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Inefficiency

 
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tommyd49



Joined: 30 Dec 2005
Posts: 3

PostPosted: Fri Dec 30, 2005 8:49 am    Post subject: Inefficiency Reply with quote

This is my first post here.

Could someone please tell me why rotary engines have inefficiency problems? I have heard it is something to do with not all the fuel being burnt as the chambers are too large.

Thanks
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Blake
Been there, done that


Joined: 12 Jul 2005
Posts: 135
Location: Portland, OR

PostPosted: Fri Dec 30, 2005 1:12 pm    Post subject: Re: Inefficiency Reply with quote

tommyd49 wrote:
This is my first post here.

Could someone please tell me why rotary engines have inefficiency problems? I have heard it is something to do with not all the fuel being burnt as the chambers are too large.

Thanks


There are several efficiencies by which engines are comparred. Rotary engines may be slightly less efficient thermodynamically, but they excel in volumetric efficiency. The result is that they turn a bit more of the fuel to heat than output, but manage to generate a LOT of output for the discplacement. More power with moderate thermodynamic inneficiency yeilds relatively poor fuel economy.

The reason for the lower thermodynamic efficiency is due to the long, thin, rectangular combustion chamber shape. The theoretically ideal combustion chamber is a sphere, for obvious reasons. Piston engines are much closer to that shape. On the other hand, the piston engines have a lot of pumping losses, a restrictive and power-robbing valvetrain, inefficient reciprocating motion, etc.

Keep in mind that the differences are not that large, particularly with the new RENESIS Rotary Engine. Part of the fuel economy problem was actually a tradeoff of using a rich mixture to satbilize the idle and low rpm/load drivability. The RENESIS actually does a much better job expelling CO, so the low speed/load combustion is much improved with a stoich-mix. Taking just one example. The Mazda6 with the 10:1 compression 3.0L V6 producing 215hp, gets 19/27 MPG. The RX-8, with a 10:1 compression 1.3L 13B Rotary Engine producing 238hp, gets 18/24 MPG.

Basically, we rotary engine guys get more Smiles-Per-Miles while our piston pals get better Miles-Per-Gallon. I'm happy with that tradeoff!
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tommyd49



Joined: 30 Dec 2005
Posts: 3

PostPosted: Sat Dec 31, 2005 1:50 pm    Post subject: Reply with quote

Why not have a small 6.5B rotary engine to get better mpg?
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Jingyee
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Joined: 11 Jul 2005
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PostPosted: Sun Jan 01, 2006 3:33 am    Post subject: Reply with quote

lol more smiles-per-miles, Very Happy
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Jingyee
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Joined: 11 Jul 2005
Posts: 85
Location: Portland, OR

PostPosted: Sun Jan 01, 2006 3:35 am    Post subject: Reply with quote

tommyd49 wrote:
Why not have a small 6.5B rotary engine to get better mpg?


Correct me if I'm wrong, but the idea of the rotary engine is for performance, and although high mpg would be nice, it then, does not take priority of performance, so less mpg is expected for the trade of, more smiles-per-miles.
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tommyd49



Joined: 30 Dec 2005
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PostPosted: Sun Jan 01, 2006 7:34 am    Post subject: Reply with quote

Have they tried diesel rotary engines?
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Rotorhog



Joined: 22 Dec 2005
Posts: 9
Location: Russellville,AR

PostPosted: Sun Jan 01, 2006 12:19 pm    Post subject: Reply with quote

Gas mileage isn't the primary focus of the rotary engine, hence it's application in sports cars. After all, no generation of the RX-7 was intended to be an econobox.

You can get respectable mileage out of a rotary if you resist it's invitation to rev and go. My 84 SE was rated at 16 mpg city, 24 hiway back in the days of 55 mph. I regularly get 17 to 19 mpg in town, and with cruise set at 65 - 70 I've seen as good as 28 mpg. And that was with a stock muffler partially clogged with cataclysmic converter chunks. I have since installed a Bonez converter and RB muffler so I haven't had a chance to get on the road and see if it's any better.

