Patent Publication Number: US-6662784-B1

Title: Pump assembly, valve and method

Description:
This application is a continuation of my application for Pump Assembly and Method, Ser. No. 09/580,877 filed May 30, 2000, now U.S. Pat. No. 6,460,510. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to pump assemblies, throttling valves and pumping methods where the output of the pump assembly is controlled by throttling inlet flow to the pump. The pump assembly, valve and method may be used to pressurize engine oil used in a Hydraulic Electronic Unit Injector (HEUI) diesel engine fuel system. 
     DESCRIPTION OF THE PRIOR ART 
     Diesel engines using HEUI fuel injectors are well known. A HEUI injector includes an actuation solenoid which, in response to a signal from the diesel engine electronic control module, opens a valve for an interval to permit high pressure engine oil supplied to the injector to extend a fuel plunger and inject fuel into the combustion chamber. 
     HEUI injectors are actuated by oil drawn from the sump of the diesel engine by the diesel engine oil pump and flowed to a high pressure pump assembly driven by the diesel engine. The pump assembly pumps engine oil at high pressure into an oil manifold or compression chamber. The manifold or chamber is connected to the HEUI injectors. Except for large engines, the high pressure pump assembly typically includes a swash plate pump using axial pistons and having an output dependent upon the speed of the diesel engine. Large engines sometimes use a variable angle swash plate pump where the output can be varied independently of engine speed. 
     The pump assembly pumps oil at a rate depending on engine speed. The output must be sufficient to meet maximum flow requirements. The pressure of the oil in the oil manifold or chamber is controlled by an injection pressure regulator (IPR) valve in response to signals received from the electronic control module for the engine. The IPR valve limits the pressure in the pumped oil by flowing excess high pressure oil back into the engine sump. 
     Most HEUI injection systems use fixed output oil pump assemblies which pump oil at a rate dependent upon the rotational speed of the diesel engine and independent of the actual instantaneous flow requirements for the engine. The pump operates at full capacity at all times, even when excess high pressure oil must be flowed or relieved back to the sump immediately to limit the pressure of the oil in the manifold as required by the engine electronic control module. Considerable power is required to drive the pump assembly at full capacity all the time. The energy required to pump high pressure oil which is relieved back to the sump is wasted and decreases the fuel economy of the diesel engine. Energy is converted to heat when high pressure oil is exhausted without doing useful work. The heat in the returned oil must be dissipated, typically by a heat exchanger. Heat exchanger capacity must be increased to accommodate the additional heat load. 
     Therefore, there is a need for an improved high pressure pump assembly and method for use in a HEUI diesel engine. The pump assembly should pump engine oil into a high pressure oil manifold or chamber in a variable amount sufficient to maintain the desired instantaneous pressure in the manifold without substantial overpumping. Return of pressurized high pressure oil to the sump should be minimized. The pump in the assembly should be capable of pumping a variable output and should be less expensive and less complicated than present HEUI pumps. 
     SUMMARY OF THE INVENTION 
     The invention is an improved pump assembly, inlet throttle valve, high pressure pump and method where the output of the pump assembly is varied by controlling or throttling the input flow to the assembly. 
     The pump assembly is particularly useful in pressurizing oil used to actuate HEUI fuel injectors for diesel engines. The improved pump assembly includes an inlet throttle valve which controls inlet flow of oil from the diesel engine oil pump to the high pressure pump. The inlet throttle valve throttles or restricts the volume of oil flowing into the high pressure pump in response to signals received from the engine electronic control module. 
     The high pressure pump includes a crank which reciprocates pistons in bores. Oil supplied to the high pressure pump through the inlet throttle valve flows into a crank chamber and into the bores during return strokes, is pressurized during pumping strokes and is pumped past poppet outlet valves to a high pressure manifold. When the inlet throttle valve is fully opened sufficient oil flows into the crank chamber to fill the pumping chambers during the return strokes and oil is pumped into the manifold at full pump capacity. When the inlet throttle valve is partially closed a reduced amount of oil flows into the crank chamber, partially fills the bores and is pumped at less than full pump capacity. 
     The inlet throttle valve is controlled by an injection pressure regulator valve having a main stage valve for flowing pressurized oil from the pump outlet into the sump when necessary to limit manifold pressure, and an electrically modulated pilot stage valve. 
     The pilot stage valve includes a solenoid modulated by a signal from the electronic control module to restrict pilot flow of oil from the pump outlet. To reach the pilot stage, oil from the pump outlet must pass through a restrictive orifice within a main stage spool, thereby regulating the spool against the closing force of a spring. From the pilot stage, pilot flow passes through a downstream restrictive orifice and then returns to the engine sump along with any drain flow from the main stage of the injection pressure regulating valve. The pressure of the oil in the chamber between the pilot stage and the downstream restrictive orifice is determined by pilot flow rate. The chamber between the pilot stage and the downstream restrictive orifice communicates with the end of the inlet throttle spool and acts on the spool area to generate a force that shifts the inlet throttle valve spool in a closing direction against a spring and inlet pressure acting on the spool area to control or throttle flow of oil into the crank chamber. 
     Control or throttling of the flow of oil into the crank chamber controls the flow rate of high pressure oil pumped from the outlet into the high pressure manifold by the pump as necessary to maintain the desired pressure in the manifold. The pump assembly flows a volume of oil sufficient to maintain the desired pressure in the manifold. The pump assembly meets flow requirements while only rarely pumping at full capacity. Less power is required to pump HEUI oil. Reduction in the power required to drive the high pressure pump increases the fuel efficiency of the diesel engine. The necessity to cool sump oil is reduced. 
     The pump assembly includes two 90° banks with two single high pressure check valve piston pumps in each bank. Each pump includes a piston in a bore and a spring in the bore biasing the piston against a slipper socket and holding the slipper against a crank eccentric. The eccentrics are oriented 180° out of phase so that the pistons in the four pumps are moved through pumping strokes spaced 90° apart to provide evenly spaced high pressure oil pumping cycles during each 360° rotation of the crank. Pulses may be timed to occur during injection events. 
     Each high pressure piston pump includes a bore extending toward the axis of a crank shaft, a piston in the bore and a check valve assembly mounted in the outer end of the bore and connected to a high pressure passage. The check valve assemblies are mounted in the bores by pressing sleeves into the outer cylindrical ends of the bores and then pressing plugs into the sleeves to form high pressure joints between the plugs, sleeves and bores. The check valve assemblies are mounted without cutting threads in the bores and without the complexity of machining and contamination that are characteristic of threaded plugs. The check valve seat is retained in the sleeve by a tapered engagement that forces the sleeve radially outward to improve sealing and increase sleeve retention force. 
     Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying drawings illustrating the invention. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a representational view illustrating the pump assembly, pressure chamber and injectors; 
     FIG. 2 is a side view of the pump assembly; 
     FIGS. 3,  4  and  5  are views taken along lines  3 — 3 ,  4 — 4  and  5 — 5  of FIG. 2 respectively; 
     FIGS. 6,  7  and  8  are sectional views taken along lines  6 — 6 ,  7 — 7  and  8 — 8  of FIG. 3 respectively; 
     FIG. 9 is a sectional view taken along line  9 — 9  of FIG. 1; 
     FIG. 9 a  is an enlarges view of a portion of FIG. 9; 
     FIG. 10 is a sectional view taken along line  10 — 10  of FIG. 9; 
     FIG. 11 is a sectional view taken along line  11 — 11  of FIG. 1; 
     FIG. 12 is a sectional view taken along line  12 — 12  of FIG. 3; 
     FIG. 13 is a side view of the inlet throttle valve spool; 
     FIG. 14 is a view of the surface of the inlet throttle valve spool unwound; 
     FIG. 14 a  is a sectional view taken along line  14   a — 14   a  of FIG. 13 showing the circumferential locations of flow openings; 
     FIG. 15 is a diagram of the hydraulic circuitry of the pump assembly; 
     FIGS. 16 and 17 are views illustrating manufacture of a first check valve assembly; and 
     FIGS. 18 and 19 are views illustrating a second check valve assembly and its manufacture. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Inlet throttle controlled pump assembly  10  is mounted on a diesel engine, typically a diesel engine used to power an over-the-road vehicle, and supplies high pressure engine oil to solenoid actuated fuel injectors  12 . Input gear  14  on pump assembly  10  is rotated by the engine to power the pump assembly. Engine lubricating oil is drawn from sump  16  by engine lubrication oil pump  18  and flowed to start reservoir  19  and pump assembly inlet port  20 . The oil pump also flows engine oil through line  260  to engine bearings and cooling jets. Reservoir  19  is located above assembly  10 . 
     The pump assembly  10  displaces the oil and flows the oil from outlet port  22  along flow passage  24  to injectors  12 . Flow passage  24  may include a manifold attached to the diesel engine. High pressure compression chamber  26  is joined to flow passage  24 . The chamber may be external to the diesel engine. Alternatively, the oil manifold may have sufficient volume to eliminate the need for an external chamber. 
     Pump assembly  10  includes a cast iron body  28  having a mounting face  30  with mounting holes  32  extending through face  30  to facilitate bolting pump of assembly  10  to the diesel engine. Mounting collar  34  extends outwardly from face  30  and into a cylindrical opening formed in a mounting surface on the diesel engine with gear  14  engaging a gear in the engine rotated by the engine crank shaft. An O-ring seal on collar  34  seals the opening in the engine. 
     Crank chamber  36  is formed in the lower portion of body  28  and extends between the interior of collar  34  and opposed closed end  38 . Crank shaft  40  is fitted in chamber  36 . A journal at the inner end of the crank shaft is supported by sleeve bearing  42  mounted in body  28  adjacent the blind end of the crank chamber. A journal at the opposite end of the crank shaft is supported by sleeve bearing  44  carried by bearing block  46 . Block  46  is pressed into collar  34 . Shaft seal  48  is carried on the outer end of block  46  and includes a lip engaging a cylindrical surface on the outer end of the crank shaft. The lip extends away from crank chamber  36  to permit flow of engine oil from annular space  49  behind the seal, past the seal and back into the diesel engine. 
     During operation of pump assembly  10  engine oil is flowed into crank chamber  36  and is in contact with the inner bearing surfaces between the crank journals and sleeve bearings  42  and  44 . When the pressure in the crank chamber is greater than the pressure at the remote ends of the bearing surfaces between the journals and the sleeve bearings so that a small lubricating flow of oil seeps through the bearing surfaces and into end chamber  66  and annular space  49 . This flow of oil from the crank chamber lubricates the sleeve bearings. The oil collected in chamber  66  flows through passage  64  to space  49  where it joins oil from the other bearing. The oil in space  49  lifts lip seal  48  and flows out of the pump assembly and back to the sump of the diesel engine. The two sleeve bearings  44  and  46  form effective pressure seals for the crank chamber  36  and permit the lip of shaft seal  48  to face outwardly on the crank shaft so that it may be lifted to permit oil to flow outwardly from space  49 . The position of shaft seal  48  is opposite the position of a normal shaft seal which would normally have an inwardly facing lip which prevents outward flow. 
     During inlet throttling the flow of oil into the crank chamber is reduced and the pressure in the crank chamber may be lowered below the pressure inside the diesel engine. In this case, oil may seep into the crank chamber from space  49  and chamber  66 . Inward or outward seep flow of oil through the bearings lubricates the bearings but does not influence operation of the pump. 
     During inlet throttling of oil into the crank chamber the pressure in the crank chamber may be reduced below the pressure in the diesel engine. This is because the pumps draw a vacuum in the crank chamber. 
     Threadable fastener  50  secures gear  14  on the end of the crank shaft extending outwardly from the bearing block. 
     Crank shaft  40  carries two axially spaced cylindrical eccentrics  52 ,  54  which are separated and joined by a larger diameter disc  56  located on the axis of the crank. The disc strengthens the crank shaft. Each eccentric  52 ,  54  is provided with an undercut slot  58  located between adjacent sides of the eccentric and extending about 130° around the circumference of the eccentric. Passage  60  extends from the bottom of slot  58  to two cross access passages  62  extending parallel to the axis of the crank shaft and through the eccentric and disc  56 . The cylindrical eccentrics  52  and  54  are oriented 180° out of phase on the crank shaft so that passages  62  for eccentric  52  are located diametrically across the crank shaft axis from passages  62  for eccentric  54 . See FIG.  4 . 
     Axial passage  64  extends along the length of the crank shaft. At the inner end of the crank shaft passage  64  opens into end chamber  66  formed in closed end  38  of the crank chamber. A cross passage  68  communicates the outer end of passage  64  with annular space  49  behind seal  48 . 
