Dear Ralph: Are there advantages of the rotary intake that Rotax uses on many of their twin cylinder engines ? Are rotary equipped engines...

Dear Ralph:

Are there advantages of the rotary intake that Rotax uses on many of their twin cylinder engines ? Are rotary equipped engines more powerful than piston ported or reed intake designs ?

First let’s compare the rotary intake to a piston ported engine. The advantages of the rotary intake are that the intake port is positioned directly into the crankcase, and that the opening and closing of the intake port is controlled by a rotary valve disc instead of the piston. The use of a rotary valve enables a very short inlet path. This design gets the fuel/air mixture into the engine without any kind of obstruction that would impair the volumetric efficiency. This intake position also enhances the lubrication of the lower connecting rod bearings. The use of a rotary valve allows a longer duration of intake and the opening of the intake starts earlier. This results in better filling of the crankcase. In addition, the rotary intake closes earlier helping to avoid fresh charge spitback. The rotary valve gives the engine designer complete control of the intake duration. This basically gives the engine greater torque at lower rpms, more peak power, and easier starting, all other factors being equal, of course.
Reed intakes provide substantial improvements in torque when compared to piston ported designs, but also have disadvantages. Reeds tend to separate air from the fuel. Since the crankcase “vacuum” must first open the reed to allow intake, this force is not fully applied to the actual intake operation of fuel & air – resulting is a partial loss of intake potential. At higher speeds, the delay in closing the reeds affects the reopening of the reed (referred to as “float”). Again, potential volumetric efficiency is affected. Stiffness of the reeds, size of the cage, and potential for breakage are also variables to deal with.

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