OK, so Mazda just recently put a rotary engine in it’s new RX-8. I have a couple questions here:
How does a rotary engine work? No pitons or cylinders…I saw part of some program that explained it, but I wasn’t paying attention. I know that there is a large outer casing, and inside in a weirdly shaped tube, and it turns in a lopsides circle, but how? Where is the combustion?
What are the benefits of a rotary engine? Is it more fuel effecient for the amount of power? Do you get more power from a smaller engine? There has to be some reason they wanted to use a rotary engine other than “'cause it’s cool,” right?
Advantages
Light weight and compact.
Smooth: no reciprocating motion.
Extended power “stroke” rotation of the output shaft: 270 degrees vs. the 180 degrees of a piston.
Fewer moving parts: no valves, connecting rods, cams, timing chains. Intake and exhaust timing are accomplished directly by the motion of the rotor.
Flat torque curve because no valves are used.
Cooler combustion means fewer oxides of nitrogen. Catalytic converters lessen this advantage.
Separation of combustion region from intake region is good for hydrogen fuel.
Lower oxides of nitrogen (NOx) emissions.
Disadvantages
High surface to volume ratio in combustion chamber is less thermodynamically efficient. The Wankel’s long and narrow chamber makes for long flame travel, but this is countered by the Mazda’s two spark plugs (three on some racing engines).
Higher fuel consumption in naive designs. This is relative to the application because the high power of the engine must be considered. Thus Mazda has been successful with the RX-7 sports car, where its fuel economy is comparable to other cars in its class. Only 16 years after the first engine ran, the 1973 oil crisis devastated the RCE before it had sufficiently developed to become more economical. Thus the engine has a more negative reputation regarding fuel consumption than is actually deserved.
I don’t pretend to be mechanically inclined in any way, so I can’t give any detailed answer about why a rotary engine would be better or worse than a standard engine. However, the fact that it only appears in one production car should tell you something. What that is, I am not sure, it may be related to manufacturing costs or maintence costs or maybe even patent and licence issues. If there were some tangible bonus or something that could be marketed as a bonus than the rotary engine would appear in more cars even if they were at the higher end of the price spectrum.
There were a number of moderately successful rotary powered motorcycles around the same time as the decline of the two-stroke. The two-stroke and rotary designs had to contend with rapidly progressing four-stroke technology. Emissions and fuel economy took much of the blame, but performance and consumer preference for four strokes ruled the market.
I owned the RE-5. A thoroughly unimpressive motorcycle. Ugly, heavy and ugly. Did I mention that it was ugly? It’s only true redeeming feature was it uniqueness. And that wore off pretty darn fast. One of the few bikes I’ve ever sold.
The rotaries can handle a lot of boost from a turbocharger also. On the other hand, the rotors are very expensive so rebuilding one hits the wallet pretty hard.
Ooh! Ooh! I missed this thread. I’ve had an RX-8 for three months now, so I admit I am biased. It’s great.
The technical aspects of how the Wankel engine works have been covered, so a few things from personal experience:
Yes and no. Yes because the nominal capacity of the engine is only 1.3 litres, yet it develops as much as 230 horsepower (mine is only the 190hp version). No because it still gets through as much petrol as you’d expect for that sort of power, if not more. I’m averaging about 23 miles per (UK) gallon, which is not good, especially since petrol is fast approaching £1 per LITRE thanks to our government’s tax policies… But I digress.
The great thing about the rotary engine is the performance. OK, it’s not the fastest acceleration away from the lights (0-60 in 6.4 seconds) but it just KEEPS ON PULLING. The 230hp version redlines at 9500rpm, and that’s artificially limited. It could apparently go up to 14000rpm with no problem, because of the inherent balance and smoothness of the rotary design. My 190hp car redlines at 8000, but the limiter doesn’t kick in till a bit higher than that. You can happily reach 80mph in 3rd gear, with the engine still pulling hard. The power output has been compared to that of an electric motor, because the torque vs rpm graph is so flat.
I was able to test all these things to the full a couple of days ago, cos Mazda threw in a free track day for all new purchasers On a track hitting speeds up to 105mph I never needed top gear. And the engine was probably running at at least 6500rpm for more or less the whole time on the track, without ever complaining.
Another link for you, specifically about the “Renesis” engine in the RX-8, which is greatly refined compared with the original clunky Wankels…
I’m not quite sure if you were implying that this is a new development. If so, no. Mazda has been putting rotaries into its RX series for at least a couple of decades. Some Mazda buff could tell you how far back the tradition goes.
