This invention relates to a rotary engine that functions without internal gears, off-centered rotors, reciprocating pistons, crankshafts, trochoidal chamber, chains, nor belts. (While no claim can be made on missing parts, the fact that such conventional parts are missing is one of the reasons that make this invention different from prior arts, and, therefore, it is so listed.) The preferred embodiment, comprising multipurpose rotors, paired dams, combustors, turbines, etc., could be made small enough to run a lawn mower or large enough to fly a big air plane. It uses a modified form of combustors, wherein combustion takes place amid ample air, thus promoting a fuller combustion of fuel at a lower temperature than in conventional rotary and reciprocating piston engines. Consequently, this invention has a surprisingly low amount of CO, NO.sub.x, and unburnt hydrocarbons in its exhausts. Each rotor has a compressing head, oil reservoir, turbine blades protruding from its radial side, and cooling means in its central part, and serves as a compressor, flywheel, lubricator, engine cooler, and turbine. Exactly at the end of each compressing cycle, each rotor releases its compressed gasses to the combustors, wherein gasses are either mixed and ignited, or directly ignited. The resulting expanding gasses passing through the combustors' exhaust nozzles strike the turbines, causing the latters to spin. Each turbine is attached to a rotor's radial side, and each rotor, in turn, is keyed to the central power output shaft. SUMMARY The present invention relates to a rotary engine that has the good features of rotary and air-breathing turbo-jet engines, but without the off-centered geared rotating parts, jet disturbances, space taking axial and radial compressors, and cumbersome heat exchangers. It has fewer mechanical parts than the majority of the conventional hot-air, free piston, gas turbine, and reciprocating piston engines, and consequently it is lighter and takes less space than the majority of such engines of comparable power. It is designed so that it could be constructed small enough to run a lawn mower, or large enough to power a car, a boat, or an electric generator of considerable size. In designing the present engine, I have kept foremost in mind a power source that will not become a threat to the environment. It is well known that the conventional off-centered rotary, free piston, and reciprocating piston engines have too much of the three major pollutants in their exhausts: (1) carbon monoxide and (2) unburnt hydrocarbons, which are due to a great extent to the incomplete combustion of fuels, and (3) nitrogen oxides which are caused by too high a combustion temperature that is very difficult or impossible to avoid in such engines. It is also known that the combustions in the combustors of gas turbine and turbojet engines are fuller and occur at a lower temperature than in the very limited combustion spaces of the conventional off-centered rotary and the reciprocating piston engines because more combustion supporting air and space are available in the formers, and consequently there is very little carbon monoxide, unburnt hydrocarbons, and nitrogen oxides in the exhausts of combustor-equipped engines. By using a new type of combustor, this invention provides a comparatively large amount of air and space to sustain a fuller combustion at a lower combustion temperature than are possible in the limited combustion spaces of reciprocating piston and off-centered rotary engines now on the market, thereby keeping the amount of carbon monoxide, unburnt hydrocarbons, and nitrogen oxides in its exhausts at a very low level that could hardly become a serious threat to environment. The present engine is made in a substantially cylindrical configuration, which comprises, among others, a casing that is divisible into two cylindrical parts, a pair of rotors that move in separate but parallel paths, paired dams or gates, and combustors. The two rotors, with a stationary cylindrical power chamber sandwiched between them, are keyed to a common power output shaft. The forward wall of each rotor serves as the piston head, and the heads of the two rotors are separated from each other by 180.degree., which means that each piston head will reach a given dam or gate in its path at a time different from that of the other. Besides being a compressor, each rotor has a central section that serves as an engine cooler and ventilator, a peripheral part that contains an oil reservoir that serves as a supplier of lubricants to the various contacting surfaces, a circle of turbine blades protruding perpendicularly from its radial side, and sufficient weight that could make it function as an energy conserving and rotation equalizing flywheel. The said stationary power chamber contains the combustors, and separates the paired dams and the paired rotors into the left and the right dams, and the left and the right rotors. Each rotor's piston head compresses gasses in its enclosed path against a leak-proof gate called dam. A dam is mounted and securely attached to each end section of a common shaft with the plane of each dam securely fixed in a position perpendicular to that of the other. Consequently, if the right rotor's piston head, after completing its side of the compression, pushes the right dam into a horizontal or open position so that it could pass under it, the left dam will, at the same moment, become vertical or closed so that the left piston head could compress gasses in its path against the closed dam, and vice versa. By virtue of the said perpendicularity of the paired dams and the way the parts, the dams' shaft in particular, are structured and arranged, the said dams open and close at the right time without any human or electronic help. Compression increases as each piston head approaches a closed or vertical dam, and at the maximum or near maximum compression, the outlet ports in each rotor's radial side, by virtue of the rotor's rotation, align with the openings in the stationary passageways that lead to the power chamber, thus giving the compressed gasses smooth channels that lead into the combustors. At low or lower medium speed, the combustors fire at regular or comparatively short intervals, but the rotation of the output or power shaft continues to be comparatively smooth due to the rotors that double as flywheels. At the upper medium and high speeds, the intervals between the compressions and firings become so short that the resulting rotation of the engine, for all practical purpose, will be very near or about the same as those of the continually firing jet engines. The said combustors use compressed air and mix it with fuel, or use compressed mixture of air and fuel, or even hydrogen. Ignition takes place in the cone located in the paraboloidal nose of each combustor. The resulting expanding gasses or jet strikes the turbines mounted and attached to the radial side of each rotor, which is keyed to the central power output shaft. Thus, the kenetic energy of the expanding gasses is converted directly into usable rotary motion. DETAILED DESCRIPTION For a fuller description of the parts, assembly, and operation of the said engine, references are made to the following specification and to the accompanying drawings wherein like reference numbers are utilized to refer to identical components throughout the several drawings. It is understood that this invention is not to be construed as limited to the particular forms described or illustrated herein, since the forms were chosen for the purpose of the disclosure, and, therefore, should be regarded as illustrative rather than restrictive.