob White <bob@bob-white.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Displacement - Again? Timing of the Work
Message-Id: <20051017163801.bf909bed.bob@bob-white.com>
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Excelent!  I can see that I need to clean up my terminology however.

Bob W.


On Mon, 17 Oct 2005 17:33:07 -0400
Doug Mueller <rotaryrx6@cox.net> wrote:

> Hi could I add 2 cents to the peanut gallery? this is from Fred Swain whi=
ch=20
> pretty much tells it all. It is long but good.
>=20
> "The rotary engine is a 6 stroke internal combustion engine. I know, peop=
le=20
> will probably start screaming at me for this so let=92s get into a little=
=20
> explanation as to why and how typical mathematical formulas for piston=20
> engines don't work.
>=20
> First of all, lets get the terms "stroke" and "cycle" defined (Some of yo=
u get=20
> your heads out of the gutter!) since everyone commonly gets these terms=20
> interchanged. They are not the same thing. Every internal combustion engi=
ne=20
> whether it is a 2 stroke, 4 stroke, diesel, gasoline, propane injected, e=
tc. is a=20
> 4 cycle engine. Why? All of these engines take in air (intake), compress =
the air=20
> (compression), ignite the air whether by spark plug or glow plug (ignitio=
n),=20
> and expel it out the tailpipe (exhaust). There you go 4 cycles. Simple is=
n't it.=20
> The term "stroke" in this context refers to how many times the crankshaft=
 or=20
> eccentric shaft makes a piston go up or down to complete the cycle.
>=20
> The connecting rods and pistons are just an extension of the offset lobes=
 of=20
> the crankshaft. This is also true in regards to a rotor and eccentric sha=
ft.=20
> When the lobe rotates upward, the piston goes up. When the lobe rotates=20
> down, the piston goes down. Every time it moves one way is considered a=20
> stroke. In a 2 stroke engine, all 4 phases or cycles of the combustion pr=
ocess=20
> are completed in only 2 strokes of the piston, 1 up and 1 down. This is o=
nly 1=20
> complete revolution of the crankshaft. In a 4 stroke engine, it takes 4 s=
trokes=20
> of the piston, up, down, up, down to go through the complete combustion=20
> process. This is 2 complete revolutions of the crankshaft. It's all a ver=
y simple=20
> mathematical relationship.
>=20
> Now lets go look at the workings of a rotary engine. If we look at a rota=
ry=20
> engine eccentric shaft and compare it to a piston engine crankshaft, we s=
ee=20
> essentially the same piece. Both have lobes and because of this both engi=
nes=20
> will have a stroke length, even the rotating rotary. It doesn't matter if=
 it is a=20
> piston going back and forth or a rotor going round and round. The cranksh=
aft=20
> motion remains the same. On a rotary engine, the rotors are spinning at=20
> exactly 1/3 the speed of the eccentric shaft. From the time that the air=
=20
> entering one chamber goes through the combustion phases to the time it=20
> leaves the engine from the same chamber (rotor face), the eccentric shaft=
 has=20
> gone around 3 complete times unlike a 4 strokes 2 times or a 2 strokes 1=
=20
> time. If we do the math we see that the lobes of the eccentric shaft must=
 have=20
> gone up and down 6 times (up, down, up, down, up, down). Since it does th=
is=20
> process the exact same way every time for every rotor face, it is a 6 str=
oke=20
> engine. That=92s right the rotary engine is a 6 stroke! Do not confuse th=
ese=20
> strokes with the 4 internal cycles that every engine has!
>=20
> Let's sum this up in a simple chart to visually explain how this works:
>=20
> 2 stroke engine (up, down) - 1 complete crankshaft revolution.
> 4 stroke engine (up, down, up, down) - 2 complete crankshaft revolutions.
> 6 stroke (rotary) engine (up, down, up, down, up, down) - 3 complete=20
> crankshaft (eccentric shaft) revolutions.
> See a pattern? All of these engines though are still 4 cycle engines! The=
y are=20
> different stroke engines though so the amount of work they do per time is=
=20
> very different. A 2 stroke engine does twice the work per amount of time =
that=20
> a 4 stroke does. Don't believe me? Go race 2-80cc motorcycles, 1-2 stroke=
=20
> and 1-4 stroke and see who wins! This must mean that the rotary engine=20
> does the least amount of work per time than both other engine types. Yes =
it=20
> does. But, unlike a piston engine, it uses 3 sides of it's piston (rotor)=
 at a=20
> time. In reality it makes no difference if we have 1 rotor with 3 usable =
faces or=20
> 6 rotors with 1 usable face each as in a piston engine.
