The utility model relates to a variable cylinder master-auxiliary two-motor vehicle engine. The utility model is characterized in that a master engine(3) which takes four cylinders as a basic quantity is used for normal driving operation; an auxiliary engine(1) which has N cylinders is used for auxiliary power; the master-auxiliary engine cylinder body are integrated into a whole through a bolt and are both arranged on an engine rack(4); and the power output end of the auxiliary engine(1) is connected with a crankshaft belt pulley(5) of the master engine(3) through an eccentric automatic positioning coupling(2). When the rotation speed of the auxiliary engine(1) excesses the idle speed after being started, the eccentric inclined tooth of the eccentric coupling(2) is threw out in the effect of centrifugal force to connect the crankshafts of the master power and the auxiliary power together to form an engine. The auxiliary engine(1) is closed when power is not needed. The coupling automatically separates the rest master power operation. All the electric devices are arranged and loaded on the master engine(3). Two electric spraying accelerator supplying wires are both arranged in parallel on a same accelerator pedal, so that the two power rotation speed can be controlled in-phase. A double-device water pipe serial connection circulating cooling manner is adopted to sink the heat of the engine.