Why Use a Wankel Engine?

[jimmydasaint]

The Honda RX-8 had a couple of unique features, including a Wankel (rotary) engine, in contrast to the piston driven engine that most schmoes drive (like me). However, what is the attraction of a rotary engine. Can it possible be more efficient than piston driven cars? And if not, why use an engine that is even less efficient when so many of us are trying to reduce our carbon footprints?

 

 

 

Rotary engine

[John Cuthber]

"Can it possible be more efficient than piston driven cars?"

Yes, at least in principle- because the "piston" doesn't have to keep stopping and starting again.

 

Also there's the comedy value of the name.

[Phi for All]

Also there's the comedy value of the name.

Shouldn't the Wankel be the one with the up and down stroke?

 

However, what is the attraction of a rotary engine. Can it possible be more efficient than piston driven cars?

Iirc, Mazda used to use the Wankel in their RX-7, and I think part of the appeal was that you were creating rotary motion that could more efficiently turn wheels, rather than turning reciprocating piston motion into rotary motion to turn wheels.

[TonyMcC]

You cannot call the Wankel Rotary a unique feature concerning the Honda car. It was invented in 1957 and has been used in various cars since then. Follow the link and scroll down to Engine Technologies - Wankel Engine. http://en.wikipedia.org/wiki/List_of_automotive_superlatives

[SMF]

I don't think Honda used a Wankel engine in any production car. These engines had a very good horsepower to weight ratio, were not very efficient, and tended to wear out quickly. I had a Mazda RX7 and it was fun to drive but hard to keep in tune. SM

Edited April 12, 2011 by SMF

[Hal.]

I'll tell you why a rotory engine is great . Someone sat down and designed the F****** thing to move you , the kids , the wife and whoever else is in the car , down the road , up the road , back down the road , up the road , ad infinitum ( get it ) !

[Danijel Gorupec]

Yes, at least in principle- because the "piston" doesn't have to keep stopping and starting again.

 

I am sorry but I can't understand if you are joking or are just serious... I can see many bad things that happen from stopping and starting the piston in piston engine, but efficiency loss is not one of them. At least not directly.

 

...

 

About the Mazda Renesis engine... A very narrow consumer group is interested in Wankel, and I think this is where Mazda is aiming. These consumers prefer 'something different' more than pure logic. Mazda Renesis runs at high revs, has specific sound and has specific power/torque curve - this can also be appealing to some people. Marketing could also be reason why Mazda is still offering Wankel engine... In addition, Mazda is the only producer of (useful) Wankel engines in the world and they are probably too proud to end production.

 

 

 

 

 

[InigoMontoya]

And if not, why use an engine that is even less efficient when so many of us are trying to reduce our carbon footprints?

Because not everybody is trying to reduce their carbon footprint. Some folks like light weight cars with zip. For those people, Wankel's have advantages.

[lemur]

I can see many bad things that happen from stopping and starting the piston in piston engine, but efficiency loss is not one of them. At least not directly.

There's no energy conserved by having the drive train all in continuous motion instead of oscillating? Doesn't it consume energy to stop and change direction numerous times per second at high speed?

[Danijel Gorupec]

There's no energy conserved by having the drive train all in continuous motion instead of oscillating? Doesn't it consume energy to stop and change direction numerous times per second at high speed?

 

I think, no. Energy is not consumed by direction changing. (Of course, such engine will have greater vibrations, and will also have to be heavier/ more robust - therefore probably less efficient, but only indirectly).

[Xittenn]

Mazdas RX Series are Race Car Spec vehicles and have little to nothing to do with 'carbon footprint,' although they certainly do not lack in this department and can be readily upgraded to increase performance. Wankel engines are quite powerful and this is best demonstrated by the success of the 787B and the R26B engine. I wouldn't be surprised if a hybrid comes out in the long run that cuts the competition down.

 

I like Mazda and would love to have one for myself : ) Nobody will mind, I'm sure, if you edit your OP to say Mazda as opposed to Honda, not that there is a problem with Honda; there simply isn't a Honda RX-8 and Honda has never produced a vehicle that contained a rotary engine(pretty sure.)

[lemur]

I think, no. Energy is not consumed by direction changing. (Of course, such engine will have greater vibrations, and will also have to be heavier/ more robust - therefore probably less efficient, but only indirectly).

I'm not sure how to definitively establish the answer to this. Either way, the pistons would cause more wear on the various bearings involved, I would think.

