This invention relates to fuel injectors for internal combustion
engines, and in particular outwardly opening solenoid actuated fuel injectors for
direct injection of gasoline.
One known type of fuel injector is that having an outwardly
opening pintle, biased towards the closed position by a spring and opened by an
actuator such as a solenoid or piezo-stack. The pintle is typically slidable within
one or more guides. Particularly in the case of solenoid actuated injectors, the
pintle opening is limited by an end stop which is typically the top surface of one
of the guides.
The flow rate of fuel through the injector is largely dependent
upon the stroke of the pintle. For a typical fuel injector having a pintle stroke
of between 30µm and 40µm and a fuel supply pressure of 200 bar, a 3% variation
in the flow rate is experienced for each micron variation in the pintle stroke.
Hence there is a high sensitivity to pintle stroke variation, requiring very high
manufacturing tolerances of the end stop, pintle and associated components to achieve
the required flow rate. Furthermore, variation in the pintle stroke over time due
to wear and/or differential thermal expansion can lead to undesirable variation
in the fuel flow rate.
What is required is the ability to fine tune the flow rate
of the injector in use, ideally through a feedback control arrangement enabling
variation of the pintle stroke to achieve the desired flow rate.
Known outwardly opening piezo-electric actuated fuel injectors
generally comprise a valve body having a tip portion defining a spray aperture,
a pintle or valve stem extending within the tip portion for axial movement between
an extended and a retracted position, the pintle having an external head engageable
with the spray aperture to close the spray aperture when the pintle is in its retracted
position, a return spring biasing the pintle towards its retracted position, an
actuating means in the form of a piezo-stack, acting upon the pintle to urge the
pintle to its extended position when the piezo-stack is energised.
The pintle stroke of a piezo-stack actuated fuel injector
can be varied by varying the voltage supplied to the piezo-stack to vary the elongation
of the piezo-stack and thus the movement of the pintle. The piezo-stack driver voltage
can thus be used to correct a lean or rich shift of the flow rate from the fuel
injector to be corrected.
However, piezo-electric fuel injectors are very costly
to produce compared to solenoid actuated injectors and require complex and costly
control systems for operation of the piezo-stack.
By contrast, solenoid actuated fuel injectors are much
cheaper to produce. However, known solenoid actuated fuel injectors having pintle
stroke limited by a hard end stop cannot provide the required pintle stroke variation
to correct such lean or rich shift of flow rate.
According to a first aspect of the present invention there
is provided a method of controlling the pintle stroke of an outwardly opening solenoid
actuated fuel injector, said method comprising controlling the current applied to
the solenoid to provide a predetermined relationship between applied current and
pintle stroke.
In one embodiment the applied current may be varied to
vary the impact and hold force of the pintle against an end stop having a predetermined
stiffness.
In an alternative embodiment the applied current may be
varied to vary the force applied by the pintle against a biasing means acting against
the actuating solenoid.
According to a further aspect, the present invention provides
a fuel injector comprising an injector body having a tip portion defining a spray
aperture; a pintle extending within the tip portion; guide means within the tip
portion guiding the pintle for axial movement between an extended and a retracted
position, the pintle having an external head engageable with the spray aperture
in said retracted position to seal the spray aperture; resilient means biasing the
pintle to said retracted position; solenoid actuating means for selectively moving
the pintle into said extended position;
control means being provided for varying the peak/hold current applied to the solenoid
to vary the pintle stroke as a function of peak/hold current applied to the solenoid.
In one embodiment an end stop is provided within the injector
body means against which a portion of the pintle is abuttable to define the extended
position of the pintle, said end stop having a predetermined stiffness to provide
a predetermined relationship between pintle stroke and impact force applied to said
end stop by said pintle as the pintle moves to its extended position under the action
of the solenoid actuator.
Preferably said end stop is provided on a sleeve provided
within the tip portion of the injector body, said sleeve having a predetermined
stiffness to provide the required deflection of the end stop. Said sleeve may comprise
an apertured tube or may be in the form of a resilient member such as a coil spring
or a multi-wave spring having a constant or progressive spring rate.
In an alternative embodiment biasing means having a predetermined
spring rate is provided within the injector body for biasing the pintle to its retracted
position, the stroke of the pintle being defined by the position at which the force
applied to the pintle by the solenoid actuating means is balanced by the force applied
to the pintle by the biasing means. Said biasing means may comprise said resilient
means biasing the pintle to said retracted position.
Preferred features and advantages of the invention will
be apparent from the claims and from the following description.
An embodiment of the invention will now be described, by
way of example only, with reference to the drawings, in which:
- Figure 1 is a cross section of a fuel injector according to one embodiment of
the present invention.
As shown in the drawing, the fuel injector comprises an
injector body 1 having a tip portion 2 having a spray aperture 3 at a distal end
thereof. A pintle 5 extends within the tip portion 2, the pintle 5 having a head
portion 6 engageable with a valve seat 4 surrounding the spray aperture 3 to close
the spray aperture 3.
The pintle 5 is axially moveable within the injector body
1 between a retracted position wherein the head portion 6 engages the valve seat
4, as shown in Fig 1, and an extended position (not shown) wherein the head portion
6 is spaced from the valve seat 4. A return spring 7 is mounted within the tip portion,
biasing the pintle 5 towards its retracted position.
An end stop 8, defined by an upper end of a tubular sleeve
9 or pintle guide mounted on the injector housing 1 cooperates with a collar 10
on the pintle 5 to limit the extension of the pintle 5 to define the stroke of the
pintle 5.
A solenoid actuator having an electromagnetic coil 11 and
a moveable armature 12 is operable to urge the pintle 5 to its extended position.
The sleeve 9 may be provided with apertures 20 or be otherwise
formed (such as in the form of a coil spring having a constant or a progressive
spring rate) to provide a predetermined degree of elasticity such that the end stop
8 can undergo axial deflection upon impact by the pintle 5.
The stiffness of the sleeve 9 is selected to provide a
desired relationship between deflection of the end stop 8 and the force exerted
on the pintle by the solenoid to provide a stable and predictable relationship between
solenoid actuating maximum or hold current (i.e. the current applied to the solenoid
at full pintle opening) and pintle stroke. For example, the stiffness of the sleeve
may be predetermined to provide an increase of 1µm in pintle stroke for each
additional Newton of solenoid force exerted against the end stop via the pintle.
In order to provide feedback control of the pintle stroke
for each individual injector, the engine speed signal and a lambda sensor signal
can be interpreted by the engine's electronic control unit to determine the actual
solenoid motion characteristics.
In an alternative embodiment, not shown, the elastic stop
may be arranged to be in contact with the pintle in both its retracted and extended
positions, so that the end stop and associated sleeve provide a biasing force against
the pintle in the opposite direction to the opening force provided by the actuating
solenoid (said opening force being a function of applied current).
In such embodiment, the return spring 7 may be combined
with the sleeve. Thus the effect of the elastic end stop and return spring can be
combined into a single closing spring having progressive spring rate.
In such embodiment, the stroke of the pintle is determined
by the position at which the force applied to the pintle by the actuating solenoid
(which is a function of the applied current) is balanced by the combined opposing
force of the end stop/sleeve and the return spring or by the combined closing spring.
Various modifications and variations to the described embodiments
of the inventions will be apparent to those skilled in the art without departing
from the scope of the invention as defined in the appended claims. Although the
invention has been described in connection with specific preferred embodiments,
it should be understood that the invention as claimed should not be unduly limited
to such specific embodiments.