>=20
> Here's a little info on how to properly figure out displacement on a rota=
ry=20
> engine. Everyone argues that it is really a 1.3 liter while others argue =
that it is=20
> really a 2.6 liter engine. They are both wrong! If we look at how a pisto=
n=20
> engines volume is calculated we arrive at a displacement based on total s=
wept=20
> volume of every piston added together. It is not based on rpm. On a rotar=
y,=20
> displacement is figured using one rotor face in one complete revolution t=
hen=20
> multiplied by 2. This only leaves the total for 2 combustion chambers tho=
ugh=20
> and the rotary has 6! Since the volume of a 13b rotary is rated at 1.3 li=
ters=20
> (only 2 combustion chambers) it really adds up to 3.9 liters!!! I can hea=
r it=20
> now, "...but we only have 2 rotors!" So what! Like I said it makes no dif=
ference=20
> if there are 2 rotors with 6 faces or 6 rotors with one face each. the to=
tal is=20
> always 6 and the base numbers are only based on 2 chambers. The rotary=20
> merely does 3 times the work in a package 1/3 the size. It's just a 3.9 l=
iter=20
> engine crammed into a 1.3 liter body. Just so none of you start a fight o=
ver=20
> this, I will explain this later so don't chastise me yet!!!
>=20
> In case anyone is curious I did some math to determine what the 13B rotar=
y=20
> would be sized at if it were a piston engine. The results are pretty neat=
. First=20
> of all the rotary would be a 3.9 liter, 6 cylinder engine. It would be a =
6 stroke.=20
> Each cylinder would be 6.54" across (damn big piston!) but the stroke len=
gth=20
> would only be 1.18" in length peak to peak. Not much there. Interesting i=
sn't=20
> it. Now just imagine a way to make all this work with only 2 intake runne=
rs!
>=20
> In all fairness to the terms I have used, the word "stroke" can be interc=
hanged=20
> with the word "cycle" since both technically have the same definition. Th=
e=20
> terms "periods", "quarters", or "phases" can also be used correctly. I me=
rely=20
> wrote it the way I did to get a certain mental picture going.
>=20
> I have already dealt with why the rotary engine is really a 6 stroke engi=
ne and=20
> why displacement is really 3.9 liters and not 1.3 liters. Now I need to e=
xplain=20
> why the rotary engine doesn't have the torque or horsepower of a good 3.9=
=20
> liter engine or why it doesn't get the gas mileage of a 1.3 liter engine.=
 The=20
> world has always wondered so here's why.
>=20
> Remember that I stated that the true displacement of the rotary engine, i=
f=20
> figured out according to the way piston engine volumes are calculated, is=
=20
> according to the total number of rotor faces and not the number of rotors=
,=20
> nor does it have anything to do with rpm. This added up to 3.9 liters for=
 a 2=20
> rotor 13B engine and not the published spec of 1.3 liters. They just cram=
med=20
> all 3.9 liters into a 1.3 liter body. If the engine is really a 3.9 liter=
 engine then=20
> why doesn't it have the low end torque of a 3.9 liter engine? This has a =
very=20
> simple answer. Lack of leverage. OK, what the hell does that mean?
>=20
> First of all we must figure out what a lever is. It is a device that mult=
iplies=20
> mechanical advantage over an object to do the same amount of work with a=
=20
> smaller amout of effort. Another way to look at it is to do a greater amo=
unt of=20
> work with the same amount of effort. It's the same thing. Let's look at=20
> leverage differences as an example in a piston engine.
>=20
> What happens to a piston engine when we make it a "stroker"? Ignoring a h=
ost=20
> of other variables, it gains torque. It also gains horsepower but they ar=
e both=20
> a fixed mathematical ratio between each other and you can't increase or=20
> decrease one without the other. Why did it gain torque? Greater mechanica=
l=20
> advantage or leverage over the crankshaft. The reason being is that on a=
=20
> =93stroker=94 crankshaft as opposed to the stock crankshaft, the lobe cen=
terline is=20
> farther out from the rotational centerline of the crankshaft. This increa=
ses the=20
> leverage that the piston has over the crankshaft. Don't believe me? Try t=
his.=20
> Get a short pole and hold it at the end straight out away from your body.=
=20
> Attach a 10 lb weight to it exactly 1 foot away from your hands. The weig=
ht is=20
> exerting exactly 10 ft. lbs. of torque on your hands. Now move that weigh=
t=20
> out away from you to 2 feet away from your hands. Now the same weight is=
=20
> exerting 20 ft. lbs. of torque on your hands. You have just in essence ma=
de a=20
> "stroker". Now let's get back to the engine.
