fdracer

A rotary motor is supposed to put out more power than a piston engine of equal displacement, right? But it also runs much hotter, indicating that more energy is wasted as heat as opposed to horsepower. How can this be? Let's say (disregarding the turbos, because turbocharging a rotary has the same effect as turboing a piston engine) that you have a rotary and a piston motor which both displace 1.3L. Now, in one combustion cycle you can only combust 1.3L of air in either engine. For a rotary to extract more power out of that 1.3L of air it must convert fuel and air more efficiently into horsepower. However, the extra heat that it produces suggests that the opposite is true. So where does the extra power come from?

pweizman

There is less energy lost in reciprocating mass in a rotary engine.

The motion is more efficient. In a piston engine the pistons are changing directions very rapidly....this takes energy. A rotary is almost pure rotational motion.

A rotary probably also has less friction on its moving parts, since there are only 3 moving internal parts...

Later,
Patrick

KZ1

although they run hotter I haev heard this is from the limitations to coolant pasages. This being true the rotary will produce less heat than piston, it is just not cooled as effectively.

as stated before the motion is much more efficent, in addition to that the rotary completes the 4 cycles in 2 cycles. instead of 4. I know is sounds wierd but basically it simultaneously, for example, does intake and power stroke at same time. thus taking have amount of time to complete the entire process

I am by no means an expert, just throwing out info from reading rotary sites. but I think I have the basic idea. Some of the experts will hopefully chime in.

TailHappy

Another thing that I've been trying to work out in my head is that each face of the rotor fires every time it goes by the spark plugs. So you have 3 explosions * 2 rotors for each turn of the shaft. On a piston engine strokes are wasted for intake and exhaust. So the rotary is most like a 6-cyl or maybe more, right? Definitely an ingenious design....

TailHappy

Ooops, you beat me to it, KZ1!

jimlab

Part of why a rotary produces more power for its displacement is rpm. Potential power is limited by the amount of oxygen and fuel that an engine can ingest over time. Increase rpm and more air and fuel are ingested (in a naturally aspirated application) as volumetric efficiency increases and approaches 100%, meaning that each "cylinder" is completely filled during each intake cycle.

With the right manifold design, you can actually create positive intake pressure with a naturally aspirated engine as volumetric efficiency surpasses 100%. My NA 396, for example, is 103-104% volumetric efficient over part of its rpm range. Obviously, a forced induction engine will have greater than 100% volumetric efficiency when under boost, which means that even more oxygen is ingested by the engine during each cycle.

The smaller the engine, the more rotations you'll need to make an equivalent amount of horsepower (burning the same amount of fuel and oxygen), comparatively speaking. It's not necessarily a matter of how much oxygen and fuel the engine can burn on each rotation (displacement limited), but how much can it burn over time. More revolutions increases this amount.

A 1.3 liter rotary can therefore make as much horsepower as a larger piston engine because the piston engine usually won't be turning as fast. If you had an amount of air of fixed volume, and each engine was pulling in air from a container of equal sized volume, the rotary engine turning higher rpm (and with three combustion faces per rotor) would use up its alloted oxygen before a 4-cylinder engine of the same displacement. This is because the 4-cylinder is only filling all of its cylinders every other rotation, which is the other main reason why the rotary engine produces more power for their displacement than an equivalently sized piston engine. More combustion events per rotation.

However, both of the factors of the rotary engine which produce power (rpm and more combustion events per rotation) are also why it consumes far more fuel over time than even larger piston engines. In addition, fuel is literally used to cool the combustion chamber and prevent detonation, meaning that a lot is wasted. The rotary engine is "sloppy" in this regard, completing its combustion process in the exhaust pipe, not inside the engine. Therefore exhaust temperatures and fuel useage are usually significantly higher than a piston engine.

The rotary engine may have a high horsepower to displacement ratio, but it should be remembered that hp/liter ratios don't necessarily win races. As someone so aptly said, the rotary engine is most efficient at creating noise and heat from fossile fuels.

peejay

What "extra power"? There isn't any.

A 13B moves 1.3l of displacement every rotation. However a 1.3l four-stroke piston engine only moves 0.65l of displacement every rotation, because four-stroke boingers require *two* rotations to process their rated displacement, instead of just one for the rotary. This is similar to the difference between 2-stroke and 4-stroke boingers.