Regardless, I can live with it. The fun factor always wins out!
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TCooperDesigns



Joined: 20 Feb 2006
Posts: 4
Location: UK

PostPosted: Mon Feb 20, 2006 8:14 am    Post subject: Reply with quote

tommyd49 wrote:
Have they tried diesel rotary engines?


Yes, way back in the late 60s and early 70s, Rolls Royce worked on a diesel design for use in tanks. The basic unit used one large rotor and one smaller. Air entered the larger rotor where it was compressed and fed to the smaller where it was further compressed. Then combustion and expansion took place followed by passing the exhaust back to the larger rotor for further expansion. An amusing design; presumably not a great success as it appears to have been dropped.
http://www.der-wankelmotor.de/Motoren/Rolls-Royce/rolls-royce.html
http://www.millville.org/Workshops_f/kess_mech/tools/1tools/engines.html
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TCooperDesigns



Joined: 20 Feb 2006
Posts: 4
Location: UK

PostPosted: Mon Feb 20, 2006 8:32 am    Post subject: Re: Inefficiency Reply with quote

Blake wrote:
tommyd49 wrote:
This is my first post here.

Could someone please tell me why rotary engines have inefficiency problems? I have heard it is something to do with not all the fuel being burnt as the chambers are too large.

Thanks


There are several efficiencies by which engines are comparred. Rotary engines may be slightly less efficient thermodynamically, but they excel in volumetric efficiency. The result is that they turn a bit more of the fuel to heat than output, but manage to generate a LOT of output for the discplacement. More power with moderate thermodynamic inneficiency yeilds relatively poor fuel economy.

The reason for the lower thermodynamic efficiency is due to the long, thin, rectangular combustion chamber shape. The theoretically ideal combustion chamber is a sphere, for obvious reasons. Piston engines are much closer to that shape. On the other hand, the piston engines have a lot of pumping losses, a restrictive and power-robbing valvetrain, inefficient reciprocating motion, etc.

Keep in mind that the differences are not that large, particularly with the new RENESIS Rotary Engine. Part of the fuel economy problem was actually a tradeoff of using a rich mixture to satbilize the idle and low rpm/load drivability. The RENESIS actually does a much better job expelling CO, so the low speed/load combustion is much improved with a stoich-mix. Taking just one example. The Mazda6 with the 10:1 compression 3.0L V6 producing 215hp, gets 19/27 MPG. The RX-8, with a 10:1 compression 1.3L 13B Rotary Engine producing 238hp, gets 18/24 MPG.

Basically, we rotary engine guys get more Smiles-Per-Miles while our piston pals get better Miles-Per-Gallon. I'm happy with that tradeoff!


Be wary of quoting engine capacities with Wankels.
For example : a 13B engine has 2 rotors, each rotor having 3 combustion chambers, each displacing 654cc. So what is the capacity?
Bear in mind that the rotor turns at 2/3 shaft speed. Therefore the engine displaces 4 x 654 ccs of air for every 2 turns of the shaft - this is the same displacement as a 2.6 litre 4 cycle piston engine. (The wankel is also a 4 cycle or 4 stroke engine) So the 13B equates to a 2.6 litre engine. This is borne out by the power output. Fuel consumption however reflects the fact that Wankels lose a significantly larger proportion of their heat to the coolant and oil than a piston engine. Lower combustion temperature than in a piston engine results in less power per unit volume of air consumed and flame quenching on the relatively cool housing wall results in some unburned fuel passing into the exhaust where it then burns. This is why Wankels have a very high exhaust manifold temp.

The low combustion temperature means that a relatively high CR can be used without the risk of detonation even when using low octane fuel. The high exhaust temp combined with low tendency to detonate seemingly makes the Wankel a good candidate for turbocharging. We ran an early NSU Wankel (CR 9:1) to boost pressures in excess of 30 psi without experiencing detonation, without intercooling, but using 105 octane fuel.

Remember also that in some countries, cars are taxed according to their engine capacity, so if you can kid the governement that the engine is smaller than it really is, you pay less tax.
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Blake
Been there, done that


Joined: 12 Jul 2005
Posts: 135
Location: Portland, OR

PostPosted: Mon Feb 20, 2006 2:45 pm    Post subject: Re: Inefficiency Reply with quote

TCooperDesigns wrote:
Blake wrote:
tommyd49 wrote:
This is my first post here.

