GB2171768A - Vacuum intensifier in vehicle braking system - Google Patents

Abstract

A vehicle braking system in which a vacuum operated brake booster used to reduce the brake pedal effort is connected to a source of low vacuum, usually the engine inlet manifold (22), via a vacuum intensifier (24) arranged to raise the level of vacuum relative to the source such that up to a maximum attainable value, the level is raised by a predetermined ratio that is independent of any variations in the level of vacuum available at the source. The intensifier (24) disclosed is a reciprocating pump preferably a double acting pump having two actuating chambers (8 and 9), of which one is connected to vacuum source (22) and the other to atmospheric pressure. The connections are reversed at the end of each stroke by a mechanically actuated control valve. <IMAGE>

Description

SPECIFICATION Vacuum intensifier This invention relates to a vehicle braking system, and to a vacuum intensifier particularly, but not exclusively for use therein.

A large proportion of petrol-engined passenger cars are conventionally fitted with vacuum operated brake boosters to reduce the brake pedal effort, the source of vacuum being the induction tract of the petrol engine.

However, with increasingly stringent exhaust gas emission regulations and fuel economy requirements, the methods used to meet these conditions are reducing the available levels of differential pressure (vacuum).

Conventional braking system boosters are designed for differential levels of 0.7 to 0.8 bar (i.e. approx. 0.2 to 0.3 bar absolute pressure). To enable satisfactory brake performance at lower differential levels would entail undesirable increase in booster dimensions.

Previous attempts to alleviate this problem have been described in Patent specifications Nos. U.K. 1,533,918 and EP 0107342.

The former specification relates to the insertion of an electrically driven pump between the inlet manifold and the brake booster to increase the available differential pressure, whilst the latter describes a reciprocating pump actuated by pulsations in the exhaust manifold of the engine. Both of these units have the advantage of producing a usable vacuum level when none exists in the induction manifold of the engine. This potentially unnecessary advantage is however, achieved at the expense of greater complications.

In the case of U.K. 1,533,918, the cost of manufacturing the pump will be only slightly less expensive than that required to produce a conventional vacuum exhauster such as used with diesel engines. In addition, a one-way valve and vacuum operated switch are required.

The pump as described in EP O 107342 is complex and very dependent on the frequency and level of pulsations in the exhaust manifold. Also, because hot exhaust gases are used, components will need to be of expensive materials able to withstand high temperature.

With the object of avoiding the disadvantages referred to above, we propose in accordance with this invention, a vehicle braking system of the kind having a vacuum operated brake booster to reduce the brake pedal effort and connected to a source of low vacuum, usually the induction tract of the engine, via a vacuum intensifier operable to raise the level of vacuum relative to the said source such that up to a maximum attainable value the level is raised by a predetermined ratio independent of any variation of the low level vacuum available at the source. Intensification of the available low vacuum enables the use in vehicle braking systems of conventional vacuum operated boosters.

Also, in accordance with the invention and particularly though not exclusively, for use in the vehicle braking system we propose a vacuum intensifier for raising the level of vacuum available relative to an existing low vacuum source, comprising a reciprocating vacuum pump having a reciprocable member driven by alternately applying said low level vacuum and a higher pressure which may be atmospheric pressure, to an actuating chamber acting on the reciprocable member.

In a preferred embodiment capable of raising the level of vacuum by a ratio of 2:1, the vacuum pump is double acting, comprising two reciprocable members working in separate cylinders and connected or linked for movement together. Each member whether a diaphragm or a piston divides the cylinder into an actuating chamber and a pumping chamber fitted with inlet and outlet valves connected respectively to the vacuum load, for example, a vacuum reservoir or conventional vacuum operated booster and the existing source of low vacuum. The application of the two pressures such that the low vacuum is applied to one actuating chamber and the said higher pressure to the other, is reversible at the end of each stroke by control means responsive to movement of the reciprocable members.The application of the two pressures may be controlled by a spool valve operable by the output pressures of two mechanically actuated pilot valves but is preferably controlled by a control valve operable directly by movement of the reciprocating member at the end of each stroke.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagram of a vehicle braking system having a vacuum operated booster and vacuum intensifier; Figure 2 is a cross-section through one embodiment of vacuum intensifier according to the invention; Figure 3 is a cross-section taken on on line 3-3 in Fig. 2; Figure 4 is a cross-section taken on line 4-4 in Fig. 3 and showing a control valve arrangement; Figure 5 is a circuit diagram of the intensifier shown in Figs. 2 to 4; and Figure 6 is a part cross-sectional view of direct acting control valve arrangement.

In Fig. 1, 20 represents a conventional petrol engined vehicle braking system of the kind fitted with a vacuum operated brake booster to reduce the pedal effort. As in conventional systems, the source of vacuum is the engine induction manifold 22, but in accordance with the present invention, a vacuum intensifier 24 for raising the level of vacuum is provided between the induction manifold 22 and the brake booster 25. Intensification of the vacuum is achieved by alternately applying the low level vacuum and atmospheric air, which is available via a connection C1 with the air filter 26 of the engine, to actuating chambers acting on the reciprocating member of a vacuum pump having an air inlet connection C3 with the brake booster 25 and an outlet connection C2 with the engine manifold 22.The intensifier to be described in more detail with reference to Figs. 2 to 5 yields a nominal 2:1 ratio of input to output differential pressure (relative to atmospheric) up to a maximum of 1.0 bar.

