[0001] The present invention relates to apparatus for controlling the fuel flow in a carburettor
engine for motor vehicles.
[0002] More particularly, the invention provides electro-pneumatic apparatus of the type
comprising
- an electrically-controlled cut-off device for cutting off the fuel supply to the
engine,
- a control device comprising a rigid casing communicating with the induction manifold
of the engine, an electrical switch having a movable control member mounted in the
casing, and a barometric absolute pressure sensor disposed in the casing and cooperating
with the movable member to energise and de-energise the cut-off device through the
switch when predetermined pressure conditions exist in the induction manifold and
hence in the casing.
[0003] In known apparatus of the above-defined kind, the microswitch of the control device
must necessarily have substantial hysteresis in order to avoid spurious switchings
at pressures around the operating threshold of the switch. In fact, the detected pressure
has a considerable undulation ("ripple") it is also found that every time the fuel
supply is restored after a cut-off period, the pressure has a characteristic damped
oscillation of low frequency (about 1Hz) with an amplitude of about 15-30 mm Hg. In
order to avoid repeated switchings of the microswitch (which would reduce its life)
and the annoyance caused to the driver as a result of the repeated restoration and
cut-off of the supply, it is necessary for the mechanical hysteresis of the switch
between its switchings for cutting off and restoring the supply to be more than 30
mm Hg. If one considers that present good-quality microswitches of a particular type
have hysteresis values which can vary from one to another by one to three times, it
follows that the hysteresis in the example given above could vary from 30 to 90 mm
Hg. Hence, with some microswitches the electro-pneumatic apparatus for controlling
the fuel flow could be sensitive enough but in some cases might not cut off the flow
even under conditions of extremely strong deceleration.
[0004] In order to avoid these disadvantages at least partly, in some known apparatus provision
is made of a sleeve acting as a calibrated passage between the induction manifold
and the casing of the pressure sensor device: this sleeve acts to damp the oscillations
in the pressure detected and, by reducing the ripples, allows the use of microswitches
with smaller hysteresis values. The filtering action of this calibrated passage, which
is provided principally to reduce the amplitude of the low-frequency oscillations
in the pressure during restoration of the fuel supply after a cut off, ends up by
introducing an undesirable delay (about 1 second) in the restoration of the supply.
In order to avoid this delay, it is necessary to raise the operating threshold of
the device, the cut off being effected at higher rotational speeds of the engine.
Thus, the benefits (reduction in fuel consumption) which it is expected to achieve
by virtue of the fuel flow cut-off device are reduced.
[0005] The object of the present invention is to provide electro-pneumatic apparatus for
controlling the fuel supply which is free from the aforesaid disadvantages and thus
has a faster operating speed both during cut off and restoration of the fuel supply
and which is also very sensitive, allowing the use of cheaper microswitches with substantial
hysteresis values and rendering superfluous the need to filter the oscillations in
the pressure detected.
[0006] The object is achieved according to the invention by means of apparatus of the type
specified above, characterised in that a differential pressure sensor sensitive to
variations in the pressure in the casing is also mounted in the casing and cooperates
with the absolute pressure sensor so that the movement of the control member of the
switch is substantially equal to the sum of a first movement proportional to the absolute
pressure in the casing and a second movement substantially proportional to the derivative
of the pressure in the casing.
[0007] Further characteristics and advantages of the apparatus according to the invention
will become apparent from the detailled description which follows with reference to
the appended drawings provided purely by way of non-limiting example, in which:
Figure 1 is a partially sectioned view of a carburettor for the engine of a motor
vehicle, provided with electro-pneumatic control apparatus according to the invention,
also shown in section, and
Figure 2 is a series of graphs showing examples of the pressure changes in the control
device and the movements of the control member of the switch in the apparatus of the
invention.
[0008] In Figure 1 a carburettor of known type is indicated 1, includes a throttle valve
2 and is provided with a cut-off device 3 in the idling duct 4. The cut-off device
3 is, for example, an electromagnet including a movable member 3a acting as an obturator,
and an excitation winding 3b.
[0009] In the induction manifold 5 of the engine there is formed a pressure outlet 6 connected
to a control device 7 through a tube 8.
[0010] The control device 7 comprises a rigid casing 10, for example of plastics material,
having an aperture 11 connected to the tube 8. The casing 10 defines a chamber 12
which communicates with the induction manifold 5 through the aperture 11, the tube
8, and the pressure outlet 6 formed in the manifold. An aneroid-type barometric capsule
13 and a capsule 23 having a calibrated hole 24 are located within the chamber 12.
The capsules 13 and 23 are disposed mechanically in series.
[0011] In one wall of the casing 10 there is a further aperture 15 which is threaded and
in which an adjusting screw 16 is engaged with its end within the casing 10 fixed
to a wall of the aneroid capsule 13.
[0012] A control rod 14 is applied to the centre of the outer face of the capsule 23 which
does not face the aneroid capsule 13.
