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