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  #1  
Old 09-30-2007, 08:22 PM
solkoe solkoe is offline
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Does anyone still make a Wankel engine?

This engine used to be in the Mazda RX7. I wonder if it is still available in some new cars. If not, when was the last time it was used and why is it not used anymore?
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  #2  
Old 09-30-2007, 08:33 PM
waterj2 waterj2 is online now
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Originally Posted by solkoe
This engine used to be in the Mazda RX7. I wonder if it is still available in some new cars. If not, when was the last time it was used and why is it not used anymore?
It's still used in the successor to the RX7, the RX8. I think that's the only automotive use, though.
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  #3  
Old 09-30-2007, 08:35 PM
Throatwarbler Mangrove Throatwarbler Mangrove is offline
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Uh, yeah, Mazda made a successor, called the RX-8

It's a little odd that you know of one and not the other. Been watching The Fast and Furious I gather?
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  #4  
Old 10-01-2007, 12:45 AM
Harmonix Harmonix is offline
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Originally Posted by Throatwarbler Mangrove
It's a little odd that you know of one and not the other. Been watching The Fast and Furious I gather?
Maybe it's just me, but I don't very many of the the fast and the furious crowd knowing the rotary engine by the Wankel name.
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  #5  
Old 10-01-2007, 02:33 AM
Colophon Colophon is offline
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I had an RX-8 for three years but sold it a few months ago. Lovely car, but too damn thirsty when you're paying almost £1 a litre for petrol.
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  #6  
Old 10-01-2007, 02:42 AM
Sage Rat Sage Rat is online now
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The rotary engine, from what I read, had issues meeting emissions standards because the shape of the combustion chamber exposed more grease to the flame. The RX-8 was eventually released of course, but I assume that it was a good bit of a pain to get working up to standards and ultimately only a very small number of people would buy a car because it has a funny combustion chamber.
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  #7  
Old 10-01-2007, 09:14 AM
Mr. Slant Mr.  Slant is offline
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Quote:
Originally Posted by solkoe
This engine used to be in the Mazda RX7. I wonder if it is still available in some new cars. If not, when was the last time it was used and why is it not used anymore?
It's used in the RX8.
The only real downside I know of is that the gas mileage SUCKS. Much worse than it should, given the weight and horsepower of the vehicle in question.
The RX8 has serious engine relaibility issues, but I don't know if that's really a problem with rotary designs in general.

Last edited by Mr. Slant; 10-01-2007 at 09:19 AM..
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  #8  
Old 10-01-2007, 09:46 AM
flex727 flex727 is offline
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Originally Posted by Mr. Slant
The RX8 has serious engine relaibility issues, but I don't know if that's really a problem with rotary designs in general.
I owned an RX-3 once upon a time. I was quite a bit younger and more irresponsible then and stupidly allowed the engine to run completely out of oil. I drove it until the engine seized up and stopped. The next day I came back, put in 5 quarts of oil, and it started right up and ran well. It did have seal damage and burned oil after that though. It was a remarkable engine with extremely high HP for the small weight and displacement. Only 3 moving parts in the engine itself. I wouldn't call it a gas hog, but it was certainly not fuel efficient.
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  #9  
Old 10-01-2007, 10:44 AM
Rick Rick is online now
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Quote:
Originally Posted by Mr. Slant
It's used in the RX8.
The only real downside I know of is that the gas mileage SUCKS. Much worse than it should, given the weight and horsepower of the vehicle in question.
The RX8 has serious engine reliability issues, but I don't know if that's really a problem with rotary designs in general.
There is a second downside, and that is the engine is very dirty. The emission levels in a rotary are much higher than in a comparable Otto cycle engine.
IIRC HC and NOX are especially high.
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  #10  
Old 10-01-2007, 01:15 PM
MrFloppy MrFloppy is offline
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Yup, the RX8 is the only one. Almost bought one so I know a bit about it. Comes in two versions. A 4 port 197HP version with an auto tranny and a 6 port 240hp version which only comes with a stick.

It is a 2 rotor design and unlike the last gen RX7 is not turbocharged.

Reliablility is questionable. One thing I learned when researching was that when you shut it down, you have to hold it at 3000RPM for a few seconds or it won't restart. This may have been fixed for 2008. It goes through a quart of oil every 1000 miles and fuel economy is not good especially considering the engine has an equivalent displacement of only 1300cc.

