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Mazda rx8 wankel efficiency nearly there?
7

Mazda rx8 wankel efficiency nearly there?

Mazda rx8 wankel efficiency nearly there?

(OP)
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/

RE: Mazda rx8 wankel efficiency nearly there?

3
Fuel economy is dependant on the overall design of the vehicle, not just the engine.

As I am not familiar with the actual changes to the RX8, the following is speculation based on my general knowledge of cars and engines, including rotary.

I suspect the RX8 has obtained it's extra fuel economy by attention to many small details of the engine and the overall vehicle, including things like friction within the engine, engine management systems, port timing, gear ratios, gear change points in an auto, aerodynamics and weight reduction. if all these are done to their next model with an Otto cycle engine, it might also gain a bit.

The Wankel or rotary design has several built in limitations vs an Otto cycle. The most significant of these is the surface area to volume ratio of the combustion chamber. A single rotor is worse than a twin in this regard.

Another double edged sword in the inherent design is the valve or port timing. As the ports are fixed, and the timing is dependant on the piston moving past the port, there is less energy lost in valve train inertia, but also, there is no possibility to incorporate "v"tech without a rotary valve as well as the piston port. I expect the rotary valve would have seal or friction problems.

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.

RE: Mazda rx8 wankel efficiency nearly there?

Pat, wouldn't the turbo help with efficiency be increasing total pressure ratio of cycle? Most units are set up to boost power, not sfc.

Must admit I think the weight/size saving alone on a Wankel make it worth considering. I'm actually suprised we haven't seen a flood of aero engines, either from converted Mazdas or new designs, based on the Wamkel principle. If it is tubocharged it basically becomes a very low maintenance 2-stage gas turbine!

Any takers? Pat, maybe some of your aus buddies might like to consider a new market - especially if it could be retrofitted to existing piston apps (eg R22 chopper or Grob Demona powered glider)...

Mart

RE: Mazda rx8 wankel efficiency nearly there?

A turbo will help a rotary just the same as it helps a piston engine.

Rotaries are great for power density, and continuous high speed operation once warmed up, but the higher surface area ratio will inherently give them a disadvantage regards to thermodynamic efficiency, and therefore fuel economy, all other factors being equal.

For short flights their extra power to weight should be an advantage, however at some point, the extra fuel requirements will offset that power to weight advantage. That point will depend on a number of factors

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.

RE: Mazda rx8 wankel efficiency nearly there?

That's true, but the same could be said of gas turbines.

There seems to be a grey area, at around the 500HP mark, where helicopters and turboprops have to accept the lower efficiency of a "small" gas turbine. Above 1000HP gas turbines efficiency becomes similar to a diesel (eg RR Trent ~40%). In choppers especially, weight is everything.

For that matter I wonder how the rotary would fair if treated to the same exotic materials as turbine blades have had. OK single crystal is out, but a ceramic coated titanium liner would reduce cooling loss and necessity. Maybe even a diesel version of a rotary, with cooling air around the edges. We are talking aero engines here, so installed cost will be higher to begin with. Maybe this is more R&D than re-engineering, but I still like the rotary concept.

While at BMW, I was actually very dissapointed with the lack of desire to innovate away from piston engines. These are definately a good example of a well engineered, but heavy, powerplant. I came away with the distinct impression that piston designs have won out, since it's the best design that can be engineered from steel.

Having seen carbon fibre reinforced aluminium pistons (I think they were cast in an oxygen free environment), I feel that auto engines materials could do with another look. Aluminium and magnesium alloys are just the start. Ford were looking at composite 2-strokes, at one point. Rover (pre BMW) developed an adiabatic ceramic coated engine, which needed no cooling water!

Sorry for the rant, but I just feel that materials play as much a part in engine development as design...

Mart

RE: Mazda rx8 wankel efficiency nearly there?

Agreed

I was talking about state of the art, not potential new developments.

I don't know if it is possible to build the seals in a rotary that will withstand the detonation inherent in a diesel.