[jimmydasaint]

Mazdas RX Series are Race Car Spec vehicles and have little to nothing to do with 'carbon footprint,' although they certainly do not lack in this department and can be readily upgraded to increase performance. Wankel engines are quite powerful and this is best demonstrated by the success of the 787B and the R26B engine. I wouldn't be surprised if a hybrid comes out in the long run that cuts the competition down.

 

I like Mazda and would love to have one for myself : ) Nobody will mind, I'm sure, if you edit your OP to say Mazda as opposed to Honda, not that there is a problem with Honda; there simply isn't a Honda RX-8 and Honda has never produced a vehicle that contained a rotary engine(pretty sure.)

 

My mistake. It was the Mazda car. The apparent mileage in town is 19 mpg, and in the country, 21 mpg. With current petrol/gasoline prices, you need to be blessed with great wealth to drive these around. Mazda engines

 

I also read about small or model aeroplanes with Wankel engines...Wankel aircraft

Edited April 13, 2011 by jimmydasaint

[physics confusion]

Hi

A Wankel motor is very unlikely to become more efficient than the 'otto reciprical engine' design. Wankel motors are used because the are small and powerful (in terms of torque), and because they run very smoothly (without the momentum from the pistons 'stopping' causing movement).

[Ophiolite]

Energy is not consumed by direction changing. (Of course, such engine will have greater vibrations, and

You don't think vibrations waste energy?

[Moontanman]

Yamaha and Suzuki produced Wankel powered motorcycles, the Yamaha never went past the prototype phase but the Suzuki was put into production, it wasn't very reliable and didn't last long in production, three years or so, For some reason the Suzuki wasn't very powerful and it's reliability problems doomed it. The Suzuki was a single rotor engine and the Yamaha was a twin rotor engine ( a Yanmar design) and was supposed to be very powerful but the lack of success of the Suzuki and air pollution standards looming on the horizon doomed the bikes as sure as it doomed two strokes. later innovations lowered the pollution problems of the Wankel but were too heavy to put on a motorcycle. Now is you want to talk motorcycles lets start another thread, i have some doozies that were never never put into production and if you had one you could ask any amount for a Wankel Yamaha or Yamaha GL 750

Edited June 11, 2011 by Moontanman

[chilehed]

However, what is the attraction of a rotary engine. Can it possible be more efficient than piston driven cars?

No, for a number of reasons. The large swept surface of the combustion chamber causes problems with high surface temperatures at some locations (puts a limit on compression ratio) and low surface temps at others (pulls heat out of the charge which reduces its availability to do work). Also, even if the tip seals were as effective as the rings on a piston, they allow a huge leak path as they pass by the spark plugs. And the odd shape of the combustion chamber makes it hard to attain high compression ratios, and causes issues with quenching early in the cycle.

 

And if not, why use an engine that is even less efficient when so many of us are trying to reduce our carbon footprints?

Because they have few moving parts, and a high power/weight ratio. And some people think they sound cool. Edited June 28, 2011 by chilehed

[jimmydasaint]

No, for a number of reasons. The large swept surface of the combustion chamber causes problems with high surface temperatures at some locations (puts a limit on compression ratio) and low surface temps at others (pulls heat out of the charge which reduces its availability to do work). Also, even if the tip seals were as effective as the rings on a piston, they allow a huge leak path as they pass by the spark plugs. And the odd shape of the combustion chamber makes it hard to attain high compression ratios, and causes issues with quenching early in the cycle.

 

Because they have few moving parts, and a high power/weight ratio. And some people think they sound cool.

 

That was my motivation for the OP about the Mazda. I had heard, over a meal, from an engineer that the tip seals allowed a lot of leakage and a large loss of energy, leading to a lower efficiency than piston driven engines. I don't think the manufacturers ever denied this, and I suppose that 'you pay your money and make your choice'.

[chilehed]

That was my motivation for the OP about the Mazda. I had heard, over a meal, from an engineer that the tip seals allowed a lot of leakage and a large loss of energy, leading to a lower efficiency than piston driven engines. I don't think the manufacturers ever denied this, and I suppose that 'you pay your money and make your choice'.

Yep. Once at the midnight drags I saw some guy show up in a beat-up looking RX-7, and all the V8 guys laughed at him until he sandbagged them out of a few hundred bucks. Thing hauled ass.