The RX8 is an eccentric vehicle with an eccentric engine. It did handle very very well though.

Anyone (especially Euro dopers) remember the NSU Ro80?
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  #11  
Old 10-01-2007, 01:25 PM
Rick Rick is online now
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Quote:
Originally Posted by MrFloppy
Reliablility is questionable. One thing I learned when researching was that when you shut it down, you have to hold it at 3000RPM for a few seconds or it won't restart.
To the best of knowledge this is not true. I have driven them, and this is not a requirement. What you don't want to do is have the battery go dead. I saw a Mazda instructor struggle to get one of his RX 8 running again after the computer lost it's memory.
They use oil by design. Oil is injected into the lip seals to keep them working. The engine has a 7 qt oil capacity as a result.
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  #12  
Old 10-01-2007, 01:35 PM
MrFloppy MrFloppy is offline
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Ran out of edit time.

A work colleague of mine who dabbles with remote control model aeroplanes informed me that there is a 5cc mini wankel engine. I found it here: http://www.osengines.com/engines/osmg1401.html

Also, the seat belt tensioners in some VWs and most Benzs, uses a small wankel engine to pull the belts in during an accident.

Last edited by MrFloppy; 10-01-2007 at 01:40 PM..
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  #13  
Old 10-01-2007, 01:38 PM
Myglaren Myglaren is offline
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Quote:
Originally Posted by MrFloppy
Yup, the RX8 is the only one. Almost bought one so I know a bit about it. Comes in two versions. A 4 port 197HP version with an auto tranny and a 6 port 240hp version which only comes with a stick.

It is a 2 rotor design and unlike the last gen RX7 is not turbocharged.

Reliablility is questionable. One thing I learned when researching was that when you shut it down, you have to hold it at 3000RPM for a few seconds or it won't restart. This may have been fixed for 2008. It goes through a quart of oil every 1000 miles and fuel economy is not good especially considering the engine has an equivalent displacement of only 1300cc.

The RX8 is an eccentric vehicle with an eccentric engine. It did handle very very well though.

Anyone (especially Euro dopers) remember the NSU Ro80?
Yes, although I have only ever seen one in my then hometown of Whitby, Yorkshire. Damned if I can remember who owned it but my dad knew them and being a fitter was very interested in the unusual engine design.
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  #14  
Old 10-01-2007, 01:41 PM
MrFloppy MrFloppy is offline
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Originally Posted by Myglaren
Yes, although I have only ever seen one in my then hometown of Whitby, Yorkshire. Damned if I can remember who owned it but my dad knew them and being a fitter was very interested in the unusual engine design.
NSU still holds the licensing for rotary engines. I believe to this day you will find 'NSU' stamped on the Mazda Renesis engine.
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  #15  
Old 10-01-2007, 01:43 PM
Throatwarbler Mangrove Throatwarbler Mangrove is offline
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fuel economy is not good especially considering the engine has an equivalent displacement of only 1300cc.
I would like to correct this misconception. A vehicle's fuel economy has very little directly to do with the displacement of its engine - a 5.7L Corvette gets the same EPA fuel economy rating as a 2.1l Honda S2000, and the Vette is a heavier, larger car to boot. There is no reason why a large engine cannot do the same work with the same amount of fuel as a small one. In the case of the rotary, this has to do with the compression ratio - two rotors == two combustion chambers, and 1300cc is pretty big for what is essentially a 2 cylinder engine. The low compression ratio in the rotors makes for inefficient combustion and increased emissions.

Without going into a lot of detail, my mechanic gave me the easy explanation for Rotary reliability: The engine revvs very high and has little torque, which translates into fewer reciprocating parts that none the less must move much more than an equivalent piston engine. Thus a rotary that's gone for 200,000km has gone through the same amount of movement and stress as a piston engine with 400,000km. It came up while we were mulling over my friend's RX-7, that I had convinced him to buy(That will teach him to ask me for car advice again).

That's FAR from an exact scientific explanation and there's a lot more involved, but it gives you a general idea. The RX-7 in particular also had a very advanced (for the time) sequential twin-turbocharger system, which added great deal of complexity to an already less-than-reliable engine.