To be really effective, ceramic coatings need to be thick, to be durable, they need to be thin. If new ceramics are developed that will allow adhesion without de-lamination over a wide temperature range with a thick coating, a lot of problems will be solved, and all engines, wither rotary or piston will have substantial increases in their thermodynamic efficiency.

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.

RE: Mazda rx8 wankel efficiency nearly there?

Good point well made, Pat.

Mart

RE: Mazda rx8 wankel efficiency nearly there?

(OP)
Thanks for all the interesting information. I noticed over the weekend that the rx8 is actually significantly better on maximum power and fuel economy than some of the supposed competition here in the UK. (I was looking particularly at an 165bhp AlphaRomeo which is apparently in direct competition but used significanly more fuel on all measures than the 190 bhp rx8. Main reviewer objection on the rx8 was poor low end torque.

So my main point is that during the '70s many thought the Wankel would take over entirely since it weighed less and apparently cost less to make. Then we had the "oil shock" and it suddenly disappeared from view; reason, it used too much fuel. Well that no longer appear to be always the case.

The main change with the Rx8 engine is the the exhaust port has been moved to the side flank which as far as I know is entirely new. Always previously,(Mercedes, NSU, Mazda, Ford, Citroen, GM - they all tried the Wankel) the exhaust port was peripheral and the inlet port was sometimes peripheral and sometimes in the side flank. The new port arrangement allows zero overlap at the expense of volumetric efficiency but the engine can now runs smoothly on very lean mixture (40 % less fuel on idle). The arrangement also traps and recycles unburnt end-gas that would otherwise have have "fallen" out of the old peripheral exhaust port. Meanwhile the volumetric efficiency is recovered by inlet tuning and exploiting the clean unobstructed porting that is attainable with Wankel engines. So what Mazda have done with the basic topology is in fact very new and deserving of re-evaluation IMHO.

Some particular pointsof interest in the interesting posts:-

1) Surely the surface to volume ratio is improved going to one rotor ( surface = linear^2 , volume = linear^3) with the same overall displacement and stays the same (obviously) going to one rotor with half total displacement. Problem with the former might be lower max speed due to flame travel time.

2) If you could raise the thermal efficiency using insulating materials surely you could use atleast say solid ceramic on the piston crown in a conventional engine. I thought I read somewhere that this doesn't work because with a high surface temperature the boudary layer becomes turbulent leading (counter-intuitively) to higher heat transfer in the piston crown. (Analagous perhaps to how snow can keep you relativeley warm if its -40 outside. Or the fact that double glazing is no more effective with separations greter than a certain critical dimension since convection takes over as the dominant heat transport mechanism.

geoff

RE: Mazda rx8 wankel efficiency nearly there?

Out of interest, for another post, what are the main failure modes of a rotary engine? I can't think of any serious ones, since it can't throw rods, jam valves, shed belt etc. Lubrication or electrical failure is a favourite, but will be no worse than piston. I imagine that these units will continue to run in the absence of lubrican't right up to the point where they lose compression...

Mart

RE: Mazda rx8 wankel efficiency nearly there?

Rotor tip seals from detonation

Gouging of end plates from distortion.

Pulling end plate studs.

End plate seals if the thermal shock is high. i.e., full load on a dead cold engine.

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.

RE: Mazda rx8 wankel efficiency nearly there?

But these won't cause sudden catastrophic failure of the engine, just a bad headache when the maintenance comes up. I'm thinking along the lines of: what mode of failure would cause a forced landing in an aircraft?

Mart

RE: Mazda rx8 wankel efficiency nearly there?

A couple of thoughts:

VTECH-like technology is not only possible with Mazda's rotary engine, but you could say that part of it has already been done.  Back in Mazda's earlier days with that engine, a so-called "bridge-porting" technique was developed to increase power output (at higher RPM's).  The location of that secondary opening determines its port timing.  This mod involved the creation of a fork off the existing port with a "bridge" of metal between it and the original port at the chamber wall to prevent a seal from falling into the port.  Take that same approach, but keep the secondary port entirely separate and give it its own software-controlled throttle and you're now controlling the rpm band during which the extra inlet area (cf: valve lift) and timing is active.  With the RX-8 engine, something similar could be applied to both intake and exhaust.

