Triggering wheel for fluid transfer device Field of the Invention The invention relates to a fluid transfer device, especially to a triggering wheel for the fluid transfer device. 5 Description of prior art The working prcess of the internal combustion engine is roughly such, the piston moves downward, the mixed combustible gas goes into the cylinder; the piston moves upward to compress 10 combustible gas, the spark plug ignites combustible gas to explode the gas, the exploded gas with high temperature and high pressure drives the piton moving downward, the linear movement of the piston is converted to the rotational movement of the crankshaft by the connecting rod; the crankshaft goes on rotating because of the 15 inertia, the rotational movement of the crankshaft is converted to the linear movement of the piston by the connecting rod, the piston moves upward to push exhaust gas out of the cylinder. In brief, inhale- compression -work- exhaust. 20 So, there is provided a device using reciprocal movement of the piston to transfer fluid. The working process of the device is that, the piston moves upward to creat a negative pressure area in the receiving chamber, fluid is compressed into the receiving chamber through the input passage by the effect of the atmosphere. The 25 downward movement of the piston within the receiving chamber compresses fluid therein out of the receiving chamber through the output passage. A single cycle of motion is separated into an input phase and an 30 output phase. The receiving chamber communicates with the input 1/6 passage during the input phase, the receiving chamber communicates with the output passage during the output phase. A cartridge freely hermetically rotating inside the receiving chamber achieves the swithcing between the input phase and the output 5 phase, it is a problem to achieve that the rotation of the cartridge interacts with the movement of the piston. Summary of the present invention The object of the invention is to provide a triggering wheel for the 10 fluid transfer device so as to solve the above problem. A triggering wheel for the fluid transfer device comprises a disc-shaped fixed sheet with a transmission bore; the triggering wheel further comprises a triggering block uniformly arranged in a 15 ring array, the axis of the array is the axis of the fixed sheet, the triggering block is axially clamped between two fixed sheets, the triggering block is coaxially provided with an interior arcuate surface and an exterior arcuate surface, both the head side surface and the tail side surface of the triggering block tangentially connects with 20 the interior arcuate surface and the exterior arcuate surface through the arcuate surface, the interior and exterior arcuate surfaces are coaxial with the fixed sheet. Preferably, the quantity of the triggering block is 2. 25 Specifically, the fixed sheet and the triggering block are fastened together by rivet. Advantageous Effects 30 The intermittent interaction between the triggering block and the triggering end achieves the interaction between the upward-and-downward movement of the piston and the rotation of the cartridge. 2/6 Because the interior and exterior arcuate surfaces are coaxial, and the axises of the interior and exterior arcuate surfaces are coaxial with the axis of the fixed sheet, a tangent condition between the exterior arcuate surface and the triggering end will be achieved, 5 such tangent condition makes the cartridge retain at the present position, that is leaving the receiving chamber communicated with the input passage or the output passage. Brief description of the drawing 10 In the following, the invention will be described in greater detail by means of some embodiments with reference to the accompanying drawings, in which Fig.1 is a sectional front view of the fluid transfer device; Fig.2 is a 3d-drawing of the triggering wheel; 15 Fig.3 is a front view of the triggering wheel without a piece of fixed sheet; Fig.4 is an exploded 3d-drawing of the triggering wheel. 1. the receiving chamber; 2. the piston; 3. the cartridge; 4. the input passage; 5. the output passage; 20 6. the fixed sheet; 7. the triggering block; 8. the triggering end; 9. the first passage; 10. the second passage; 11. the interior arcuate surface; 12. the exterior arcuate surface; 13. the arcuate surface. 25 Detailed description of the preferred embodiment Referrign to Fig.1, it is a fluid transfer device of the invention, inorder to increase the transferring efficiency of the device, the device includes four piston units, four piston units are uniformly arranged in a ring array, the axis of the array is the axis of the 30 cartridge 3. Three of the four input passages 4 are sealed by plugs, and three of the four output passages 5 are sealed by plugs, the transfer device only keeps one input passage 4 and one output passage 5. 