When I look at a picture of a rotary engine, there is something I don't understand.

http://upload.wikimedia.org/wikipedia/commons/c/c3/Wankel_engine_diagram.png

At the ignition, the gas expands, but it looks to me as though some of the energy is directed perpendicular to the desired direction, some of the force looks as if it is traveling straight left from the spark plugs. It looks to me as if much of the useful energy is being absorbed by the shaft and the rotors only turn with the resultant power that has nowhere else to go.

I have a feeling there is something I'm missing, can anyone tell me?

Have you seen MM's video? Understanding takes more than a simple picture.;)

im sure inertia helps combat the leftward push you are talking about.

im sure inertia helps combat the leftward push you are talking about.

That's originally what I thought, and it definitely helps, but I don't think there is any way to avoid significant energy loss through the shaft. I have been trying to draw a newly shaped rotor/chamber design that would better avoid this problem, but I'm coming up empty.

This geometry isn't all that different from a piston engine near TDC, when the crank is pointing straight up. Not much leverage right then, and it's not the point where power is transmitted to the output. The air/fuel mixture is burning and building pressure, not expanding much right at that time.

Power is transmitted to the shaft during the stroke as the volume of the burned mixture expands.

The Wankle never has a position with great leverage, like a piston engine at 90 degrees after TDC, which is why they're not known for torque. (Just like short stroke piston engines aren't torque monsters.) But the power that you get out them is still the energy associated with the burned mixture expanding through its volume, just like in a piston engine. The Wankel's inefficiency comes from the surface areas involved with the combustion chamber and the separate intake and compression regions.

Ken

This geometry isn't all that different from a piston engine near TDC, when the crank is pointing straight up. Not much leverage right then, and it's not the point where power is transmitted to the output. The air/fuel mixture is burning and building pressure, not expanding much right at that time.

Power is transmitted to the shaft during the stroke as the volume of the burned mixture expands.

The Wankle never has a position with great leverage, like a piston engine at 90 degrees after TDC, which is why they're not known for torque. (Just like short stroke piston engines aren't torque monsters.) But the power that you get out them is still the energy associated with the burned mixture expanding through its volume, just like in a piston engine. The Wankel's inefficiency comes from the surface areas involved with the combustion chamber and the separate intake and compression regions.

Ken

I see what you're saying. But if that's true, than couldn't the problem be solved by designing an engine with elongated rotors for more leverage? OR by placing the spark closer to the bottom seal of the rotor?

I see what you're saying. But if that's true, than couldn't the problem be solved by designing an engine with elongated rotors for more leverage? OR by placing the spark closer to the bottom seal of the rotor?

It's not a problem. It's just the way the thing works, what happens when. I don't know enough about rotaries to know if they can play with epitrochoid proportions the way the piston guys play with bore/stroke ratios. But the geometry, plug positions and ignition timing were worked out by people who know what they're doing. They'd have accounted for flame speed, gas momentum, leverage on the eccentric shaft, etc. The rotary works!

You really need to consider the entire power stroke, not just what happens at one position. Think about Diesels, where all the combustion takes place close to TDC.

Ken

I see what you're saying. But if that's true, than couldn't the problem be solved by designing an engine with elongated rotors for more leverage? OR by placing the spark closer to the bottom seal of the rotor?

The force that is perpendicular has nowhere to go because the rotor can't move that way. Work only occurs if there is movement -- i.e., force through a distance. The rotor does move, and the energy of combustion drives that movement. Consider a piston. There's plenty of force against the cylinder walls trying to expand the cylinder. That energy is not wasted when the cylinder refuses to expand. :)

Due to the necessary geometry, elongated rotors would simply mean a larger rotor housing, and a larger engine. Very large rotary engines have been built, but they suffer the same fundamental issue -- the combustion zone is moving. The larger the rotary engine the lower the rpm because otherwise the air/fuel mixture moves faster than the flame front can propagate.

From your last question I get the feeling that you are thinking of force being directed outward from the spark plug. The plug simply begins the combustion process. The location and timing of that initial spark have to take into account the movement that occurs. This is already timed in such a way as to optimize the energy transfer as the rotor continues its movement and the gasses burn and expand.

The force that is perpendicular has nowhere to go because the rotor can't move that way. Work only occurs if there is movement -- i.e., force through a distance. The rotor does move, and the energy of combustion drives that movement. Consider a piston. There's plenty of force against the cylinder walls trying to expand the cylinder. That energy is not wasted when the cylinder refuses to expand. :)

Due to the necessary geometry, elongated rotors would simply mean a larger rotor housing, and a larger engine. Very large rotary engines have been built, but they suffer the same fundamental issue -- the combustion zone is moving. The larger the rotary engine the lower the rpm because otherwise the air/fuel mixture moves faster than the flame front can propagate.

From your last question I get the feeling that you are thinking of force being directed outward from the spark plug. The plug simply begins the combustion process. The location and timing of that initial spark have to take into account the movement that occurs. This is already timed in such a way as to optimize the energy transfer as the rotor continues its movement and the gasses burn and expand.

Question answered. Thank you :)

makes sense to me:)