     Pump assembly  10  includes four high pressure check valve piston pumps  74  arranged in two 90° oriented banks  70  and  72 . Each bank includes two pumps  74 . As shown in FIG. 3, bank  70  extends to the left of the crank shaft and bank  72  extends above the crank shaft so that the pump assembly has a Vee- 4  construction. One pump  74  in each bank is in alignment with and driven by eccentric  52  and the other pump in each bank is in alignment with and driven by eccentric  54 . The four check valve pumps are identical. 
     Each check valve piston pump  74  includes a piston bore  76  formed in one of the banks and extending perpendicularly to the axis of the crank shaft. A hollow cylindrical piston  78  has a sliding fit within the inner end of bore  76 . The piston has a spherical inner end  80  adjacent the crank shaft. End  80  is fitted in a spherical recess in a slipper socket  82  located between the piston and the eccentric actuating the pump. The inner concave surface of the slipper socket is cylindrical and conforms to the surface of the adjacent cylindrical eccentric. Central passage  84  in the spherical end of the piston and passage  86  in the slipper communicate the surface of the eccentric with variable volume pumping chamber  88  in piston  78  and bore  76 . The variable volume portion of the pumping chamber is located in bore  76 . 
     A check valve assembly  90  is located in the outer end of each piston bore  76 . Each assembly  90  includes a sleeve  92  tightly fitted in the end of bore  76 . A cylindrical seat  94  is fitted in the lower end of the sleeve. Plug  96  is fitted in the sleeve to close the outer end of bore  76 . Poppet disc or valve member  98  is normally held against the outer end of seat  94  by poppet spring  100  fitted in plug  96 . A central boss  99  projects above valve member  98  and is fitted in spring  100 . 
     A piston spring  102  is fitted in each piston  78  and extends between the spherical inner end of the piston  78  and a seat  94 . Spring  102  holds the piston against pump slipper  82  and the slipper against an eccentric  52 ,  54 . Rotation of crank shaft  40  moves the slots  58  in the surfaces of the eccentrics into and out of engagement with slipper passages  86  to permit unobstructed flow of engine oil from the crank chamber into the pumping chambers  88 . Rotation of the crank shaft also moves the pistons  78  up and down in bores  76  to pump oil past the check valves. During rotation of the crank shaft the piston springs  102  hold the pistons against the slippers and the slippers against the eccentrics while the slippers oscillate on the spherical end of the pistons. The eccentric and slipper of each pump form an inlet valve for flowing oil into the pumping chamber during return strokes of the piston. The inlet valve is closed during pumping strokes. 
     The diesel engine rotates crank shaft  40  in the direction of arrow  256  shown in FIGS. 3,  4  and  5 . FIG. 4 shows the position of piston  78  in bank  72  when fully extended into bore  76  at the end of a pumping stroke. Upon further rotation of the crank spring  102  and internal pressure move piston  74  away from the fully extended position. The energy of the trapped, pressurized oil is thereby recovered, and the pressure of the trapped oil drops. Continued rotation of the crank moves slot  58  into communication with passage  86  in the slipper socket  82  to permit flow of oil into the opened pumping chamber  88  during the return stroke of the piston. FIG. 5 illustrates the return stroke with uninterrupted communication between slot  58  and the pumping chamber of pump  74  in bank  70 . 
     Inlet port  20  opens into inlet throttle valve  104  located in body  28 . See FIG.  12 . Valve  104  controls the volume of engine oil pumped by the four pumps  74  by throttling the flow of oil flowed from oil pump  18 , through passage  110 , to the crank chamber  36  and into the check valve pumps  74 . 
     The inlet throttle valve  104  includes a bore or passage  106  extending into the body from mounting face  30  to closed end  108 . Oil inlet passage or port  110  surrounds the center of bore  106  and communicates the bore with crank chamber  36 . See FIG.  4 . Hollow cylindrical spool or wall  112  has a close sliding fit in the bore permitting movement of the spool along the bore. Outer end  114  of the spool is open and inner end  116  is closed to form a piston. A cylindrical wall extends between the ends of the spool. Retainer  118  is fitted in the outer end of bore  106 . Inlet throttle spring  120  is confined between the ring  118  and the inner end  116  of the spool to bias the spool toward the closed end  108  of the bore. Locating post  122  extends inwardly from the closed end of the spool to the end of the bore. Chamber  125  surrounds post  122  at the closed end of the bore. Passage  124  communicates injector pressure regulator valve  192 , described below, with chamber  125  at the inner end of bore  106 . Post  122  prevents spool  112  from closing passage  124 . Closed spool end  116  prevents flow between chamber  125  and the interior of the spool. The spool at all times extends past passage  110 . 
     As shown in FIGS. 13 and 14, four large diameter flow openings  128  extend through the wall of the spool adjacent open end  114 . Four pairs of diametrically opposed and axially offset flow control openings  130 - 136  are formed through the wall of the spool at short distances inwardly from flow openings  128 . Small diameter flow control opening  130   a  is diametrically opposed to small diameter flow opening  130   b.  As indicated by line  138 , the outer edge of opening of  130   a  lies on line  138  at the inner edge of openings  128 . Opening  130   b  is shifted a short distance inwardly from opening  130   a.  The shift difference may be slightly more than ¼ the diameter of the openings. A second set of small diametrically opposed openings  132   a  and  132   b  are formed through the spool. Opening  132   a  is shifted the same distance inwardly from opening  130   b  and opening  132   b  is located inwardly slightly more than ¼ the diameter of opening  132   a.  A third set of small diametrically opposed openings  134   a  and  134   b  are formed through the spool with opening  134   a  located inwardly from opening  132   b  slightly more than ¼ the diameter of the opening and opposed small diameter opening  134   b  located inwardly from opening  134   a  slightly more than ¼ the diameter of the opening. Likewise, small diameter flow passage  136   a  is located inwardly from opening  134   b  slightly more than ¼ the diameter of the opening and diametrically opposed small diameter flow opening  136   b  is located inwardly from small diameter opening  136   a  by slightly more than ¼ the diameter of the opening. 