>=20
> Now we know that the greater the stroke length, the greater the engine=20
> torque. As I stated, the rotary engine only has an effective stroke lengt=
h of=20
> 1.18". My weed eater has that! There is not very much mechanical advantag=
e=20
> over the eccentric shaft. This still doesn't explain everything though.
>=20
> Remember, I stated that if the 13B rotary were a piston engine it would h=
ave=20
> pistons 6.54" across. Now we just discovered another enemy of efficiency,=
=20
> flame front speed. When the spark plug ignites the mixture in the engine,=
 it=20
> doesn't just ignite everything all at once. The spark ignites at the plug=
 and=20
> then has to travel outward away from the plug at a certain rate of speed.=
=20
> While this only takes milliseconds, this amount of time gets more critica=
l the=20
> higher the rpm gets due to the shorter amount of available time. The resu=
lt is=20
> that as rpm's rise efficiency decreases. The larger the area of the pisto=
n, the=20
> farther the flame front has to travel and the greater the chance that all=
 of the=20
> mixture does not get ignited when it should. Just can't go far enough fas=
t=20
> enough. Today=92s rotaries have 2 sparkplugs per chamber to help combat t=
his=20
> problem. Varying their ignition time in relation to each other even helps=
=20
> somewhat with power and emission. That's right they don't necessarily fir=
e=20
> together even though they are in the same chamber. This can get complex s=
o=20
> I will not deal with it at this time. Some race engines even have 3 plugs=
 per=20
> chamber to improve efficiency and ignition wave front speed. On piston=20
> engines, Mercedes has capitalized on this and uses 2 plugs per cylinder i=
n=20
> some of their higher end cars. Do they know something others don't?
>=20
> There is also one more aspect that affects it. Remember that the rotary i=
s a 6=20
> stroke engine. A 2 stroke engine does twice the amount of work per amount=
=20
> of time that a 4 stroke engine does. A 4 stroke engine does 50% more work=
=20
> per amount of time that a 6 stroke does. The rotary engine does less work=
=20
> per eccentric shaft rotation than your typical 4 stroke counterpart. All =
of=20
> these characteristics combine to make an engine that has relatively littl=
e low=20
> end power and needs to be revved up to be truly powerful.
>=20
> I make it sound like we should have less torque than a 1.3 liter engine d=
ue to=20
> the above reasons. This isn't true though. Remember that we still have a =
3.9=20
> liter engine even though it only uses 2 lobes on the eccentric shaft. We=
=20
> should not expect to develop the torque numbers of a 1.3 liter engine. It=
=20
> should settle in somewhere around 50% less than a 3.9 liter engine which=
=20
> would put it around equal to a 2.6 liter engine in power.
>=20
> These traits of the rotary engine are also why the engine gets worse gas=
=20
> mileage than your typical 1.3 liter engine. Hell it gets worse gas mileag=
e than=20
> your typical 2.6 liter engine. Another aspect that affects this is port t=
iming=20
> and duration. If we had a piston engine of 2.6 liters in size that had th=
e same=20
> intake and exhaust timing as the rotary then it would get comparable gas=
=20
> mileage to the rotary. The 12A/13B rotary though have much more exhaust=20
> duration than intake duration due to the peripheral exhaust port location=
.=20
> This contributes to several factors which decrease efficiency. Exhaust ga=
s=20
> dilution is one of them. For each stroke there is a small amount of overl=
ap.=20
> The exhaust ports and intake ports are open to the same chamber at the=20
> same time for a short amount of time as measured in degrees of eccentric=
=20
> shaft rotation. The higher the rpm's the less important this becomes sinc=
e air=20
> velocity will generally keep the gasses where we want them to go. At lowe=
r=20
> rpm's though, the intake and exhaust air velocity is not very high. This =
will=20
> cause some exhaust to go back through the combustion chamber again.=20
> When this happens volumetric efficiency decreases and there is less room =
for=20
> fresh air to fit inside the combustion space. Also this re-circulated exh=
aust=20
> gas is very hot. A hotter air molecule is larger than a cold one which me=
ans a=20
> fewer number of molecules can fit in the same area per amount of pressure=
=20
> exerted on them. Another aspect of the rotary's peripheral exhaust port=20
> configuration that contributes to less low end power and greater fuel=20
> consumption is its incredibly long duration or time it is open for.=20
> Unfortunately when we make the port bigger we also change it's timing. We=
=20
> don't have the luxury of being able to mill out a head to accept a larger=
 valve=20
> while still being able to use the same cam. The timing is really only opt=
imized=20
> for high rpm use. We are leaving it open for too long which gets back to =
the=20
> whole overlap problem. Again, all of this is just a generalization and ca=
n be=20
> affected by how well the intake and exhaust flow and how well they can=20
> scavenge. The affects of scavenging, intake design, Helmholtz effect, and=
=20
> proper exhaust design are all out of the scope of this article. So just a=
ssume=20
> it is an even world.
