Abstract:
A system for pressurizing transmission control elements includes an accumulator for containing pressurized fluid, first and second check valves, a booster valve supplied with accumulator pressure, and an actuator that causes the booster valve to open a fluidic connection between the accumulator and the control elements through the check valves in response to an engine restart signal

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a hydraulic system having a hydraulic accumulator for an automatic transmission, and more particularly to an accumulator that maintains hydraulic pressure for immediate engagement of the transmission following an engine restart. 
         [0003]    2. Description of the Prior Art 
         [0004]    In order to improve the fuel economy of vehicles that use an internal combustion engine for propulsion, some vehicles employ a stop-start strategy wherein, when the vehicle is stopped at a traffic light, for example, the engine is automatically turned off. Then when the driver releases the brake pedal to apply pressure to the accelerator pedal, the engine is automatically started to allow the vehicle to accelerate. For such vehicles that also have an automatic transmission, a need arises to maintain hydraulic pressure in the transmission so that the transmission can engage immediately after the engine is automatically restarted, which will allow the vehicle to accelerate without hesitation. 
         [0005]    In order to address this need, some vehicles employ an electric pump for pressurizing the hydraulic fluid rather than a conventional engine-driven pump. While this allows for the maintenance of hydraulic pressure while the engine is shut off, it may be more costly and require more packaging space than is desired. It also may require an additional battery to power the electric motor. A system of this type may also require continuous operation even when a pressure charge is not required due to a delay between a pump-motor startup and hydraulic pump prime time. 
         [0006]    Another solution is to use a spring-loaded piston accumulator with a mechanical latch. This approach calls for control system redesign to package it internally. Such a spring-loaded accumulator may require one hundred percent capacity fluid charge in order to mechanically latch the accumulator&#39;s piston, which may present an issue when frequent engine stop-start events occur. 
       SUMMARY OF THE INVENTION 
       [0007]    A system for pressurizing transmission control elements includes an accumulator for containing pressurized fluid, first and second check valves, a booster valve supplied with accumulator pressure, and an actuator that causes the booster valve to open a fluidic connection between the accumulator and the control elements through the check valves in response to an engine restart signal 
         [0008]    The system maintains a full hydraulic charge pressure while the engine is off (under stop-start operating conditions), thus allowing the transmission to engage immediately when the engine is automatically restarted by electronically commanding the solenoid. The control assembly can operate to minimize oil leakage from the accumulator and eliminate need for frequent recharges due to use of a poppet valve and a sealed pressure balance valve. 
         [0009]    The control system can release stored fluid with a minimum delay after electronically commanded due to use of spring compressed by the pressure balance valve. 
         [0010]    The control system operates over wide range of hydraulic pressures simultaneously using a low force solenoid and a pressure balance booster valve. 
         [0011]    The control system minimizes hydraulic losses between the hydraulic accumulator and the transmission hydraulic passage by using hydraulic valves with large passages and sealing elements. 
         [0012]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0014]      FIG. 1  is a schematic diagram of a hydraulic accumulator and hydraulic control system connected to a transmission hydraulic passage; and 
           [0015]      FIG. 2  is a schematic detailed view of a booster valve. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]      FIG. 1  illustrate a hydraulic accumulator system  10 , which includes a hydraulic accumulator assembly  12  and accumulator control system  13 , preferably mounted inside a transmission housing under a sealed cover. The cover is  15  typically used to seal a transmission control system from environment in front wheel drive transmissions. 
         [0017]    The hydraulic accumulator assembly  12  is formed integrally with a gas chamber  14 , which includes a piston  15  with a seal  16  installed inside a hydraulic cylinder  17 , which is permanently separates the hydraulic fluid from the  20  compressed gas in gas chamber  14 . 
         [0018]    The hydraulic accumulator assembly  12  communicates hydraulically to the accumulator control system  13  through a screw thread  18 , sealed by O-ring  19  and an accumulator passage  20 , which connects hydraulic fluid in cylinder  17  to an accumulator hydraulic control passage  21 . 
         [0019]    The accumulator hydraulic control passage  21  communicated with the accumulator control system  13 , which includes a solenoid  22 , a pressure booster valve  23  (also known as a booster valve), a pilot check valve  24 , a high pressure check valve  25  and a line check valve  26 . 
         [0020]    The pilot check valve  24  consist of a spring  34 , ball  33 , ball seat  35  and  30  a pilot pin  36  partially located in the pressure booster valve  23 . 
         [0021]    The pressure booster valve  23  includes a piston  28 , a cylinder  29 , and a spring  30 . 
         [0022]    The high pressure check valve  25  includes a small ball  37 , a high force spring  38  and valve bore guides. 
         [0023]    The line check valve  26  consists from a low force spring, a ball approximately the same size as the ball  33  of the pilot check valve and check valve bore guides similar to the pilot check valve bore. 