     During opening and closing movement of the spool  112  in bore  106  the flow openings  128 - 136  move past inlet passage  110 . During initial closing movement of the spool from the fully open position shown in FIG. 12 large flow openings  128  are rapidly closed. Further closing movement moves the small diameter flow openings  130   a - 134   a  past and  134   b - 136   b  partially past the oil inlet passage  110  to reduce the area of the opening flowing oil into the crank chamber. Travel of spool  104  is stopped when it contacts retainer  118 , allowing minimum flow through the pump for cooling and lubrication. The overlapping positions of the small diameter flow passages assures that the flow opening is reduced smoothly. 
     The opposed pairs of passages  130   a,    130   b;    132   a,    132   b;    134   a,    134   b;  and  136   a,    136   b;  reduce frictional loading or hysteresis on the spool during shifting as the spool is moved back and forth in bore  106 . Each of the pairs of openings are diametrically opposed and are either open or closed except when the openings are crossing the edge of oil inlet passage  110 . The diametral opposition of the slightly axially offset pairs of openings effectively balances radial pressure forces and reduces binding or hysteresis during movement of the spool. Reduction of binding or hysteresis assures that the spool moves freely and rapidly along the bore in response to a pressure differential across inner end  116 . The opening of passage  110  completely surrounds spool  112  and helps reduce hysteresis. The circumferentially spaced and opposed openings  128  also help reduce hysteresis. 
     Binding or hysteresis is further reduced by locating axially adjacent pairs of diametrically opposed flow openings circumferentially apart as far as possible. For instance, as shown in FIG. 14 a,  openings  132   a  and  132   b  are located at 90 degrees to openings  130   a  and  130   b  and openings  136   a  and  136   b  are located 90 degrees to openings  134   a  and  134   b.  Openings  132   a  and  132   b  are, of necessity, located at 45 degrees to openings  134   a  and  134   b.  Further, all of the “a” openings are located on one side of the spool and all of the “b” openings are located on the opposite side of the spool valve. This arrangement reduces binding and hysteresis by assuring that the side loadings exerted on the spool as the small diameter flow passages are opened or closed are balanced and offset each other. 
     In one valve  104 , bore  106  has a diameter of 0.75 inches with the spool having an axial length from outer end  114  to inner  116  of about 1.65 inches. The large diameter flow openings  126  have a diameter of 0.312 inches and the small diameter flow openings  132   a - 136   b  each have a diameter of 0.094 inches. The small diameter flow openings are axially offset, as described, with adjacent openings at approximately 0.025 inches, slightly more than ¼ the diameter of the openings. 
     When the engine is shut off valve spool  112  is held against closed bore end  108  by spring  120 , as shown in FIG. 12, and large holes  128  and a few of the small diameter passages open into inlet passage  110 . During starting of the diesel engine an electric starter rotates the crank shaft of the engine and auxiliary components including the oil pump  18  and pumps assembly  10  relatively slowly. In order for the engine to start it is necessary for pump  10  to provide flow to increase the pressure of oil in the flow passage  24  to a sufficient high level to fire the injectors  12 , despite the slow rotational speed and corresponding limited capacity of pump  10 . At this time, the inlet throttle valve is fully open and passages  128  open into passage  110 . Oil from the oil pump  18  flows with minimum obstruction into the crank chamber and is pumped into passage  24 . 
     The rotational speed of the diesel engine increases when the engine starts to increase the pressure of the oil in passages  156  and  232 . When pressure reaches a desired level as determined by current to solenoid  220 , pilot relief valve  195  will open, allowing flow into passage  124  and chamber  125  and shift spool  112  to the left from the position shown in FIG. 12 to an operating position where large diameter openings  128  are closed and oil from pump  18  flows into the crank chamber through the small diameter passages  132 - 136  which open into inlet passage  110 . Increased pressure in chamber  125  shifts the spool further to the left to a partially closed position in which the small diameter passages  132 - 134   a  have moved past the inlet opening  110  and passages  134   b,    136   a,    136   b  are partially open and only minimal flow of oil to the crank chamber is allowed. 
     Pressure shifting of spool  112  moves the flow control openings or holes  128 - 134   a  past inlet passage  110  to reduce the cross sectional flow area through valve  104  and reduce or throttle the volume of oil flowed into the crank chamber. 
     Oil flowed into the crank chamber is pumped by the check valve pumps  74  into outlet openings  150  extending through sleeves  92 . Openings  150  in the pumps  74  in bank  70  communicate the spaces in the pumps above the poppet discs with high pressure outlet passage  152 . The outlet opening  150  in the pumps  74  in bank  72  communicate the spaces above the poppet discs with high pressure outlet passage  154 . Angled high pressure outlet passage  156  joins passages  152  and  154 , as shown in FIG.  9 . 
     A makeup ball check valve  158  is located between passage  156  and passage  160  opening into crank chamber  36 . See FIG.  6 . Gravity and the pressure of oil in the outlet passages normally hold valve  158  closed. Spring  162  is fitted in a cross passage above the check valve to prevent dislodgement of the ball of valve  158 . When the diesel engine is shut off and cools, pressure drops and oil in the high pressure flow passages and manifold  24  cools and contracts. Engine crank case pressure acting on the fluid in reservoir  19  lifts the ball of valve  158  and supplies makeup oil from the crank chamber to the high pressure flow passages to prevent formation of voids in the passages. 
     High pressure mechanical relief valve  168  shown in FIG. 8 is located between banks  70  and  72  and extends parallel to the axis of the crank shaft. The valve  168  includes a passage  170  extending from mounting face  30  to high pressure outlet passage  156 . Valve seat  172  is held against step  173  in passage  170  by press fit sleeve  175 . The step faces away from passage  156 . Valve member  174  normally engages the seat to close the valve. Retainer sleeve  176  is press fitted into passage  170  at face  30 . Spring  178  is confined between the retainer and the valve member  174  to hold the valve member against the seat under high pressure so that valve  168  is normally closed. When pump assembly  10  is mounted on a diesel engine the outlet opening  180  in sleeve  176  is aligned with a passage leading to the engine oil sump. An O-ring seal is fitted in groove  182  to prevent leakage. Opening of the mechanical relief valve  168  flows high pressure oil from the outlet passage  156  back into the engine sump. Valve  168  has a high cracking pressure of about 4,500 pounds per square inch. 
     The cross sectional area between sleeve  175  and valve member  174  is selected so that when the valve is open the force from pressurized oil acts on the cross sectional area of valve member  174 . Increased flow through the relief valve requires increased displacement of valve member  174  from seat  172 , thereby requiring greater force as spring  178  is deflected against its spring gradient. The flow restriction between valve member  174  and sleeve  175  is chosen so that the supplemental force from increasing flow will offset the increased spring force, and relief pressure will be relatively independent of flow rate through the relief valve. 