Regarding catastrophic failures, I do recall something about harmonics ganging up on some gear in the earlier engines (I forget whether it was a rotor internal gear or one on the shaft) and causing it to fail ("pulverize" was the descriptive term used in a Car & Driver article).  But perhaps that's been studied enough since and re-design has eliminated that as an issue.

Norm

RE: Mazda rx8 wankel efficiency nearly there?

(OP)
As far as I understand the "variable valve timing" is already a feature of the "high output" version on the rx8 Renesis engine and it does indeed have 3 inlet ports under intelligent control. I believe one of them connects to a tuned inlet tract.

I imagine its much easier to implement than with poppet valve and cam shafts etc.

The various reliability concerns are rather out of date as far as I know. Anyone out there who owns a Mazda and could comment on reliability?

I suppose sports car engines don't last as long anyway becasue of the way they are driven.

Geoff

RE: Mazda rx8 wankel efficiency nearly there?

"...harmonics ganging up on some gear in the earlier engines ..."pulverize"...re-design has eliminated that as an issue."

Eeeek! I certainly hope so, sounds like a very nasty mode of failure. I've got enough battered pistons and brocken gear teeth on my mantle piece already...

Mart

RE: Mazda rx8 wankel efficiency nearly there?

Hmmm, certainly interesting. I couldn't understand why there seemed to be two concepts. Is one a development engine, while the other is the ideal final product?

The smooth lined version looked very neat, albeit more complicated than wankel. The version with "rollerskates" looked like an emissions nightmare, with all sorts of horrible crevice volumes to leak away heat and hide incomplete products of combustion.

Apart from power to weight (again I don't really see what it has over a wankel) why is is it a better engine concept?
I'm certainly not convinced about the 32 strokes per cycle - i understand the theory, but don't see both sides supporting combustion (with cooling scavenging etc) in practice. Presumably the concept is the ability to support very high compression ratios.

Curious about others thoughts on it though...

Mart

RE: Mazda rx8 wankel efficiency nearly there?

(OP)
Take a look at

http://quasiturbine.promci.qc.ca/QTpasWankel.html

where they compare their engine with the Wankel. You have to hope they understand their own engine better - this is  90% gibberish.

1) They seem to think the Wankel phasing gear transmits some of the power. Obviously not true since each rotor face is symmetrical about the centre bearing so the rotor cannot experience any torque as a result of gas pressure (speed of sound equalizes pressure in tiny a fraction of the rotation period) . The main function of the gear is to accelerate decelerate the rotor during engine speed change.

2) The bit about "unusable dead time" is nonsense. There is no intrinsic connection between 4-stroke and 4 cylinders (or chambers). By this logic a single cylinder 4 stroke has 75% dead time.

3)"the Wankel geometry divides the contour perimeter in 3 sections of long lengths,making the combustion chamber quite extended, while the Quasiturbine divide it in 4 sections of shorter lengths"

 - oops they forgot to mention the width - back to primary school guys. You can see just by comparing the animations at the top of the page that, if anything, the "quasiturbine" is far worse in terms of surface area to volume ratio.

I could go on ...

Geoff

RE: Mazda rx8 wankel efficiency nearly there?

(OP)
As I appear to have been rather has on the "quasiturbine" in the last post - I should perhaps add that the points I made were only my personal opinion - I could be wrong.

Geoff

RE: Mazda rx8 wankel efficiency nearly there?

"...where they compare their engine with the Wankel..."

Agree with your points Geoff - from my skim...

Quasiturbine is a neat concept, but I don't see why it is better than a wankel. The only advantage I can see is compression ratio, but a turbo (which RX8 is - thanks Pat) obviates that point.

I'd certainly like to see a prototype quasiturbine running - ideally without the rollerskates. I think the only real test is a dyno test, as everything else is too theoretical or too subjective...

