Abstract:
A vacuum management system for an engine with variable valve lift includes a vacuum control valve at the entrance to the intake manifold to increase vacuum within the manifold as needed and preferably only when it can be done without impairing fuel economy or engine performance. Vacuum may then be used for any of various vacuum-assisted devices and functions, for example, boosting a vehicle braking system. The numerical relationships among important operating parameters are determined in a laboratory, and a programmable engine control module (ECM} is provided with algorithms and tables of such values by which the ECM is able to vary valve lift and vacuum control valve position to provide optimum flow across the intake valves and optimum manifold vacuum under all engine operating conditions.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to internal combustion engines; more particularly, to such engines wherein devices for variably controlling the lift of intake valves are the primary throttling means of the engine; and most particularly, to a system for providing and managing manifold vacuum in such an engine to optimize fuel economy and enable vacuum-assisted devices such as a brake booster.  
         BACKGROUND OF THE INVENTION  
         [0002]    Fuel-injected internal combustion engines are well known, especially for automotive applications. Torque output of such an engine is typically controlled by moderating airflow into the engine via a throttle device. The throttle, usually a butterfly valve disposed at the entrance to the engine intake manifold, may be directly actuated by a driver&#39;s foot pedal or may be electronically governed through a digital or analog controller. Under typical driving conditions, the engine is substantially throttled. Because the engine is a positive displacement pump, a vacuum is created in the intake manifold downstream of the throttle valve.  
           [0003]    Recently, some engines are known to be provided with means for varying the lift of one or more engine cylinder intake valves to improve fuel economy (also known as variable valve actuation, VVA, and referred to herein as variable valve lift, VVL). Typically, the lift of a plurality of valves in a multiple-cylinder engine is reduced or modulated during operating periods of low engine load to reduce fuel consumption, the amount of lift being directed by an engine control module (ECM) responsive to various performance inputs, operator pedal position, and programmed algorithms.  
           [0004]    In some such engines, it is known to control engine torque by directly utilizing the variable valve lift means to controllably throttle the flow of air into each of the individual cylinders, thereby obviating the need for any conventional throttle valve at the inlet to the intake manifold.  
           [0005]    A first unfavorable consequence of eliminating a manifold throttle valve is that the air pressure within the manifold is substantially the same as atmospheric pressure outside the engine; i.e., there is no useful level of manifold vacuum. However, a variety of standard engine and other automotive subsystems have evolved over many years which utilize vacuum as the source of actuation. The engine intake manifold has previously been a “free” source of vacuum for operating such devices and functions, which may include brake boosting, evaporative canister purging, exhaust gas recirculation, and HVAC systems among others. Providing an auxiliary vacuum pump for auxiliary automotive devices adds cost to a vehicle, consumes valuable onboard space, and parasitically decreases fuel economy. Engine functions, such as improving fuel preparation for cold starting, inducing exhaust gas recirculation into the intake manifold, and reducing cylinder-to-cylinder air volume differences at light engine loads, require manifold vacuum and cannot be accomplished by addition of an auxiliary vacuum pump.  
           [0006]    A second unfavorable consequence of eliminating a manifold throttle valve is that fuel economy typically is sub-optimal when there is no manifold vacuum.  
           [0007]    It is a principal object of the present invention to provide a substantially non-parasitic system for creating and managing vacuum for operating vacuum-assisted devices and functions in a vehicle powered by a VVL-equipped engine wherein primary throttling has heretofore been provided exclusively by variable valve lifting.  
           [0008]    It is a further object of the invention to provide such a system whereby fuel economy is improved.  
           [0009]    It is a still further object of the invention to provide a failsafe means for operating a VVL-equipped and throttled engine in the event that the VVL control fails and the valves assume a full-lift mode.  
         SUMMARY OF THE INVENTION  
         [0010]    Briefly described, a vacuum creation and management system for an engine with variable valve lift includes a vacuum control valve at the entrance to the intake manifold connected to a programmable engine control module (ECM) to increase vacuum within the manifold as needed. Vacuum may then be used for any of various vacuum-assisted devices and functions, for example, boosting a vehicle braking system. Numerical values for important operating parameters are determined in a laboratory, and the ECM is provided with algorithms and tables of such values according to which the ECM varies valve lift and throttle valve position to provide optimum manifold vacuum under all engine operating conditions. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    These and other features and advantages of the invention will be more fully understood and appreciated from the following description of certain exemplary embodiments of the invention taken together with the accompanying drawings, in which:  
         [0012]    [0012]FIG. 1 is a schematic diagram of a known VVL-equipped fuel-injected internal combustion engine wherein the sole throttle means is the variable valve lift means;  
         [0013]    [0013]FIG. 2 is a schematic diagram of the engine shown in FIG. 1, showing a manifold vacuum control system in accordance with the invention;  
         [0014]    [0014]FIG. 3 is a diagram showing various engine and automotive functions which either require or are conventionally adapted for vacuum actuation; and  
         [0015]    [0015]FIG. 4 is a schematic diagram of an engine similar to that shown in FIG. 2, showing the various flow relationships which may enter into a system for managing vacuum to an optimum level in such an engine.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    Referring to FIG. 1, a fuel-injected engine  12  with variable valve lift (VVL) means  14  for actuation of intake valve  15  includes a programmable engine control module  20  (ECM). (It should be understood that engine  12  is a multiple-cylinder engine and that valve  15  is individually representative of a plurality of engine valves in a plurality of engine cylinders.) Intake manifold  18  is connected for air flow  19  to engine head  22  via runner  24  which supports a conventional fuel injector  25 . Head  22  supports intake valve  15  and exhaust valve  23 . The ECM is electrically connected to VVL lift means  14  via first lead  30  for varying the lift of intake valve  15 . Engine throttling and consequent torque control is provided by varying the lift of the intake valves via ECM  20  in response to engine load request from an electronic pedal module  27  connected via second lead  28  and responsive to positional input of accelerator pedal  29  from operator  31 . ECM  20  may be further connected to other engine and vehicle inputs and may be provided with algorithms for determining the instantaneous performance of engine  12 . Typically, during operation of engine  12  as shown in FIG. 1 there is substantially no vacuum in manifold  18 .  
