Mazda rx8 wankel efficiency nearly there? 7

[harveygp]

Looking at the fuel consumption of the rx8 in relation to the power output, it seems to be comparable with piston engine sport cars of similar (high) performance.

Doesn't that suggest that say a single rotor version of half the output (say 110hp) but in average use having a wider open throttle and only half the frictional loss would give a very acceptable power unit for a small saloon with good economy too.

The improvement over the previous rx7 engine seems very impressive but has been got through careful attention to detailed understanding. So the basic engine topology has much more to offer that anyone would have guessed a few years ago.

Should the industry in general be taking a new look at the Wankel topology? Maybe it already is?

Any comment from those who know a bit more than me/

 

[zzonbi]

Not exactly, because it is much more smoother and suffers less wear.

But yes, you can play with various combinations of capacity if you correlate them with rpm (and torque, implicitly).

Blackbox guessing it's rather turbine or electric (or something with lots of small cylinders).

 

[facty]

Theoretically speaking, this engine has 6 working volumes which each go through a 4-stroke cycle. Forget about output shaft speeds its cyclic speeds that are the only relevant factor.

Consider trying to mathematically model this engine and this point becomes very clear.

 

[NormPeterson]

In the end it depends on the equivalence factor used. If it is just output shaft speed, the engine is like a 2.6 at 10000rpm, . . .

Why would you consider the rotational speed of anything else? As far as drawing useful work is concerned, it's the speed of the main output shaft that matters. Not that of any of the internals. We wouldn't factor 4-stroke cycle piston engine displacement by two if we decided to drive the vehicle off the camshaft sprocket, would we?

Norm

 

[dgallup]

While I agree that the Renasis engine is a big improvement over previous generation Wankels, I take great issue with the statement that kicked of this thread:

"Looking at the fuel consumption of the rx8 in relation to the power output, it seems to be comparable with piston engine sport cars of similar (high) performance."

The RX-8 gets significantly worse real world milage than comparable piston engines. Look at Road & Tracks long term test on the RX-8 and 350Z. The Z is heavier and more powerful so it should get a little worse milage but it got 20.5 mpg and the RX-8 got 16.3. This is average milage for the life of the test, obviously not identical conditions but I think most people would get similar results.

I've owned both an RX-7 (84 GSL-SE, first fuel injected 6 port 13B) and 350Z (2003). In repeated highway measurements over multiple tanks of fuel with the curise set at 80 mph and average trip speeds of about 70 mph (including stops), I got 24 mpg with the RX-7 and 28 with the 350Z. However, that RX-7 only weighed 2400 lbs vs the 350X at 3300 lbs.

 

[harveygp]

Norm,

That's just the point though, where you take the power is immaterial. So in your example it's still a 4 cylinder four-stroke - that's what you're saying, isn't it? The power output is the same whichever output you choose.

So now the question is what is a Wankel engine equivalent to?

I think you just agreed that if you go down the impulses per crank/cam shaft route,you get a different answer depending on which of the "internals" you choose.

That's why you need to avoid the mechanical details.

Facty was correct "its cyclic speeds that are the only relevant factor"

.... otherwise you can make the Wankel specific power output / capacity be pretty much whatever you want, just choosing your favourite definition.

 

[harveygp]

Greg

"Please see FAQ731-376 for tips on how to make the best use of Eng-Tips."

I read it and asks you to remember that you are not infallible and you could be wrong.

I think your argument goes:-

you have only considered the issue "Simplistically"
... but you are right anyway,
and anyone who disagrees with you must be a marketing man

 

[GregLocock]

"anyone who disagrees with you must be a marketing man" Don't be such a condescending git. Sure I was being simplistic, because it is a simple problem. By all means explain /mathematically/ why it is any more complex. I've given a simple way of approaching the black-box equivalence. Where is it wrong? Why is any other approach better, unless you are a Wankel fan-boy?




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[facty]

NormPeterson,

You're on the wrong track - at the end of the day its not the speed of the output shaft that really matters its the fact that the power must be consistent throughout the complete enngine system!

Consider an engine coupled to a gearbox. The rotational speed of the engine will be different than that of the output shaft from the gearbox, but the constant factor is the Power - the power at the flywheel is the same as the power at the output shaft (less frictional losses which we can agree are zero for this discussion). In this simple case it will be the torque that chenges - hence the need for a gearbox.