Best way to describe a rotary is that it’s halfway between a reciprocating engine and a turbine engine. It has a lot of the advantages of a turbine:[ul]
[li]Smooth rotation[/li][li]Mechanically simpler (no valves, cams etc.)[/li][li]Much higher p/w ratio[/li][/ul]
But without the extreme stresses that a true turbine has (i.e. extremely high RPMs and temperatures).
[QUOTE=bouv]
OK, so Mazda just recently put a rotary engine in it’s new RX-8. I have a couple questions here:
How does a rotary engine work? No pitons or cylinders…I saw part of some program that explained it, but I wasn’t paying attention. I know that there is a large outer casing, and inside in a weirdly shaped tube, and it turns in a lopsides circle, but how? Where is the combustion?
[QUOTE]
It seems to be covered but this sight has some nice animations of how the rotary (wankel) and other engines work.
What discourages others from building rotory engines:
-less thermodynamically efficient
-tough to meet smog control standards
-slighty harder than piston engine to improve economy/mpg
But they are nuisances. Bottom line is that there is no great benefit to tooling up a factory and getting engineers to design your very own rotary. In other words, you must ask the begging question:
"Why would anyone want to build a rotary engine?" What could justify the cost of development and tooling up for it?
There would be almost no inherent competetive advantage over other car mfgrs.
You need to think about clearly effective enhancemens to design…think about AWD, Anti-lock brakes, airbags, comp control, port injection, etc…as these CLEARLY delivered competitive advantages that threatened to leave mfgrs in the dust should they ignore these developments. The rotary lacks the distinct mark that these engineering improvements have…and the cost of development would hardly justify it, especially when radical develop efforts/costs are focusing on alternate fuels.
I used to work with a carpenter from Germany; he himself drove VW pickups and a Mercedes-Benz diesel sedan. He’d seen the commercial on TV with the lines: Piston engine goes poink, poink, poink, poink…
And the Mazda goes Hmmmmmmmmm…
We stopped at a filling station, where a wrecked Mazda had been dragged onto the lot. Kurt looked at it and told me, “That Mazda went PppppppTT!”
Just to add one more thing… at the Mazda-sponsored track day they showed some promotional videos over coffee before we started. The main slogan I remember was “Daring to be different”. I think that just about sums it up. There’s no real need for them to go rotary, but once they went down that route in the 60s they just decided to run with it, and see how far they could take the rotary design. I guess they also thought (correctly) that there will always be a market for something perceived as “different”.
As far as the RX-8 goes, the smaller and lighter engine meant they could design the whole car around a specially designed chassis, with the engine set further back (well behind the front wheels) to give better 50-50 weight distribution. The car as it stands simply wouldn’t work with a regular piston engine.
The RX7, nor any production car with a rotary engine I’m aware of, never saw particulary high boost levels in stock trim (never more than 10 PSI, I believe). The boost levels it used were comperable to those of most common turbo piston engines, and in some cases significantly less (The Mitsubishi Evo VIII for example runs 19 PSI). Furthermore, running large amounts of boost in these engines is generally dangerous as with more boost comes more heat, and a higher chance of accidental fuel ignition (detonation). A typical piston engine can tolerate some detonation, but to a rotary engine, detonation spells death.
For any sports car, a unique selling point such as a rotary engine is a Good Thing. Hence the reason Mazda put it in their RX-8 rather than in their saloons/hatchbacks.
One advantage Colophon didn’t mention (well it applies to UK only) is lower road tax - its 1300cc engine puts it in a lower road tax class than any of its competitors. This saves RX-8 owners £55 a year (hey, it’s better in your pocket than the chancellor’s, right?) UK Road Tax Rates
just whats ive noticed, rotary’s are a bit of a cult figure.
Infact, a lot of great drag cars use rotarys, ones with 8 second quarter mile times and the like.
The high revs somehow helps a car in a dragging situation.
Also note the rotary gives off a true purrrrrrrrrrr sound, unlike other sports cars, which give off a put put put (but sometimes cant notice anyway). Is this better? Well, auto enthusiasts could (and would) crap on for hours I suppose
(mine is only the 190hp version). No because it still gets through as much petrol as you’d expect for that sort of power, if not more. I’m averaging about 23 miles per (UK) gallon, which is not good, especially since petrol is fast approaching £1 per LITRE thanks to our government’s tax policies… But I digress.