Last edited by Throatwarbler Mangrove; 10-01-2007 at 01:45 PM..
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  #16  
Old 10-01-2007, 01:47 PM
Contrapuntal Contrapuntal is offline
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I'm pretty sure the Mazda RX8 has a Wankel engine.
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  #17  
Old 10-01-2007, 02:30 PM
sqweels sqweels is offline
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I got 'cher "wankel" right here!
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  #18  
Old 10-01-2007, 02:36 PM
Rick Rick is online now
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Quote:
Originally Posted by MrFloppy
Also, the seat belt tensioners in some VWs and most Benzs, uses a small wankel engine to pull the belts in during an accident.
Again, to the best of my knowledge this is also not true. Do you have a cite for this?
Thinking off the top of my head, I can't see how this could be true. The start up time for any internal combustion engine would exceed the time frame for deployment of the seat belt tensioners. We are talking <40 milliseconds for the tensioner to operate. Everyone I have ever seen is either pyrotechnic or mechanical (a big ass spring)
So once again, cite?
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  #19  
Old 10-01-2007, 02:39 PM
MrFloppy MrFloppy is offline
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Originally Posted by Throatwarbler Mangrove
I would like to correct this misconception. A vehicle's fuel economy has very little directly to do with the displacement of its engine - a 5.7L Corvette gets the same EPA fuel economy rating as a 2.1l Honda S2000, and the Vette is a heavier, larger car to boot.
That is only because in order for Chevrolet to meet CAFE and other EPA fuel economy standards (and still be able to sell a ton of Corvettes), the 6th gear on the C5 and C6 Vette is almost absurdly low. Like an overdrive with overdrive so that at 65mph the engine is barely turning 1500RPM. That makes for good fuel economy so long as you stay at 65mph in top gear.

You're not seriously trying to tell me that a 430HP 6.2l V8 has a 'real world' fuel economy that is better than a 2.2l 4 banger now are you?

Fuel economy is dependant on many things and one of the major factors is the displacement of the engine.

Last edited by MrFloppy; 10-01-2007 at 02:41 PM..
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  #20  
Old 10-01-2007, 02:51 PM
steronz steronz is offline
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Not that I can add much at this point, but my basic understanding of why rotaries suffer in the reliability department is because you're trying to maintain a tight seal in an irregular shape. Machining round parts is relatively easy - drills make nice round holes in engine blocks, and lathes make nice round pistons to fit inside them. Slap on a few cheap piston rings, and you have a nice setup that will withstand lots of abuse and even a little bit of wear/warping.

Rotary chambers are mostly oval, but not quite... it's an oval with the sides tucked in a bit, sort of like an overinflated 8. You then have to get a triangle to maintain a constant seal inside that shape. The seals at each point of the triangle are called apex seals, and because of the extreme precision required to keep a seal with that bizarre shape, they wear out a lot. When rotaries fail, it's not usually in a big kablooey, time to get a new car sort of way. They just start burning more and more oil until you can't take it anymore and rebuild it. The trick to getting a rotary to last past 100,000 miles seems to be lots of proper maintenance and lots of luck.
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  #21  
Old 10-01-2007, 02:53 PM
Figaro Figaro is offline
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Originally Posted by MrFloppy
You're not seriously trying to tell me that a 430HP 6.2l V8 has a 'real world' fuel economy that is better than a 2.2l 4 banger now are you?
I don't have a horse in this race, but given the absurdly high RPMs required to extract horsepower from the S2000 engine, I wouldn't be surprised if it burned fuel like crazy when driven hard.
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  #22  
Old 10-01-2007, 03:01 PM
Throatwarbler Mangrove Throatwarbler Mangrove is offline
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That is only because in order for Chevrolet to meet CAFE and other EPA fuel economy standards (and still be able to sell a ton of Corvettes), the 6th gear on the C5 and C6 Vette is almost absurdly low. Like an overdrive with overdrive so that at 65mph the engine is barely turning 1500RPM. That makes for good fuel economy so long as you stay at 65mph in top gear.
1) The Honda meets the same regs.

2) What you just said proves my point exactly - with the right gearing, a big engine is just as efficient as a little one. The only difference is that with a smaller engine, you CAN'T do what the corvette does because there isn't enough torque.

Quote:
You're not seriously trying to tell me that a 430HP 6.2l V8 has a 'real world' fuel economy that is better than a 2.2l 4 banger now are you?
No, I'm telling you that when driven the SAME way, the big engine does get better fuel economy, and the EPA figures prove it. Whether Corvettes and Hondas are driven the SAME way in the real world is another question entirely.