[0013] Two electrically-conductive blades 17 and 18 extend into the chamber 12 through the
wall of the casing 10.
[0014] The blade 18 is connected to one end of the excitation winding 3b of the cut-off
device 3, the other end of this winding being connected to earth.
[0015] The blade 17 is intended for connection to a direct current voltage supply V.
[0016] The free ends of the blade 17 and 18 face each other in the chamber 12 and carry
respective contact members 17a and 18a, as shown in Figure 1.
[0017] The rod 14 carried by the capsule 23 is in contact with the free end of the blade
18.
[0018] As stated above, the capsule 13 is of aneroid type, that is, it is sealed with a
residual internal pressure (theoretically) of zero, while the capsule 23, however,
has a calibrated hole.
[0019] Each time there is a variation in the pressure in the induction manifold 5, the pressure
in the casing 10 varies correspondingly and, due to the deformation of the capsules
12 and 23, the rod 14 which controls the microswitch formed by the blades 17 and 18
moves. The movement of the rod 14 is substantially equal to the sum of a movement
due to the capsule 13 and a movement due to the capsule 23. The first movement is
proportional to the absolute pressure in the induction manifold 5 at any instant,
while the second is a function of the variation in the pressure and, in particular,
is nil when the pressure is constant. As will become more apparent from the following,
the action of the apertured capsule 23 is substantially of the derivative or differential
type. Indeed, imagine, for example that the pressure in the manifold 5 and hence in
the chamber 12 undergoes a practically instantaneous and considerable increase: while
the aneroid capsule 13 changes from a condition characterised by a certain state of
compression to another condition of greater compression and stays there, the capsule
23 feels the effect of the difference between its internal pressure and the external
pressure for a short period of time and hence undergoes a transitory compressive action.
This compression is exhausted at the moment when the pressure within the capsule reaches
equilibrium with the external pressure by means of the calibrated hole 24. The capsule
23 is thus subject to a compressive action for a period of time which depends on the
diameter of the hole 24 and the volume of the capsule. In the case given by way of
example above, the rod 14 thus feels the permanent action of the aneroid capsule 13
and the transitory action, in the same direction and sense, of the differential capsule
23. To a certain extent, the latter temporarily reinforces the action exerted on the
rod by the aneroid capsule.
[0020] A similar situation, though in the opposite sense, occurs when there is a reduction
in the absolute pressure in the induction manifold.
[0021] Figure 2 shows graphs which enable the behaviour of the apparatus according to the
invention to be explained more clearly. In this Figure, the curve p represents the
absolute pressure in the induction manifold of the engine and hence in the chamber
12, and the broken-line curve p
i shows the corresponding changes in the pressure within the diaphragm 23. Furthermore,
in the lower graph of Figure 2, the curves x
1 and x
2 represent the movements of the rod 14 and the blade 18 due to the aneroid capsule
13 and the differential capsule 23, respectively. The curve x represents the resulting
movement given by the sum of x
1 and x
2.
[0022] The respective threshold levels at which the fuel flow to the engine is restored
and cut off are indicated p
M(x
M) and p
m(x
m).
[0023] The instant to represents the moment at which the accelerator pedal is released with
the engine running. The instant t
1 is the moment at which the cut-off device 3 is de-energised and hence the fuel flow
is interrupted to achieve fuel cut-off. The instant t
2 represents the moment at which the fuel supply is restored.
[0024] In the absence of the differential capsule 23, and hence when the movement of the
blade 18 is controlled solely by the aneroid capsule, the fuel flow would be cut off
at the instant t
l, and the fuel flow would be restored at the instant t'
2. From the lower graph of Figure 2 it can thus be deduced that, by virtue of the action
of the differential capsule 23, the cut off occurs with an advance δ
1 and the restoration occurs in its turn with an advance δ
2.
[0025] If, in operation, the absolute pressure P
c in the induction manifold does not fall below the threshold P
m, the fuel flow could still be cut off provided that the reduction in the absolute
pressure occurs very rapidly, as normally happens when the accelerator pedal is released.
In this case in fact, by virtue of the derivative action of the differential capsule
23, the control rod 14 could move temporarily below beneath the position x , causing
the fuel flow to be cut off.
[0026] The restoration would then occur only when, by virtue of the action of the aneroid
capsule 13, the rod 14 rises above the magnitude x again, this naturally occuring
in the restoring the supply without reopening of the throttle valve since it would
otherwise be this reopening which would immediately cause the restoration of the fuel
supply since the control rod would rise immediately as a result of the derivative
action of the differential capsule 23.
[0027] The apparatus of the invention has numerous advantages. In the first place, spurious
switchings of the microswitch 17,18 are eliminated, particularly in the release phases,
even without the attenuation of the low-frequency oscillations in the pressure in
the induction manifold. Moreover, the cut off and restoration of the fuel supply occur
more rapidly. Finally, the apparatus has a very high sensitivity.