FIELD OF THE INVENTION

This invention relates to hydraulic braking systems, and in particular to hydraulic brake boosters for such systems. BACKGROUND OF THE INVENTION

There is a growing demand for hybrid trucks to reduce energy consumption and related operating costs. Some hybrid trucks utilize electric batteries as their main power source. But batteries are too heavy and too inefficient for many truck applications .

An alternative hydraulic hybrid truck technology utilizes compressed gas as a power source. A large amount of energy can be stored in a fluid accumulator, and the accumulator can be cycled may times without loss of performance or efficiency. Truck motion is maintained by driving the truck wheels by extracting energy from the accumulator. The energy can be returned to the accumulator during braking by a recuperative braking system. In the recuperative braking system, pumps are connected to the truck wheels. The pumps apply braking force by re-pressurizing the compressed gas in the accumulator. The EPA has shown that hydraulic hybrids are capable of reducing energy consumption by 50% over conventional diesel -powered trucks in stop and go driving conditions.

Hydraulic hybrid trucks still require conventional friction braking systems in addition to recuperative braking systems. The friction braking system is used when the truck is moving at slow speeds, or to apply braking when the truck is stopped. The friction braking system also operates as a backup to the recuperative braking system.

Conventional friction braking systems utilize a hydraulic brake booster that directs high-pressure brake fluid stored in a hydraulic accumulator against a piston in response to a driver pressing on the brake pedal. The piston presses against a piston in a master brake cylinder to apply the brakes .

Gray, Jr. et al . Patent Application Publication US2009/0320464 discloses a hydraulic brake booster suitable for use with friction braking systems used on hydraulic hybrid trucks or other hybrid vehicles. The brake booster draws brake fluid from the brake accumulator only when the friction brake is being applied. By closing the brake accumulator when the friction braking system is not being used, energy efficiency increases and operating costs are reduced.

Although the brake booster disclosed in the Gray, Jr. et al . application has advantages over prior hydraulic brake boosters, it also has disadvantages. One embodiment utilizes a long, movable spool valve having multiple lands that open and close fluid passages to operate the booster. The spool valve is difficult to seal, and fluid leakage reduces energy efficiency. A second embodiment utilizes a movable piston between a spool valve and a check valve. This embodiment is difficult to manufacture and repair.

Thus there is a need for an improved hydraulic brake booster for use with friction braking systems of hybrid vehicles. The improved brake booster should be easier to seal, easier to manufacture, and easier to repair.

BRIEF SUMMARY OF THE INVENTION

The invention is an improved hydraulic brake booster for use with friction braking systems of hybrid vehicles. The improved hydraulic brake booster of the present invention is easier to seal and easier to manufacture.

A hydraulic brake booster in accordance with the present invention includes a housing defining a piston chamber, and a piston movable in the piston chamber. The piston divides the piston chamber into a low-pressure chamber and a high-pressure chamber on opposite sides of the piston. A piston bore extends along an axis within the piston, the piston bore having a bore wall and an inlet configured to receive flow into the piston bore from a source of high-pressure fluid such as the brake accumulator. A check valve is in the inlet, the check valve configured to check flow from the high-pressure fluid source through the inlet, and a spool valve is in the piston bore. The spool valve has an outer periphery with a sealing surface.

The spool valve is axially movable in the piston bore between a first position away from the check valve to a second position pushing against the check valve and thereby opening the check valve, the spool valve and the check valve defining a fluid chamber between them in the piston bore. A first fluid line extends from the piston bore to the low-pressure chamber, the first fluid line having an opening in the wall spaced from the check valve. A second fluid line extends from the fluid chamber to the high-pressure chamber. The opening in the first fluid line is in fluid communication with the fluid chamber when the spool valve is in the first position, and the sealing surface in a sealing relationship with the wall and sealingly separating the opening in the first fluid line from the fluid chamber when the spool valve is in the second position.

By sealing the piston bore directly with the spool valve, the construction of the spool valve is simplified. Since the spool valve has only a single valve land, leakage is reduced.