         [0017]    Referring to FIG. 2, a vacuum system  10  in accordance with the invention includes an engine  12  and components substantially as shown in FIG. 1. In addition, a controllable vacuum control valve  16  is disposed in the entrance  17  to intake manifold  18  and is connected to ECM  20  by third lead  26  for sensing the rotary position thereof and for actuating valve  16  to move to a different rotary position in response to an algorithm in the ECM. Manifold  18  is further provided with a pressure sensor  33  connected to ECM  20  via fourth lead  35  for sensing pressure (vacuum) therein.  
         [0018]    Referring to FIG. 3, in Venn diagram  36 , regional boundary  38  encompasses automotive and engine functions benefiting from an engine manifold vacuum system; regional boundary  40  encompasses those automotive functions which may be performed by an auxiliary vehicle vacuum pump in the absence of a manifold vacuum control system such as a system in accordance with the invention. Power brake vacuum assist  42  and HVAC control  44  are readily though expensively accommodated by an auxiliary vacuum pump, and power brakes may also be accommodated electrically without vacuum assist. Vacuum purge  46  of a fuel tank emissions canister might also be accommodated at a cost of more vehicle expense and complexity.  
         [0019]    The functions within region  38  but outside of region  40  are engine functions requiring manifold vacuum and cannot be accommodated by either engine  12  in FIG. 1 or an auxiliary vacuum pump. Functions  48 ,  50 , and  52  may each be optimized when the engine intake valves are operated at a slightly higher lift permitted by the presence of a slight vacuum in the manifold.  
         [0020]    Function  48  refers to improving atomization of fuel when an engine is cold, which improves fuel efficiency and reduces tailpipe emissions.  
         [0021]    Function  50  refers to improving the uniformity of air and fuel flow to the cylinders.  
         [0022]    With no manifold vacuum, under low load conditions the valves may be nearly closed;  
         [0023]    small absolute differences in manufacture or wear of valves can cause large percentage differences in fueling and even torque pulses in an engine. Providing a manifold vacuum requires a higher valve lift for the same flow, thereby increasing the open area of the valve throat and reducing the percentage flow differences between valves.  
         [0024]    Function  52  refers to improving fuel economy by causing a slight amount of internal recycling of engine exhaust back through the opening intake valve at the end of the exhaust stroke. It is well known in the art that dilution of fresh fuel/air mix with exhaust gas can improve thermal efficiency and reduce NOX formation; indeed, such is the basis of external exhaust gas recirculation (EGR).  
         [0025]    Functions  54 ,  46 , and  56  also require a manifold vacuum.  
         [0026]    Function  54  is the well-known external recirculation of a portion of the engine exhaust (EGR) into the intake manifold, as just recited, and requires a positive pressure differential between the exhaust and intake manifolds.  
         [0027]    Function  46 , noted above, is the stripping of collected adsorbed fuel from a charcoal-filled canister in communication with a vehicle fuel tank. Fuel vapors are collected by the canister during refueling and are stripped into the engine subsequently, most conveniently in response to intake manifold vacuum.  
         [0028]    Function  56  refers to prevention of a full-torque condition in engine  12  of FIG. 1 in the event that the VVL system fails and the intake valves commence operation at maximum lift. The presence of vacuum control valve  16  in accordance with the invention permits the ECM to instantaneously convert engine control to conventional electronic throttle control of valve  16  by operator  31 , thereby avoiding a runaway vehicle.  
         [0029]    Referring to FIG. 4, a vacuum management system  58  in accordance with the invention includes a brake booster vacuum assist  60  connected to manifold  18  via a vacuum tube  62  including a check valve  64 ; a canister purge valve  66 ; an EGR valve  68 ; and a mass flow sensor  70  in entrance  17  to manifold  18 .  
