Rotor Speed vs. Piston Speed

One of the many reasons we cannot achieve 100% efficientcy from gasoline or other liquid internal combustion fuels is the fact that the fuel does not atomize completely. Another is the fact that the mixture does not have enough time to burn completely. A 4 stroke piston will have a certain "dead time" at TDC and BDC where the piston's position will not change in relation to the cylinder. During the intake stroke, the piston will accelerate to 3500ft./min. (on an engine with a 3 in. stroke spinning 7000rpm) and come to a dead stop at the bottom of its bore (hence bottom dead center). Meanwhile, the air fuel mixture has made its way into the cylinder and has been superheated by the hot cylinder walls, aiding in atomization of the fuel. The same can be said about the expansion, or power stroke. The dead time of the piston at BDC would give the mixture just that much more time to burn before being swept out as exhaust gas.

Rotaries however, have no dead time. The rotor itself is in a constant eccentric motion and the 4 strokes are always being performed in their own respective portions of the housing, causing unequal heat distribution. (but thats something that cannot be helped in the rotary)
And as i am writing this i realize that any change to the rotors speed at these specific points in its thermodynamic cycle, would have adverse effects on events in the other working chambers. For instance, slowing the rotor's speed at the end of the intake stroke for one face of the rotor, would be slowing the compression stroke on the next, leaving much more time for the mixture to be exposed to heat soak from that portion of the housing. I suppose it cannot be helped. Just some ideas that spawned in my head while reading Kenichi Yamamoto's book.

But doesnt the e-shaft spin 3x for every 1 full rotation of the rotor? I guess what im getting at is, is this one point where a piston engine would be more efficient, and why rotaries get poor gas mileage as well as bad emissions? Completely open for discussion, honestly, this book has got me thinking around in circles ( or peritrochoids if you will) and i could easily be overlooking something.

 

http://www.rotaryrefs.net/rotaryrefs...tarybooks.html

the graph is on pg. 16

 

Not all fuel is used for combustion...like at full power in a turbo car you are shooting for 12.5:1 AFR. That means...there is unburned fuel in the exhaust. It was used to cool combustion so it did not detnoate.

However, if you could run full power at 14.7:1 ......

James

 

^exactly, although some turbo cars even go into low 11's. But thats the track im on, i guess what im confused about is when the rotor actually slows down. at the most narrow point of the peritrochoid? Of course the perfect internal combustion engine has yet to be built (if ever) im just trying to understand the fundamental differences between operation characteristics of rotary vs. piston.

 

The reason that Piston engines have better fuel mixing and combustion is only slightly aided by having more compact combustion chambers.

If you examine something called the leidenfrost effect you will see that upon fuel hitting a super hot cylinder wall or piston surface the fuel takes longer to atomize into the air. There is a very thin layer under the fuel droplets that prevents them from atomizing quickly and this is dependent on the surface temperature. In fact, the greatest aiding component in swirl velocity and turbulence in a piston engine to create fuel atomization is created by flow past the intake valve. At the edges of the valve there are very high velocity vortices which are created and die out towards TDC as they are compressed by the pistion crown. This helps tremendously in the combustion efficiency and speed by creating a much more uniform mixture in the cylinder of air and fuel. If you take a look at the new advances for VVT (variable valve timing) there are different methods that can be used with independent valve control (electromagnetic or hydraulic usually) efficiencies can be increased, along with decreasing emmisions simply by the use of different valve timing strategies, and perhaps the best strategy for induction charge being the ability to have the valve at full lift for maximum flow based on the piston speed.

 

Extra time at TDC may allow more complete combustion, but no power is produced there. That said, the rotary has more dwell time, for same rpm, vs piston engine.

Look at fig 2.9 in your link. If both crves were plotted for same crank rotation, the rotary's volume curve peaks would be more spread out at tdc and bdc, meaning more dwell time.

Although the rotor keeps moving, the dV/dt will hit zero at bdc and tdc, just like a piston engine.

Good questions.

 

Hey you know there is a very good site for basics, especially for understanding the movement of the rotary engine aside from all the combustion effects and etc.

www.rotaryengineillustrated.com

if you haven't checked it out it has some pretty good animations and basic information.

 

Yeah, ive checked out that site quite a bit. It hasent been updated for a while. Ive been thinking about this some more, and is it true that for one full cycle of the rotor, the e-shaft has spun 3x? If so, i think that would explain kevink's point, being that a piston will go through a full cycle in 360deg. of crank rotation. That would mean that a rotor would take a bit longer to complete a cycle (in rpms).

But kleetuz, your saying that the hot clylinder walls are not as good for atomization vs. the side housing's "cool" intake port side? I figured if the air and fuel were superheated upon entering the chamber, that they would expand and mix better.