In a preferred embodiment of the invention the piston includes a brake shaft. The brake shaft includes a brake sleeve having an open end, and a tubular valve retainer inserted into the open end of the brake sleeve. The valve retainer is a tubular member, the interior bore of the valve retainer forming the piston bore. This construction further simplifies manufacture, assembly, and repair of the hydraulic brake booster.

The invention is described in the attached five sheets of drawings that illustrate an embodiment of the hydraulic brake booster in accordance with the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a vertical sectional view of a hydraulic brake booster in accordance with the present invention, the brake booster in a retracted condition;

Figure 2 is similar to Figure 1 but illustrates the brake booster in a ready condition;

Figure 3 is similar to Figure 1 but illustrates the brake booster in a braking condition;

Figure 4 is similar to Figure 1 but illustrates additional details of the brake booster;

Figure 5 is a sectional view taken along line 5-5 of Figure 4 ; Figure 6 is an enlarged portion of Figure 4 illustrating the fluid chamber between the check valve and spool valve of the brake booster;

Figure 7 is an enlarged portion of Figure 5 illustrating the fluid chamber between the check valve and the spool valve of the brake booster;

Figure 8 is a partial sectional view taken along lines 8- 8 of Figure 5 ; and

Figure 9 is similar to Figure 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Figure 1 illustrates a hydraulic brake booster 10 for a motor vehicle in accordance with the present invention. The brake booster 10 includes a housing 12 that defines a piston chamber 14 and an axial bore 16 extending from the piston chamber 14. A movable piston 18 sealingly divides the piston chamber 14 into a low-pressure chamber 20 and a high-pressure chamber 22. A piston spring 24 in the piston chamber 14 biases the piston 18 to its retracted position shown in the figure. A vent 26 in the housing wall permits flow of low pressure fluid out of the low-pressure chamber 20.

The piston 18 includes a piston shaft 28 that extends out of the housing 12 and actuates a piston in a brake master cylinder (not shown) and a brake shaft 30 journaled in the housing bore 16. A piston bore 32 defined by an annular bore wall 34 extends axially into the brake shaft 30 but does not open into the piston chamber 14. A first fluid line 36 extends from a first opening 38 in the bore wall 30 and opens into the low-pressure chamber 20. A second fluid line 40 extends from a second opening 42 in the bore wall 34 and opens into the high-pressure chamber 22. The fluid lines 36, 40 and the piston bore 32 define a pressure equalization line that, when open, fluidly communicates the low-pressure and high- pressure chambers 20, 22. As shown in the figure, the wall openings 38, 42 are axially spaced from one another, with the wall opening 38 closer to the open end of the piston bore 32.

An inlet line 44 in the brake shaft 30 flows into the piston bore 32 near the inner end of the piston bore 32. The inlet line 44 is fluidly connected to a source of high- pressure working fluid in an accumulator 46. The accumulator 46 is supplied as needed by a pump 48 driven by the motor 50 of the motor vehicle. A normally-closed check valve 52 in the inlet line 44 blocks the flow of high-pressure fluid from the accumulator 46 into the piston bore 32.

A spool valve 54 is movable within the piston bore 32. The spool valve 54 is mechanically connected to the brake pedal 56 of the motor vehicle. The spool valve 54 is movable towards the check valve 52 when braking force is applied to the brake pedal 56 and moves away from the check valve 52 when the braking force is relieved. The spool valve 54 and the check valve 52 define between them a fluid chamber 58 in the piston bore 32. The axial length of the fluid chamber 58 varies with movement of the spool valve 54 towards and away the check valve 52.

The spool valve 54 includes a cylindrical valve body 60 and an axial push rod 62 extending into the fluid chamber 58. The valve body 60 includes a number of circumferentially- spaced flow channels 64 that fluidly communicate the fluid chamber 58 with the outer periphery of the valve body 60. The illustrated flow channels 64 are formed as axial grooves or flutes on the outside of the valve body 60 that face the fluid chamber 58 and extend partially the length of the body 60. Immediately adjacent the flow channels 64 is a sealing surface or valve land 66 on the outer periphery of the valve body 60.

