Sorry if this has been brought up before but they seems to be more efficent than piston.
And wny is it only Mazda uses them?

I thought they are much less efficient...

They produce a lot more bhp per liter of displacement than a piston engine, but they pose very specific issues. I'm sure someone can elaborate on those.


I remember reading that the rotary engine in the Mazda RX-792P IMSA racecar gave off so much heat through the exhaust, that the car was being constantly set on fire.

Blimey, it would probably cook the rear of the car by lap 5

Mechanically, the rotary is more efficient than a piston engine. It's not even close thermally, though. The Wankel is so inefficient with respect to heat usage, that it will never be a popular option. It's only real advantage is packaging, and when you take into account the radiator and fuel tank sizing necessary, that advantage is minimized.

Combustion chamber area to volume ratio is high which means heat "escapes" rather than does useful work.

I have heard that before and wondered if more, smaller rotors would help to alleviate that.

No, the surface area to volume ratio gets worse (higher) as you scale an engine down. The higher speeds of smaller engines helps (less time for heat transfer) but everything else (friction etc) works against thermal efficiency.

They do? F1 engines have been making 300+ bhp/litre.

Well this at least explains the general trend of your posts. In the universe to which I belong, volume scales with linear dimensions^3, and areas with linear dimension^2. In your universe, of which you write so authoritatively on a regular basis, apparently this is not so. In future please could you remind us which universe you are writing about when you answer posts, it would reduce the confusion.

Awesome....

Whoops - brainsnap. Of course surface area to volume ratio gets WORSE as you scale an engine down. (In the popularly accepted universe)

And funnily enough rotary engines get more efficient the smaller they are and less efficient the larger they are.

bit harsh on the rotary to compare the rotary road car motor to an f1 motor don't ya think? besides what did the Le man rotary make? i think it was about 700bhp from 2.6L wasn't it?

Just for information, I ran a 'Works' Norton rotary engine in the early nineties in a hillclimb single seater, 588cc producing 143hp on pump fuel. This isn't a million miles away from F1 efficiency particularly considering this was 17 years ago.

I don't think so.

The first thing you have to do is decide just how to calculate the capacity of the rotary.

So why didn't the big capacity rotary catch on?

For reasons other than thermal efficiency I guess.

hmm I see. From memory most rotors have dishes in them too. This must affect the cc surface area a bit too.


How many RPM? That must that have sounded something different.

The dish is intended to serve as a small, compact combustion chamber - low surface area. Unfortunately as soon as the rotor moves away from TDC the remainder of the rotor and housing surfaces are fully exposed, revealing lots of "sliver" shaped regions where combustion is likely to be quenched. Direct injection would help ensure these regions were devoid of fuel but problematic with the "chamber" being a moving target.

700 hp from 2616 cc and 180 kg, that's 268 hp per litre and 3.9 hp per kg. While the former is high by naturally aspiranted four stroke piston engine standards, although a F1 engine is able to beat it, it's low by two stroke piston engine standard, and the rotary use its full displacement one every rotation of its shaft just like the two stroke. The latter, power to weight, is not particulary impressive.

For a road car engine the power per displacement doesn't really matter, power output in relation to cost and fuel consumption are much more important.

Except a F1 engine is not designed to run 24 hours non-stop.

A mate of mine who built Group C Rotaries and 6 of the entrants for the last Group C Bathurst to run carried his "accidently" saw some rotors that were from Moffats (read Mazda works) RX7 and rather than a single radius they had a double radius with slight tapering from the seals to the very small chambers, probably an attempt to wave the fuel towards the center I guess. He was going to try it himself but of course they knocked Group C on the head.

He supplied engines to a little less than 300hp, after that the gearboxes would go pop except for Moff's gearbox (it's said Moff had 320hp) and a lot of RX7 runners were unhappy about that with Mazda. It's not the power or revs that makes the boxes go pop, it's the uneven running down low bucking the driveline and the more power (greater port timing/overlap) the worse they are down low.