         [0024]    The accumulator passage  20  is connected simultaneously to the pilot check valve  24  apply passage  39  and the booster valve apply passage. The high  10  pressure check valve  25  passage  41  is connected on the transmission main pump  42  supply side, to the transmission line pressure passage  43 , and to the downstream side to the accumulator passage  20 . 
         [0025]    The pilot check valve  24  hydraulic exhaust passage  44  is connected to the line check valve  26  release passage  45  and through valve  26  to the  15  transmission line pressure passage  43 . 
         [0026]    The housing of solenoid  22  is attached to the body of the accumulator control system  13  so that solenoid armature pin  46  has a minimum clearance between pin&#39;s end and the mating face of pilot pin  36 . 
         [0027]    The pressure balance valve piston  28  has a seal  47  creating a sealed  20  hydraulic area “A”. The pressure balance valve piston  28  is restricted in the axial direction by a sliding flange  48  and the booster valve spring  30 . There is a small clearance between the piston  28  and the cylinder  29  in the as-installed condition. The sliding flange  48  is retained on the piston  28  by a shoulder  49 . 
         [0028]    The booster valve spring  30  compression is restricted by the piston  28   25  shoulder  50 . The maximum travel of spring  30  is preferably smaller than the maximum travel of booster valve piston  28 . 
         [0029]    The pressure balance valve cylinder  29  has a side bracket  51  with a hydraulic passage sealed by an O-ring  52  against the accumulator control system  13  and connected with the hydraulic passage  40 . The side bracket  51  is attached  30  to accumulator control system  13  by fasteners. 
         [0030]    The pressure booster valve  23  is enclosed into an internal opening formed inside the pilot pin  36  with the side bracket  51  protruding through a slot  53  formed into the pilot pin  36  internal opening walls. The slot  53  depth is sufficient to allow full booster valve piston stroke “A”. 
         [0031]    The booster valve pilot pin  36  spool  54  is free to move through an opening  55  made in the accumulator control body in a way that aligns the pilot pin  36 , the booster valve spool  54  and the ball  33  of the pilot check valve. The end of booster valve pilot pin  36  should have a minimum allowable clearance with the ball surface. 
         [0032]    The pressure balance valve piston  28  surface area Pa is equal or smaller than the area of the contact ring of the ball  33  of the pilot check valve and the seat in the accumulator control system  13 . This will provide a counter hydraulic force toward the ball to balance the hydraulic force holding the ball  33  on the seat and minimize the force required from the solenoid  22 . 
         [0033]    The spool  54  cross sectional area Rpa should be equal to or smaller than that of the piston  28  surface area Pa to generate piston return force sufficient to re-seat the ball  33 . 
         [0034]      FIG. 2  illustrates an optional configuration, which includes an orifice  56  that provides additional means of allowing the ball  33  shown in  FIGS. 1 to 20  reseat under high residual pressure conditions in the passage  44 . 
         [0035]      FIG. 2  also shows a simplified configuration where the booster valve spring  30  compression is not restricted by the piston  28 , as shown in  FIG. 1 . 
         [0036]    In operation the main pump  42  pressurizes the accumulator assembly  12  through the check valve  25  when the engine is initially started. The ball  33  of  25  pilot check valve  24  and seal  47  tightly seal the pressure in accumulator  12  until the engine restart signal occurs. At this point the booster valve piston  28  is pressurized by accumulator  12  through booster valve apply passage  40  towards the ball  33  to balance the hydraulic forces on the ball. The spring  30  is compressed, storing energy to extend the ball  33  displacement beyond the immediate  30  displacement of the pressure balance valve piston  28 . 
         [0037]    The solenoid  22  is energized when the engine restart signal is generated, adding enough force to unseat the ball  33  against the bias spring  34 . The booster valve spring  30  further displaces the ball  33  beyond the stroke of the solenoid  22  to ensure a large flow opening into the exhaust passage  44  and then out to the transmission control element, i.e., clutches and brakes, ensuring their rapid refill as the main pump  42  is coming up to speed. 
         [0038]    The accumulator assembly  12  holds the system pressure at a predefined level until the pressure at the outlet of the main pump  42  exceeds that held pressure and closes the line check valve  26 . The main pump  42  pressure is controlled below the crack pressure of check valve  25  to prevent its flow reaching either the accumulator assembly  12  or the ball  33  until the ball  33  has reseated. 
         [0039]    Electric current supplied to the solenoid  22  shuts off at a predetermined time to allow the ball  33  to reseat. Residual pressure in passage  44  acts on the end of the pilot pin  36  counterbalancing the pressure balance valve piston  28  forcing it toward the solenoid  22 . This and the spring  34  will assist in moving the pressure balance valve piston  28  toward the solenoid  22  against the friction of the seal  47  and allowing the ball  33  to reseat. 
         [0040]    The main pump  42  pressure is raised at a predetermined time to recharge the accumulator assembly  12  and reset the remainder of the accumulator control system  13 . 
         [0041]    In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.