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The prime mover (PM) rotates the gear-reducer gears through a V-belt drive. The two most common PMs are electric motors and internal combustion (IC) engines. The decision concerning which to use depends on a variety of considerations, which includes the following: * Availability of the power source (electricity or combustible fluid) * HP required to pump the well * Efficiency of the system * Ability to control the PM to match the on/off potential operation of the pumping unit * Availability of field and/or service personnel capable of maintaining and repairing the equipment * Condition of the gas (sweet or sour) or availability now and in the future of the gas or liquids (i.e., propane or diesel) if an IC engine is used * Current and future expected cost for the power source * Anticipated full-cycle total cost (including initial capital, operating, maintenance, downtime, and repairs) for the duration of the well These considerations, as well as other factors, have been discussed in numerous publications.[1][2][3][4][5][6][7][8][9][10] The characteristics of these engines are summarized here, and the detailed comparisons and field experiences have been published elsewhere.[11][12] Four-cycle, high-speed engines (greater than 750 rpm): * Best suited for portable test installations vs. permanent installations * Lower initial cost * Lower weight per HP * Wide speed and power range * Operates on a variety of fuels * Large speed variations occur during pumping cycle because of a small flywheel effect * Operates on a fixed throttle with the governor mechanism acting only as an overspeed device * Has relatively short life because of the fast moving parts and the close tolerances required * Requires frequent oil changes * Requires frequent maintenance * Major repairs require that the engine be removed from the pumping unit These test data should be requested and furnished to the purchaser from the manufacturer. The data should include the manufacturer's curves showing the torque, maximum brake HP, and the rated-brake HP vs. engine speed. These are important to know the speed range in which the engine would be able to operate. A general guide for installation and maintenance of gas engines is API RP 7C-11F,[14] which covers all three types of engines and includes a troubleshooting section. This practice should be used as a starting point for engines unless the specific manufacturer's operating manual details otherwise. Additionally, there are a number of published papers on installation, care, operation, and lubrication of engines as prime movers for pumping units.[15][16][17][18][19]
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