Abstract:
An altitude compensating turbocharger wastegate control system operates by bleeding air from an intake air line extending from an intake manifold of an engine to a wastegate actuator to raise the intake manifold pressure at which the wastegate is opened when the engine is operating at high altitudes. The control system operates in response to a comparison of barometric air pressure sensed by a pressure sensor and a number of predefined ranges of sensed pressure in a memory of an engine controller to determine whether no, full, or an intermediate amount of bleeding should take place in order to maintain the desired turbocharger output at high altitudes.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an altitude compensating wastegate control system for a turbocharger which automatically compensates for decreased air density and barometric pressure at high altitudes by bleeding off, in preselected steps, the pressure in an air line supplying intake manifold pressure to an actuator of the turbocharger wastegate, thereby raising the intake manifold pressure level required before the wastegate of the turbocharger opens. The preselected steps are selected by the control system in response to sensed barometric pressure to produce different bleed configurations without requiring a dedicated feedback circuit. 
     THE PRIOR ART 
     Although the intake manifold pressure requirements of a turbocharged engine are fairly constant across the engine speed range at full load, the output pressure is highly dependent on engine speed because the compressor of the turbocharger is driven by a turbine powered by flow from the exhaust manifold. Inasmuch as a turbocharger must produce an adequate manifold pressure in the engine at lower speeds, a turbocharger wastegate is used to maintain an upper limit on intake manifold pressure of the engine at higher engine speeds by bypassing a portion of the exhaust flow around the turbocharger turbine to avoid engine damage due to excessive cylinder pressure. Operation of the wastegate is controlled by a spring controlled actuator which is fed pressurized air from the intake manifold, so that pressurized air fed to the actuator will cause the normally-closed wastegate to open as the spring force is overcome. 
     However, the low altitude parameters for controlling wastegate operation are not desirable for controlling engines at higher altitudes, where air density is lower because there will be less oxygen per piston stroke available for combustion resulting in less power and poor combustion. In fact, higher intake manifold pressure and increased mass air flow are desirable. 
     Previously, various altitude compensation systems for controlling turbocharger wastegate operation at high altitudes have been taught. Such systems are disclosed, for example, in U.S. Pat. Nos. 3,195,805, 4,476,682, 4,492,519, 4,790,139, 4,848,086, and 5,036,663. 
     As an example, U.S. Pat. No. 4,476,682 includes a bleed valve comprising an evacuated bellows exposed to atmospheric pressure and located in an intake manifold pressure sensing line. The bellows expands as altitude increases and releases pressure from the sensing line to atmosphere, reducing pressure in the sensing line and, thereby raising the intake manifold pressure at which the wastegate opens so that turbocharger operation does not decrease. 
     The electronic systems disclosed in the other references require feedback for operation, requiring the presence of a separate circuit and/or sensor for providing such feedback. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the invention to provide an altitude compensating wastegate control system for a turbocharger that does not require feedback or a separate circuit and/or sensor. 
     This object, as well as others, is met by the wastegate actuator control system of the present invention which utilizes available input from a barometric pressure sensor for operating a bleed-off valve system associated with the air supply line from the intake manifold which controls the operation of the wastegate actuator. The control system conFIG.s the bleed-off valve system in one of a plurality of predefined stepped settings, to compensate for altitude effects, the altitude being derived from sensed barometric air pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become more apparent upon perusal of the detailed description thereof and upon inspection of the drawings in which: 
     FIG. 1 is a schematic diagram of a turbocharged engine incorporating the altitude compensating wastegate control system for a turbocharger made according to the teachings of the present invention. 
     FIG. 2 is a chart defining a plurality of predefined settings for the wastegate control system of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1 in greater detail, there is illustrated therein a simplified diagram of an altitude compensating wastegate control system for a turbocharger generally identified by reference numeral  10  made in accordance with the present invention. 
     As is known in the engine art, a turbocharger  12  for a diesel engine  13  comprises a compressor  12   a  engaged with an intake air manifold  14  and a turbine  12   b  engaged with an exhaust air manifold  16  of the engine  13 . The turbine  12   b  is driven by exhaust gas flow and is connected to the compressor  12   a  so that the compressor  12   a  is driven to provide a larger volume of pressurized air flow to the engine  13 , as required for optimum engine operation. The supply of pressurized air from the turbocharger  12  must be controllable to accommodate various engine operating conditions from low speed and low load conditions, involving low exhaust and low intake air flow, to high speed, high load conditions, involving high exhaust and high intake air flow. 
     Conventionally, a wastegate  20 , generally incorporated as part of the turbocharger  12 , may be positioned across an outlet  22  of an exhaust bypass  24  which branches off the exhaust passage  16  at a point upstream of the turbine  12   b . The wastegate  20  is normally closed but is actuated at high intake manifold pressures, as necessary, to siphon off air fed to the turbocharger  12  to avoid excessive intake manifold pressure in the engine  13  which can cause adverse effects on the engine by increasing the peak cylinder pressure. 
     One of the variables affecting the operation of engine  13  is the altitude at which the engine  13  is operating. The altitude affects the barometric pressure and the air density and, as the pressure and density decrease, there must be compensation to optimize the operation of engine  13  by increasing the amount of turbocharging so that a sufficient amount of air flow will be present to ensure adequate combustion and delivery of the desired engine power. 