I purposely chose the S2000 because it's also a sports car and is set up with performance oriented gearing, cams etc. Of course in terms of performance and power it doesn't hold a candle to the Vette. Comparing it to a Honda Civic would be pointless, since the Civic is not physically capable of performing like a sports car.

Do you remember the Top Gear episode where Jeremy Clarkson ran the Vette from 0-100 entirely in 5th gear(or something like that)? The big V8 engine has so much torque on tap that you can happily do that all day, and get great fuel economy, if you don't mind economy car-like performance. You don't have a choice with the Honda.

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Fuel economy is dependant on many things and one of the major factors is the displacement of the engine.
See cite above. What you are actually seeing is correlation, not causation - Most vehicles with big engines are either sports cars or large, heavy utility vehicles/trucks. These vehicles generally are less fuel efficient, but that isn't because of the displacement of the engine.

Since we're on the topic of RX-7s, one of the most popular aftermarket "modifications" to the RX-7 is to simply remove the factory rotary engine and turbo setup entirely and replace it with a store-bought GM or Ford V8. Here is one of the bigger outfits specializing in this. The weight and balance of the car remains almost unchanged, power and torque are increased significantly, and fuel economy and reliability are both much improved. This is additionally attractive due to the low cost and relative abundance of used RX-7s with non-functioning rotary engines.

Last edited by Throatwarbler Mangrove; 10-01-2007 at 03:04 PM..
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  #23  
Old 10-01-2007, 03:09 PM
Throatwarbler Mangrove Throatwarbler Mangrove is offline
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Also, the seat belt tensioners in some VWs and most Benzs, uses a small wankel engine to pull the belts in during an accident.
Didn't see this one. I've taken the seats apart on my Benz and you are completely incorrect.
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  #24  
Old 10-01-2007, 03:12 PM
steronz steronz is offline
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I'm gonna back Throatwarbler up and say that the displacement is almost irrelevant. What matters is how much mass you're moving, and how quickly you do it. Given a car of the same weight, a 6.2L V8 and a 2.2L I4 will burn almost the same amount of fuel if driven at the same speeds with the same coefficient of drag, intake and combustion chamber design notwithstanding. Granted, the 'vette will likely burn a lot more gas because it can go faster, but as the proud owner of a high revving honda, I can say that fuel economy is not its strong point when you drive it like it ought to be driven.

As a fun little game, you can go out to edmunds and start comparing EPA ratings for the same car with differently sized engines. I don't have a whole lot of time to do it, but the base model 2008 accord weighs in at 32xx lbs, has a 2.4L 4 cyl, and gets 21/31. The 3.5L V6 weighs 35xx and gets 19/29. What do you think is a bigger factor there -- the 1.1L of displacement and extra 2 slugs, or the extra 300 lbs that the V6 necessitates carrying around? I think if you were to compile the data and do the math, it would back up the hypothesis that smaller displacement engines get better mileage only because they weigh less.
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  #25  
Old 10-01-2007, 03:15 PM
MrFloppy MrFloppy is offline
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Originally Posted by Throatwarbler Mangrove
Didn't see this one. I've taken the seats apart on my Benz and you are completely incorrect.
Well you need to get with it. Buy a new one.

http://www.mercedestechstore.com/pdf...2010-30-02.pdf

I hope that link works.
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  #26  
Old 10-01-2007, 03:20 PM
Throatwarbler Mangrove Throatwarbler Mangrove is offline
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Well, I wouldn't really call that an "Engine", but OK, fair enough. (Mine was the "Original cable" type)
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  #27  
Old 10-01-2007, 03:30 PM
Rick Rick is online now
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Originally Posted by MrFloppy
Well you need to get with it. Buy a new one.

http://www.mercedestechstore.com/pdf...2010-30-02.pdf

I hope that link works.
While the system uses a wankel type rotor, to call it an engine is stretching the definition of what most people consider an engine.