     High pressure outlet passage  156  opens into stepped bore  166  extending into body  28  above the inlet throttle valve  104  and transversely to the axis of crank shaft  40 . See FIG.  9 . Drain passage  190  extend from the outer large diameter portion of stepped bore  166  to chamber  66 . See FIG.  11 . 
     Injection pressure regulator (IPR) valve  192  is threadably mounted in the outer portion of stepped bore  166 . The valve  192  is an electrically modulated, two stage, relief valve and may be Navistar International Transportation Corporation of Melrose Park, Ill. Part No. 18255249C91, manufactured by FASCO of Shelby, N.C. 
     IPR valve  192 , shown in FIG. 9, has an elongated hollow cylindrical body  193  threadably mounted in the large diameter portion of stepped bore  166  and a base  196  on the outer end of body  193 . The IPR valve includes a main stage mechanical relief valve  194  located on the inner end of body  193  and a pilot stage electrically modulated relief valve  195  located in the outer end of body  193 . Body  193  retains spring  162  in place. An o-ring and a backup ring  198  seal the inner end of body  193  against the reduced diameter portion of the bore. A cylindrical valve seat  200  is mounted inside body  193  adjacent base  196  and includes an axial flow passage  202 . 
     Main stage valve  194  includes a cylindrical spool  204  slideably mounted in body  193  and having an axial passage including restriction  206 . Spring  208 , confined between valve seat  200  and spool  204 , biases the spool toward the inner end of bore  166  to the position shown in FIG.  9 . The spring holds the spool against a stop in body  193  (not illustrated). Oil from high pressure outlet passage  156  flows into the inner end of body  193 . 
     Collar  212  is fixedly mounted on body  193  and separates the large diameter portion of bore  166  into inner cylindrical chamber  214  extending from the step to the collar and outer cylindrical chamber  216  extending from the collar to base  196 . A narrow neck  218  on the collar spaces the collar from the base. Small diameter bleed passage  219  extends through collar  212  to communicate chambers  214  and  216 . See FIG.  9 A. 
     If a transient over pressure occurs in the high pressure passages, the pressure of the oil shifts the spool  204  of the main stage valve  194  to the left or toward seat  200  against spring  208 . Movement of the spool is sufficient to move the end of the spool away from the spring and past a number of discharge passages  210  extending through body  193 . High pressure oil then flows through passages  210 , into the chamber  214 , through drain passage  190  to chamber  66  and then back to the sump of the diesel engine, as previously described. 
     The pilot stage valve  195  includes a solenoid  220  on base  196 . The solenoid surrounds an armature  222  axially aligned with base  196 . The lefthand end of the armature engages retention block  224  retained by a tube affixed to body  193 . Solenoid leads  226  are connected to the electronic control module for the diesel engine. A valve pin  228  contacting armature  222  extends toward the flow passage  202  in valve seat  200  and has a tapered lead end which engages the seat to close the passage when the armature is biased towards the seat by solenoid  220 . 
     High pressure oil from passage  156  flows into body  193 , through restriction  206 , and through passage  202  in seat  200  to the end closed by valve pin  228 . The electronic control module sends a current signal to the solenoid to vary the force of the pin against the valve seat and control bleed flow of oil through the passage  202  and internal passages in the IPR valve, including slot  230  in the threads mounting the IPR valve on body  28  and leading to chamber  216 . The oil from chamber  216  flows through restriction  219  to chamber  214  and thence to the engine sump as previously described. Chamber  216  is connected to chamber  125  by passage  124  so that the oil in chamber  216  pressurizes the oil in chamber  125  of the inlet throttle valve. IPR valve  192  is shown in detail in FIG.  9  and diagrammatically in FIGS. 10 and 11. 
     FIGS. 16 and 17 illustrate a method of assembling check valve assembly  90  in the outer end of a piston bore  76  during manufacture of assembly  10 . First, piston  78  is extended into open bore  76  and spring  102  is fitted in the piston. The piston engages a slipper  82  on an eccentric  52 ,  54 . Then, sleeve  92 , having a tight fit in bore  76 , is pressed into the bore. 
     As illustrated in FIG. 17, the interior surface  91  at the inner wall of sleeve  92  is tapered inwardly and increases the thickness of the sleeve. The outer wall of seat  94  is correspondingly tapered outwardly. The seat  94  is extended into the sleeve so that the tapered surfaces on the end of the sleeve and on the seat engage each other. The seat is then driven to the position shown in FIG. 16 to form a tight wedged connection with the sleeve. This connection deforms the sleeve against the wall of the bore and strengthens the connection between the sleeve and the bore  76 . Reduced diameter collar  101  on the inner end of the seat extends into the center of spring  102  to locate the spring radially within pumping chamber  88 . 
     Next, poppet disc  98  is positioned on spring  100 , the spring is fitted in plug  96  and the plug is driven into the open outer end of sleeve  92 . Driving of plug  96  into the sleeve forms a strong closed joint between the plug and the sleeve and strengthens the joint between the sleeve and the wall of bore  76 . A circular boss  99  on the top of poppet disc  98  extends into the spring  100  so that the spring holds the poppet disc in proper position against seat  94 . 
     FIG. 18 illustrates an alternative check valve assembly  240  which may be used in check valve pumps  74  in place of check valve assembly  90 . Assembly  240  includes a sleeve  242  driven in the outer end of a bore  76  as previously described. Sleeve  242  includes a tapered lower end which receives a seat  244 , with a tapered driven connection between the seat and sleeve, as shown in FIG.  19 . The outer end  246  of the sleeve extends above the top of body  28  when the sleeve is fully positioned in the bore  76 . 
     Plug  248  of assembly  240  is longer than plug  96  and includes an angled circumferential undercut  250  at the outer end of the plug extending out from body  28 . The interior opening of plug  248  has the same depth as the corresponding opening of plug  96 . 
     After sleeve  242  and seat  244  have been driven into the passage, poppet disc  252 , like disc  98 , is mounted on spring  254 , like spring  100 , the outer end of the spring is extended into the bore in plug  248  and the plug is driven into the sleeve to the position shown in FIG.  18 . Undercut groove  250  is located above the surface of body  28 . The upper end of the sleeve is then formed into the undercut groove to make a strong connection closing the outer end of the bore. 