Therefore, if you want to compare apples to turds (rotary to piston), you have to either divide the boinger's displacement by two, or multiply the rotary's displacement by two.

A 13B processes the same amount of air per revolution as a 2.6l boinger, and a 12A processes the same amount of air as a 2.3l boinger.

Due to heat losses and combustion chamber inefficiency, rotaries do require more fuel for a given amount of HP, which is why they burn so much fuel, even if you think of a 13B as a 2.6.

Orange!FD

I wish people would stop repeating this little myth. It doesn't matter how much mass is "Changing directions very rapidly", it doesn't take any energy at all to do this. Any energy used in accelerating the piston and rod is simply transferred back and forth to the crankshaft in a piston engine over the course of a revolution. Loss of energy comes from friction and heat transfer, period.

I doubt whether a wankel has less frictional losses than a piston engine, either. I don't know, but I doubt it.

(I hope this doesn't burst anyone's bubble over Mazda's unique and excellent engine.)

dis1

Wrong!! By this logic it also doesn't take any energy to accelerate. Anytime something changes direction energy is used. Think about this... If you go in to space with a object and set it spinning it will spin forever because there is no friction and energy isn't needed to keep it spinning. Now think about something that needs to go up and down. Each time the object stops and starts moving again force must be applied. Force is energy. This is a very simple concept and is taught in every HS physics class.
cvs

xiaomingming50

orange fd is completely correct
think about a pendulum
if you let it swing, it would oscillate forever without air resistance
the same for a piston
without friction, if you released it from top dead center, it would drop down, and go back up, with the crankshaft and the rods causing all the centripetal acceleration on the system
friction is the only thing causing energy loss

and as for your space metaphor, your system takes place in an isolated environment without the effects of gravity
the piston's up/down motion is directly related to gravity

Tom93R1

iTrader: (2)

And this is related to why pistons in a boxer engine wear flat on one side, or not

xiaomingming50

oh yea i was thinking about boxer engines on my drive home from class
for those, if you had a force pushing the piston sideways, and a force holding the piston up horizontally, but no frictional force during its oscillation movement, then the rods would be doing all the centripetal acceleration on the piston

and remember (for those of you who've taken high school physics) centripetal acceleration is perpendicular to the velocity vector of the object it's acting upon, and work is force times displacement times cosine of the angle in between, and since they are at 90 degrees, the net work done is always zero, although the rods will suffer a lot of stress, and break if the stress is too great (where rod/stroke ratios come in)

Orange!FD

Exactly, xiaomingming. Dis1, you should actually attend at least one HS physics class before you start telling us how simple the concepts are.

Force does not equal Energy. Now *that* is simple.

dis1

Ya, you got me there but it takes energy to provide force. As for the pendulum that's actually more like a circle than up/down like a piston. However I do see the point you are trying to make. I think it might boil down to the fact that there is more than one piston so that the forces cancel each other out, which is what xiaomingming50 was saying (I think). At any rate this point is nothing more than one of efficiency. We all know that rotaries aren't that efficient. They are better at making more power from less displacement, which was explained by jimlab.
cvs

Orange!FD

Uhh, no, it doesn't. Your car is pushing down on something right now to the tune of around 3000 lbs. Is energy being expended in order for it to do this? No, it isn't.

Again, no, it has nothing to do with "forces canceling each other out", and no, it isn't what xiaomingming was saying.

The round-and-round vs up-and-down argument has not the slightest thing to do with efficiency. This particular rotary engine isn't anything too wonderful in the "power per displacement" arena, either. Power vs weight, or power vs size is good, but power vs displacement is not.

fdracer

Thanks for the info guys. The way I see it is that one crankshaft revolution in the rotary is equivalent to 2 revolutions in the piston engine, in that it basically combines the intake and power strokes. So a 1.3 rotary would produce roughly the same hp as 1.3L piston engine at double the rpm. However, being fuel inefficient, does the rotary consume even more fuel than a piston motor would at double the rpm? If this is so, what is it about the wankel combustion process that causes this inefficiency and what can be done design-wise to improve this? Also what, exactly, causes the extreme heat found in rotaries? If it is, as KZ1 stated, due to limitations in the coolant passages, is there anything an engine builder can do to remedy this (i.e. porting out bottlenecks in the water passages, etc.)?

peejay

The inefficiency and extreme heat loss are due to the high surface-to-volume ratio inherent in the Wankel design. A "perfect" combustion chamber would be spherical, because that has the lowest surface to volume ratio - minimal surface area for combustion heat to be lost. Rotaries have the highest surface/volume ratio out there, it's astronomical and insane. LOTS of opportunity for combustion heat to get lost to the cooling system.