[side comment] man what a strange way to design a pre-tensioner. Like we used to say, Mercedes Benz over-engineered like no other car in the world.[/sc]
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  #28  
Old 10-01-2007, 03:32 PM
Throatwarbler Mangrove Throatwarbler Mangrove is offline
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I think if you were to compile the data and do the math, it would back up the hypothesis that smaller displacement engines get better mileage only because they weigh less.
I'm not so sure, because

1) Smaller displacement on the SAME engine actually means LESS weight. The 7.0l Corvette engine weights about 30lbs less than the 5.7l Corvette engine, because the engine block is the same, except the 5.7L has an extra 1.3ls of metal.


2) Even physically smaller engines don't always weigh less. If you look at that Hinson site, you can see that the aluminum 5.7L Corvette engine (LS1) weights LESS than the 4 Cylinder Nissan KA series engine, since the Nissan is made of cast iron. The owner of the BMW Tuner shop here in Calgary drives an E36 (1991-99, I think?) BMW M3, with the old iron BMW I6 engine replaced with an aluminum Corvette engine - same result. The Corvette engine weighs less, sits lower in the engine bay and further back from the front axles, thus improving handling, and gets better fuel economy. All the extra horsepower is just gravy.

3) All things considered, the difference in engine weights is pretty minor compared to the weight of the whole vehicle, even taking into consideration the heavier, beefier transmission and drivetrain in a higher powered car.

Last edited by Throatwarbler Mangrove; 10-01-2007 at 03:33 PM..
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  #29  
Old 10-01-2007, 04:31 PM
scr4 scr4 is online now
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Originally Posted by Throatwarbler Mangrove
A vehicle's fuel economy has very little directly to do with the displacement of its engine -
Then what exactly is the point of variable displacement engines?

And why does a Prius get better highway mileage than the Corolla? The way I understood it, hybrid cars don't use the electric motor during sustained highway driving, and the only reason they get good highway mileage is because they have smaller engines than comparable size non-hybrid cars.
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  #30  
Old 10-01-2007, 04:35 PM
Mr. Slant Mr.  Slant is offline
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The point of variable displacement engines is to create a very minor increase in fuel economy.
They *do* help. Now, how *much* do they help?
The best way to get good gas mileage is to never use the brakes.
The second-best way to get good gas mileage in town is to have a light car.
The second-best way to get good highway gas mileage is to have a car with little wind resistance.
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  #31  
Old 10-01-2007, 04:50 PM
ralph124c ralph124c is offline
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Its amazing that MAZDA poured so much money into the Wankel engine-given that GM, M-B, ans AUDI decided to drop the effort. I wonder if a steam engine based on a wankel would work? I did drive an RX-7 once-it really flew-and the turbine like feel (and lack of vibration) was amazing. Incidentally, a few years ago, automakers were looking at the two-stroke engine for cars-what became of that?
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  #32  
Old 10-01-2007, 05:09 PM
Myglaren Myglaren is offline
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Incidentally, a few years ago, automakers were looking at the two-stroke engine for cars-what became of that?
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SAAB used to have an excellent 3-cylinder 2-stroke. Sounded crap but superb to drive.
SAAB 96
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  #33  
Old 10-01-2007, 05:10 PM
Kevbo Kevbo is offline
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A few things:

-Mazda makes ( or until recently made..not sure) a Japan-only luxury car called a Cosmo that uses a dual rotor, dual turbo induced Wankle. They are available in the US as "Japanese take out" motors.

-In addition to the already mentioned O&S model airplane engine, there is a german built Wankle go-cart engine that is currently making big waves in that sport.

-Big gasoline engines suffer from excessive pumping losses when operated at light load. The 'vette avoids this by using tall gearing that keeps the engine loaded, albeit at low rpm.

-Both Wankle and reciprocating engines are examples of Otto-cycle engines. The fact that a Wankle uses different areas of the epitrichoid housing for intake, compression, expansion, and exhaust notwithstanding.
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  #34  
Old 10-01-2007, 05:14 PM
Throatwarbler Mangrove Throatwarbler Mangrove is offline
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-Mazda makes ( or until recently made..not sure) a Japan-only luxury car called a Cosmo that uses a dual rotor, dual turbo induced Wankle. They are available in the US as "Japanese take out" motors.
To add a little more to that, the Cosmo is actually available with the 2.0l 20B triple rotor engine, in the highest trim level. Both the 20B and 13B used in the RX7 used the same sequential twin turbo system.
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  #35  
Old 10-01-2007, 05:38 PM
steronz steronz is offline
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Quote:
Originally Posted by Throatwarbler Mangrove
I'm not so sure, because

1) Smaller displacement on the SAME engine actually means LESS weight. The 7.0l Corvette engine weights about 30lbs less than the 5.7l Corvette engine, because the engine block is the same, except the 5.7L has an extra 1.3ls of metal.


