Abstract:
A controller for a vehicle pneumatic system loads a compressor responsive to engine power absorption by the engine and air storage facility air pressure readings up to a pressure limit. The controller is responds to power demand on the engine in excess of an engine speed related global maximum and air pressure readings down to a global minimum for unloading the compressor. The controller provides for loading and unloading the compressor to maintain storage facility pressure within an operating maximum and an operating minimum which lie between the global maximum and global minimum pressures. The controller responds to air pressure readings reaching or falling below the global minimum for loading the compressor and to air pressure readings reaching or exceeding the global maximum for unloading the compressor.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The technical field relates generally to motor vehicle compressed air systems and more particularly to control over an internal combustion engine driven compressor in a compressed air system. 
         [0003]    2. Description of the Technical Field 
         [0004]    In order to meet demands for improved motor vehicle fuel economy engineers have taken steps to reduce energy consumption attributable to parasitic motor vehicle systems such as power steering systems, air conditioning systems and pneumatic systems. Pneumatic systems have long been used with motor vehicle air brake systems and recent developments have expanded the scope of their application on vehicles. A pneumatic system architecture utilizes a piston in cylinder type compressor pump which is mechanically engaged to the thermal/internal combustion engine. The compressor pump feeds a compressed air storage system (represented here as a simple tank) through a check valve. The compressor draws air through an intake valve which may be closed to allow compression of air during a piston compression stroke and opened to allow air to be drawn into the compressor during a piston down stroke. 
         [0005]    Air pressure in the storage system is kept within minimum and maximum bounds by discontinuing delivery of air to the storage system when pressure in the storage system reaches a maximum allowed level and resuming air delivery when pressure falls to a minimum allowed value or is below the minimum allowed value, as may occur on engine start where the vehicle has sat for some time. In simple systems, when fully pressurized, the compressor pump may continue to operate and compressed air discharged to the environment. In such an arrangement the compressor pump absorbs up to two or more horsepower from the engine. 
         [0006]    In order to avoid the waste of driving a compressor pump when pressure in the storage system is at its maximum allowed value, some systems provide for the compressor to be mechanically disengaged or unloaded. Mechanical disengagement can involve use of a clutch and can impose a penalty in terms of added weight and relative complexity compared with systems for unloading the compressor. Unloading a compressor interrupts the compression cycle. To do this a compressor unloader holds the compressor&#39;s intake valve open when compressed air storage facilities are fully pressurized. The air compressor piston continues to cycle, but simply draws and pushes air in and out through the intake valve. Little or no compression of air occurs and the load imposed by the air compressor on the engine is reduced to relatively minor frictional losses. 
         [0007]    The compressor unloader itself is a piston which is displaced to hold open the compressor intake valve. Pneumatic compressor unloaders utilize compressed air from system storage the flow of which is triggered when air pressure in storage reaches its maximum allowed value. When air pressure in the storage system falls to or is below the minimum allowed value the compressor unloader is discharged and the air compressor intake valve operates to allow the air compressor to compress air. 
       SUMMARY 
       [0008]    A vehicle pneumatic system includes an engine, an air compressor coupled to the engine to be driven by the engine, an air storage facility connected to receive air pumped by the air compressor, an air compressor unloader, an air pressure sensor for generating pressure readings from the air storage facility, sensors for indicating power output from or absorption by the engine, a sensor for indicating power requested from the engine and a tachometer for measuring engine speed. A controller is provided responsive to indication of power absorption by the engine and air pressure readings from the air storage facility for turning the air compressor unloader off. The unloader remains off up to a global pressure limit. The controller is further responsive to indication of power demand in excess of a engine speed related maximum limit for turning the air compressor unloader on. It can remain on down to a global pressure minimum. The controller is responsive to air pressure readings reaching or falling below the global minimum for turning the air compressor unloader off and to air pressure readings reaching or exceeding the global maximum for turning the air compressor unloader on notwithstanding the engine state as long as it is turning and generating or absorbing power. The controller is otherwise responsive to pressure air pressure readings and indication of engine operation for turning the air compressor unloader on and off to maintain air pressure in the air storage facility within a normal operating maximum and a normal operating minimum. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of a truck which is equipped with an engine driven pneumatic system. 
           [0010]      FIG. 2  is a high level block diagram of major vehicle systems for the truck of  FIG. 1 . 
           [0011]      FIG. 3  is a block diagram of a pneumatic system compressor control system. 
           [0012]      FIG. 4  is a state diagram. 
