Patent claim: Fuel injector 100 comprisinga fuel valve 110 comprising a valve seat 130 and a movable valve needle 125 which cooperate for controlling a flow of fuel through the injector 100a calibration spring 155 andan electromagnetic actuator 115 comprising a solenoid 140 and a movable armature 145 whereinthe calibration spring 155 exerts a pressing force on the valve needle 125 for pressing the valve needle 125 in a closing direction towards the valve seat 130when the solenoid 140 is electrically energized the electromagnetic actuator 115 is operable to transfer a lifting force to the valve needle 125 by means of engagement with the armature 145 for lifting the valve needle 125 from the valve seat 130 to a fully open position against the pressing force of the calibration spring 155 wherein the calibration spring 155 and the electromagnetic actuator 115 are configured such that the lifting force equals the pressing force in the fully open position.

Related passage: The amount of lift of the valve body 114 is small because the fuel passage crosssectional area As is small the flow rate of the fuel flowing in the vicinity of the seat diameter ds increases and a pressure loss occurs in the fuel passage 502.In general since the pressure loss increases in proportion to a dynamic pressure v22 p is a density of fluid and v is the flow rate the pressure loss is more increased as the flow rate is larger.Also when the flow rate is increased a reduction in a static pressure due to the Bernoullis theorem is increased with the result that the pressure in the vicinity of the seat diameter ds is decreased.The pressure on the tip of the valve body 114 is reduced due to the reduction in the static pressure in the vicinity of the seat diameter ds and the pressure loss.The hydrodynamic pressure exerted on the valve body 114 is a product of a differential pressure between a pressure upstream of the valve body 114 for example a contact position with the spring 110 and a pressure on the tip and a pressure receiving area for example area of an outer diameter on the tip of the valve body.Therefore as the pressure on the tip of the valve body 114 is lower the hydrodynamic pressure exerted on the valve body 114 becomes larger.Also when the amount of lift of the valve body 114 is small the flow rate of the fuel flowing in the vicinity of the seat diameter ds becomes higher.Therefore the pressure downstream of the seat diameter ds cannot be increased due to the reduction in the static pressure under the Bernoullis theorem the differential pressure between the upstream side of the valve body 114 and the tip becomes larger and the hydrodynamic force exerted on the valve body 114 becomes larger.As the amount of lift is larger the fuel passage crosssectional area As between the valve body 114 and the valve seat 118 becomes larger thereby decreasing the flow rate on the seat diameter ds.As the flow rate in the vicinity of the seat diameter ds is decreased the reduction in the static pressure due to the Bernoullis theorem is suppressed.Therefore the pressures in the vicinity of the seat diameter ds and on the tip of the valve body 114 located downstream of the seat diameter ds are increased the differential pressure between the upstream side of the valve body 114 and the tip thereof is reduced and the hydrodynamic force exerted on the valve body 114 is decreased.A difference between the fuel injection device exerted on the valve body 114 when the valve is closed and the maximum value of the hydrodynamic force exerted on the valve body 114 after the valve has been openedis increased.As a result a range in which a relationship between the hydrodynamic force exerted on the valve body 114 and the amount of lift of the valve body 114 becomes a positive correlation can be increased.The range of the amount of lift in which the valve body 114 is stabilized in the state of the intermediate lift between the valve closing position and the target lift position can be enlarged.Also the shape of the tip of the valve body 114 may be configured so that the area of a tip outer diameter dp of the valve body 114 where the pressure is reduced when the valve body 114 is opened is larger than the area of the seat diameter ds in the valve closed state where the valve body 114 contacts with the valve seat 118.With this effect the range where the static pressure is decreased due to the Bernoullis theorem can be increased when the valve body 114 is opened.Therefore the hydrodynamic force exerted on the valve body 114 when the valve is opened can be increased as compared with the hydrodynamic force exerted on the valve body 114 when the valve is closed.Also the shape of the top of the valve body 114 may be configured by a spherical surface R.With this configuration the range where the fuel passage between the valve body 114 and the valve seat 118 becomes a slight gap in the valve open state can be increased.Therefore the area of the valve body 114 that receives the reduction in the pressure can be enlarged and the hydrodynamic force exerted on the valve body 114 can be increased.With thisadvantage the range of the amount of lift where the valve body 114 is stabilized in the state of the intermediate lift can be increased.When the spring constant of the spring 110 is set to be larger the amount of compression of the spring 110 in the valve opening state where the needle 102 contacts with the magnetic core 107 is larger than that in the valve closing state where the valve body 114 contacts with the valve seat 118.Therefore the load of the spring 110 becomes larger.This effect makes it possible to increase the range of the amount of lift in which the force exerted on the valve body 114 in the valve closing direction has a positive correlation with the amount of lift.