     Gear  14  rotates crank shaft  40  in the direction of arrow  256  shown in FIGS. 3,  4  and  5 , or in a counterclockwise direction when viewing mounting face  30 . Rotation of the crank rotates eccentrics  52  and  54  to reciprocate the pistons  78  in bores  76 . In each high pressure pump  74  spring  102  holds the inner spherical end of piston  78  against a slipper  82  to hold the slipper against a rotating eccentric as the piston is reciprocated in bore  76 . During return or suction movement of the piston toward the crank shaft the inlet passage leading from crank chamber  36  to the pumping chamber  88  is unobstructed. There are no check valves in the inlet passage. The unobstructed inlet passage extends through passages  62 , passage  60 , slot  58  and passages  86  and  84  in the slipper and inner end of the piston  78 . The unobstructed inlet passage permits available engine oil in the crank chamber to flow freely into the pumping chambers during return strokes. The inlet passage is opened after piston  78  returns sufficiently to allow trapped oil to expand near the beginning of the return stroke and is closed at the end of the return stroke. 
     FIG. 4 illustrates check valve pump  74  in bank  72  at top dead center. Oil in chamber  88  has been flowed past poppet valve  98  and the valve has closed. The closed pumping chamber  88  remains filled with oil under high pressure. Passage  86  in slipper  82  is closed and remains closed until the crank rotates an additional  18  degrees beyond top dead center and slot  58  communicates with passage  86 . During the  18  degree rotation from top dead center piston  78  travels from top dead center down two percent of the return stroke and the pumping chamber and compressed fluid in the chamber expand to recover a large portion of the energy of compression in the fluid. The recovered energy assists in rotating the crank shaft. Recovery of the compressed energy of the fluid in the pumping chamber reduces the pressure of the fluid in the chamber when the pumping chamber opens to the crank chamber so that the fluid does not flow outwardly into the slot  58  in the crank shaft at high velocity. Recapture of the energy in the compressed fluid in the pumping chamber improves the overall efficiency of the pump by approximately two percent. 
     If the slot in the crank were moved over opening  86  at or shortly after top dead center, the high pressure fluid in the pumping chamber would flow through the opening and into the slot at a high velocity. This velocity is sufficient to risk flow damage to the surfaces of passage  84  and  86  and slot  58 . Opening of the pumping chamber at approximately 18 degrees after top dead center permits reduction of the pressure in the pumping chamber before opening and eliminates high flow rate damage to the surfaces in the pump. The pumping chamber opens sufficiently early in the return stroke to allow filling before closing at bottom dead center. 
     It is important that the inlet passage is unobstructed during cold startup. While the passage is open, available engine oil, which may be cold and viscous, in the crank chamber flows into the pumping chambers during return strokes as the volume of the pumping chambers increases. The circumferential length of slots  58  and the diameter of passages  86  are adjusted so that the pumping chambers in the pistons are open to receive oil from the crank chamber during substantially all of the return stroke. 
     The poppet valve for the pump is held closed during the return stroke by a spring  100  and high pressure oil in the outlet passages. In FIG. 5, pump  74  in bank  72  is at the bottom of the return stroke. Oil has flowed into pumping chamber  88  and the inlet passage communicating with the crank chamber is closed at bottom dead center. Pump  74  in bank  70  has moved through part of its return stroke and the inlet passage to the pumping chamber  88  is in unobstructed communication with the crank chamber. Oil may flow from the crank chamber directly into slot  58  to either side of a slipper  82  or may flow into the slot through passages  60  and  62 . 
     The unobstructed inlet passage is open to flow available oil into the pumping chamber during the entire return stroke of the piston, with the exception of the first two percent of the stroke following top dead center. Provision of an unobstructed inlet passage to the pumping chamber during essentially the entire return stroke increases the capacity of the pump and facilitates flowing cold, viscous oil into the pumping chamber during starting. 
     After each piston completes its return stroke the pumping chamber is filled or partially filled with available oil from chamber  36 , depending upon the volume of oil flowed to the crank chamber through inlet throttle valve  104 . Continued rotation of the crank shaft then moves the piston outwardly through a pumping stroke. During the pumping stroke slot  58  on the eccentric driving the piston is away from passage  86  in the pump slipper and the inlet passage leading to the pumping chamber is closed at the eccentric. Outward movement of the piston by the eccentric reduces the volume of the pumping chamber and increases the pressure of oil in the chamber. A void in a partially filled chamber is collapsed as volume decreases after which pressure builds. When the pressure of the oil in the chamber exceeds the pressure of the oil in the high pressure side of the poppet disc  98  the disc lifts from seat  94  and the oil in the pumping chamber is expelled through the opening in the seat into the high pressure passages. Pumping continues until the piston reaches top dead center at the end of the pumping stroke and commences the return stroke. At this time, spring  100  closes the poppet valve and the pressure in the pumping chamber decreases below the pressure of the oil in the high pressure passages. 
     During operation of pump assembly  10  sleeve bearings  42  and  44  are lubricated by bleed flows of oil from crank chamber  36 . The oil flowing through bearing  44  collects in the space  49  behind seal  48 , lifts the seal, flows past the seal and drains into the sump of the diesel engine. Oil flowing through bearing  42  collects in end chamber  66 , together with any oil flowing through passage  190  and into the chamber from the pilot and main stages of the IPR valve. The oil in chamber  66  flows through the axial bore  64  in the crank shaft, through cross passage  68 , lifts and passes the seal  48  and then drains into the sump of the diesel engine. The bearings  42  and  44  may be lubricated by oil flowing into chamber  66  under conditions of inlet throttling when pressure on the crank chamber  36  is below atmospheric pressure. 
     FIG. 15 illustrates the hydraulic circuitry of pump assembly  10 . The components of injection pressure regulator valve  192  are shown in the dashed rectangle to the right of the figure. The remaining components of pump assembly  10  are shown in the dashed rectangle to the left of the figure. 