As will be seen from the Figures, the basis of the vacuum intensifier is the reciprocation of two pumping members which may be pistons or, as illustrated, diaphragms 5 and 6, which are connected by a push rod 7 and work in opposed cylinders. Each cylinder is divided by the diaphragm into an actuating chamber 8, 9 and a pumping chamber 1, 2 which is fitted with an outlet valve 4 connected to the low level vacuum in the engine inlet manifold (22 Fig. 1) and an inlet valve 3 connected to the brake booster (25 Fig. 1).

In order to produce motion of the push rod 7, atmospheric and available low level vacuum pressures are alternately applied to the two actuating chambers 8 and 9 acting on the diaphragm 5 and 6.

The application of these pressures is controlled by two pilot valves 10 and 11 (which are actuated mechanically by the movement of the diaphragms), and a spool valve unit 12 (see Fig. 3) which in turn, is actuated by the output pressure from the valves 10 and 11.

In operation, the spool is biased into position (ii) by the spring 12B when the pressures in chambers 12K and 12G are both equal to the low level of vacuum. With the spool in position (ii) spool 1 2C directs atmospheric pressure into chamber 9 (via port 12F) and spool 12D directs the low level vacuum to chamber 8 via port 12E.

These differential pressures will result in the diaphragm assembly 5, 6 and 7 moving to the right in Fig. 4, and at full-stroke, operating pilot valve 10.

Operation of valve 10 applies atmospheric pressure to chamber 12K resulting in pistion 12A moving to position (i). This directs low level vacuum into chamber 9 (via port 12F) and atmospheric pressure to chamber 8 (via port 12E) resulting in the main reciprocating member moving back to the left.

Thus the cycle is completed with chambers 1 and 2 alternately charging and discharging to perform the pumping function.

In place of the pilot valves 10 and 11 shown in Fig. 4, a direct acting control valve arrangement such as shown in Fig. 6 may be used. This avoids the need for a spool valve assembly (cf. 12 in Fig. 3) to control the alternate application of the low level vacuum and atmospheric pressure to the acting chambers 8 and 9 of the vacuum pump.

The arrangement of Fig. 6 includes a plunger 20 mounted for sealed sliding movement in end supporting walls 22, 24. Each wall 22, 24 has formed therein a chamber 26, 28 in communication via drillings 30 with an inlet port 32 connected to the low level vacuum source. Between the two end walls is a partition wall 34 separating outlet ports 24 and 36 respectively to the actuating chambers 9 and 8 and defining chambers 38 and 40 in communication via a passage 42 and port 44 with the atmospheric supply 46.

The plunger is movable between two operating positions defined by engagement of a spring-loaded detent or ball 48 in one ar other of two V-shaped grooves 50, 52 in a control boss 54 on the plunger 20, by direct contact with opposite ends of the plunger of the reciprocating pump member (5, 6, 7) at the ends of its stroke.

Mounted on the plunger 20 respectively in the chambers 38 and 40 are face sealing valve members 56 and 58 arranged such that when one member 56 seats against the end wall 22 the other seats against the partition wall 34 and when the other member 58 seats against opposite end wall 24, the member 56 seats against the opposite side of the partition wall 34. Hence in the position shown in Fig. 6 chamber 9 is connected to atmosphere and chamber 8 to the low level vacuum and in the other plunger position the connections are reversed.

Although the intensifier described above is limited to a ratio of 2:1, it will be understood that, where other ratios are appropriate a device similar in principle, but with diaphragms of different areas, may be used.

Claims (7)

1. A vehicle braking system of the kind having a vacuum operated brake booster to reduce the brake pedal effort and connected to a source of low vacuum via a vacuum intensifier wherein the vacuum intensifier comprises a pump operable to raise the level of vacuum relative to the source such that up to a maximum attainable value, the level is raised by a predetermined ratio independent of any variation in the low vacuum available at the source.

2. A vehicle braking system of the kind having a vacuum operated brake booster to reduce the brake pedal effort and connected to a source of low vacuum via a vacuum intensifier operable to raise the level of vacuum relative to the source, wherein the vacuum intensifier comprises a reciprocating vacuum pump driven by alternately applying the low level vacuum and a higher pressure to an actuating chamber on one side of the reciprocat ing member of the pump.

3. A system according to claim 2 and comprising control means operable directly or indirectly by the reciprocating member at the end of each stroke, to connect the actuating chamber alternately with the low level vacuum and a higher pressure.

4. A system according to claim 2 wherein the vacuum pump is double acting, and comprises two reciprocable members connected for movement in unison and each defining on opposite sides thereof a pumping chamber having an inlet valve and an outlet valve connected respectively to the vacuum load and the low level vacuum source, and an actuating chamber.

5. A system according to claim 4 wherein application of the two pressures such that the low vacuum is applied to one actuating chamber and the higher pressure to the other is reversible at the end of each stroke by control means responsive to movement of the reciprocable members.

6. A system according to claim 3 or claim 5 wherein the control means comprises a spool valve operable by the output pressures of two mechanically actuated pilot valves.

7. A system according to claim 3 or claim 5 wherein the control means comprises a control valve operable directly by movement of the reciprocating member at the end of each stroke.