Rotary vs. Piston

If an ordinary piston engine has a compression ratio of 10 to 1, and the rotary engine does not do compression. One might assume that a rotary engine would need to be 10 times as large to generate the same power.

We can stipulate that the rotary engine does need 10 times as much working displacement as a piston engine to generate the same power. But that doesn't mean the rotary engine needs to be 10 times as large to generate the same power. At best the 4 stroke piston engine generates power 25% of the time. The rotary engine should be able to generate power around 90% of the time.

Combining the compression ratio of the piston engine with the better power generating time of the rotary engine should bring us to the point where the rotary engine needs to be about 2.7 times as large as the piston engine.

But the piston engine requires a large crankshaft to convert the linear motion of the pistons to circular motion. It also requires a rotating cam to control the valves. I imagine the crankshaft and the cam together are actually as large as the pistons.

The Round8 engine needs no crankshaft or cam. It does need some gears to synchronize the rotation of the vanes and the rotating valve plate, but it should be possible to design these gears so as to not make the engine much larger than the combustion chambers.

That might bring us to the point where the rotary engine needs to be about 1.6 times as large as the piston engine to generate the same amount of power.

But there is another factor. The rotary engine is inherently more efficient than the piston engine. At the end of the compression stroke, the air in the piston has been compressed to about 150 pounds per square inch. Ignition brings the pressure up to about 1500 psi. During the power stroke, the volume of the piston is expanded by a factor of ten, bringing the pressure in the cylinder down to 150 psi. Then the exhaust valve opens and the energy of the 150 psi is just blown out the exhaust and wasted. The release of this energy makes so much noise that is has to be muffled to make it tolerable. The muffling wastes energy by making the pressure in the exhaust manifold higher than the ambient pressure outside the engine.

The above assumes an engine is running at full throttle. At half throttle, the ignition pressure would only be 750 psi, and the wasted exhaust pressure would only be 75 psi.

The rotary engine can be programmed to bring the combustion chamber pressure to near ambient when the exhaust port is activated. It might be desirable to use more fuel and create more noise and waste in situation requiring an unusual amount of power.

Also the operation of the crankshaft and cams are inherently wasteful. Because the motions involved are linear instead of rotary, the components are subjected to large variations in the direction and intensity of acceleration, which increases the amount of friction and wear on bearings and components, and requires them to be more robustly designed and heavier.

Because of the high compression, the piston engine, must have a very tight seal. To achieve that tight seal the piston rings must push hard against the cylinder walls. That creates a lot of friction between the cylinder walls and the piston rings and wastes power.

Perhaps these factors could bring the rotary engine and the piston engine to rough parity in power per unit volume.

But power per unit mass is a different matter. The rotary engine could be much lighter than the piston engine because the loads being handled by the critical components are only one tenth as large as in the piston engine, because there are a lot fewer components, and because all motions are circular with the exception of the butterfly valve. With a lighter engine putting out the same power, the mechanism that the engine powers can also be lighter and more efficient. The mechanism that connects the engine to the device it powers can be less robust, so the weight savings will extend beyond the difference between the mass of the piston engine and the rotary engine.

Like the piston engine, the rotary engine could be turbocharged. This is seldom done in commercial piston engines, so it might not be practical. On the other hand a rotary engine that has a turbo charging capability when extra power was needed might be useful.

The above calculations are a first draft and need to be checked, elaborated, and verified by empirical tests.