One thing I like about rotaries is the easy hp you can get out of a 12A, 220 - 240hp for a home workshop is quite realistic from a standard motor.

13B's need expensive wide side gears and lightweight carbon apex seals which also require the rotors to be machined and they will go bang if stressed (usually the side gears although they spit seals out of the exhaust port occasionally).

Not far from it at the moment.

The engine revved to 12,000 RPM. It had iron rotors and at the time the 'John Player' works team were looking at developing titanium rotors to try and get more rpm, but I don't know if they ever did this as Norton were then in their death-throws.
Exhaust temperatures were the biggest problem, with the whole system including the silencer glowing red, and noise was also an issue not helped by the stainless steel silencer packing disappearing rather quickly!

Still very far. The engines that did the most running this season did less than 2000 km each AFAIK. Le Mans-winning cars have been averaging 5300 km in 24 hours in the last few years.

Of course, with 2000 revs knocked off the sprint race redline, Peugeot's A32 (or whatever it was) 3.5 V10 did survive 24 hours.

The engine is on the heavy side even for a Le Mans engine.

Seems to me the most recent quote I read about F1 engines was something to the effect that - "my boss would be very angry if our current 2000km engine was capable of 3000km". Any race engine that has a lifespan significantly beyond that dictated by the rules suggest there's some power left on the table. Toyota is claiming they've run 2500km with some engines depending on tracks.

the engine was also essetially two road car motors on a common shaft... ok heavily modified ones but none the less road based so it aint bad in my book

Paddy's axe Jeremy.

The CNC machined centered housings were obviously from billet aluminium with electron coating liners.

I believe the rotors were in Ti, the side gears would have been proven 13B Mazda Comp. wide ones with special attention no doubt and seals would have been in ceramic.

The 3 side housings to the middle were probably based on standard castings, heavily modified. The rear and front side housing would have been special castings for sure.

Then theres the crank...

So Paddy replaced his axe handle with carbon fibre and the head with Ti but it's still the same axe he bought at the hardware store?

i take your point, and by no means was i trying to say it was a complete road car motor, but my point was more that it was a compromised design, I'm sure a better job could have been done with a clean sheet of paper.

and a 4 stroke 1 litre road bike engine is somewhere in the region of 2.7 hp per KG but that is including the gearbox.. if you take that out, we are in the high 4's..

I have no technical knowledge.

So a statement / question.

I would say that the engineers in F1, given any engine to develop are smart enough to push the envelope past what has ever been done before, they will come up with new and interesting ways in how to deal with the drawbacks / weaknesses of a given design. While getting every single iota and BHP from the advantages / strengths of a given design.

I accept that there are thermal issues with a wankel.

But should we not presume that the engineers of F1 could do more with a wankel, than have ever been done before? And even though it is not currently legal, can we not presume that a wankel could at least be developed to be the 2nd worst engine on the grid?

It would only be at the back of the grid and only on the grid because of it's great HP per $ figure as has been proven for the last 30 years in many open racing classes and club days filling up spots mid field back.

There seems to be some misconception about F1 Teams and their abilities compared to Manufacturers - Mazda had 30+ engine test beds going night and day developing the Rotary and no F1 Team can match that. Todays F1 is so reliable in all component area because of the more recent "Manufacture's Era"






Couldn't disagree more, the Rotary that you buy today, production materials aside, is as highly developed as it can be. No engine in history has had the effort, time and money put into it from a single manufacturers perspective that the Rotary has from Mazda.

More than 20 years ago I had an original Mazda hardback book about it's development, it's gone now but amazing what they went through. Very frank too about their early failure rates. There was a good one put out too about Moffats ATCC RX7 - had some of that book's contents in it.

i agree with the first point... but the second point??? to say that a road car motor is as highly developed as it can be is a bit hard to swallow. i realise more effort than is normal was put in by Mazda especially in the early days but in terms of hours spent developing the motor would have to pale in comparison with the time put into the piston engine over its history. to say all that can be done has been done and there is no potential left in particular if you where to build a dedicated race motor. i'm not saying they are ultimately better by the way, just i'm sure if someone did a motor from scratch they would get it lighter and with more power than anything derived from a road car engine

Well...