 

[facty]

And Greg,
As "simplistic" as this may all be, you still haven't declared exactly how you view this engine re displacement, no. of relevant working volumes and shaft speeds!

This is an important topic as there seems to be a great difference of opinion worldwide on how to specify a Wankel/Rotary engine against its reciprocating counterpart.

 

[GregLocock]

Last bit first. It is not an important topic. Nobody's life will be saved if we get it right. I imagine that at most a thousand people worldwide will even be materially affected by any change in equivalence ratio between rotaries and reciprocating engines.

Equivalence formulae for engines are always arbitrary. Choosing swept volume as the basis for equivalence is a historical tradition, not an engineering requirement. At a system level I am interested in things like power, installed mass, installed volume, and emissions, and fuel consumption, and cost, and reliability. The details of which bit of metal sees burning gas quite how often are a minor detail.

In most racing series, demonstrably, equivalence formulae don't work anyway - the front runners all end up running the same engine configuration.

I agree that the choice of output shaft rpm as a defining characteristic is somewhat arbitrary, but at a system level it is at least relevant.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Come on guys, play nice.

This has drifted away from the original question and is really only important re class rules in racing, or bulls**t for bench racing.

It is political and arbitrary and not engineering. This is an engineering forum for professional engineers in their work, not a notice board for the bias ravings of a one eyed brand or type enthusiast.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[facty]

On the contrary - this is an extremely significant design issue - how can an engineer design an engine without knowing these basic facts.

This is not intended to be a "pushy" issue trying to promote or fight the corner of one engine configuration over the other, but merely an attempt to promote discussion between the experts in this forum.

So can anyone shed any light on this very real issue of mathematically Modelling and subsequently Designing a Wankel Engine?

 

[patprimmer]

I would start with calculating the capacity (measured however you like), rotor diameter, rotor width, compression ratio, airflow and port timing of existing rotaries, measure their power, decide what I needed, then make a model using the data obtained doing the above.

It won't matter whether you call it 1 litre or 10 litres swept volume. what matters is that you are consistent with your methodology, and you consider the parameters that matter for the reasons they matter, for instance, rotor dia and width are important for the effects they have on surface area to volume, surface speeds, capacity, manufacturing cost of the capacity, packaging, weight, etc etc, just as the bore, stroke and number of cylinders is important in reciprocating piston design.

I still don't see a need to argue.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[goldedge]

Hi isn't the best way to rate equivalence of performance
ie weight + drag (at x speed) divided by fuel consumed?

I have a carbureted non turbo RX-7 race car, on the open road at 140kph it's reasonably economical at 160 - 170 kph the carb secondaries are well open and fuel consumption is approximately doubled from 140k

In short v8 performance = v8 fuel consumption. having said that on a race track at similar speed it uses way less fuel than a v8 and is not that much slower.

The renesis engine in the RX-8 is way more advanced in terms of power and fuel economy than the rx-7.
So the real equation is in my opinion based on vehicles of comparable mass and drag at the the same speeds divided by fuel consumption?

Regards
Michael

 

[dgallup]

And the real answer, based on extended road tests reported in the press is that the RX-8 is a gas hog!

 

[harveygp]

Could you give some figures or a link to the press reports and extended road tests you mentioned?

I seems to to remember putting up some figures that showed an Alpha Romeo V6 to be an even bigger gas hog.

BTW, that post and quite a few others seem to have dissappeared from the board.

Any one know how/why that happened?

 

[dgallup]

http://www.autoweek.com/article.cms?articleId=102151

Check Road & Track, Car & Driver, etc. Almost all the consumer auto mags did long term tests.

 

[harveygp]

Hmm.. I see what you mean.

Still the reputation the rotary has is that its worse than the worst of piston engines by some margin and I'm not sure that's any longer true. With further innovation it can improve further.

(Maybe the 350Z you compared with previously is a particularly good example. From the figures I saw BMW is even better. As I said previously Alpha Romeo looked worse.)

 

[patprimmer]

In my experience, motoring journalist are good journalists. Their arts degrees prepares them for that.

They are rarely good engineers and rarely know how to do objective controlled tests. Their arts degrees does not prepare them for that.

Some argumentative threads, or plainly amateur threads might have been removed as this forum is for "professional Engineers" in work related context.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[harveygp]

Pat,

The posts I had in mind very much didn't seem to fit in either catagory, hence my curiosity.