Also, a spherical combustion chamber is ideal because, if you can start the burn at the center of the sphere, it is a very short distance to the farthest reaches of the chamber. Rotaries have very wide, VERY long chambers, with lots of nooks and crannies that are hard for the flame to get to. This results is a decent chunk of the air/fuel mix simply not burning, and with a peripheral exhaust port, it just gets thrown right out the exhaust pipe. The fire is still burning as the port opens, too, because the burn is so slow. This results in high EGTs (heat energy wasted through the exhaust) and high noise.


The "ideal" engine, as far a current technology goes, would be a multivalve piston engine with large quench areas and an undersquare (stroke larger than bore) chamber, and a properly designed piston (flat-top or dish if you can keep the C/R high enough). But then you lose the inherent simplicity of the rotary. "Given two viable solutions, the simpler one is always the best"

DamonB

A huge and profound difference between a piston and a wankel is how the power is transmitted to the output shaft. Keep two things in mind:

A piston is only contributing to power output on the power stroke; for the other three cycles it is just along for the ride and it's mass consumes power from the other cylinders which ARE in the power stroke.

A piston has a nice linear ride inside the cylinder walls. The connecting rod however is attached to an offset throw on the crankshaft. If you could see through the crankcase and look down the centerline of the crankshaft you would notice that the big end of the connecting rod is swinging back and forth as the crank spins, while the small end is merely oscillating up and down with the piston. As far as mechanical advantage between the piston and the crank is concerned it is at its maximum during the power stroke when the connecting rod and crank throws are at 90 degrees to eachother. At all other times the relationship between the piston and crank is much less efficient. This means that during rotation of a boingers crank, the torque is actually varying wildly in each cylinder as it progresses through the power stroke.

The reason we always so rotarys are more efficient is in a purely mechanical sense (because they are not presently as fuel efficient. That is due to valve issues which is a whole other story...). Lets think of each three faced rotor as a piston. During one trip around the crank case each rotor will comlete 3 power strokes and never spend any time not contributing to intake, compression, power or exhaust; it is a nice fluid motion. Remember that the gearing between rotor and eccentric is 1 to 3. This gearing of the rotor to the eccentric shaft also allows the same mechanical advantage for the rotor's entire revolution. A piston on the other hand has to transmit its power through the connecting rod and into the crank. When the piston is nearing TDC and BDC its mechanical advantage to the crank is practially nil; the inertia of the shaft or output from other cylinders supplies the power to rotate the crank through. A very wide range of the piston's motion through the cylinder is wasted as far as it's percentage of contribution to power due to the widely varying changes in mechanical advantage as the degrees of deflection vary between the rod and crank throws.

A fluid or rotary transmission of power is always more efficient than an oscillating one. When something oscillates, a pendulum for example, it swings one way, stops, swings the other, stops, etc. In order for the pendulum to swing the other way it MUST stop for an instant. During that instant is is wasting time so to speak whereas the rotors in a wankle just follow a path without ever stopping to reverse the other way. The wankle also does not have to fight inertia within the motor to keep yanking these masses back and forth; the masses move in the same direction (with the exception of the "wobble" around the eccentric). This is why we call a rotary more efficient.

That said; boingers are so highly developed and have simpler engineering issues (like sealing combustion chambers for instance) that they can make more power more fuel efficiently with less heat generated. I would imagine a rotary does have less internal friction, but remember it also has to drag the sideseals as well as the apex seals all the way around the chamber. That is more than you realize.

I love the rotary for its engineering, simplicity and elegance. But a "low tech" small block Chevy can still moe than compete with it in the real world.

Orange!FD

DamonB, aside from the last two sentences, your post is complete and unmitigated BS, in the tradition of Dis1 and pweizman. You need to read what's been written here by peejay, xiaomingming, and me. If, after reading it, you find problems with that material, then feel free to unleash your ignorance.

DamonB

I am sorry; I was attempting to illustrate the geometric differences in how power is transmitted to the crank between a piston engine and a rotary engine. It was not a pro or con.

What part(s) am I completely stupid or ignorant on?