Figure 1 illustrates the hydraulic brake booster 10 in its inactive position, with no brake force being applied to the brake pedal 56 and the piston 18 in its retracted position. The spool valve 54 is located in a first, or inactive, operating position spaced away from the closed check valve 52 so that the fluid chamber 58 is at its maximum volume. The spool valve 54 is aligned with the first channel wall opening 38 such that the spool valve flow channels 64 are directly opposite the opening 38 and fluidly communicate the fluid line 36 with the fluid chamber 58. The valve land 66 is on a first side of the wall opening 38 adjacent the open end of the piston bore 32.

In this operating condition the equalization line defined by the fluid lines 36, 40 and the fluid chamber 58. The equalization line is open in Figure 1 and the piston chambers 20, 22 are in fluid communication with one another. Any leakage of high-pressure fluid from the accumulator 46 past the check valve 52 and into the fluid chamber 58 will thereby be in fluid communication with both piston chambers 20, 22 and will not cause piston movement.

Figure 2 illustrates the hydraulic brake booster 10 in its ready position, with sufficient brake force being applied to move the spool valve 54 a first distance towards the check valve 52. The spool valve 54 is still spaced away from the check valve 52, and the check valve 52 remains closed. The valve land 66 has moved from the first side of the wall opening 38, across the wall opening 38, and to the opposite, second side of the wall opening 38. The bore wall 34 on the second side of the wall opening 38 and the valve land 66 are cooperatively sized to form a sealing relationship that seals or plugs the fluid chamber 58 from the wall opening 38. This effectively closes the equalization line and closes the fluid communication between the high-pressure and low-pressure piston chambers 22, 20 through the equalization line. The flow chamber 58 remains in fluid communication with the high- pressure piston chamber 20 through the second fluid line 40.

Figure 3 illustrates the hydraulic brake booster 10 in a braking position. Increased braking force has been applied to the brake pedal 56 to move the spool valve 54 into pressing engagement against the check valve 52. The illustrated spool valve 54 moves about 50 -thousandths of an inch (0.050 inches) from its ready position shown in Figure 2 to its braking position shown in Figure 3. During this movement of the spool valve 54, the spool valve 54 forces fluid from the fluid chamber 58 into the high-pressure chamber 22 through the fluid line 40. This causes the piston 18 to move slightly away from its retracted position, but the movement of the piston 18 is not sufficient to initiate braking.

In its braking position, the spool valve 52 presses against the check valve 52, forcing the check valve 52 open. The illustrated check valve 52 is formed as a poppet-style valve and includes a movable plug or valving member 68 that cooperates with a fixed valve seat 70 to open and close the check valve 520. The valving member 68 is mechanically biased against the valve seat 70 by a valve spring 72 captured in the inlet line 44 between the valve seat 70 and the brake shaft 30. The valving member 68 includes a number of circumferentially- spaced, full-length axial grooves 74 that enable fluid flow past the valving member 68. The valving member 68 further includes a push rod 76 that extends away from the valve seat 70 and into the fluid clamber 58.

The valve spool push rod 62 presses against the end of the valving member push rod 76, pushing the valving member 68 away from the valve seat 70 and thereby opening the check valve 52. High-pressure fluid flows from the accumulator 46 and into the flow chamber 58. The degree of opening of the check valve 52 increases with displacement of the valving member 68 until the valving member is displaced to an operating position representing the fully-opened condition of the check valve 52. The high-pressure fluid in the fluid chamber 58 is in fluid communication with the high-pressure chamber 22 through the second fluid line 40, urging the piston 18 to the left as shown in Figure 3 and causing the piston shaft 28 to engage and actuate the braking piston of the master brake cylinder.

The high-pressure fluid in the fluid chamber 58 applies hydraulic force against the spool valve 54, urging the spool valve 54 to move away from the check valve 52. This is felt by a driver as increasing resistance at the brake pedal, providing tactile feedback to the driver.

