Test your engine knowledge - Rotary vs Piston Comparison
 

I came here because I know that there are a few of you out there that are full of general engine knowledge and wants to share it!

I'm writing a report attempting to prove that a rotary engine is superior to a piston engine and so I need to do an engine comparison. I've searched far and wide for stats on a rotary engine but have been hard pressed to find the right information.

Since I know there are many different generations and years of RX-7's, I just need a "good" model. I want to set up a comparison between an RX7's performance (non turbo) stock, with a comperable piston engine car. Horsepower, RPM, Weight, Volume, Compression Ratio, etc.

I also want to set up another comparison, between high performance vehicles. A Rotary race engine and a piston race engine of similar class cars.

Now, if there is anyone who thinks they're smart enough to throw some values out at me, I'd be interested to see what you know.

P.S. If you know of any websites where I could obtain some of this info, it would be greatly appreciated too.

Less moving parts than a piston engine of similiar HP. Most people that think the rotary engine is to complicated simply aren't smart enough to understand it or don't want to understand it. Rotarys are commonly used in experimental aircraft due to reliability and good power to weight ratio. The engines are capable of amazing HP for their size. Try this site for good aviation info.


http://home.earthlink.net/~rotaryeng/ACRE.html

Anyway that I can get some hard numbers? For example Car (X) versus RX7 (Y) - Car X has xxx HP at 5000 rpm, Car Y has xxx HP at 7500rpm. 4 Cylinders vs. 2 Rotors, etc. I need to be pretty detailed with my description and I'm not familiar enough with cars to know which two cars are "compariable" by performance.

Here's a start for the high performance part. It's a site about the 787B Le Mans car. It has stats about the 4 rotor 700hp N/A 2.6 liter engine. In 1991 I doubt there were any 2.6 liter piston engines making 700 naturally aspirated horses. http://www.monito.com/wankel/lemans.html
You can look around for other cars specs in the 1991 Le Mans series. Supercars.net is a good site with cars statistics.

I don't know of any specific comparison sites but I think a very good comparison to make would be the 84-89 Nissan 300ZX vs. 86-91 rx7. Both cars are similar size, similar performance, both came in N/A and turbo. The difference is the Nissan had a 3.0 V6 that weighed the car down and compromising the handling compared to the rx7. Maybe even throw the mkIII Supra in there. Should not be difficult at all to find stats on those 3 cars.

Thanks
 

Thanks for the great start, I'll look into those suggestions. If anyone else has any additional comments, that would be wonderful. :) I knew there were people that could help me!

You can't go into a research project with a set outcome. That's not research, that's propaganda.

compare swept working volume per stroke and size/weight

mike

:) my thoughts exactly....but who cares...rotary power:D

Well if you do the RESEARCH, you'll find that rotaries are very thermally inefficient engines due to the high surface/volume ratio. More combustion heat is lost to the cooling systems, meaning less power gets to the E-shaft. (And meaning rotaries must have larger, heavier cooling systems) In a similar vein, the combustion chamber shape is very poor and it's difficult to achieve complete combustion. (A "perfect" combustion chamber would be spherical, and the burn would initiate in the center of the sphere. The closest we can get to that is an undersquare pentroof/hemi boinger with flat or slightly dished pistons. That, and the walls would be the same temperature as the burn so heat cannot be lost to the cooling system)

This is why rotaries have poor BSFC, and lower HP/l (given the same torque curve) for a similarly sized piston engine. It's also why the EGTs are so much higher - more energy is thrown away out the exhaust .

The rotary does have three advantages though... simplicity, better space utilization (in theory anyway, in practice the manifolds and accessories make it as bulky as many V6s and some small V8s) and smoothness in small-engine applications like chainsaws and lawnmowers. In large-engine applications, like cars, where you have multiple cylinders, it's not that hard to make the piston engine smoother.

Very much on the money, the RE does suck cock compared to many boingers now days. But it is cheap! and it is reliable for the power it makes for its size.