Could someone please tell me why rotary engines have inefficiency problems? I have heard it is something to do with not all the fuel being burnt as the chambers are too large.

Thanks


There are several efficiencies by which engines are comparred. Rotary engines may be slightly less efficient thermodynamically, but they excel in volumetric efficiency. The result is that they turn a bit more of the fuel to heat than output, but manage to generate a LOT of output for the discplacement. More power with moderate thermodynamic inneficiency yeilds relatively poor fuel economy.

The reason for the lower thermodynamic efficiency is due to the long, thin, rectangular combustion chamber shape. The theoretically ideal combustion chamber is a sphere, for obvious reasons. Piston engines are much closer to that shape. On the other hand, the piston engines have a lot of pumping losses, a restrictive and power-robbing valvetrain, inefficient reciprocating motion, etc.

Keep in mind that the differences are not that large, particularly with the new RENESIS Rotary Engine. Part of the fuel economy problem was actually a tradeoff of using a rich mixture to satbilize the idle and low rpm/load drivability. The RENESIS actually does a much better job expelling CO, so the low speed/load combustion is much improved with a stoich-mix. Taking just one example. The Mazda6 with the 10:1 compression 3.0L V6 producing 215hp, gets 19/27 MPG. The RX-8, with a 10:1 compression 1.3L 13B Rotary Engine producing 238hp, gets 18/24 MPG.

Basically, we rotary engine guys get more Smiles-Per-Miles while our piston pals get better Miles-Per-Gallon. I'm happy with that tradeoff!


Be wary of quoting engine capacities with Wankels.
For example : a 13B engine has 2 rotors, each rotor having 3 combustion chambers, each displacing 654cc. So what is the capacity?
Bear in mind that the rotor turns at 2/3 shaft speed. Therefore the engine displaces 4 x 654 ccs of air for every 2 turns of the shaft - this is the same displacement as a 2.6 litre 4 cycle piston engine. (The wankel is also a 4 cycle or 4 stroke engine) So the 13B equates to a 2.6 litre engine.


Yes, I know this. The engine is 1.3 liters displacment (true capacity) but equates to 2.6 liters by the piston engine terms (double capacity) . That's why I compared the output to a 3L V6!

Quote:
This is borne out by the power output. Fuel consumption however reflects the fact that Wankels lose a significantly larger proportion of their heat to the coolant and oil than a piston engine. Lower combustion temperature than in a piston engine results in less power per unit volume of air consumed and flame quenching on the relatively cool housing wall results in some unburned fuel passing into the exhaust where it then burns. This is why Wankels have a very high exhaust manifold temp.


I'm not sure I agree entirely...or perhaps it's just the way you word it. Yes, they suffer thermodynamically (heat vs. output) but where the heat goes (coolant, oil, exhaust, atmosphere, etc) is only important in terms of heat management. A lot of heat goes into the housings (coolant cooled) and rotors (oil cooled) because of the surface area involved. Due to the shape of the combustion chamber and the gas flow inside (faster than the flamefront can travel!) results in dead areas where the unburnt fuel congregates, which then burns in the exhaust and causing higher EGTs. But, it's all just heat being dealt with. I would not characterize the chamber walls as being any cooler than a piston engine, though there is so much more surface area. Rotaries have a "hot" side that never sees a fresh blast of cool air, and the combustion itself yields a great deal of heat.

Perhaps we are saying the same thing in different ways...you sound very knowledgable.

Quote:
The low combustion temperature means that a relatively high CR can be used without the risk of detonation even when using low octane fuel. The high exhaust temp combined with low tendency to detonate seemingly makes the Wankel a good candidate for turbocharging. We ran an early NSU Wankel (CR 9:1) to boost pressures in excess of 30 psi without experiencing detonation, without intercooling, but using 105 octane fuel.