         [0030]    System  58  takes into account the following flows, pressures, temperatures, positions, ratios, and relationships:  
         [0031]    F T =throttle valve flow  72 , or flow past valve  16   
         [0032]    F P =purge valve flow  74 , or flow past valve  66   
         [0033]    F E =EGR valve flow  76 , or flow past valve  68   
         [0034]    F V =intake valve flow  78 , or flow past intake valves  15   
         [0035]    T A =temperature  80  of the atmosphere outside the engine  
         [0036]    T M =temperature  82  within manifold  18   
         [0037]    T E =exhaust gas temperature  84   
         [0038]    P A =atmospheric pressure  86   
         [0039]    P B =brake booster pressure  88   
         [0040]    P E =exhaust gas pressure  90   
         [0041]    P M =manifold pressure  92   
         [0042]    {acute over (ω)}=engine speed  
         [0043]    θ=position of vacuum control valve  16   
         [0044]    X P =position  94  of purge valve  66   
         [0045]    X E =position  96  of EGR valve  68   
         [0046]    I=lift  98  of intake valves  15   
         [0047]    Thus:  
           F   V   =F   T   +F   E   +F   P   (Eq.1)  
         = f ( P   M   , {acute over (ω)} , I, T M )  (Eq. 1a)  
         [0048]    Flow across intake valve  15  is the sum of flows across vacuum control valve  16 , EGR valve  68 , and purge valve  66 , and is a function of manifold pressure, engine speed, intake valve lift, and manifold temperature.  
           F   T   =f (( P   M   /P   A ), θ,  T   M )  (Eq. 2)  
         [0049]    Flow across vacuum control valve  16  is a function of pressure drop across valve  16 , the position of valve  16 , and the manifold temperature.  
           F   E   =f (( P   M   /P   E ),  X   E   , T   E )  (Eq. 3)  
         [0050]    Flow across EGR valve  68  is a function of pressure drop across valve  68 , the position of valve  68 , and the temperature of the exhaust gas.  
           F   P   =f (( P   M   /P   A ),  X   P   , T   M )  (Eq. 4)  
         [0051]    Flow across purge valve  66  is a function of pressure drop across valve  66 , the position of valve  66 , and the manifold temperature.  
           P   M     —     DESIRED   =f ( P   B   , F   E     —     DESIRED   , F   P     —     DESIRED )  (Eq. 5)  
         [0052]    The desired manifold pressure (vacuum) is a function of brake booster pressure  88 , the desired EGR flow  76 , and the desired purge flow  74 .  
           F   T     —     DESIRED   =F   V     —     DESIRED −( F   E   +F   P )  (Eq. 6)  
         [0053]    The desired flow across valve  16  equals the desired flow across intake valve  15  minus the flows  76 , 74  across EGR valve  68  and purge valve  66 .  
         Θ DESIRED   =f (( P   M     —     DESIRED   /P   A ),  F   T     —     DESIRED )  (Eq. 7)  
         [0054]    The desired angular position of valve  16  is a function of the ratio of the desired manifold pressure to atmospheric pressure and the desired flow across valve  16 .  
         [0055]    For simplicity, ECM  20  is omitted from FIG. 4; however, it should be understood that ECM  20  is in communication with manifold pressure sensor  33  and with similar appropriate means (not shown) for measuring and transmitting T A , T M , T E , P A , P B , P E , {acute over (ω)}, θ, X P , X E , and I to ECM  20 .  
         [0056]    In a control method in accordance with the invention, all of the above relationships are measured on a test engine under simulated use conditions in an engine laboratory, and the relationships are numerically quantified and mapped, primarily for optimum fuel efficiency. From these data, algorithms are developed in known fashion and programmed into ECM  20 .  
         [0057]    The primary objective of vacuum management system  58  is to provide optimum flow across intake valves  15  at an optimum manifold pressure, P M     —     DESIRED , at any given time, taking into account all of the above factors and relationships. The ECM algorithm considers all of the above parameters, decides on an engine condition based primarily on load (inputted by operator  31 ), engine speed, and manifold temperature, and establishes a height of valve lift  98  and a desired flow across valves  15 , F V     —     DESIRED , for those conditions. The algorithm then establishes the desired air flow  72  across vacuum control valve  16 , F T     —     DESIRED , in accordance with Eqs. 6 and 1, at the desired manifold pressure, P M     —     DESIRED , and sets the position θ of valve  16  in accordance with the parametric maps provided from the laboratory determinations. As engine conditions change, for example, when purging of the fuel canister is complete and valve  66  is closed, ECM  20  automatically varies the lift of valves  15  and the position θ of vacuum control valve  16  to maintain the optimum flow and manifold pressure.  
         [0058]    A vacuum management system in accordance with the invention, such as system  58 , provides insurance against an inadvertent full-torque event. Engines throttled solely by VVL means, like engine  12  in FIG. 1, are vulnerable to unexpected full-torque conditions if the VVL control system fails and the intake valves assume a full-lift mode. The presence of vacuum control valve  16  in accordance with the invention permits ECM  20  to be programmed to switch throttle control to valve  16 , responsive to pedal input from operator  31  in essentially a conventional engine operating mode, until the VVL control means can be repaired.  
         [0059]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.