Mart

RE: Mazda rx8 wankel efficiency nearly there?

If you dig around there is a video of the chainsaw somewhere.  The last time I saw it it bogged down my computer.  I would say the claims are theory, however it's a new theory to me and that's what I find most interesting(Kind of like the Coates Rotary Valve Heads of the early nineties, Wonder what happened to him?).  I would like to see more prototypes.  I like to seeing both positive and negative comments about it from you guys, maybe where it could be improved, etc.

Nothing's Perfect

Jomor

RE: Mazda rx8 wankel efficiency nearly there?

I should point out that the rotary in the RX8 is not turbocharged.

RE: Mazda rx8 wankel efficiency nearly there?

If the big problem with the wankel is surface to volume ratio, couldn't that theory be run in reverse, as an improvement to the piston? I'm a total n00b about this stuff, but I'm guessing that a long thin piston suffers from too much surface area when fully extended, and a short stubby one has too much when fully compressed, so real ones pick a height/width ratio at some compromise value. If a 'piston' could be designed with a compressible chamber which stayed the same shape during its entire cycle, couldn't it in theory have near optimal surface/volume the entire time? Would that significantly improve engine efficiency?

RE: Mazda rx8 wankel efficiency nearly there?

(OP)
My view is that that the surface to volume thing may be out-of date dogma thats amounts to a lazy way to rubbish the Wankel concept. If you look at the fuel efficiency of the RX8 it is a huge advance on the RX7 and comparable (or better) than piston engine powered cars in the same power range and market segment. More important, the efficiency must surely now be way better than the state of the art with piston engines back at the time(about 40 years ago)folks first put forward the idea that surface to volume represented a fundamental barrier to acceptable efficiency for the Wankel topology. Clearly, with the benefit of hindsigh, that view was wrong then and maybe wrong now. Since that time I would guess at least 100X the resource has into making the piston topology more efficient compared with the resource expended on the Wankel topology.

RE: Mazda rx8 wankel efficiency nearly there?

Interesting HarveyGP. I had wondered how much truth was in the surface area to volume thing. An interesting point has been raised in a thread on 2-strokes that the power output is limited to head temp in a piston engine anyway. I imagine that the rotary piston width is very well optimised with this in mind.

Since the "big end" is epicycloidal, the seals can run very hot with no real wear. I imagine that cooling circuits are also optimised to flow in opposite direction to rotation, so as to assist heat transfer. As you say Mazda is keeping up the research that everyone else forgot. Pity someone like Ford doesn't give it another look...

Mart

RE: Mazda rx8 wankel efficiency nearly there?

New guy here with a question... I have a new RX8. Its got roughly 8800 miles on it. I added a bottle of octane booster to the tank about a week ago. I ran it through hitting a top speed of 135+/- before i ran out of road. After the tank was drained I refuled and started noticing a hrad idle. The exhaust was "puttering" and the car sounded as if it were going to stall. It has been doing this for a week now, even after 2 refuelings and a bottle of injector cleaner. Could the octane booster have caused a problem with my cat or an O2 sensor, even though it claimed the booster was safe on those componants? PLEASE HELP.

RE: Mazda rx8 wankel efficiency nearly there?


As a former RX-7 owner, I can relate a few real world points. Mine averaged about 25 mpg with a 4 speed at somewhat elevated speeds (5000 rpm is too smooth to intrude as a "speed alarm"). The main failure mode is having the end seal finally wear enough that it flips out and at the least drops the compression between two of the lobes on that rotor. No warning or symptoms.

I always considered my 11A engine to be a 2.2L six, and found the fuel economy and power levels to roughly coincide with piston engines of the same displacement.

Someone else beat me to the comments about "VTEC"-ing a rotary.

RE: Mazda rx8 wankel efficiency nearly there?