DamonB

I agree with peejay. I agree with xiaomingming. You? I don't see any real input; you appear to be riding coattails. I would like to hear your views as opposed to just pointing out others discrepencies. You have made statements, but no supporting evidence.

Last I checked I passed physics and AP physics in high school. Then it was Physics I, Physics II, Statics and Dynamics... etc while studying for an ME degree at the University of Texas at Austin. Last I checked it was one of the top 10 engineering schools in the country. Maybe it's slipped, or maybe I just got lucky to pass.

Dis1

DamonB

I think what dis1 was trying to say is that anything with mass poccesses potential energy. That potential is directly related to how far above the surface an object is because of the acceleration due to gravity. If you drop a brick from the top of a ladder onto the sidewalk the brick breaks. Drop it from the top of a tall building and it makes a hole in the sidewalk. The brick that was dropped from the building had no more mass, but its potential was higher due to the fact it was released much higher from the sidewalk.

There is no energy released while the car sits on the driveway, but the potential DOES exist.

Orange!FD

You should probably check the postings again, then.

Ok, I've said all this once, but I'll try to relate it to the specific terms that you used in your remarks. That might make things easier.

The mass of the piston doesn't consume any power. At all. Friction and heat dissipation are the only things that can consume power. Apparently this is such a simple fact that it isn't even recognized as one.

There's nothing *wasted* because of this variation in mechanical advantage. Whatever power isn't transmitted by the rod at a given crank position is eventually transmitted elsewhere in the rotation. Why? Because the hot gases in the combustion chamber only respond to volume changes and heat transfer. You can't get energy out of hot gases without cooling them down or allowing them to expand, or both.

Well you should stop calling it that, because it isn't true, by any definition of the word "efficient" that I've ever heard. A fluid or rotary transmission of power is not any more efficient than an oscillating one. There's no such thing as "wasting time" in a mechanism. The wankel has to "fight inertia" just like any other rotating mechanism, simply because it is rotating. If the rotor, for example, weren't made of pretty strong stuff, it would fly apart from the inertial forces inherent in spinning it around an axis, wobble or no.

I think it's starting to become clear that semantics are more the problem than anything else, here. If you'd insert the word "elegant" every time you used the word "efficient", then I wouldn't have the reaction I did.

No comment. Except, why does it say "Dis1" there?

DamonB

dis1 is a typo; shouldn't be there.

Alright Orange!FD, I agree with you on the semantics part. What I was trying to point out is that as far as the conversion of a linear motion (piston in cylinder) into a rotational one (crankshaft) the piston is at a mechanical disadvantage because maximum torque occurs only when the connecting rod and crank throw are at a 90 degree angle. Yes, the combustion gases keep expanding as they are burning and therefore continue to drive the piston through the entire cycle. It still seems to me though that when the connecting rod is at very acute angles to the crank throw that it has very little leverage and therefore does not transmit its power as "elegantly" as a wankle does since the rotor's mechanical advantage against the eccentric does not vary as it rotates.

I am trying to illustrate that during the piston's trip down the cylinder during the power stroke that most of the power is actually delivered to the crank in a relatively short distance due to the geometry of the rod/crank connection. The piston spends a good part of time traveling but not contributing a whole lot of force to the crank. Whereas a rotor's motion is always contributing to crank output; it does not spend time changing direction.

Even if I am doing a horrible job of explaining the geometry problem we do agree that since a piston must stop once at TDC and once at BDC that it is IMPOSSIBLE for it to contribute meaningful output to the crank because the piston is in fact stopped? In the case of a rotor even when a particular face is not under power, its inertia at the very least carries it in a direction that contributes to crank output because the rotor does not reciprocate; it always travels in one direction.

rookie7

ok my turn now. A rotary engine will complete 1 power stroke in 1 revolution and the reason is because the rotor rotates 1/3 the speed of the eccentric shaft. A 2 rotor engine will complete 4 power cycles in 2 revolutions, the same as a 4 cylinder does in 2 revolutions. the difference in power produced between a 1.3L rotary and a 1.3L piston engine comes from the fact that the rotary engine's power stroke lasts for 270 degrees of eccentric shaft revolution as compared to 180 degrees of crankshaft revolution for a piston engine.

you can compare mechanical, thermal, and volumetric efficiencies, but none of those are sufficient to explain the difference you see in horsepower between a rotary and a piston engine. it is the difference between power stroke duration that is the main factor.