I would say in a power/size/cost relationship it is up there with anything + with new technologies like stratified charge compatability (not realy a performance thing) the RE has some merit.

From a passionate and ecentric point of view nothing matches driving a car powered by a unique power plant.

Hard to show you a conclusive benifit though? Like the R26B is a 5.2Lt engine (when looking at what "work" it does in RPM) It is not cheap ! The RENISIS only makes 10bhp more than a S2000 2lt engine ! and it's a 2.6lt There are many other power units out there that rev far higher than a RE as well. And no RE matches the BSFC of mid range piston engines.

The main things it has going for it are simplicity and (at least here in Aust) being CHEAP. Oh and the sound is Excellent :) ....and they shoot flames !

I tried to leave the subjective points out, because that's just opinion... but subjectively no engine can beat the sound of a built 2-rotor! (Call me crazy but I'm not keen on the way 3-rotors sound)

As far as cost... again that all depends on where you live and who you know. It would be far cheaper for me to build a small-block ***vy engine than a rotary of similar power output. But then with the SB* I'd have a bellybutton engine that didn't sound as nice. And really, modifying cars is something I do for pleasure, not business, so I'll deal with less power/dollar in exchange for a more pleasurable experience.

Thank you
 

I would like to thank everyone who posted their suggestions. I was able to form a fair comparison for my report. Thanks again for all the help.

Since rotary engines are compartively smaller than a piston engine producing similar power, it allows for better weight distribution. The rx-7 FD has near 50/50 weight diust and a low centre of gravity since the engine can be mounted low down and far back.

A stock 89 RX7 pumps out 122.3 hp per liter, where a 2001 Integra typre-R does 108.5 hp per liter.

*sigh* Here we go again.

HP/l means absolutely nothing. And even if it did, no, the "stock '89 RX-7" doesn't put out 122.3hp/l.

I wouldn't write a paper on something you know very little about.

but isnt that the point of doing a "research" paper? Doing it on something you know little about? Hell, isnt it always to a good idea to do a report on something you know little or nothing about? Wheres the effort in people these days?

I guess the learning/teaching system has begun to suck more cock than it did when i was back in school.

The scientific method generally suggests that you form a hypothesis, then do research that will either prove or disprove your hypothesis. This is not propaganda, provided it is done with a fair amount of intellectual honesty.

And don't forget that rotary engines are far better suited to alternative fuels (such as hydrogen) than reciprocating engines. Heck, if it weren't for emissions, the suckers would burn 70 octane gasoline, too!

A/.

Right, scientific method suggests that you do reasearch that proves or disproves the hypothesis.

However...
One must always walk into research with an open mind. It sounded like the original poster wanted to prove his feelings no matter what, which is propaganda.

Honestly, a built piston engine has a lot more potential than a built rotary engine...the fastest Mustang in the world will probably always be faster that the fastest RX-7 in the world. I think RX-7 owners are RX-7 owners because they love the rotary, the RX-7, and just being different from the norm. Pistons are more practical car engines IMO. But then again, maybe that's because everyone knows pistons (since the automobile was invented) yet, like someone stated earlier, so many are afraid to know rotaries.

Someone should jump in and discuss the Volumetric Efficiency differences.

well, according to that website, you should classify rotary displacement as 1.5 times the number to get an equivalent piston number. Well, that makes the RENESIS a bit under/over 2.0L with a 10 hp boost over Honda's S2000 motor, plus the 2.6L R26B just became a 3.9L piston equivalent displacement motors, that makes 700 hp, which isn't too shabby. Though there are drawbacks to using the rotary, as there are with any engine, you should still point out the pluses as well as the minuses.

Nevermind about equivalent displacement and multipliers...

Ok bitches, how about this one then...
Divide WEIGHT by HP.
Divide ENGINE DIMESIONS (i.e. how much space the engine actually occupies) by HP.
Average the two.

Which engine comes out with the better numbers?

Oh, I thought so...


-Ted