I would like to point out the engine is actually a Umblebee (sp?), invented in the UK in the early 1900's. Wankle took out his 'patent' the day Umblebee's expired. It was not until the kinematics was inverted by the head of NSU's design (forgot his name) did Wankle's engine become useable.
As far as aircraft use, check out Midwest engines who are building a version of the Norton motorcycle engine for aircraft. I have a Norton rotary and building a copy of their NRS588 race bike. BSA originally bought the license (which were based on power output) and built the prototypes. Norton inherited it and developed the liquid cooled version.
First difficulty when comparing to recipricating engines is measuring the displacement. The rotor turns 1/3 the crank (eccentric shaft) speed. I believe Mazda uses 1/2 the swept volume of the rotor. That, I believe is the reason for their quoted high power/displacement. They are lighter however without the valve gear. Comparable to a two stroke. This one uses periferal ports and air cooled rotors. I think Mazda were side ports which had more control over the exhaust/intake overlap.
As far as piston surface area, F1 engines are about 2:1 bore stroke. If area was a problem they would not have such large piston areas. Of course, efficiency is not a prime directive of F1.

RE: Mazda rx8 wankel efficiency nearly there?

(OP)
If I remember correctly, according to the book by Jan Norbye, there was a lot of debate in the early days about how one should compare engine sizes until someone called R.F.Ansdale "proved" at a Society of Automobile Engineers (?) that the Wankel was equivalent to a 2-stroke (!) of the same swept volume. The guy was either loosing it or had some vested interest, in my humble opinion.

I'm firmly with TOHCan in his last posting. To me it's obvious that the mechanical details (shaft speed etc.)are irrelevant and that to make a fair comparison one should look at the number of simultaneous volumes being swept on a four stroke cycle. So TOHCan is absolutely right. A 2 rotor Wankel is should be compared to a six cylinder piston of capacity six times the swept volume associated with each rotor face. That's not how Mazda spec their engine though.

On a similar theme, many journalists rave about how the Mazda "redlines" at 9000 rpm. But that is the shaft speed and equates to the same number of power impulses as a conventional six cylinder engine at 6000rpm - so nothing special really.

It's a shame that these stupidities persist because it makes ready comparison rather difficult.

(BTW, I think the NSU guy was called Walter Froede)

RE: Mazda rx8 wankel efficiency nearly there?

Am I ill informed, or is there no talk about a hybrid wankel?
I would think that the wankel is the ideal engine for an electric hybrid.

I'm told the wankel will never match a piston because the
lobes don't capture enough of the expanding gas.
Add a big turbo to get back the efficiency, and add an electric
motor to eliminate tutbo lag and add low-end torque.

What am I not seeing?
 

RE: Mazda rx8 wankel efficiency nearly there?

The surface area problem can be corrected with ceramic coating. Same problem occurs with large round or oval pistons, the surface area picks up more heat from the combustion which has to be cooled by oil jets on the lower surface.
Some believe the Wankle is neither two or four stroke. However, it evacuates the chamber through the exhaust port before opening the intake to refill the chamber like a four stroke. A two stroke uses the incoming charge to help blow out the exhaust which a Wankle does not.
The exhaust/intake overlap emmissions can be eliminated using the same technique as Lotus with their new two stroke. Move the port to later in the cycle and use a supercharger to provide the quantity of flow.
From all accounts of the Norton race bike, low end torque was not a problem and had a wider power band than the piston engines. Adding a turbo eliminates it from most racing classes.
Until batteries become more efficient, electric hybrids will be less efficient than mechanical hybrids.
Harveygp - thanks for the name.

RE: Mazda rx8 wankel efficiency nearly there?

microhenry

What you are not seeing, is that if you do all these things to a piston engine, they also improve, and thereby maintain an advantage, if developed to the same extent with the same technology.

The inherent disadvantage of the rotary engine is it's high surface to volume ratio. This still applies no matter what else you do to improve it.

There is no argument that it has very good power density, which can result in a lighter car which can help up to the point that this becomes offset as you also need to carry more fuel to maintain range. What range you require becomes critical to the comparison.

Ceramic coatings reduce the losses in piston engine and rotary engines, but the rotary has more scope for improvement as it has higher losses. This does not mean it will end up better in this regard, just worse by a smaller margin.

