Boricua

Does anyone understand why the Renesis, or any rotary for that matter, gets such horrible mileage. This has been an issue that to this day I still do not understand. I mean I absolutely love the Rotary engine, but I'm also somewhat of an environmentalist, and I just don't see how a 1.3 liter engine can achieve such poor mileage. In fact, I would have expected the opposite. Any engineers or technical gurus that can explain this for me? I'd love to find out more info on the issue.

dragula53

short answer: It is an inefficient design.

long answer:
http://travel.howstuffworks.com/rotary-engine.htm

rx8wannahave

I think I've seen that link already but I don't think it went into detail on WHY the rotary is ineffcient.

I too would like an explanation...basically...like this is why a PISTON is and this is why a ROTARY aint.

Gomez

Put very simply, fuel in equals power out. How many other 1.3 litre naturally aspirated engines produce this much power? If you want power, there is a trade off....

This car produces more power than my old 5 litre Ford Windsor.....and has around the same fuel consumption. I'm not complaining

dmp

Piston Engines of similar displacement, making similar power will consume at LEAST as much fuel.

Boricua

That is true. But by the same token, there are many 4 and even 6 cylinder engines that get almost the same, or at least similar, power outputs and still get decently good mileage. Gomez: I see your comparison to the Windsor, but thats comparing a new engine (Renesis) to an old engine. I would imagine that the newer engine would be more efficient in the sense of fuel consumption. And I too would also like to know why exactly the rotary design is just inherently inefficient.

dragula53

the article explains it..

fuel burnt goes to 2 things, basically. heat and motion (or expansion or whatever you want to call the oomph).

in a rotary, a lot more of the burnt fuel goes to heat than a regular piston engine.

My explanation sucks, and is extremely elementary, but that's pretty much it.

Boricua

Interesting. So let me put it this way. What if you took a rotary and piston engine with the same displacement and same power output. How would the mileage figures compare between the two?
I know I'm pushing this issue alot, but I'd really like to understand the reasoning behind this.

Boricua

Ok, I kinda see what your saying. But I still wonder if the engineers can do something about that (putting more of the fuel towards motion rather than heat).

dragula53

the rotary would be worse.

it is unfortunate, but true.

design limitations and stupid stuff like thermodynamics.

valpac

iTrader: (2)

from http://auto.howstuffworks.com/rotary-engine10.htm


Challenges
There are some challenges in designing a rotary engine:
Typically, it is more difficult (but not impossible) to make a rotary engine meet U.S. emissions regulations.

The manufacturing costs can be higher, mostly because the number of these engines produced is not as high as the number of piston engines.

They typically consume more fuel than a piston engine because the thermodynamic efficiency of the engine is reduced by the long combustion-chamber shape and low compression ratio.



Any more in depth would be too much science. Thermodynamics sux.

dragula53

engineering can go a long way.. It has already come a long way.

The renesis actually produces quite a lot more power than the original lower displacement rotary engines(and the later naturally aspirated engines with the same displacement). And it gets better mileage, too.

There may still be room to get better, but as mazda has the only production vehicle with a rotary engine...

dragula53

and yeah, the article covers a bunch of that :P

and mazda isn't exactly the 800 lb gorilla of the auto industry. money spent on R&D of the rotary isn't as much as it could be.

brillo

I thik the answer that your looking for is that the rotor combusion chamber isn't as efficent as a piston because it isn't as uniform in shape (piston engines have perfect cylinders as combustion chambers) the chamber size may change as the piston moves up and down, but its alot more symetrical than the rotory chamber whic is sorta a half circle, and is constantly moving.

This explains why the fuel is not as perfectly combused as your average piston engine, especially given the complexity of todays fuel managent systems.

Now, some new piston cars are introducing direct injection engines (where fuel is injected directly into the chamber) which are both more powerful and efficient than you average fuel injection setup. I think this could help us as well. Its obviously more expensive, but offeres the greatest degree of control (the new Mazdaspeed 6 as this type of setup).

Honda is claiming their new 2.0 Direct Injection Engine for the next civic puts out like 150HP and 150lb TQ and ges better mileage, which is pretty damn good.

Hope that helps.

dragula53

and yeah. so fuel isn't fully burnt in a rotary either, due to odd combustion chamber shape. some of this is offset by having a second spark plug. And from what I understand, unburnt fuel gets carried on to the next cycle because of the relocation of the exhaust ports to the side housings, instead of just dumped into the exhaust. Which presumably is part of why the renesis both gets better mileage and has lower emissions.

valpac

iTrader: (2)

the rotary engine has been inefficient from its beginnings. The combustion volume is long and narrow. Thats why two spark plug fires sequentially trying to ignite all the fuel in the rectangle. The propogation of the burn does not "reach" the corners or the combustion chambers due to its shape. The fuel also has a hard time fully igniting because the rotor is just moving way too fast. There isnt the same latency (time at top dead center) on a rotor as a piston engine, therefore it will NEVER burn all the fuel during its combustion stroke (in its current configuration). Get used to it, as it's not going away.

dragula53

yeah, what he said

rx8wannahave

Well, I don't know about that because the Vette's 400HP V8 get's the same city and KILLS the Renesis in highway fuel economy. You gotta love that old Chevy pushrod V8!

Other than that, thanks for explaining it a little further guys. I understand alot better now and I'll go re-read the article again. I wish more money, effort, and research was given to the rotary engine but I can't really defend it too much when you have proven, cheaper, easier, and highly researched piston engines doing the job just fine.

Heck, now with an electric engine attached the old piston looks like it might have just extended it's life some more. I think it would be very interesting to see a Renesis with an Electric Motor Assist engine attached to it. (...and a SC just for kicks...LOL)

Boricua

Wow, all interesting conclusions. Thanks alot. If there's more, please add it. I love to learn about this stuff! Since I'm not that skilled with engineering, I have one quick question; can the thermodynamics issue be dealt with. Rather, can that problem be solved?

dragula53

the issues could be dealt with by changing the design of the engine... chuckles

or you could just change the laws of thermodynamics.

either one would work just fine, I think.

rx8wannahave

LOL...yes, but only GOD can change that...LOL

I'm just messing with you, not making fun...what do I know...

dragula53

If you think that the EPA was inaccurate when rating the RX-8, drive a 'vette in a spirited manner for a couple days. You will see the ugly side of 10 miles per gallon.

the corvette can achieve some obscene mileage on the highway.. far better than cars that have 100+ less horsepower than them. it has an extremely good engine. This isn't a good comparison, as it annihilates many other cars' mileage... like the STi, the Evolution 8, umm... well just about any sporty-ish car.

All in all, the rotary's mileage isn't so terrible. You can make any car get really bad mileage. Rotaries are just more fun to make get bad mileage.

rx8wannahave

Yeah, I bet the EPA was wrong again but I do think that the Vette's V8 has better fuel economy. If you take some city and highway into account, I think the Vette MIGHT avg better. But...I don't own a Vette to say anything more than that...LOL

dragula53

I doubt you will get better mileage in a 'vette.

Nor does mileage affect the total cost of ownership significantly.

the 'vette is $20k+ more than the rx-8. Not in the same category of vehicle.

You can quite probably own 2 rx-8's for the cost of one 'vette. mmm yep. $52,000 for a z06.

IcemanVKO

There are 3 combustions per rotation of the Rotor in the engine, but only 1 rotation of the transfer shaft per combustion.

In some ways rotary engines don't lose as much energy to piston inertia.