     The diesel engine oil pump  18  flows engine oil from sump  16  to start reservoir  19 , inlet port  20  and, through line  260 , to bearings and cooling jets in the diesel engine. The start reservoir  19  is located above the pump assembly  10 . The reservoir includes a bleed orifice  21  at the top of the reservoir. When the reservoir is empty the bleed orifice vents air from the enclosed reservoir to the engine crank case permitting pump  18  to fill the reservoir with engine oil. During operation of the engine reservoir  19  is filled with engine oil and the bleed orifice spills a slight flow of oil to the sump. When the engine stops, the pressure of the oil in the reservoir  19  falls and the bleed orifice allows air at engine crankcase pressure to permit gravity and suction flow of oil from the reservoir through inlet port  20  and into the crank chamber  36 . In this way, oil from reservoir  19  is available for initial pumping to the injectors during cranking and startup of the diesel engine, before the oil pump  18  draws oil from sump  16  and flows the oil to the pump assembly. 
     Oil flows from port  20  to the inlet throttle valve  104 . Oil from the inlet throttle valve  104  flows to the four check valve pumps  74 , indicated by pump assembly  241 . Rotation of pump crank shaft  40  flows pressurized oil from assembly  241  to high pressure outlet passage  156  and through high pressure outlet port  22  to flow passage  24  and fuel injectors  12 . 
     The high pressure outlet passage  156  is connected to the inlet of pump assembly  241  by makeup ball check valve  158  and passage  160 . The high pressure outlet line  156  is connected to high pressure mechanical relief valve  168  which, when opened, returns high pressure oil to sump  16  to limit maximum pressure. 
     Two stage injection pressure regulator valve  192  includes main stage mechanical pressure relief valve  194  and pilot stage electrically modulated relief valve  195 . The mechanical pressure relief valve  194  is shown in a closed position in FIG.  9 . In the closed position, spool  204  closes discharge passages  210 . Shifting of the spool shown in FIG. 9 to the left opens passages  210  to permit high pressure oil from passage  156  to flow through passages  210 , passage  190  and thence back to the diesel engine sump, as previously described. 
     The pressurized oil in passage  156  biases spool  204  in valve  194  toward the open positioned and is opposed by spring  208  and the pressure of fluid in chamber  232  in the IPR valve. Chamber  232  is connected to high pressure passage  156  through internal flow restriction  206  in the spool. 
     The pressure of the oil in chamber  232  acts over the area of the hole in seat  200  on one end of the valve pin  228  of pilot stage of valve  195  to bias the pin toward an open position. Solenoid  220  biases the pin toward the closed position against seat  200 . A pilot flow of oil from valve  195  flows through slot  230  in the threads mounting base  196  in the outer portion of bore  166 , into chamber  216 , through orifice  219  into the chamber  214  and then to the engine sump. Pressurized oil in chamber  216  is conducted by passage  124  to chamber  125  of the inlet throttle valve  104  to bias spool  112  to the left as shown in FIG. 12, away from closed end  108  of bore  106 . Spring  120  and pressure of the oil from pump  18  bias the spool in the opposite direction. The position of the spool depends on the resultant force balance. 
     Operation of inlet throttled control pump assembly  10  will now be described. 
     At startup of the diesel engine start reservoir  19  contains sufficient oil to supply pump  10  until oil is replenished by the diesel engine oil pump. Bleed orifice  21  allows the reservoir to be at engine crank case pressure. The oil may be cold and viscous. The high pressure manifold  24  is full of oil at low pressure. Spring  120  in inlet throttle valve  104  has extended spool  112  to the fully open position shown in FIG.  12 . 
     Actuation of the starter motor for the diesel engine rotates gear  14  and crank shaft  40 . Engine oil pump  18  is also rotated but does not flow oil into the pump assembly immediately. 
     During starting, gravity and engine crank case pressure flow engine oil from reservoir  19  into port  20 , through the open inlet throttle valve and into crank chamber  36 . The oil in the crank chamber is drawn by vacuum freely into pumping chambers  88  through the unobstructed inlet passages in the crank shaft, slippers and inner ends of the piston  78 , despite the viscosity of the oil. During starting, the pump assembly flows oil into manifold  24 . Pressure increases to a starting pressure to actuate injectors  12 . The starting pressure may be 1,000 psi. The reservoir  19  has sufficient volume to supply oil to the pump assembly until the oil pump establishes suction and flows oil to the assembly. During starting and initial pressurization of manifold  24 , valves  194  and  195  are closed. 
     When the diesel engine is running pump assembly  10  maintains the pressure of the oil in manifold  24  in response to current signals to solenoid  220  from the electronic control module. The signals are proportional to the desired instantaneous pressure in the high pressure outlet passage and manifold  24 . Pump assembly  10  pumps a volume of oil slightly greater than the volume of oil required to maintain the desired instantaneous pressure in manifold  24 . When the pressure in manifold  24  must be reduced quickly, excess high pressure oil is returned to the sump through valve  194 . For instance, significant flow may have to be returned to the sump through valve  194  when the engine torque command is rapidly decreased. 
     During operation of the engine a bleed flow of high pressure oil flows through restriction  206  and into chamber  232  at a reduced pressure and acts on the inner end of the main stage valve spool  204 . When the pressure in passage  156  is increased sufficiently to cause a transient over pressure, the force exerted on the high pressure end of spool  204  by oil in high pressure passage  156  is greater than the force exerted on the low pressure end of the spool by spring  208  and the oil in chamber  232 , and the spool shifts to the left as shown in FIG. 9 to open cross passages  210  and allow high pressure oil to flow through the crank shaft and back to sump  16 , reducing the pressure in passage  156 . 
     The solenoid force in pilot stage valve  195  is opposed by the pressure of oil in chamber  232  acting on the pin  228  over the area of the opening in seat  200 . When the electronic control module requires an increase of pressure in the manifold  24  the current flow to solenoid  220  is increased to reduce the pilot flow of oil through valve  195 , through orifice  219  and then through the shaft to the engine sump. Reduction of pressure in chamber  125  permits spring  120  to shift spool  112  to the right toward the open position as shown in FIG.  14 . Oil expelled from chamber  125  flows through passage  124  into chamber  216 , through orifice  219  and through the crank shaft to the engine sump. 
     Shifting of spool  112  toward the open position increases the flow openings leading into the crank chamber to correspondingly increase the volume of oil flowed into the crank chamber and pumped by the high pressure poppet valve pumps into manifold  24 . The inlet throttle valve will open at a rate determined by the forces acting on spool  112 . The pressure of the oil in bore  106  acting on the area of the spool and spring  120  bias the spool toward the open position. These forces are opposed by the pressure of the oil in chamber  125  acting on the area of the spool which biases the spool in the opposite direction. The spool moves toward the open position until a force balance or equilibrium position is established. When an equilibrium position of the spool is established, the pilot flow rate through bleed passage  219  is too low to develop a differential pressure across orifice  206  sufficient to shift spool  204  against spring  208  and open valve  194 . Increased flow of pumped oil into the manifold increases the pressure of oil in the manifold. 