The fuel efficiency advantage is in the reduced inertia in the rotor vs a piston, but a rotor at constant rpm still has some losses as the rotor moves back and forth across the dog boned shape housing as it rotates.

I have no emotional support for either design concept, as I have had a lot of fun driving both rotary and reciprocating piston powered cars and boats.

By the way, suck, squeeze, bang blow adds up to 4 cycles in my book, no matter what mechanical configuration or valving you use to achieve it.

In a 4 stroke, all 4 are reasonably separate processes, and all occur within the combustion and positive displacement area. In a 2 stroke the suck only occurs in the exhaust system, and suck and blow occur virtually simultaneously outside the positive displacement area.

This clearly, at least in my mind makes a rotary a 4 stroke.

The capacity should be calculated by the displacement of air during one complete cycle or 4 strokes of the engine. This means a twin rotor should be treated as a 6 cylinder, and rotor rpm not crank rpm is the real issue in determining engine speed.

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.

RE: Mazda rx8 wankel efficiency nearly there?

Patprimmer, perhaps it was just the way you stated it, but the rotary does not move back and forth across the housing. The rotor is mounted by a bearing to the eccentric shaft and connected to the housing by a gear. The rotor center turns in a circle and rotates backward from the eccentric shaft. The curve enscribed by the tips is an epicycloid.
The same can be done with a four sided rotor and three lobed housing or any higher number.
I agree that a fair measurement of its capacity is the total swept volume, whether the volume is moving radially or along a single axis and regardless of the number of shaft revolutions it takes to accomplish it.

RE: Mazda rx8 wankel efficiency nearly there?

JrGman
As another former RX-7 owner,
Did you need the octane booster to the the car to that speed
The octane boost would change the flame shape and may have overheated some of the seals.
Did you bypass the rpm interlock? I assume it has one just like the rx7 at 7000 rpm (loud buzzer and ignition cutout)
Though distasteful and annoying it is what forces you to make the engine last longer.
Have you looked at the tip seals? Remove the plugs and bump the starter until you see the lobes,  there may be a relubing procedure to get them to reseal again. Or you may have breakdown and rebuild it.  The manual direct from mazda for the rx7 was complete including how to replace all those seals (24 components per rotor)
 
Harveygp,  have you driven a RX? they unlike any piston engine, the power is smooth and does not fall off telling you it is time to shift, the engine will continue to produce more power above the redline than spec'd.  Why the journalists were raving about the redline was not about the number but about the power and the feel of the power at redline.

I always thought the engine was comparable to a 4 cylinder you get two bangs per revolution of the crank (the crank is spinning at 3 times rotor speed)

Hydrae

RE: Mazda rx8 wankel efficiency nearly there?

(OP)
Hydrae
I'm very entusiastic about rotaries, so regarding the redline, I'm only saying that if we could compare apples with apples more easily, we would be able to get a better idea of how (say) the RX8 compares with other cars in the class.

For example, currently if a journalist were to compare bhp at 6000rpm of (say) a BMW six cylider and (say) RX8, that would be unfair to the RX8 since it has 1/3 less power impulses per second.

Crank position is a red herring in terms of determining the equivalent number of cylinders, which most of the recent postings seem to concurr should be six, based on the number of phased four stroke cycles simultaneously repeating. This is important, not just academic, since it would clearly be a nonsense to compare a six cylinder engine with a four cylinder engine using the same piston sizes.

We need to compare like with like to get a true idea of whether for example the intrisic porting advantages and zero valve gear losses  of the Wankel outweigh (or not) the disadvantages of higher surface to volume ratio and seal frictional losses.

The engine size confusion has probably had the more damaging effect on the fortunes of the Wankel topology, long term, since it resulted in the engine being outlawed from many forms of motor sport. It's clearly ludicrous to say the Mazdas are equivalent to 1.3 litres.

For example, just pasted the following from a review, but what does it mean? Completely hopeless interms of comparison.