2) Even physically smaller engines don't always weigh less. If you look at that Hinson site, you can see that the aluminum 5.7L Corvette engine (LS1) weights LESS than the 4 Cylinder Nissan KA series engine, since the Nissan is made of cast iron. The owner of the BMW Tuner shop here in Calgary drives an E36 (1991-99, I think?) BMW M3, with the old iron BMW I6 engine replaced with an aluminum Corvette engine - same result. The Corvette engine weighs less, sits lower in the engine bay and further back from the front axles, thus improving handling, and gets better fuel economy. All the extra horsepower is just gravy.

3) All things considered, the difference in engine weights is pretty minor compared to the weight of the whole vehicle, even taking into consideration the heavier, beefier transmission and drivetrain in a higher powered car.
That's all true, I should have said something along the lines of "if a smaller engined car does get better gas mileage, it can more easily be explained by the decreased weight of the drivetrain than the decrease in raw displacement." It just seems that people are so used to saying "I got the smaller engine (I4 vs V6, V6 vs V8, etc) in my vehicle because it gets better gas mileage," that they lose sight of WHY that's true.

Incidentally, your point about bigger engines not always being heavier is very true, which is why a corvette doesn't weigh a whole lot more than an s2k. My limited experience has shown that a little bit of extra engine weight tends to cascade, as 50 more lbs for a V6 turns into an extra 75lbs in the beefier transmission, + heavier axles, +heavier subframe, and all that extra weight means bigger, heavier front brakes, which means bigger, heavier wheels, etc etc.
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  #36  
Old 10-02-2007, 06:09 AM
Colophon Colophon is offline
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Originally Posted by MrFloppy

Reliablility is questionable. One thing I learned when researching was that when you shut it down, you have to hold it at 3000RPM for a few seconds or it won't restart. This may have been fixed for 2008. It goes through a quart of oil every 1000 miles
That's absolutely not true. As I say, I ran one for three years, bought new in 2004 (only 5 or 6 months after they came out in the UK) and never had any reliability problems whatsoever. I never had to "hold it at 3000rpm" before shutting it down. The only precaution I was told to take was not to start the engine and then shut it down again without letting it run long enough for the idle speed to drop down to normal (say 2-3 mins). I forgot about this a few times but never had any problem, but I was told it could lead to the engine flooding. Also the oil use was not nearly as high as you say. I checked the level after every two or three tanks of petrol and it rarely needed topping up. I'd put in a litre about every 2000 or 3000 miles or so.

Quote:
and fuel economy is not good especially considering the engine has an equivalent displacement of only 1300cc.
I certainly have no arguments there! I would get about 18-19mpg (and these are UK gallons!) on short journeys. From a 61 litre tank the longest I ever got, on a motorway journey, was 305 miles!

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The RX8 is an eccentric vehicle with an eccentric engine. It did handle very very well though.
Also agree. It handled like a dream and was very fun to drive. I miss it.
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  #37  
Old 10-02-2007, 06:59 AM
Xema Xema is offline
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Originally Posted by MrFloppy
Fuel economy is dependant on many things and one of the major factors is the displacement of the engine.
Well, the largest piston engine in the world also boasts the lowest specific fuel consumption.
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  #38  
Old 10-02-2007, 09:42 AM
Dag Otto Dag Otto is offline
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Originally Posted by Rick
While the system uses a wankel type rotor, to call it an engine is stretching the definition of what most people consider an engine.
I was curious about this too, but it most definitely is an engine. It converts the energy from the combustion of a chemical fuel into a mechanical force. Where it differs from what most would consider an engine is the number of cycles it is designed for. It's pretty clever how the second and third cycles are ignited.


[OT]
Looking through that pdf, I see that the crash sensor control module on my 1992 W124 is made by Becker. I hope that they did a better job on that then the pile of junk that they call a stereo in that car.
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  #39  
Old 10-02-2007, 09:43 AM
scr4 scr4 is online now
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Quote:
Originally Posted by Xema
Well, the largest piston engine in the world also boasts the lowest specific fuel consumption.
Yes, but I bet it would be horribly inefficient if you try to extract only 30 horsepower out of it.