     To compensate for the decreased air density and barometric pressure at higher altitudes and to maintain an optimum flow of air from the turbocharger  12  to the engine  13 , the wastegate  20  must remain closed until a higher intake manifold pressure is attained. In the control system  10  of the present invention, this is accomplished by decreasing the flow of pressurized intake air to an actuator  26  of the wastegate  20 . 
     It will be understood that, in this embodiment, the actuator  26  includes a rod  28  that mechanically engages the wastegate  20  at one end  30  and engages a diaphragm  32  extending across the actuator  26  at another end  33  thereof. The diaphragm  32  is spring-biased toward the wastegate  20  in a manner such that extension of a biasing spring  34  maintains the wastegate  20  closed. 
     Extension of the biasing spring  34  is controlled by the intake manifold air pressure that is conducted into an opposing air chamber  36 , within actuator  26  to oppose the force of the spring  34 . The air is supplied to the chamber  36  via an air line  38  connecting to the intake air manifold  14  at a position downstream of the turbocharger  12 . When the intake air pressure in line  38  is low, the wastegate  20  is closed by spring  34  and all of the exhaust gas within the exhaust manifold  16  is used to drive the turbine  12   b  of the turbocharger  12 . 
     When the intake manifold pressure is sufficiently high to overcome the opposing force of the spring  34 , the wastegate  20  is opened, allowing a portion of the exhaust flow to bypass the turbine  12   b  of the turbocharger  12 , slowing the operation the compressor  12   a  and decreasing the pressure and flow of air to the intake manifold of the engine  13  and to the air line  38 . 
     To provide altitude compensation, the control system  10  of the invention provides at least two bleed lines  40   a  and  40   b , each having a respective valve  42   a ,  42   b  therein, for serially bleeding off the pressure in intake air line  38  into the ambient environment. The bleed lines are configured so that the first upstream bleed line  40   a  has an orifice diameter smaller than that of an orifice  40   c  in the intake air line  38  and so that the second, downstream bleed line  40   b  has an orifice diameter smaller than that in the upstream bleed line  40   a.    
     It will be understood that the intake air line  38  branches off the intake air manifold  14  at a point downstream of the turbocharger  12  and thus the line  38  carries pressurized air exiting the turbocharger  12 , and that the turbocharger  12  is working harder at such high altitude. Because of a false high pressure created in the intake air line  38  by the increase in turbocharger  12  output, if no bleed of the actuator air line  38  were possible, the wastegate  20  would open, decreasing turbocharger output to the engine  13  with an inherent loss of power and efficiency thereof. 
     Each valve  42   a  and  42   b  is controlled by an electronic driver  44   a  and  44   b , respectively, with the actuation of each driver  44  being controlled by an engine control module  50  that processes a barometric pressure signal from a pressure sensor  51  and which incorporates a comparator  52  and a memory  54 . The memory  54  includes therein predetermined, specific parameters for defining reference pressures for low, medium, and high altitude operation of the wastegate control system  10 . 
     Thus, the logic of the comparator  52  can access the baseline reference parameters, determine altitude as a function of sensed barometric pressure and, based on these values, can compare these sensed values to predefined values of operational parameters for the system  10  stored in memory  54  and determine whether bleeding of the pressure in the air line  38  is required, and, if so, what degree of bleeding is required. If bleeding is required, the valves  42   a  and  42   b  are opened, singly or in combination, to provide the appropriate degree of bleeding, all without feedback from the engine sensors. 
     Turning now to the chart  100  of FIG. 2, it will be seen that the memory  44 , in the present embodiment includes parameters for four possible state settings. 
     State 1 exists when the sensed barometric pressure is less than that defined in the memory  54  as the reference low altitude pressure. In this state, since the air pressure indicates that the engine  13  is operating at low altitude, the valves  42  in the bleed lines  40  are maintained closed, with no bleeding taking place. Thus, all air in the intake line  38  is fed to the wastegate actuator chamber  36 , and the wastegate  20  is operational. 
     State 2 exists when the sensed barometric pressure is above the reference low altitude pressure but below a reference medium altitude pressure in the stored memory  54 . In this state, a small degree of bleed would be required to raise the intake manifold pressure at which the wastegate  20  opens and thereby compensate the engine to provide similar air flow from the turbocharger  12  as at low altitude. To provide such small degree of bleed, the comparator  52  would activate the driver  44   a  for valve  42   a , allowing bleeding of air through the smallest orifice  40   a  available. 
     State 3 exists when the sensed barometric pressure is above that defined as a reference medium altitude pressure in the stored memory  54 . Here, a slightly greater degree of bleed is required and the comparator  52  activates the driver  44   b  for valve  42   b , allowing bleeding through the next larger orifice  40   b  available. 
     State 4 exists when sensed barometric pressure is above the reference high altitude pressure in the stored memory  54 . Here, a maximum amount of bleed would be required to keep the wastegate  20  closed until a higher manifold pressure is obtained, and both valves  42   a  and  42   b  would be opened. 
     As described above, the present invention provides a number of advantages, some of which have been described above and others of which are inherent in the invention. Also, modifications may be proposed without departing from the inventive teachings herein. Accordingly, the scope of the invention is only to be limited as required by the accompanying claims.