     If the main stage IPR valve  194  is closed when solenoid current is increased, valve  194  will remain closed. If the main stage valve  194  is partially open, the increase in solenoid current will partially close valve  195 , increase the pressure in chamber  232  and close valve  194 . 
     When the pressure of oil in manifold  24  is increased the pressure in chamber  232  will increase, pilot flow through passage  219  will resume and resulting pressure increase in chamber  125  will stop opening movement of the inlet throttle spool. If the inlet throttle spool overshoots the equilibrium position and the pressure of the oil in the manifold exceeds the commanded level, the main stage IPR valve  194  may open to flow oil from the manifold and reduce pressure in the manifold to the commanded level. 
     A sharp decrease in the solenoid current decreases the force biasing the valve pin  228  toward seat  200  to permit rapid increase in pilot flow and flow to inlet throttle valve chamber  125 . The increased pressure on the closed end of the spool shifts the spool in a closing direction or to the left as shown in FIG. 12, reducing flow of oil into the crank chamber. The pumping chambers do not fill completely and output of high pressure oil flowed into the manifold is decreased. 
     Inlet throttle response may lag behind a steep drop in solenoid current because of the time required to consume oil in the crank chamber when solenoid current is decreased. In this event, the opening of pilot valve  195  decreases the pressure in chamber  232  and the main stage IPR valve  194  opens to permit limited flow from the manifold to the sump and reduction of the pressure of the oil in the manifold. 
     During equilibrium operation of the diesel engine solenoid  220  receives an essentially constant amperage signal and pilot oil flows through valve  194  to chamber  214  through orifice  219  uniformly, but is influenced by pressure fluctuations from injection and piston pulsations. The resulting pressure in chamber  125 , fed by passage  124 , acts on the closed end of spool  112  and is opposed by the force of spring  120  and inlet pressure acting on spool  112 . An equilibrium balance of forces occurs so that the flow of oil into the crank chamber is sufficient to maintain the desired pressure in manifold  24 . 
     Inlet throttle controlled pump assembly  10  flows the required volume of engine oil into manifold  24  to meet HEUI injector requirements throughout the operating range of the diesel engine. During starting, when the engine is cranked by a starter, the inlet throttle valve is fully open and the high pressure check valve piston pumps  74  pump at full capacity to increase the pressure of the oil in the manifold to the starting pressure for the engine. During idling of the engine, at a low speed of about 600 rpm, the spool in the inlet throttle valve is shifted to the closed position where only flow control openings  134   b,    136   a  and  136   b  are partially open and a low volume of oil is pumped to maintain a low idle manifold pressure of 600 psi. If the minimum flow allowed by the inlet throttle spool is not utilized by the injectors, the main stage IPR valve  194  opens to allow the excess oil to return to the sump. 
     Pump assembly  10  flows the high pressure oil into manifold  24  and compression chamber  26 , if provided. The high pressure oil is compressed sufficiently so that the flow requirements of the injectors  12  are met by expansion of the oil. The flow requirements for the injectors vary depending upon the duration of the electrical firing signal or injection event for the injectors. The control module may vary the timing of the injection event relative to top dead center of the engine piston, according to the desired operational parameters of the engine. The large volume of oil compressed by assembly  10  assures that a sufficient volume of compressed oil is always available for expansion whenever an injection event occurs, independent of the timing of the event signal. 
     Large volume manifolds and compression chambers increase the cost of diesel engines. The volume of the internal manifold may be reduced and external chamber may be eliminated by providing the diesel engine with a HEUI pump assembly  10  having a number of high pressure pumps  74  sufficient to provide a high pressure pumping stroke during the occurrence of each injection event for each engine cylinder. For instance, the pumping stroke for each high pressure pump may be timed so that a sufficient volume of high pressure oil is flowed into a pressure line leading to the injectors when an injection event occurs so that a sufficient volume of pressurized pumped oil is available to fire the injector. As an example, assembly  10  includes four high pressure pumps  74  each having an approximately 180° pumping stroke with the strokes occurring one after the other during each rotation of crank shaft  40 . The pump assembly could be mounted on an eight cylinder diesel engine with rotation of the assembly crank shaft timed so that output flow into a line leading to the injectors peaks when each ejector is fired. In this way, it is possible to provide a flow pulse in the line at the proper time and of a sufficient volume to fire the injectors, without the necessity of a large volume manifold or compression chamber. In other four stroke cycle engines, one high pressure pump may pump oil during injection events for each pair of cylinders. 
     Control pump assembly  10  includes an inlet throttle valve and a hydraulic system, including electrically modulated valve  195 , for controlling the inlet throttle valve to throttle inlet flow of oil to pump assembly  241  shown in FIG.  15 . If desired, the hydraulic regulator may be replaced by an electrical regulator including a fast response pressure transducer mounted in high pressure outlet passage  156  to generate a signal proportional to the pressure in the passage, a comparator for receiving the output signal from the pressure transducer and a signal from the diesel engine electronic control module proportional to the desired pressure in the high pressure passage and for generating an output signal proportional to the difference between the two signals. The electrical system would also include an electrical actuator, typically a proportional solenoid, for moving the spool in the inlet throttle valve to increase or decrease flow of oil into the pump assembly  241  as required to increase or decrease the pressure in the high pressure passage. The electrical control system would include a pressure relief valve, like valve  194 , to flow oil from passage  156  in response to transient overpressures and a mechanical relief valve like valve  168 . The electrical regulator would control the output pressure as previously described. 
     Pump assembly  10  is useful in maintaining the desired pressure of oil flowed to HEUI injectors in a diesel engine. The assembly may, however, be used for different applications. For instance, the pump may be rotated at a fixed speed and the inlet throttle valve used to control the pump to flow liquid at different rates determined by the position of the spool in the inlet throttle valve. The spool could be adjusted manually or by an automatic regulator. The pumped liquid could flow without restriction or could be pumped into a closed chamber with the pressure of the chamber dependent upon the flow rate from the chamber. 
     While I have illustrated and described a preferred embodiment of my invention, it is understood that this is capable of modification, and I therefore 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 claims.