fig1 the pin 98 of each vane is slightly smaller in diameter than the width of cam groove 96 so that the pins slidably follow along the groove as the rotor rotates this may be appreciated from the different positions of the vanes shown in fig1 the pins 98 cause the vanes to reciprocate axially as the rotor rotates compared to prior art engines of the type mentioned above the provision of a guide cam and follower in the form of cam groove 96 and pins 98 means that the force to move the vanes is removed from the cam surfaces 42 and 44 thus the strength of materials on the cam surfaces may be reduced so that lighter materials such as aluminum can be employed in addition liquid lubrication can be applied to the cam grooves and pins to reduce friction and wear previously the load had to be carried by the cam surfaces which had much more marginal lubrication and consequently higher rates of wear and fictional losses the lubricant can be introduced into the cam groove located on outer housing 18 of the stator either through the rotor and drained out the through the outer housing or through the outer housing and drained out through other openings in the outer housing or back through the rotor the cam groove can be machined directly into the outer housing as in the illustrated embodiment of fig1 or can be machined into an insert.