Thanks for the input everyone.
Brett

 

No, that's not exactly what I mean. The higher temperature does aid in atomization, but not as much as the turbulence created by flow past the valves. However think about the cool induction port for the rotary, this helps to keep the mixture temperatures lower, which can help to increase charge density and thus power output, but if you need a combination of both to be most effective. Also, because the higher the temperature the less controlled burn you get and more you have to retard your ignition point. Well for one full cycle of the rotor yes the output shaft rotates three time, but looke at what happens for one full rotation of the rotor, ignore the planetary rotation to think about this better, all three chambers go through a power expansion cycle. So there are 3 power expansions for 1 rotor rev and 3 shaft rotations for 1 rotor rev. So 1 power expansion for each shaft rotation, for each rotor. This is the advantage of capacity. For a piston engine the power stroke occurs for only once for 4 travels through its combustion volume. Again it is not quite fair to make this comparison to a piston engine of equivalent displacement, there is more to it.

 

hmmm, yes, more food for thought. Thanks again guys.

 

illogical 1 piston = 1 rotor logic.

1 rotor = 3 pistons on a 50% slower crank.

 

Brett please don't take this the wrong way as I mean nothing bad by it. Have you ever seen a rotary taken apart and played with one or are you just learning all you can about them? The reason I ask is if it would help you to understand anything about them by actually looking at an engine taken apart, I can stop by SGP with an old housing and rotor.

 

^haha, yes i have taken one apart and am waiting for the cash to rebuild. But i didnt "play with it" by turning the crank and watch the rotor spin if thats what you mean. The whole idea of this thread was to learn more about what makes the rotary fundamentally different from the piston engine from a combustion efficientcy standpoint. It is certainly possible to take one apart and play with it without knowing exactly how it works, and as you can see im still learning. I just hope I still remember when it comes time to put her back together. LOL

 

The dv/dt?

 

derivative (d) of volume (v) with respect (/d) to time (t); the rate of volume change.

the fact that a piston "stops" or reverses directions at the top and bottom of its cyles realy means nothing in and of itself. both a piston and rotary engine change their volumes (dv/dt) in a sinusoudal wave; even though the rotor doesnt stop or reverse directions, it's volume, and by extension change in volume (dv/dt) WILL "stop" (or reach a relative max and minimum) and cylce just like that of a piston engine.

i think more of the differences you are looking for between piston and rotary lie in the relative timing of intake and exhaust, and the shape and flow characteristics of the combustion chambers, not in the pumping mechanism itself, since these are essentialy identical at the basic level.

 

^now i see, even though the rotor itself doesnt ever come to a stop, its volume will reach a point where it tops or bottoms out, and mimics that of tdc and bdc of a piston engine.

 

exactly

 

I believe kleetuz is refering to a film boiling situation. by definintion this is where the surface temperature in contact with the fluid (cyl. wall, piston, rotor housing or rotor) is at least 120C above the saturation temperature of that fluid (gasoline) at that pressure. at this delta T (DT) the heat flux (heat transfered to the gas) is greatly reduced.

in fact this being the only variable in consideration the ideal DT is around 30C. this is where the greatest amount of heat is tranferred to the gas to aid in boiling.

lets assume gas at boils at 100C (i quick internet search yeilded 40-200C for the boiling point of gas at atmo pressure). that means the ideal surface temps of the rotors and chambers is 130C. the boiling curve (attached) depics the fact that if these surfaces are near 220C or above the heat transfer is both small and somewhat constant until DT's in the 600C+ range are seen where radiation effects become substantial.

so the question is what are these surface temperatures?

kleetuz's point is very valid if the rotary intake chamber has DT's in the 20-40C range and the piston engine is above this.

cool stuff...
justin

 

here is the boiling curve:

http://batman.mech.ubc.ca/~mech475/M...20transfer.pdf

DT-> difference between the surface temperature and the satuation temp of the fluid at the given pressure.

D-> DT=5C
D'->DT=10C
C-> DT=30C
B-> DT=120C

justin

 

Sorry to barge in, but thanks. I never understood that before until the way you just said it, even though I'd read about it before.
Ron

 

this is why i like rx7club.com. very informative stuff, keep it up.

 

Yup, ive noticed that there are a lot of engineer types here, and its really cool to have input from these guys. Engine theory is awesome. Sometimes when im working on a car, i just stop and think that the object sitting before me is a product of decades and decades of dedicated study, trial, and error.
Once again, thanks to everyone who participated and threw some wood on the fire. Was it captain planet who said knoweledge is power?

 

I think i understand where you are going with this. So let me try to dumb it down and see if I hit the mark. Your saying that if the surface temperature of the working chamber is slightly higher than the saturation point of the liquid, it will substantially reduce the amount of heat transfered and hinder atomization?

 

I don't think it will necessarily reduce the amount of heat transferred by an excessive amout, but it will take a little longer to transfer the engery and atomize the charge creating a good mixture, this is why for many years injection was done half on the closed intake valve and the other half of the fuel when the valve opens. It is somewhat of an atomization timing issue if you will, the charge has to be ready at the ideal mixture (uniform) at the right time. Because timing is everything.

 

I havn't read through all the other replies yet for my 2 sense but 80% of the power in a piston engine happens in the first 20 degrees of crank rotation therefor the extra burn time at BDC really only has an effect on emissions and almost nothing to do with power output.