Again, I don't quie agree with the idea of low combustion temperature and "relatively high CR". The compression ratios we run are considered quite low in piston terms. The combustion temps are not particularly low but the area into which it discipates is large. Spark plugs in particular are a real hot spot that is hard to deal with, so detonation is a real problem.

Your example is quite impressive. 30psi without intercooling is quite amazing. No matter how efficient the compressor, heat will be generated by the act of compressing, so the intake temps would have to be very high...that is a recipe for disaster for most rotaries. I am curious what your intake temps were. But, regardless, if you retard the timing enough, virtually anything can be pulled off. Ignition timing is critical. What were you running? What kind of output did you see? Was this the 1L engine in the Ro-80? We have a lot of experience with Mazda 3rd gen FD engines running 30+ psi, with output in the 700 rwhp range...both "high" (9.7:1) and "low' (9.4:1) CR. I would never characterize them as having a "low tendency to detonate". More like prone to detonate and having to try every trick in the book to keep them from fulfilling that destructive tendency. The part about rotaries that make them love turbocharging so much is all that free heat energy in the exhaust, yearning to be put to work. At that point, it becomes a game of keeping it from blowing up by running low compression (9.4:1 or lower), controling intake temps, running "cold" plugs, supplying plenty of fuel and mapping the ignition timing very, very carefully. One mistake and the jig is up.

Of course, we are talking about apples and oranges. Don't take my perspective as anything disputing your experience. I would love to hear more about your experiences, which sound extensive.

Quote:
Remember also that in some countries, cars are taxed according to their engine capacity, so if you can kid the governement that the engine is smaller than it really is, you pay less tax.


Certainly. But there is also a convining argument that it is the rotary that is properly rated and the piston engines are rated double. All they do it take the displacment of each cylinder and multiply, ignoring that it takes a full cycle to realize the capacity of all cylinders. If you rated a rotary engine that way, it would be a 3.9L. But if everything is based on one rotation of the crank, then rotaries are properly quantified and piston engines are doubled...which is the basis of Capacity (the way you rate an air pump). It's just because piston engines came first that their ratings are out of whack with the more logical system. So, we must resort to doubling our true displacment (equals capacity) to be equivalent with a piston engine (doubles capacity).
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Pineapple Racing, Inc.
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Graviman



Joined: 24 Feb 2006
Posts: 6

PostPosted: Sat Feb 25, 2006 8:26 am    Post subject: Reply with quote

Quote:
My 84 SE was rated at 16 mpg city, 24 hiway back in the days of 55 mph. I regularly get 17 to 19 mpg in town, and with cruise set at 65 - 70 I've seen as good as 28 mpg.


Eeek! My 1.9 litre piston VW diesel gets 55 mpg for an average speed of 70mph over 500 miles. Then again i've always driven diesels, so perhaps i have no soul...

Mart
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rx8-tx



Joined: 11 May 2006
Posts: 3

PostPosted: Thu May 11, 2006 7:06 pm    Post subject: Re: Inefficiency Reply with quote

tommyd49 wrote:
This is my first post here.

Could someone please tell me why rotary engines have inefficiency problems? I have heard it is something to do with not all the fuel being burnt as the chambers are too large.

Thanks

Like Blake mentioned earlier, the RE has a high VE (volumetric efficiency), meaning that it'll ingest an amount of air close to or higher than 100 its 'theoretical' capacity. Now....what capacity? Let's don't get into that argument right now, let's just say that for all intents and purposes (and to level the playing field) we will consider that it should have the intake characteristics of a 4 stroke 2.6 liter engine.

I've done the math myself, with the help of a CANScan logger on my own RX-8 (we are talking about a NA Renesis here), and I've come up with a VE of ~80% below 4500 rpm, and over 90% thereafter -until I let off the gas well past 8000 rpm. Mind you, it theoretically hit over 100% for a spread of some 3000 rpm. The full publised article is here: Renesis Volumetric Efficiency

Now we can safely state that the RE is reletively efficienct at sucking in air. However, it is not thermally efficient. Why? Combustion area is the trick. Now that particular topic is a wee-bit over my head. You an probably find some really good really material on Paul Yaw's website: http://www.yawpower.com

Cheers!
RX8-TX
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