Fëakhelek

I had a thought and I hope that one of the forum's Rotary Gurus can tell me if I am on the right track. I believe that in the now obsolete piston engine the length of the piston rod affects torque since it acts as a lever on the crank shaft. This is just a theory, but I think the reason that a Rotary has fairly low torque is that the lobes(?) on the eccentric shaft take the place of rods in producing leverage and they are much shorter than a piston rod.

My question is this: Would it be possible to increase torque in a Renesis (or any Rotary) by putting bigger lobes on the eccentric shaft? To clarify, I am talking about a change to engine design, no an aftermarket mod.

Hopefully one of the big-brains can throw me a clue.

wakeech

certainly, that would work.
making the rotor (and associated housings, shaft, seals) larger in the radial direction, and narrowing the width of the rotor to maintain the same 1308cc displacement of the engine would yeild you a greater leverage on the eccentric shaft, and significantly longer cycles (most notably the combustion one). the other thing about this would be that your ports would be dispropotionately enormous to the volume you're trying to fill through the inspiration cycle, if you kept them at scale (which obviously you wouldn't for greater volumetric efficiency).

the tradeoffs would be a significantly larger (in height and length, whilst reducing in depth), and (probably) heavier motor for the same displacement, a lower redline from the greater rotatioal inertia in the rotors, and probably even worse thermal efficiency as the combustion chamber is made even wider in area and shallower in depth. ah yes, trying to get a decent compression ratio without having to worry about the rotor smacking the perhipheral housing on the ignition side (which really means either side), AND having a decent combustion chamber around the spark plugs (not having to fill too much of the "bathtub") would be another problem.

do not forget though: torque has not to do with leverage alone.
torque is the rotational force created by applying a force to a lever. the lever in this case is the rotor, and the force is the positive pressure from the combustion of complex hydro-carbons (gasoline) and oxygen rich gases (air). the force comes from the amount of pressure acting on the rotor, times the surface area of the rotor being acted upon. thus, if we increase the surface area of the rotor itself (wider, in this example, 'cause its simple), thus increasing the displacement of the engine too, allowing more surface area to be acted upon by (assumingly) the same amount of pressure (proportionately equal increase in area and air+gas to combust), we've just increased the amount of force acting upon the lever, and thus torque.

(hey, all you real rotary gurus, does this make me a guru too now?? )

cueball

I do not consider myself a guru, but that probably qualifies you for it.

MWG

This is a quote wakeech "do not forget though: torque has not to do with leverage alone.
torque is the rotational force created by applying a force to a lever. the lever in this case is the rotor, and the force is the positive pressure from the combustion of complex hydro-carbons (gasoline) and oxygen rich gases (air). the force comes from the amount of pressure acting on the rotor, times the surface area of the rotor being acted upon. thus, if we increase the surface area of the rotor itself (wider, in this example, 'cause its simple), thus increasing the displacement of the engine too, allowing more surface area to be acted upon by (assumingly) the same amount of pressure (proportionately equal increase in area and air+gas to combust), we've just increased the amount of force acting upon the lever, and thus torque.

(hey, all you real rotary gurus, does this make me a guru too now?? "


I not sure if that would be true because the amount of force would only depend on the amount of gas and air combusting and acting on the other molcules to raise and create more pressure from there action of bumping in to each other. thefore increasing torque by a increased amount of force acting upon the lever. I dont think it matters if the lever is two inches thick or four inches thick. it would depend on the amount of force applied. I think the reason you would see some increase in torque from making the rotor wider or adding another rotor is because you have a ratio of dispacement to vacum. The more size you get more vacum you get. therefore more power. now the more rotors you have the more controlled the force can be applied. That why V8 are usally more powerful than V6 they spead out the force. That is why the renesis has finer mist injectors more molcules to act with. and better emssions.

Fëakhelek

I think the crucial assumption Wakeech makes is that you are increasing the area of the rotor being acted upon and maintaining the same pressure. Remember that pressure is force over area (distance squared - wish I could do a superscripted 2). Of course if you simply increase area then pressure decreases (since force is spread over a greater area) and you gain nothing (or lose due to increased mass being moved). The assumption is that if you increase the area of the rotor surface you are also increasing the volume of the combustion chamber. I think as long as the compression ratio stays the same then Wakeech is right.

Jreyenga

no.

The only fundamental way to increase torque is more displacement or higher compression. There are dynamic things like less friction, better fuel burning/mixing, better volumetric effeciency, and what not, but the torque arm length will only change the torque for other reasons.

The reason long stroke motors have been known as torqy motors is because they generally have a lower rod length to stroke ratio(so that they can fit that long stroke in the motor), which results in more acceleration of the piston at top dead center(than a motor with the same stroke but a long connecting rod). The higher acceleration, and thus velocity of the piston near top dead center creates more "suck" at the low lift levels of the intake cycle. This generally means you get lots of turbulence, better V.E., and better fuel mixing at this point. However, because of the higher accelerations, it's even harder to make a long stroke motor with short rods rev high.

