1.  Field of the Invention
The present invention relates to a rotor-type pump suitable for a hydraulic pump producing the oil pressure which is required to circulate lubricant to automobile parts such as various moving engine parts or to deliver working fluid to power steering. 
2.  Description of the Related Art
Various types of oil pumps are generally known, which are used in pressure-feed systems, for delivering lubricant to an internal combustion engine or working fluid to a power steering system.  Among them, for instance, there are gear pumps, plunger pumps. 
In addition, in order to provide a hydraulic pump having higher performance, it is desired to apply a basic construction of a Wankel engine, namely, four-stroke cycle rotary piston engine, to hydraulic pumps.  The construction and operation of the Wankel engine is explained hereinafter.  The Wankel engine typically has a rotor housing having an inner peri-trochoidal curved surface, spaced side housings enclosing and sealing the housing, a drive shaft or crankshaft with a rotor journal eccentric to a center axis of the drive shaft.  A generally triangular rotor is rotatably eccentrically disposed in the rotor housing and has three rotor lobes or apexes which are circumferentially equi-distantly spaced to each other and slides on the inner peri-trochoidal curved surface of the housing as the rotor rotates on the rotor journal.  A stationary gear is secured to one of the side housings and has a bearing supporting one end of the drive shaft.  A rotor internal gear is mounted in the rotor.  The rotor fits on the eccentric rotor journal so that the rotor internal gear meshes with the stationary gear.  The rotor is guided by the stationary gear and rotates around the stationary gear.  Apex seals on the three lobes are in contact with and tightly fit against the inner peri-trochoidal curved surface to provide a tight seal.  Thus, three separate chambers are defined by the inner peri-trochoidal curved surface, respective adjacent two of the rotor lobes and outer peripheral surface portions extending between the respective adjacent two of the rotor lobes.  When the rotor rotates around the stationary gear of the side housing, the chambers increase and decrease in volume.  An intake port and an exhaust port are provided in the rotor housing or the side housing in parallel to each other.  A pair of spark plugs are so disposed in the housing as to face the two ports. 
In case of a well-known one-rotor Wankel engine, the gear ratio between the rotor internal gear and the stationary gear is set at 1:3 so that the drive shaft rotates three times every revolution of the rotor.  Thus, there are four stages, namely, induction stroke, compression stroke, power stroke and exhaust stroke, with respect to each of the chambers defined by the inner peri-trochoidal curved surface and the outer peripheral surface of the rotor during one revolution of the rotor.  Specifically, when the rotor rotates after one of the rotor lobes has cleared the intake port, the chamber between the one lobe (leading lobe), the adjacent lobe (trailing lobe) and the housing begins to increase to produce a partial vacuum, causing air-fuel mixture to flow into the rotor housing.  With a further rotation of the rotor, the chamber continues to increase in volume.  When the rotor reaches a point wherein the trailing lobe passes the intake port, the air-fuel mixture is sealed between the leading and trailing lobes.  As the rotor further rotates, the chamber decreases in volume to cause the mixture therein to be compressed.  When the compression of the mixture reaches near TDC on the compression stroke, the mixture is ignited by the spark plugs to cause the combustion.  Thus, the power stroke commences.  At this stage, the hot burnt gases push the rotor to further turn around, and expand until the leading lobe has cleared the exhaust port.  The hot burnt gases begin to discharge from the chamber via the exhaust port and the exhaust stroke continues.  Then the leading lobe has cleared the intake port again and the induction stroke restarts.  In this manner, the four stages are repeatedly executed each revolution of the rotor.  One example of the conventional Wankel engine has been disclosed in Japanese Utility Model Application Second Publication No.  64-15726. 
However, it is very difficult to apply the basic construction of the Wankel engine as previously explained, to oil pumps used in pressure-feed lubricating systems of automotive engines, for the reasons described as follows. 
In the Wankel engine, one meshing pair, namely, the stationary gear and the rotor internal gear, are provided for controlling the rotor in such a manner that the rotor eccentrically rotates around the drive shaft and follows the peri-trochoidal curved surface of the rotor housing.  Such a conventional rotor-control device composed of the stationary gear and the rotor internal gear requires a high machining accuracy of the meshing pair.  Further, the conventional rotor-control device has a complicated structure and many parts and therefore it also requires a relatively great installing space in the housing.  Accordingly, if the conventional rotor-control device is used in a rotor-type pump, the high accuracy of machining of the meshing gears and the complicated structure cause reduction of operating efficiency in the producing process, leading to increase in production costs of the rotor-type pump.  In addition, in the case of utilizing the conventional rotor-control device in the rotor-type pump, the relatively great space for installation of the meshing gears causes increase in the entire size and weight of the rotor-type pump. 
Further, since the conventional rotor-control device guides the rotor in such a manner that the lobes of the rotor always slides on the peri-trochoidal curved surface, the lobes and the peri-trochoidal curved surface are subject to frictional abrasion caused due to the sliding contact therebetween for duration of time.  This leads to reduction of the durability of the rotor and the peri-trochoidal curved surface.  For this reason, the rotor and the peri-trochoidal curved surface must be covered with abrasion-resistant member or be in entirety made of suitable anti-abrasion materials.  Therefore, in the case of using the conventional rotor-control device in the rotor-type pump, it results in increase in the production and material costs thereof. 
Furthermore, in the Wankel engine, the fuel system mixes a fine spray of fuel with air to make a combustible and compressible air-fuel mixture and the compressible air-fuel mixture is compressed on compression stroke and ignited and expanded on power stroke.  Namely, the Wankel engine is applied to a compressible fluid and so designed as to function as an internal combustion engine by way of compressing and expanding action of the compressible fluid, i.e., changes in volume in the combustion chamber.  On the other hand, oil pumps are applied to incompressible fluid such as lubricating oil for automotive moving or rotating parts or working fluid for a power steering device.