3/6 For better understanding the triggering wheel, one piece of fixed sheet 6 of the triggering wheel in Fig.1 is hidden, the triggering end 8 with a round tail end mates with the triggering block 7, the triggering end 8 is fixedly connected with the cartridge 3, the 5 triggering end 8 is located between two fixed sheets 6 of the triggering wheel. The rotation of the crankshaft drives the piston 2 moving reciprocally in the receiving chamber 1, the crankshaft transfers the power to the transmission shaft through thte belt wheel, the transmission shaft drives the triggering wheel rotating 10 through the transmisson bore configured on the fixed sheet 6. The triggering end 8 retains at the position shown in Fig.1 under the elastic force of the reset component, the end portion of the triggering end 8 is at the middle position between two triggering 15 blocks 7, and the distance between the triggering end 8 and the triggering block 7 is equal. The first passage 9 of the cartridge 3 communicates with the receiving chamber 1, the second passage 10 of the cartridge 3 communicates with the output passage 5, that is the receiving chamber 1 communicates with the output passage 5 20 with the help of the cartridge 3. The triggering wheel rotates in a clockwise direction during the piston 2 moving downward, when the triggering wheel rotates a certain angle, the piston 2 is near to the constriction of the receiving 25 chamber 1, the arcuate surface 13 of the head side surface of the triggering block 7 touches the end portion of the triggering end 8, after this, the piston 2 goes on moving downward, meanwhile the triggering block 7 pushes the triggering end 8 to rotate, the cartridge 3 rotates as the triggering end 8 rotates, the second 30 passage 10 and the output passage 5 are gradually staggered till the piston 2 moves downward to the extreme position, that is the bottom dead center, the second passage 10 do not communicate with output passage 5 at all. 4/6 The piston 2 moves from the bottom dead center to the top dead center with the help of the crankshaft, meanwhile, the triggering wheel goes on rotating, the triggering block 7 goes on driving the cartridge 3 rotating, the second passage 10 gradually turns to the 5 input passage 4 till the second passage 10 totally communicates with the input passage 4, at this time, the piston 2 is at the pistion 2, the cartridge 3, the triggering end 8 and the triggering block 7 shown in dashed line in Fig.1. 10 The piston 2 goes on moving upward, the triggering wheel goes on rotating in clockwise direction, because the triggering end 8 is tangent to the exterior arcuate surface 12 of the triggering block 7, the exterior arcuate surface 12 of the rotating triggering block 7 do not go on pushing the triggering end 8 to rotate, that is the 15 cartridge 3 stops rotating to leave the second passage 10 communicated with the input passage 4. The piston 2 goes on moving upward, the triggering wheel goes on rotating in clockwise direction, when the piston 2 moves upward to 20 the top dead center, the triggering block 7 of the triggering wheel separates from the triggering end 8, due to the elastic force of the extension spring, the cartridge 3 rotates in clockwise direction till the triggering end 8 touches the limit component, the cartridge 3 stops rotating and retains at the present position, that is the second 25 passage 10 is switched from the input passage 4 to the output passage 5 and maintained a communication with the output passage 5, so far, a single cycle of motion is done, in a sigle cycle, the first passage 9 always communicates with the receiving chamber 1, while the second passage 10 communicates with the output 30 passage 5 or the input passage 4 with the cooperation of the triggering wheel. Referring to Fig.2 and Fig.4, the triggering wheel is provided with two disc-shaped fixed sheets 6, two arcuate triggering blocks 7 and 5/6 four rivets. Two triggering blocks 7 are uniformly arranged in a ring array, the axis of the array is the axis of the fixed sheet 6, two triggering blocks 7 are clamped between two fixed sheets 6, the head and tail ends of each triggering block 7 are fastened to the two 5 fixed sheets 6 by rivet. Each fixed sheet 6 at the center portion is provided with a through hole for matting with the crankshaft and the key. Referring to Fig.3, the triggering block 7 is coaxially provided with an 10 interior arcuate surface 11 and an exterior arcuate surface 12, both the head side surface and the tail side surface of the triggering block 7 are tangentially connected with the interior arcuate surface 1 and the exterior arcuate surface 12 by the arcuate surface 13, the interior and exterior arcuate surfaces are coaxial with the fixed 15 sheet 6. 6/6