The ratio of combustion chamber surface area to volume is large on the rotary, so you lose more of the combustion energy to the cooling system. The "new surfaces" explanation seems like it would also be a significant part of the increased loss of energy to the cooling system vs. piston engines.

Other NA engines that make >100 HP/L (more than the Renesis):
- E46 M3
- Ferrari 360 Modena

-Max

 

What a welcome to the forum huh? Don't worry about it although I'd like to think that most people can tell that I usually don't shoot my mouth off without at least glancing at something and giving it a little thought.

Brian

 

Thanks for the post KevinK2. It did a better job than I can do explaining some things.

There are a few other engines out there that may produce more hp/l than the renesis but we will have to see what the compression ratio on the renesis is before we can really compare them.

One of the easiest ways to increase the thermal efficiency of an engine is to increase the compression ratio. The s2000, e46 M3 and the Ferrari 360 Modena all have engines with compression ratios of at least 11:1. (I believe the e46 M3 has some reliability problems?)

Mazda has not released anything that I could find that explicitly stated the compression ratio of the renesis. They mention that it is "high". I assume that they have increased it over the 9.7:1 of the early 90's na. One limiting factor is the reliability and efficiency of the apex seals. Mazda may have made changes to optimize the apex seal design since there will no longer be any ports on the peripheral housing.

I find myself wondering where the original poster in this discussion went. I hope he wasn't just a troll but he did start an interesting discussion on the engine that may help people understand it more.

Brian

 

Factor in the horsepower to weight ratio of these motors also. I think that is a more fair comparison since pistons and rotors are comparing apples and oranges. The RX8 produces 247 horsepower. I don't know what the motor weighs, but it would be nice to know. I am sure that we kick the hell out of piston engines in the HP to weight ratio though. Just imagine if the side plates were aluminum.

 

wow, cant even be bothered reading all this thread

inefficient? damn 1146cc pushes my car up and over 100MPH easily, and its a pretty much standard 78/79 RX-7. my 1.3L 4 cyl mazda has maybe an absolute topspeed around 100MPH

oh yeah, also know that the eccentricity of the shaft is only 15mm, meaning that the stroke is only 15mm, no wonder they dont make much torque.

 

Yeah, but which one gets better gas mileage? This thread is mostly about how much power you get versus fuel burned as much as it is about displacement. And your 1146cc rotary is equivalent in air flow to a 2292cc four-stoke piston engine.

-Max

 

thermal inefficiency

comparo with '2.6L' 944 4 cyl. I assumed spherical comb'n chamber, enlarged for same FD 9.0 compr ratio. adjusted bore and stroke from oem 2.5L, to get 2.6L with same bore/stroke ratio.

for FD rotor, R=105mm, and w=80mm. 40 cu-in face displacement. Determined contour radius is 7.67", and found larger radius for 5 cu-in chamber, then radius for 40 cu in displacement from there. also checked using flat chamber assumption ... about same results. I think the approximation of the chamber in the housing as a constant radius should be close enough for a comparo. Here are calc'd heat exchange areas, in sq inches:

944 ___ FD __ % more FD

Total @TDC

26.9 __ 51.0 __ 90%

Total @BDC

66.8 __ 86.1 __ 29%

NEW area @ BDC

39.9 __ 61.5 __ 54%

When things are the hottest near TDC, the area issue is the worst. Also a time factor. The rotary stroke is during 3/4 crank rev. while 944 is only 1/2 rev. The rotary will allow more time for heat exchange, unless typical rpms are 50% higher.

The 944 is a convenient comparo, as it fires 2 pistons per rev, and the rx7s fire 2 rotor faces per crank rev.

" ... eccentricity of the shaft is only 15mm, meaning that the stroke is only 15mm, no wonder they dont make much torque."

The effective stroke is closer to 3 x e.

 

hey pistons, take this

 

I've created a monster. I'm no troll, I have an RX-7, an 81, but it's dead at the moment. That's why I came here. I went deer hunting over the weekend and they moved my thread, I just found it today.

I would most likely compare the wankel to a piston port two stroke. They use ungodly amounts of fuel, are noisy, hard to tune......but man will they knock your socks off for the size, weight, and displacement.

