EP0209497B1

EP0209497B1 - Electro-pneumatic apparatus with a combined derivative-proportional action for controlling the fuel flow in a carburettor engine for motor vehicles

Info

Publication number: EP0209497B1
Application number: EP86830187A
Authority: EP
Inventors: Vittorio Di Nunzio
Original assignee: Fiat Auto SpA
Current assignee: Fiat Auto SpA
Priority date: 1985-07-12
Filing date: 1986-07-02
Publication date: 1988-11-09
Also published as: EP0209497A2; ES8705571A1; IT8567645A0; DE3661149D1; EP0209497A3; ES556413A0; IT1182512B; BR8603340A

Description

The present invention relates to an electro-pneumatic apparatus of the type defined in the introduction of appended Claim 1 (see EP-A-207010).
An electrically actuated fuel shut-off apparatus employing a fuel cut-off member operated by a solenoid as a function of the pressure in the induction manifold of an engine is disclosed in US-A-2 856 167.
That known apparatus comprises a switching member including a case divided by a diaphragm into a first chamber, which is sealed and substantially evacuated, and a second chamber communicating with the induction manifold. The said diaphragm controls an electric switch which in turn controls the energisation of the cut-off member.
In order to render the operation of the shut-off apparatus unaffected by a change in elevation of the vehicle whereon it is mounted, means are provided to make the vacuum in said first chamber vary as a function of the elevation.
Another electrically activated fuel shut-off apparatus is disclosed in US-A-2 933 168.
In known apparatus of the above-defined kind, the switch 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 20-40 mbar (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 40 mbar (30 mm Hg). If one considers that present good-quality microswitches of a partiuc- lar type have hysteresis values which can vary from one to another by one of three times, it follows that the hysteresis in the example given above could vary from 40 to 120 mbar (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.
In order to avoid these disadvantages at least partly, in some known apparutus 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 hystereiss 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.
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.
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.

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.

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.
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.
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.
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.
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.
Two electrically- conductive blades 17 and 18 extend into the chamber 12 through the wall of the casing 10.
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.
The blade 17 is intended for connection to a direct current voltage supply V.
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.
The rod 14 carried by the capsule 23 is in contact with the free end of the blade 18. 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.
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.
A similar situation, though in the opposite sense, occurs when there is a reduction in the absolute pressure in the induction manifold.
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 _Pc represents the absolute pressure in the induction manifold of the engine and hence in the chamber 12, and the broken-line curve p, shows the corresponding changes in the pressure within the diaphragm 23. Furthermore, in the lower graph of Figure 2, the curves _X1 and _X2 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 _X1 and x₂.
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).
The instant to represents the moment at which the accelerator pedal is released with the engine running. The instant t, 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 t₂ represents the moment at which the fuel supply is restored.
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'_i, 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 6₁ and the restoration occurs in its turn with an advance δ₂.
If, in operation, the aboslute 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_m, causing the fuel flow to be cut off.
The restoration would then occur only when, by virtue of the action of the aneroid capsule 13, the rod 14 rises above the manitude x_m 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.
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.

Claims

1. Electro-pneumatic apparatus for controlling the fuel flow in a carburettor engine for motor vehicles, comprising:

an electrically-controlled cut-off device (3) for cutting off the fuel supply to the engine,

a control device (7) comprising a rigid casing (10) communicating with the inducation manifold (5) of the engine, an electrical switch (17, 18) having a movable control member (18) mounted in the casing (10), and a barometric absolute pressure sensor (13) disposed in the casing (10) and cooperating with the movable member (18) to energise and de-energise the cut-off device (3) through the switch (17, 18) when predetermined pressure conditions exist in the induction manifold (5) and hence in the casing (10),
characterised in that a differential pressure sensor (23) sensitive to variations in the pressure in the casing (10) is also mounted in the casing (10) and cooperates with the absolute pressure sensor (13) so that, upon variation of the pressure in the casing (10), the movement of the control member (18) of the switch (17, 18) is substantially equal to the sum of a first movement proportional to the pressure in the casing (10) and a second movement substantially proportional to the variation in the pressure in the casing (10).

2. Electro-pneumatic device according to Claim 1, in which the absolute pressure sensor comprises a barometric capsule (13) of aneroid type, characterised in that the differential pressure sensor comprises a capsule (23) which has a calibrated hole (24) and is located in the casing (10) mechanically in series with the aneroid capsule (13).