>Mazda RX-8 Specifications
>
>Engine  
>1.3L displacement gas engine, 210 hp @ 7200 rpm 164 ft->lbs. @ 5000 rpm, premium unleaded fuel  

should perhaps read

2.6L (or 3.9L - not sure, anyone know?)gas engine, 210 hp @ 4800 rpm(equivalent) 246 ft-lbs(equivalent) @ 3300 rpm(equivalent).

Now we're getting somewhere, surely?
  
I had a ride in an RX8 recently and its an ambition to own one. Never driven one, but look forward to it.

RE: Mazda rx8 wankel efficiency nearly there?

patprimmer

I agree that much can be done to improve the piston engine.
The question I'm posing is: on a weight & total engine size
basis is the wankel not inherently and insurmountable
superior to a 4 stroke piston? Likewise isn't the piston
inherently and insurmountable superior to a wankel in terms
of fuel efficiency?

If the above is true, then a hybrid wankel overcomes the
efficiency deficit, whereas a hybrid piston will gain
efficiency at the expense of weight & size.

RE: Mazda rx8 wankel efficiency nearly there?

For a 360 degrees rotor cycle there are 6 pulses on the Renesis, so it's more like a 12 cylinder, not 6. The whole 1.3l volume is processed, so we have to make the equivalent V12 2.6l. However, it will only have 1/3 of the quoted 10000rpm, hence 3x the torque value. Actually the high shaft revs are an advantage as it allows a smaller transmission to follow.

The big handicap is surface/area. On the bright side simplicity, dimmensions, robustness and fewer losses, but still insufficient to overcome consumption, regardless of the claimed figures. It is still rather unnefficient. A cool concept though, still hugely underdeveloped vs piston one.

PS So the equivalent is a 2.6 V12 with 250HP at 3000rpm?? Looks suspicious? That's because it is, the actual swept volume is 2x654ccm per shaft rev, which makes the equivalent V12 8.4l ! Now the comparison is fair.

RE: Mazda rx8 wankel efficiency nearly there?

I have to agree with Harveygp - its the comparison of the cycle times that must be considered. The RX8 has 2 rotors and therefore 6 working volumes. One complete 4-stroke cycle takes place in 3 output shaft revolutions as opposed to 2 in the conventional reciprocating engine. So, if the output shaft of a Wankel engine is turning at 9000 rpm then this will equate to 3000 cycles per second. The crankshaft of a reciprocating equivalent would have to be turning at 6000 rpm to produce 3000 cycles per second.

So, it is reasonable to say that the RX8 engine with the following specs (detailed above):
210 hp @ 7200 rpm 164 ft->lbs. @ 5000 rpm, premium unleaded fuel.

could be equivalent to a 6-cylinder reciprocating engine with total volume 3.9L and the following respective specs as also given by Harveygp):
210 hp @ 4800 rpm 246 ft->lbs. @ 3333 rpm, premium unleaded fuel.

In this light it looks rather poor!! Another point worth noting is the Fuel Consumption quoted for the RX* as this also helps give the game away -: 18 to 24 mpg. Its certainly no 1.3L!!

RE: Mazda rx8 wankel efficiency nearly there?

2
Simplistically

In one minute the output shaft goes round N times
In one minute it makes X bangs
In one minute it inhales V m^3 of air at 100% VE

Why make it more complex than that?



Cheers

Greg Locock

RE: Mazda rx8 wankel efficiency nearly there?

It has 6 chambers, but fired twice as frequent. It is extremely well balanced too, just like a V12. So per rotor revolution it fires as a 12 cyl.

The engine wear is more related to rotor speed than shaft speed, 10000rpm is used mostly for bragging.

In my opinion 654ccm is the volume of a single chamber of the 6. By the time this volume is cycled out, so do the other 5 volumes. That would suggest 3.9l. However, an equivalent classic 3.9l cycles its internal components twice in this interval, so from this perspective it would take a 7.8l, working at the rotor speed.

250hp from a 7.8 is way lower than modern 100hp/l standards, but the 1/3 actual rotor speed helps explaining that.