I was actually thinking an effort like Mazda, and not Joe DSM in the back garage.

That there is more to a wankel than what is currently seen as the 'top performance'.

C'mon cheapy, rotaries dominate in classes like IPRA back home in Oz and have done for many years!

Explain this then - any decent builder can take a standard 13B add wide side gears, machine the standard rotors for carbon apex seals, expoxy the side ports and add Mazda spec PP housings (or port them yourself) and go racing at Bathurst with 320 hp wet sumped and that is with porting on the safe side. Even Mazda comp rotors are only 1/2 lb lighter than a road cars (mostly from being machined for the wider apex seals anyway.

The 26B (13B + 13B = 26B) with the full force of Mazda Racing behind it, scratch built, dry sumped, 3 spark plugs per cylinder and driving race standard ancilleries was good for 350hp per 13B a staggering 30hp on top of what you can get in your home shed.

Apparently you didn't read this that I said - "No engine in history has had the effort, time and money put into it from a single manufacturers perspective that the Rotary has from Mazda"

Now tell everyone the truth about the rules of that class.

Hi Phil

I remember your car well from the early nineties [ we competed in the Leaders champ during those years ]

The most memorable signature HOWL as you nailed it up Shelsley !

Wonderful memories

Carl

I think I can remember it too. Was it a bit temperamental but quick? Was that the car that got tipped over on it's side to get worked on sometimes?

Not sure, I suppose you're going to suggest that the parity in the rules is wrong?

I crewed on an RX-2 in the IMSA RS series. I remember a lot of guys just sent a check to Racing Beat, got an engine in a crate. Others built their own engines, doing things like polishing the side of the rotors and housings by hand, on sandpaper on a glass plate in the parts washer. We ran tracks like Daytona and Talladega, those rotaries would run and run and run week after week, while the piston engined cars needed rebuilds. Mike Meyers lost top gear at Road Atlanta about halfway through an enduro, he had a tired engine needing rebuild anyway so he decided to just keep driving, finished the race turning crazy rpm and it never blew up. I have a dim recollection that Datsun guru Don Devendorf lobbied to get the piston/rotary displacement equivalency factor redefined, I think he authored an SAE paper that I remember reading to that effect. I tried searching but couldn't find it, I did find this Ludvigsen article :
http://www.hemmings.com/hsx/stories/2008/0..._feature11.html

It's not really two production engines on a common shaft (a three piece shaft actually). It shares the basic rotor geometry with the production engines yes, but all the parts are made specifically for it. This can be done with a piston engine too, and the result will be nothing like the production engine.

Parity?

No I think all turbo cars should run dead standard or with inlet restrictors and boost monitors against full racing rotary PP's.

Anyway, getting off topic a bit - that is specific to 1 class.

I guess it really depends on what you define as efficiency. If it is as Victor says hp/l then the rotary is tops - depending how you rate the capacity of a rotary of course.

In terms of BSFC I thought rotaries were poor compared to pistons.

I shut up after my initial comment, as I didn't want to dig a deeper hole for myself, but I meant overall, and should have said so. I remember the Wankel 'epitrochoidal' engine being announced in the press, complete with photograph of the rotor in its housing, and reading that it was going to change the World...I'm still waiting.

I once wrote on a forum that no manufacturer has ever built a successful rotary engine. I think I am still right.
Mazda only makes them because they can't stop. Imagine if one day they said that it was all a mistake and they should never have started making them in the first place. They can't do it because they would look bad. All they can do is produce a small number a year and fit them to the right class of car to keep the warranty costs down and just keep doing this forever. In 1974 they made about 400,000 rotaries and now they just make a handful. They are not a success and never will be.

They are not brave enough to actually give up just like Citroen, NSU, Rolls Royce, Mercedes, Datsun, Suzuki, Chevrolet, etc. did.

Ow. have you driven an RX8? Bloody good car I reckon.