>=20
> Luckily there is a cure for this. It is called Renesis! It is the new 13B=
 based=20
> rotary engine in the new Mazda RX-8. The exhaust ports are no longer in t=
he=20
> periphery of the chamber but have rather been moved to the side housings.=
=20
> This allowed the designers to more appropriately optimize the port timing=
=20
> duration. The location also allows more port area leaving the engine. So =
now=20
> we have more area to flow air out of faster. This new location also compl=
etely=20
> got rid of the port overlap. There is actually 64 degrees of dwell. This =
amount=20
> of dwell was originally greater in the early test engine called the MSP-R=
E since=20
> it had the intake timing of the '84-'91 n/a RX-7's 6 port engine. However=
=20
> dwell is only useful if you just have enough to get the job done but not =
so=20
> much that you are getting losses from it. Because of this Mazda engineers=
=20
> learned that they could open the intake earlier than previously and still=
=20
> maintain all of the other good aspects of the new exhaust layout.
>=20
> A bigger intake port =3D more time for air to enter and a greater CFM rat=
ing=20
> through the port.
> Less turbulence through the port as well.
> Less overlap gives us less dilution of the intake air and a cooler intake=
=20
> charge.
> More available room for incoming air.
> Volumetric efficiency increases.
> Since efficiency goes up, our use of gas gets more efficient. In other wo=
rds it=20
> takes less fuel to do the same amount of work.
> What=92s the result? Better gas mileage. With today=92s gas prices this i=
s a very=20
> welcome thing. The efficiency increase also means that emissions=20
> characteristics are also improved -another bonus with today=92s laws=20
> concerning air quality.
>=20
> So after reading this you are probably wondering why in the world anyone=
=20
> would want to use one of these engines. First and most obvious is size. T=
hey=20
> crammed a 3.9 liter engine, or more appropriately a 2.6 usable liter engi=
ne=20
> into a 1.3 liter body. Second, it is just such a simple design. There are=
 only 3=20
> moving parts. Fewer moving parts have less frictional losses. Also fewer=
=20
> moving parts have less chance statistically of failure. The more it moves=
 the=20
> more chances you have for failure. Third, nothing moves back and forth. S=
o=20
> what? A piston stopping and changing direction exerts a lot of stress on=
=20
> everything from the crankshaft to the connecting rods, to the pistons, to=
 the=20
> wristpins, etc. Let=92s not also forget the stresses on the valves for be=
ing=20
> slammed open and shut as well as the temperature extremes they see during=
=20
> the combustion cycle. A body in motion tends to stay in motion. It is a v=
ery=20
> unnatural act to change direction suddenly or at all for that matter. A r=
otary=20
> just spins away in the same direction. Yes the lobes of the eccentric sha=
ft do=20
> see stress but remember that we don't have very much leverage over them.=
=20
> The rotors are also exerting some of their rotational stress on the stati=
onary=20
> gears as well so some stress is never transmitted to the eccentric shaft =
from=20
> the rotors. The lack of stroke length and pure rotational motional do mak=
e it=20
> very naturally adapted to high rpm use. If we look at really high horsepo=
wer=20
> piston race engines, their stroke length has been shortened to reduce the=
=20
> stresses to all of the engine components at high rpms. The last and most=
=20
> important reason why the rotary engine is still a popular engine despite =
its=20
> shortcomings is because it is different. There is always something to be =
said=20
> for individuality and uniqueness. If you own a piston engine it doesn't m=
atter=20
> how big it is or if it is made by Chevrolet or Honda. It is still the sam=
e device.
>=20
> Just to shoot down right now any arguments on displacement think about=20
> this:
>=20
> The 13B rotary engine is a 1.3 liter. Yes.
> The 13B rotary engine is a 2.6 liter. Yes.
> The 13B rotary engine is a 3.9 liter. Yes.
> Notice that all of these statements are TRUE!!! That's right there is a t=
ruth to=20
> all of those statements. Go read the whole thing again. To understand why=
=20
> this is so, lets define truth. Truth can be defined in a couple of ways:=
=20
> Anything that is not false (none of those statements is) or it can be def=
ined=20
> as: One's individual interpretation of presented facts. This herein is th=
e=20
> source of our debate. We can't change the facts no matter how hard we try=
.=20
> Arguing won't do it. What is debatable however, is each individual's=20
> interpretation of facts. If your interpretation doesn't match someone els=
e's,=20
> you argue about it.