The issue is displacement vs. efficiency for a given power output. A typical car needs maybe 60 horsepower to maintain 70 mph*. So, if you compare a 5-liter engine vs. 1-liter engine while each one is producing 60 horsepower, what are their efficiencies?

* A guesstimate based on the fact that my car, a 4-door sedan, has a 70-horsepower engine and can get up to 75 mph or so.
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  #40  
Old 10-02-2007, 10:06 AM
Xema Xema is offline
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Quote:
Originally Posted by scr4
I bet it would be horribly inefficient if you try to extract only 30 horsepower out of it.
Agreed - IC engines are likely to show best efficiency only in a rather narrow range of power output. The "big engines are inefficient" notion comes from using automobile engines that are substantially larger than they really need to be.
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  #41  
Old 10-02-2007, 10:23 AM
Kevbo Kevbo is offline
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Quote:
Originally Posted by Xema
The "big engines are inefficient" notion comes from using automobile engines that are substantially larger than they really need to be.
Non-hybrid IC engines really need to be sized for merging on to a busy, yet quick flowing freeway without getting yourself rear-ended, bucking a strong headwind, or climbing a steep grade at a non-obstructing pace. And yes, this makes them considerably oversized for level cruise conditions with no headwind.

If all cars had the power/weight ratio of a loaded 18 wheeler, then they would all cause the same traffic disruption that occurs when one of those behemoths is trying to merge onto the freeway, or lumbering it's way over a mountain pass.
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  #42  
Old 10-02-2007, 10:31 AM
solkoe solkoe is offline
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Quote:
Originally Posted by Throatwarbler Mangrove
Uh, yeah, Mazda made a successor, called the RX-8

It's a little odd that you know of one and not the other. Been watching The Fast and Furious I gather?
I've never seen the movie. Twenty years ago, a friend of mine had an RX7. I was fascinated by the engine. I'm not into cars so I would have no idea about an RX8.
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  #43  
Old 10-02-2007, 10:42 AM
scr4 scr4 is online now
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Quote:
Originally Posted by Kevbo
And yes, this makes them considerably oversized for level cruise conditions with no headwind.
So all car engines are operating at below optimal power most of the time. Doesn't that mean for almost every car, increasing the engine displacement would make the engine even more overpowered, and therefore less efficient?

Quote:
If all cars had the power/weight ratio of a loaded 18 wheeler, then they would all cause the same traffic disruption that occurs when one of those behemoths is trying to merge onto the freeway...
Cars have brakes, they are perfectly capable of slowing down to allow even the most underpowered truck to merge onto the freeway. If you really don't like that, you can avoid the right-most lane except when necessary.
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  #44  
Old 10-02-2007, 12:06 PM
RedSwinglineOne RedSwinglineOne is offline
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Quote:
Originally Posted by Throatwarbler Mangrove
No, I'm telling you that when driven the SAME way, the big engine does get better fuel economy, and the EPA figures prove it.
I don't believe this is true. If you look at the mpg figures for a model available with several engine choices, the smaller engine almost without exception gives better feul economy.

2007 Mazda MAZDA6 i 5-Door Touring 2.3L 156 hp I4 FWD 24/31 - 32
2007 Mazda MAZDA6 s 5-Door Value Edition 3.0L 212 hp V6 FWD 19 - 20/27

2007 Audi A3 2.0 T DSG 2.0L 200 hp I4 FWD 25/32
2007 Audi A3 3.2 DSG 3.2L 250 hp V6 FWD 21/27


2007 Dodge Caliber Sport Appearance 1.8L 148 hp I4 FWD 26 - 27/30 - 32
2007 Dodge Caliber R/T FWD 2.4L 172 hp I4 FWD 23/26

2005 Dodge Charger SE V-6 3.5L RWD 21/28
2006 Dodge Charger R/T V-8 5.7L RWD 17/25
2006 Dodge Charger SRT8 V-8 6.1L RWD 14/20

I think scr4 is correct in pointing out that engines are not efffeciant when making much less than their max hp. If our goal were only efficiency, why have 8 sets of pistons and rods, and all necessary valving and such, when 4 would create enough power to move down the road?
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  #45  
Old 10-02-2007, 12:31 PM
Mr. Slant Mr.  Slant is offline
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Those numbers prove nothing unless you also include vehicle weight with fluids in place as well as tire sizes and final drive ratio. A sub-optimal gearing choice can cause a 15% gas mileage hit all by itself.
On my car the base engine gets the 'efficient' final drive ratio while the upgraded engine gets the 'sporty' final drive ratio.
Also, in most vehicles the big engines get added to the loaded models, not the stripped ones.