If you guy know anything about the older Kawasaki two strokes, they played with a rotary valve. I think this would benefit the wankel. It's a rotating plate with holes that allign with the ports. At lower RPMs the holes are less alligned and less air/fuel can enter the motor. As you rev it up the holes allign closer untill being fully alligned at WOT.

Just a thought.

 

I think Ford purchased a portion of Mazda in the 70's, then took full control in the 90's.

 

Any idea why has the Miller Cycle didn't come into widespread acceptance? I realize it "only" had ~10-15% increase millage and power compared to a standard Otto engine.

Regarding the other stuff. How about comparing renesis to a Honda 3.0L V6 or Nissan 3.5L? Higher power output, worse gas millage. Not to mention the traditional rotary benifits. In terms of raw power the 3.5Ls with the hotter cams produce more raw power, but not power/litre. Not to mention the have been complaints/issues/etc. with the 3.5Ls not producing numbers consistant with the claimed power.

Go back to the 60s. Rotary power and millage were similar to a big V8.

 

Every turn of a 13B crank it "burns" 1.3ltrs of mixture.
Every turn of a 2.6 litre (4 cylinder) piston motor crank, it burns 1.3 litre of mixture.
Do you see where the comparison comes from now ?
As said before, inefficiency comes from combustion chamber shape.
Power to weight (and size) ratio's are good tho'.

 

http://www.monito.com/wankel/mercedes.html

 

250mph is faster even than a Mclaren F1 which holds the production car high speed record at 241mph many years later...

 

300th post! whoo hoo!!!

(sorry, had to )

 

Rotarys are very efficent a 91 N/A for example puts out around 160 HP and is around 80 CI. That is bone stock from the factory. Thats about 2 HP a cube. If piston engines were even close to that efficency level a 91 camaro with a 350 would be putting out over 700 H.P bone stock from the factory and I think in 91 a camaro with a 350 CI was putting out around 245 HP. So I guess I am not getting how the piston engine engine is more efficent? Please explain. Is it because the rotary is a thirsty little motor? Or what? Power costs fuel always. More Power usually means more fuel no matter what size engine.

 

We're talking about thermal efficiency, you're talking about specific output, two very different things. The closest layman's analogy to thermal efficiency is fuel mileage, so in simple terms, we're trying to explain using physics why rotaries are fuel inefficient. If you don't care about fuel mileage, feel free to move along .

 

different displacement rating methods. want to compare hp/cube with boinger power, 91na is 1 hp/cube. not bad, but not 2.

 

i dont think anybody has pointed out that the 13B slings fuel out of a large hole in the peripheral of its housing. the renesis fixes this and i have heard numbers of 32% more efficient.... still no torque though

Justin

 

Where should this fuel go, if not out "a large hole in the peripheral of its housing"? There wouldn't *be* any fuel there if the rotary were better at burning the air/fuel charge.

 

The shape of the combustion chamber makes it really difficult to get a complete burn. The flame front has to travel a long ways. If you read the SAE paper on the renesis, the side ports allow the chamber to act as a centrafuge, the heavier unburnt fuel is pushed to the very periphery of the housing. By placing the exhaust port slighly toward the center, the unburnt fuel is actually swept past the exhaust port and stays in the chamber to be burned in the next cycle, while still allowing the exhaust gases to exit efficiently. Its genius imho, such an awesome idea and only possible with a rotary.

 

yoosa dumbass. for every revolution a piston gets, the rotor gets two. thats because the rotor fires on each side and the piston only fires on the opposite side of the crank. god i have no patience for idiots.

 

yeah renesis is the bizzah to the b- aahm.

 

Renesis is what to whom? If there was a thought in there, it didn't come through. God I have no patience for mumbling.

 

Orange,

most engines dont burn all of their fuel completely. if you lean the engine out so that it does, you risk predetonation. due to the fact that during the explosion on the powerstroke of a rotary, it is actually burning its way out of the motor, not all of the fuel can be burnt, nor should be burnt, before the exhaust port "opens". the renesis (as previously stated) allows the ports to open, yet doesnt dump out the excess fuel

justin