>=20
> Here are the facts: The rotary engine as rated by Mazda is 1.3 liters bec=
ause=20
> each individual rotor, following one face of one rotor through the comple=
te=20
> cycle, has a swept displacement of 654cc or .65 liters. Multiply this tim=
es 2=20
> rotors to achieve 1.3. Since this only accounts for 2 of the total of 6 r=
otor=20
> faces, we multiply our answer by 3 to get an actual displacement of 3.9 l=
iters.=20
> However since the rotary engine is a 6 stroke engine and not a 4 stroke=20
> engine since it takes 3 complete eccentric shaft revolutions to fire all =
faces=20
> instead of the typical engine's 2, it only does 66% the work of a 4 strok=
e 3.9=20
> liter engine. Calculating for this we divide 3.9 by 1.5 to get a total of=
 2.6=20
> liters equivalent work to a 4 stroke piston engine. All of these, from a =
1.3 liter=20
> in physical size package.
>=20
> No one can argue that this is not correct and any response saying otherwi=
se=20
> will have been explained by what I just said. Any debate will only focus =
on=20
> one aspect and not the total facts.
>=20
> Just to put a cap on this whole thing: If at any time you try to calculat=
e proper=20
> sizing for a turbo, intake manifold runners, intake plenum size, exhaust =
size,=20
> etc, and you try to use the 1.3 liter number in your equations, you will =
be=20
> way, way, way off!!!!!!!!! There are only 2 ways to flow more air: increa=
se=20
> displacement or increase rpm. A 1.6 liter Honda engine doesn't flow=20
> anywhere even remotely near what a 13B (1.3 liter) flows per the same rpm=
.=20
> Just some food for thought."
> Doug Mueller
> RX-6 13BT
> N900DM
> Boulder City, NV
> >=20
> > From: Ernest Christley <echristley@nc.rr.com>
> > Date: 2005/10/17 Mon PM 03:52:49 EDT
> > To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
> > Subject: [FlyRotary] Re: Displacement - Again? Timing of the Work
> >=20
> > Bob White wrote:
> >=20
> > >Let me re-emphasize this:  Every detail of Ed's analysis looks exactly
> > >correct to me.  The Mazda 13B produces power and breathes about the=20
> same
> > >way a 4 cylinder 2.6L 4 cycle piston engine does, or about the same as=
 a
> > >2 cylinder 1.3L 2 cycle piston engine.
> > >
> > > =20
> > >
> > Yeah, but what if the eShaft had an integrated reduction drive that=20
> > dropped the ouput to 1/3, so that the eShaft output and the rotors had=
=20
> > the same speed.  Would it then breathe like a 3.9L 2 cycle,  or a 5.2L =
4=20
> > cycle?
> > 8*)
> >=20
> > >I also think it sound better to think of the rotary as a 3.9L engine
> > >turning 3000 rpm (rotor speed) rather than a 1.3L engine turning 9000
> > >rpm (output shaft speed).  It's too bad we can't easily couple the
> > >propeller directly to the rotors and eliminate the PSRU.  Now that
> > >would be a setup.
> > >
> > >Bob W.
> > > =20
> > >
> >=20
> > We could do that; especially easy on a single rotor.  Press in a=20
> > propeller adapter in place of the rotor bearing.  Then the wobble of th=
e=20
> > propeller would almost be enough to make you think your were flying=20
> > behind a Lycoming again!  The certified crowd would feel right at home!!
> >=20
> > (The peanut gallery hath spoken 8*)
> >=20
> > --=20
> >          ,|"|"|,                                    |
> > ----=3D=3D=3D<{{(oQo)}}>=3D=3D=3D----        Dyke Delta         |
> >         o|  d  |o          www.ernest.isa-geek.org  |
> >=20
> > --
> > Homepage:  http://www.flyrotary.com/
> > Archive and UnSub:   http://mail.lancaironline.net/lists/flyrotary/
> >=20
>=20
> Doug Mueller
> RX-6 13BT
> N900DM
> Boulder City(61B),Nevada
>=20
>=20
> --
> Homepage:  http://www.flyrotary.com/
> Archive and UnSub:   http://mail.lancaironline.net/lists/flyrotary/
>=20
>=20


--=20
http://www.bob-white.com
N93BD - Rotary Powered BD-4 (real soon)
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