Last edited by Mr. Slant; 10-02-2007 at 12:33 PM..
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  #46  
Old 10-02-2007, 12:41 PM
Throatwarbler Mangrove Throatwarbler Mangrove is offline
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Also, in the case of the Audi, you are comparing a 2.0 l turbocharged engine with a naturally aspirated VR6.

Maybe I was using a poor choice of words. My contention is that as far as the amount of fuel used to move a particular vehicle a certain distance is concerned, the displacement of the internal combustion engine used isn't really indicative of anything. You can use a big engine that will do it in fewer cycles, or a little engine that will do it in more cycles, the amount of work going through the transmission is the same, and if the transmission efficiently moves the power to the wheels, there will be little difference between the two.

You are correct in that car models with larger, more powerful engines typically use more fuel in tests such as those you cite, but as others have pointed out, the discrepancy is due to factors other than the engine displacement. How much fuel is consumed by a vehicle, I would say, is 80% dependent on the driving style, and 20% dependent on the gearing.
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  #47  
Old 10-02-2007, 12:51 PM
Necros Necros is offline
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Location: Golden, CO USA
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Quote:
Originally Posted by Throatwarbler Mangrove
Since we're on the topic of RX-7s, one of the most popular aftermarket "modifications" to the RX-7 is to simply remove the factory rotary engine and turbo setup entirely and replace it with a store-bought GM or Ford V8.
I think this mod is pretty much the antithesis of a "popular" modification, since you will get neverending crap from FD purists about it.
And I will echo the poor gas mileage, although 20 mpg for a 12 second car is not actually THAT bad. Defintely not as good as the 30 mpg I get out of my S2000...

Last edited by Necros; 10-02-2007 at 12:52 PM..
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  #48  
Old 10-02-2007, 12:52 PM
scr4 scr4 is online now
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Quote:
Originally Posted by Throatwarbler Mangrove
You can use a big engine that will do it in fewer cycles, or a little engine that will do it in more cycles, the amount of work going through the transmission is the same, and if the transmission efficiently moves the power to the wheels, there will be little difference between the two.
What about pumping loss? From the Wikipedia article I already linked to:

Quote:
In typical light load driving driver uses only around 30 percent of an engine’s maximum power. In these conditions, the throttle valve is nearly closed and the engine needs to work to draw air. This causes an inefficiency known as pumping loss. Some large capacity engines need to be throttled so much at light load that the cylinder pressure at top dead centre is approximately half that of a small 4 cylinder engine. Low cylinder pressure means low fuel efficiency.

Last edited by scr4; 10-02-2007 at 12:53 PM..
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  #49  
Old 10-02-2007, 01:07 PM
RedSwinglineOne RedSwinglineOne is offline
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Quote:
Originally Posted by Throatwarbler Mangrove
Also, in the case of the Audi, you are comparing a 2.0 l turbocharged engine with a naturally aspirated VR6.
Well that opens up a whole new can of worms, but I think it also proves my point. Why bother with an expensive turbo? Because it allows you to use a smaller, more efficiant motor for cruising, yet provides the power consumers want for passing and merging.

Quote:
How much fuel is consumed by a vehicle, I would say, is 80% dependent on the driving style, and 20% dependent on the gearing.
The auto makers test their cars in absolute best possible scenarios. I don't believe for a minute that these numbers are caused by the tester having a lead foot.
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  #50  
Old 10-02-2007, 02:27 PM
Pushkin Pushkin is offline
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Join Date: Oct 1999
Quote:
Originally Posted by MrFloppy
considering the engine has an equivalent displacement of only 1300cc
Is that the correct equivalent? I'm sure for racing they multiply it up by a few times to get a balance against ordinary piston engined cars.

Quote:
Originally Posted by MrFloppy
Anyone (especially Euro dopers) remember the NSU Ro80?
Yup, quite modern looking too by today's standards. There were also wankel motorbikes.
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