I was reading this today.... (http://www.off-road.com/hummer/tech/power.html)
And after a bit, I realized that rotary engines don't work the same way as piston engines, and thus torque is measured differently. Allow me to quote:
To get our vehicles moving, we use some kind of engine to convert chemical energy (fuel) into useful work. In an internal combustion engine, the fuel in the combustion chamber is ignited by a spark (gasoline) or from compression (diesel), creating high pressure gasses that press down on the pistons. Via the connecting rods, this force pushes the crankshaft throws or arms. Bearings control the crankshaft, allowing it to only rotate. Since each crank arm is offset from the centerline of the crankshaft (the rotational axis), we have torque. Voila.
Okay, the measure of the torque is in the turning of the crank arm... rotary engine's don't have those.

So how are we to measure the torque in a rotary, and how would we correlate it to a piston engine? While the RX-8 seems low in torque to most, I feel that in a rotary engine there would be some type of different property that would allow for a seemingly low torque number to produce good acceleration, compared to the piston engines.

So what do you think?

What difference does the type of engine make? The car isn't moved by the engine (directly)--it's moved by the wheels. So, whether piston, turbine, radial (don't think these are used in cars too often!), or rotary, they all transfer power to the wheels. We can effectively compare torque at the wheels no matter the engine type. In fact, because torque at the wheels incorporates gearing, it's a much better measure of a car's potential (straight line) performance.

The rotary's advantage is the width of its powerband, allowing Mazda to gear the car in such a way that low engine torque is not as much of a factor.

Don't have my physics book here, but I believe torque is force x lever arm.

Yes, I agreed. Torque is a measure of work, regardless of how it is produced.

Hi Herc,

That was a nice link that you found that explains some things very well. I think Nostatic did a good job of answering your questions first but here is my long winded version anyway. :)

Engine Dynos measure the output of engines whether they are rotary or piston. The Engine dyno is connected to the output shaft of the engine and measures the torque that the engine can produce at different rpms by applying a load or braking force. Engine dynos are much more accurate than the common chassis dynos that people use to find out their power at the wheels.

The reason a rotary can produce good acceleration is no different than the reason a piston engine can produce good acceleration. I think it basically boils down to hp/weight ratio and more specifically wheel torque/weight ratio. No magic rotary special sauce. A 3000 lb car with 160 ft-lb, 160 hp can't compete with a 3000 lb car with 160 ft-lb, 250 hp because the higher horsepower car can be geared much shorter to produce much more wheel torque. Engine torque is just one variable in an equation. People that say that the 153 ft-lbs peak engine torque of the S2000 is too low to provide good acceleration probably haven't felt the acceleration of the S2000 when it's at it's 153 ft-lb torque peak @ 7500 rpms. What they probably mean to say is that the s2000 torque below 5000 rpms is relatively low compared to other sports cars but they just don't know enough to say that. ;)

A little known fact about Wankel rotaries is that the rotors spin at 1/3 the speed of the output shaft. When your tachometer says you are at redline at 9000 rpm, your rotors are actually spinning at 3000 rpm. This is one reason why rotaries are incredibly smooth. It is also the reason why a 1.3L rotary has comparatively lower torque than a 2.6L piston engine. One power stroke of a piston engine turns the driveshaft approx 180 degrees while one power "stroke" of the rotary turns the driveshaft approx 270 (90 * 3) degrees. At 9000 rpms, the renesis has overlapping powerstrokes from rotors that spin at 3000 rpms or 50 times a second. A piston engine at 9000 rpms, has pistons that must stop, change direction, accelerate, decelerate and get ready to stop again 300 times every second!!! Anybody that own a S2000 really needs to feel how the RX-8 feels at redline. The difference will amaze them. :D

The things that keep a rotary from revving higher are e-shaft flex and the amount of air that is available to the engine. The renesis redline is not higher because of breathing not because of e-shaft flex. You can see that the renesis torque curve starts going down faster after the hp peak at 8500 rpm because the renesis can't get in enough air at higher speeds to make more power.

Brian

sorry Boogs, had to skim bud... up late talkin' with a chick (whoo!! what a chick) and my head hurts...

to comment on Herc's origional question, about rotaries not having a "crank", the terminology is different, but it still has an output shaft which the rotor leverages upon, and the rotor still has a stroke...

the distance from the centre of rotation to the outermost edge of the e-shaft is 40mm on the button, which traslates into a 40mm stroke...
i wrote considerably more about what this means in terms of engine characteristics in (my very favourite thread) "How the RENESIS makes more power", now in the Tech Garage section.
check it out. post in there, keep it alive me hearties... please. if you really love me, you'll do it... :(

g'night all.

I agree with Buger. People always ask me how it is to drive me S2000 with "so little torque". Usually I simply let them get into the passenger seat and show them 8500 RPM! They usually change their tune after they feel, and hear, that. The really cool part is the change in the exhaust note as the engine passes 6000 rpm and the vtec moved the cams. It drops and octave and BOOM now you have a ton of power. I am really gonna miss this car, this engine is incredible. It is as fun as my 93 3rd gen RX-7 was, and in kind of the same way. But I have another year to go on the lease, so I will listen to all of you, specifically how you who buy the car like it, and then next year I will buy the RX-8. So, keep the reviews coming. Oh, anyone want to but a nice S2000 next year? ha ha ha

Thanks Brian for sharing your knowledge on the rotary engine. Your post are always entertaining and informative.

I don't if it's just me, but everytime I read a technical post from Buger, I want to test drive the car soooo bad!

Originally posted by Buger
Hi Herc,

A piston engine at 9000 rpms, has pistons that must stop, change direction, accelerate, decelerate and get ready to stop again 300 times every second!!! Anybody that own a S2000 really needs to feel how the RX-8 feels at redline. The difference will amaze them. :D

The things that keep a rotary from revving higher are e-shaft flex and the amount of air that is available to the engine. The renesis redline is not higher because of breathing not because of e-shaft flex. You can see that the renesis torque curve starts going down faster after the hp peak at 8500 rpm because the renesis can't get in enough air at higher speeds to make more power.

<<yawn>>
good info B-dawg, and to all other noobs who wanna find out more why rotary power is better, please visit The Rotary Engine Illustrated (http://www.rotaryengineillustrated.com) and learn some stuff, m'kay?? :D is coo'.

hmmmmm.... hmmmmmmmmmmmm..... HMMMMMMMMMMMMMMMMM....:confused: i think the "How the Renesis makes more power" thread needs another chapter... want to discuss how Mazda will move the redline up further?? i thought so... :)
to the Boogmobile!! <<da na na na na na na na, da na na na na na na na>>