Assuming that the driver now maintains the position of the brake pedal 56, the piston 18 moves to the left as shown in Figure 3 while the spool valve 54 remains stationary. The valve seat 70 moves left and back into closed engagement with the valving member 68, closing the inlet line 44. The piston 18 is held in an equilibrium position away from its retracted position, and high-pressure fluid is captured in the fluid chamber 586 that continues to urge the spool valve 54 away from the check valve 52.

Releasing the brake pedal 56 enables the fluid pressure to move the spool valve 54 away from the check valve 52 and back to its first position shown in Figure 1. The flow opening 38 comes back into fluid communication with the fluid chamber 56, opening the equalization line between the low- pressure and high-pressure chambers 20, 22. The piston 18 returns to its retracted position, with fluid flowing from the high-pressure chamber 22 through the spool valve flow channels 64, through the wall opening 38, and to the low-pressure chamber 20. The flow area defined by the flow channels 64 does not substantially restrict flow from the flow chamber 58 to the wall opening 38, and so the piston 18 can return to its retracted position with essentially no resistance caused by flow restrictions in the equalization line.

Figures 4-9 illustrate construction of the hydraulic brake booster 10 in greater detail. The brake shaft 30 is formed from multiple components, including a brake sleeve 78, a valve retainer 80 in the brake sleeve 78, and a tubular shaft retainer 82. A pin 93 nonrotatably connects the brake sleeve 78 and the valve retainer 80 when the valve retainer 80 is in the sleeve 78.

The brake sleeve 78 includes an opening or tubular extension 84 that extends from a wall 86, and an axial bore 88 extending from the wall 86 into the brake sleeve closed end of the opening 84. The bore 88 defines an axial length of the inlet line 44 and contains the check valve 52.

The valve retainer 80 is a tubular member having an internal through-bore 90. The valve retainer 80 is closely received in the sleeve opening 84 and is pressed against the sleeve wall 86 by the shaft retainer 82 removably threaded into the open end of the sleeve opening 84, compressing gasket material between the brake sleeve and valve retainer. The valve retainer bore 90 and the shaft retainer bore 92 are coaxial and form the piston bore 32. The retainer bore 90 has a large-diameter section 94 that holds a spool spring 96 that urges the spool valve 54 towards the brake pedal 56, and a reduced diameter section 98 that has a bore diameter less than the diameter of the sleeve bore 88. The end of the valve retainer 80 against the sleeve wall 88 thereby presents an annular abutment surface facing the bore 88 that prevents movement of the valve seat 62 out of the inlet line 44.

Fluid lines 32, 36 are each formed as multiple circumferentially- spaced fluid lines formed in part by aligned sets of axial holes drilled into the brake sleeve 78 and the valve retainer 80. As shown in Figure 8 fluid line 32 consists of three sets of lines 32a, 32b, and 32c and fluid line 36 consists of a pair of lines 36a, 36b. The portions of the fluid lines 36 in the valve retainer 80 are formed by first drilling axial holes 36' and then drilling radial intersecting lines 36' ' from the retainer bore 98, see Figure 6. The portions of the fluid lines 32 in the valve retainer 80 are formed by drilling straight holes from the end of the valve retainer 80 that are angled to intersect the retainer bore portion 98.

Figure 9 is an enlarged view of a portion of the valve retainer 80 when the brake booster 10 is in its retracted condition. The spool valve land 66 has a smaller outer diameter as compared to the outer diameter of the adjacent portion of the spool valve body 60. The valve bore portion 98 has a corresponding smaller diameter bore portion 100 that cooperates with the valve land 66 to seal the bore 98 when the valve land 66 has moved past the line openings 38. The valve bore portion 98 transitions to a larger diameter bore portion 102 at the fluid openings 38. 0-ring seals 104 carried on the valve body 60 resist fluid leakage past the spool valve body 60.

While I have illustrated and described a preferred embodiment of my invention, I do not wish to be limited to the precise details set forth, but desire to avail myself of such changes and alterations as fall within the purview of the following features and methods.