The effect of a long torque arm is canceled out by the small piston diameter(assuming the same displacement). You can look at thermodynamics, and you just look at the change in volume to find the work that the gas in the piston does. Assuming the same displacement, both will do the same amount of work during the power stroke, and since both have a 180 degree power stroke, both generate the same average torque during the power stroke. (and by both I meant a small stroke and long stroke motor).

Now I know that I was talking about piston motors, but it's the change in volume that matters, so this applies just as well for rotories. There may be some dynamic benefit from changing the eccentricity of the eccentric shaft, but not from having a longer torque arm.

sorry for the long response.

babylou

I don't have a big brain and many say I am a fat head but piston rod (aka connecting rod) length does not affect torque for a piston engine. Crankshaft stroke does. Rod length affects the acceleration curve of the piston and piston side loads.

wakeech

precisely... didn't i say that the stroke is the distance from the centre of rotation of the e-shaft to the centre of the e-lobe?? omg, i didn't. :o i apologize for not including this, and saying that (to paraphrase) "the rotor has more torque on the e-shaft". it's like this, gents:

with a rotor that is larger in that "radial" direction (i dunno what to call it), keeping things proportional, you're gonna have a way bigger e-lobe in the middle of that aforementioned "bigger but skinnier" rotor, which means the centre of rotation of the e-shaft has a greater distance between it and the centre of the e-lobe (which is what the rotor does its work upon), thus increasing the stroke, and (to quote directly) "would yeild you a greater leverage on the eccentric shaft".

Fëakhelek

Is it possible to increase the rotary's stroke without using a bigger rotor? I kind of pictured a bigger "hole" in the center of the rotor, or is the motion of the rotor dependent on its size making that impossible?

wakeech

i'm sure that Mazda has designed the rotor to take as large an e-lobe as it possibly can... if you look at any of those cutaway pictures in that presskit about Mazda's rotary developments (c.1999) you'll see that the inner gear (which is the part in the rotor) actually impedes making the e-lobe any larger... i'm sure if somehow they figure out how to elongate the stroke even by a mm they'd do it, as it'd be a huge percentage increase!!

Jreyenga

no, really it doesn't

Trust me guys, if you take some special motor with infinitely long connecting rods and increase the stroke, but keep the same displacement, you will not make more torque, period. If you open up a thermodynamics book, you'll find out that the only thing that affects how much work a piston/cylinder generates, is the change in displacement. And since both a long stroke and a short stroke motor go through 180 degrees during the power stroke, and generate the same work over the power stroke, both generate the same average torque over that 180 degrees.

Look at it this way, if you keep the same displacement and go with a longer stroke, you have to have a smaller bore. That smaller bore means the same cylinder pressure is acting over a smaller area, so you have a smaller force, with a greater torque arm. These two cancel out.

Buger

Great post Jreyenga,

You sound like an engineer. This is an interesting subject that people far smarter than me have argued over. I previously posted the below on this subject:

"Theoretically, some people say that the oversquare and undersquare engines should have the same torque characteristics if the displacement is the same. In the real world, a piston engine with a very wide bore and short stroke has more area to have larger valves than one with a short bore and long stroke. Short explanation is that a short bore/long stroke engine can breathe comparatively better at low rpms than high and a wide bore/short stroke engine can breathe comparatively better at high rpms than low. This would explain why the long stroke engines generally have higher low end torque than the short stroke engines. Short stroke engines can also rev much higher than long stroke engines because each revolution of the driveshaft on a piston engine means that a piston goes up and down the length of it's stroke. A short stroke engine has less distance to move so more revs can happen for a given time span.
"

We agree that theoretically, displacement is the main factor. We must not forget however that in 2 different engines with the same displacement, the one with a very short stroke / very wide bore will have a much larger area to put the intake and exhaust valves. The increased intake valve area is great for high rpm breathing but does not help with low rpm intake velocity yes?

Brian

Fëakhelek

I am not a Mechanical Engineer, so I am just applying my limited knowledge of physics and speculating on things that I am sure are a science to actual professionals. I think I have a handle on this now. I was assuming that a longer stroke would imply more leverage and thus more torque. I assumed that the need to have a larger bore with a shorter stroke only applied to a piston engine. That makes sense for pistons but I wasn't sure how it applied to a rotary. Now I see that my idea on leverage was wrong. Making a longer lever (or eccentric lobe, or connecting rod) would give more leverage on the shaft but since it has to travel a greater linear distance to orbit the shaft, this cancels out.

Thanks for a great discussion. It is good to learn new things. The practical knowledge I've picked up here is much more effective than if I had to spend the time it would take to learn this from books.

wakeech

hmmmm... what about the stroke of the crankshaft??? that must have some effect on torque, yes??? 'cause the e-lobe is equivalent to teh crank arm, not the con rod......??

SureShot

Mazda has invested millions in tweaking the geometry.
All the trade-offs are fully evolved at this point.
The torque improvements are comming from side porting and intake tuning.
Next comes either "trombone" intakes & exhaust, or boost.