Simplistically

In one minute the output shaft goes round N times
It goes 10000 times.

In one minute it makes X bangs
It bangs 20000 times

In one minute it inhales V m^3 of air at 100% VE
It inhales 654*20000*10^(-6) m^3, or 13 m^3

Why make it more complex than that?
Did it get simpler?

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, but considering also wear and smothness it is rather a 7.8

And I am not sure any polygonal shape can be used as rotor. The Wankel rotor is a constant width figure, a Reuleaux triangle.

RE: Mazda rx8 wankel efficiency nearly there?

So its black box equivalent is a 10000 rpm 4 cylinder with a capacity of  2.6 litres

Any other conclusion is marketing, not engineering.

Cheers

Greg Locock

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

RE: Mazda rx8 wankel efficiency nearly there?

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).

RE: Mazda rx8 wankel efficiency nearly there?

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.

RE: Mazda rx8 wankel efficiency nearly there?

Quote:

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

RE: Mazda rx8 wankel efficiency nearly there?

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.

RE: Mazda rx8 wankel efficiency nearly there?

(OP)
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.  
 

RE: Mazda rx8 wankel efficiency nearly there?

(OP)
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

 

RE: Mazda rx8 wankel efficiency nearly there?

"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.

RE: Mazda rx8 wankel efficiency nearly there?

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.

RE: Mazda rx8 wankel efficiency nearly there?

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.

RE: Mazda rx8 wankel efficiency nearly there?

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.

RE: Mazda rx8 wankel efficiency nearly there?

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.

RE: Mazda rx8 wankel efficiency nearly there?

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?

RE: Mazda rx8 wankel efficiency nearly there?

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.

RE: Mazda rx8 wankel efficiency nearly there?

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

RE: Mazda rx8 wankel efficiency nearly there?

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

RE: Mazda rx8 wankel efficiency nearly there?

(OP)
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?

RE: Mazda rx8 wankel efficiency nearly there?

(OP)
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.)  

RE: Mazda rx8 wankel efficiency nearly there?

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.

RE: Mazda rx8 wankel efficiency nearly there?

(OP)
Pat,

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

RE: Mazda rx8 wankel efficiency nearly there?

hi,I heard a 18b two rotor engine, a three rotor 21b, both in drag cars, and the four rotor mazda lemans engine, and can't help but to hear a four cylinder engine exhaust note in the two rotor engine, a six cylinder, or a two stroke triple engine exhaust note,in the three rotor engine, and a 180 degree header,cross plane crank engine exhaust note coming from the 4 rotor engine, can that be explaned? or am i imagining things. thanks Pat

RE: Mazda rx8 wankel efficiency nearly there?

HarveyGP,

You are correct.  Some posts have been dumped.  Mine was one.  There was no emotion in it.  Just a thought I had regarding comparing recips and rotaries.  I guess it didn't sound engineery enough.  I'll have to go back to school and take the "sound engineery" communications course.

RE: Mazda rx8 wankel efficiency nearly there?

Quote:

. . . and can't help but to hear a four cylinder engine exhaust note in the two rotor engine, a six cylinder, or a two stroke triple engine exhaust note,in the three rotor engine, and a 180 degree header,cross plane crank engine exhaust note coming from the 4 rotor engine, can that be explaned?
At least partly by the general exhaust configuration.  Did all of those engines feature single exhaust systems or at least was there a portion in each through which the exhaust from all rotors passed (similar to the "X-pipe" arrangement that's commercially available for certain US V8 engines)?

Norm

RE: Mazda rx8 wankel efficiency nearly there?

the four rotor mazda lemans engine dumped its exhaust into a four into one exhaust, the other engines, the two rotor and the three rotor engine where turbocharged.It was expained to me that the two rotor engine had two eccentrics on its shaft with a 180 degree offset, the three rotor had three eccentrics on its shaft with a 120 degree offsets, and the four rotor engine had four eccentrics on its shaft with a 90 degree offset, and that explains the engines similar exhaust note to the 4, 6, and eight cylinder engines.  

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