[echaopalante6]

well, the advantage of the wankel engine is that is very small, light and produces a lot of power; and can produce high revs because it isnt doing an up and down motion like the regular piston engine. The other positive is that because a rotary or wankel engine, rotates, it reduces a lot of vibration. regular piston engines have counerweights to couner balance the force of the expanding gases after the combustion stage. The rotary engine was first used in a small lawn mower back in the early days. then Mazda re designed it and fixed a few problems it had and patented it and has used it since the Cosmo in a large number of its sport and some saloons, and yes, Mazda is the only automobile manufacturer that has used the rotary engine. The wankel engine is also more efficient in the snese that i doen't lose much energy transfer to the the flywheel, and driveshaft and to the rear wheels, because all of it is in a circular motion, just like the engine. I personally love the rotary! I think it is very different, sounds awesome and it's engineering and physics is quite fantastic! The idea of the wankel rotary was performance and power with a small and lightweight engine, without efficiency on it, it is true that you lose a little bit of gas when the apex seals go over the spark plugs, and that some exhaust gasses can get back into the combustion chamber when the selas go past the plugs, but if you think about it, the change is very minimal, as the rotor is going at high speeds. Again like i said earlier, the wankel was make with performance in mind, i still think it is a fantastic engine!

[Peter Dow]

Regarding the mathematics of the unusual shape and profile of the Wankel engine triangular rotor and combustion chamber housing, I'm reviewing a mathematics demonstration I have just come across but which has been on the internet for a few years but, like me, you may not have come across it before now.

 

 

"Wankel Rotary Engine: Epitrochoidal Envelopes" by Tony Kelman on the Wolfram Demonstrations Project.

 

 

http://demonstrations.wolfram.com/WankelRotaryEngineEpitrochoidalEnvelopes

 

This Demonstration gives an animation of an epitrochoid and associated planetary-motion envelope curve. The configuration shown has applications in the internal combustion engines invented by Felix Wankel and popularized by Mazda in RX-7 and RX-8 cars

 

Review by Peter Dow

 

If you think this video looks interesting, I highly recommend that you download the Wolfram CDF player software so that you can experiment with the features of Tony Kelman's demonstration. To quote Tony

 

"The "eccentricity ratio" changes the shapes of the curves. The "reference frame" determines what is held stationary in the animation: either the epitrochoid (blue), the envelope curve (purple), or the centers of rotation of both curves. The "inner" envelope is the triangular rotor shape used in place of a piston in a Wankel rotary engine, whereas the "outer" envelope is the continuation of the envelope curve along the opposite extreme of motion."

So selecting reference frame = epitrochoid allows the display of the familiar KKM Wankel engine and selecting reference frame = fixed centers shows Wankel's original DKM engine with rotating housing.

 

You can slow the rotation animation down as well..

 

Looking at eccentricity ratios widely different from what we see in real Wankel engines is quite a revelation too.

 

As if all that wasn't enough, you also get to download and look at Tony's open source code and in particular the maths equations he uses to generate the curves.

 

Tony suggests some extensions to his demonstration. Well I have ideas of my own - I'd like to see computations of the areas between the curves representing the combustion chambers and a calculation of compression ratios for example.

 

Unfortunately, I don't have the Mathematica developers software package which, unlike the free player I got to view the demo, you have to pay - A LOT - for.

 

Excellent demonstration! Can't praise it highly enough!

Edited May 29, 2014 by Peter Dow

[Peter Dow]

After I started this topic, I have since found another trochoids interactive demonstration webpage, this time by Christopher J. Henrich. His code is in Javascript which means it is pretty much open source, can run on most modern web browsers and therefore is ideal for me to modify.

 

So I've made a start and I'm publishing a webpage today which partially performs some of what Tony Kelman's demonstration does. I've a lot more to do yet but if you want to see how far I've got and monitor my progress, then click to my webpage using the following link.

 

My page includes links back to Christopher J. Henrich's original webpage and he is OK with me publishing this link. Anyway see for yourself.

 

Mathematics of the Wankel rotary-engine shapes Webpage by Peter Dow

[Elite Engineer]

Yes, at least in principle- because the "piston" doesn't have to keep stopping and starting again.

 

Also there's the comedy value of the name.

Comedy value? How do you Brits pronounce piston? (peestone?). Sure hope its not as ridiculous as "aloo-min-yum"

[CharonY]

I am pretty sure he was referring to Wankel.

[Elite Engineer]

I am pretty sure he was referring to Wankel.

Ya I just figured that out. I'll leave the comment though...gotta protest the way those Brits say aluminium.