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/

 

[patprimmer]

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
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[GraviMan]

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

 

[patprimmer]

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
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[GraviMan]

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

 

[patprimmer]

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
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[GraviMan]

Good point well made, Pat.

Mart

 

[harveygp]

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

 

[GraviMan]

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

 

[patprimmer]

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

 

[GraviMan]

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

 

[NormPeterson]

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

 

[harveygp]

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

 

[GraviMan]

"...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

 

[jomor]

If you like rotory engines, check this link out. He made a prototype with a zero-vibration chainsaw.

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

jomor

 

[GraviMan]

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

 

[harveygp]

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

 

[harveygp]

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

 

[GraviMan]

"...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

 

[jomor]

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