           [0013]      FIG. 5  is a chart relating air pressure in a truck pneumatic system over time. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    In the following detailed description, like reference numerals and characters may be used to designate identical, corresponding, or similar components in differing drawing figures. Furthermore, example sizes/models/values/ranges may be given with respect to specific embodiments but are not to be considered generally limiting. 
         [0015]    Referring now to the figures and in particular to  FIG. 1 , a truck  101  supporting a pneumatic system  51  (see  FIG. 2 ) is shown. Truck  101  is typically a diesel engine equipped vehicle having a pneumatic brake system. Truck  101  is supported by wheels  104  which support a chassis  102  which in turn carries a cab  105 . Cab  105  is provided with an engine compartment  109 , a greenhouse  110  and a door  103 . 
         [0016]      FIG. 2  is a high level schematic of major vehicle systems installed on truck  101 . A pneumatic system  51  includes an air compressor  60 , a compressed air storage facility  54  and one or more compressed air utilizing systems/loads  46  such as a pneumatic brake system. Control over the pneumatic system  51  may be integrated with other vehicle systems using network technology such as a controller area network (CAN). A CAN may be implemented using an SAE (Society of Automotive Engineers) J1939 compliant public data link  18 . Several controllers are coupled to data link  18  for the exchange of data and commands. 
         [0017]    Among the controllers coupled to the public data link  18  are an electronic system controller (ESC)  24 , a type of body computer, a gauge cluster controller  14 , an engine control module (ECM)  20 , a transmission controller  16  and an anti-lock brake system (ABS) controller  43 . ECM  20  supplies control signals relating engine  30  control such as signals controlling operation of fuel injectors (not shown) and receives signals from engine  30  mounted sensors such as engine speed (RPM) from a tachometer (not shown). An engine  30  speed measurement may be made by the transmission controller  16  based on signals from a transmission mounted tachometer (not shown). In this case the engine speed signal is transmitted over data link  18  and may be received by ECM  20  and ESC  24 . Fuel flow and engine speed relate to the load being placed on the engine  30  and may be used by the ECM  20  to generate signals named “MF_TOT”, relating to maximum allowed fuel flow and “MF_SLM”, relating to maximum fuel allowed for smoke limiting. ESC  24  usually provides a connection to an accelerator or throttle position sensor and generates a “throttle position (TP)” signal relating thereto. This signal may be generated autonomously by ESC  24  in response to a speed control setting. In addition, ECM  20  is connected to receive a signal from an air pressure sensor  57  which relates to the pressure of gas/air stored in the air storage facility  54 . 
         [0018]    Referring to  FIG. 3 , control elements for an engine  30  driven air compressor  60  are illustrated in greater detail. Air compressor  60  may be any type of positive displacement compressor with a pneumatically controlled ON/OFF (LOAD/UNLOAD) feature such as a compressor or “intake valve” unloader  38 . A compressor unloader  38  includes an unloader piston  39 . When compressed air is fed into the cavity  41  above the unloader piston  39  the unloader piston moves down and holds an air compressor  60  intake valve  42  open. This stops compression in and discharge of air from the air compressor  60 . 
         [0019]    Air compressor  60  is driven by a gear drive  34  connected between the crankshaft  32  of engine  30  and the air compressor  60 . In the present embodiment air compressor  60  is based on a piston (not shown) which reciprocates in a cylinder. During down strokes of the piston an intake valve  42  in an air intake inlet  44  opens allowing air to be drawn in the air compressor  60 . During compression strokes the intake valve  42  is usually closed resulting in air being compressed until pressure reaches a level allowing air to be discharged through a check valve  36  to the compressed air storage facility  54 . In some applications air intake inlet  44  may receive air from a engine intake air boost system  48  such as a turbocharger or a supercharger. 
         [0020]    In order to reduce the load imposed on engine  30  by air compressor  60  when air pressure in the compressed air storage facility  54  reaches a maximum allowed value, air compressor  60  can be “unloaded” by compressor unloader  38  which operates to hold the intake valve  42  open until pressure declines to a minimum allowed value, unless engine  30  begins to absorb power. With the intake valve  42  held open air simply cycles in and out of the air intake inlet  44 . 
         [0021]    Compressor unloader  38  is itself a pneumatic device which operates using compressed air from the compressed air storage facility  54 . Compressed air is delivered to compressor unloader  38  by a three way valve  26  from the compressed air storage facility  54 . Three way valve  26  may also be operated to discharge compressed air from compressor unloader  38  to the environment/atmosphere. Three way valve  26  is mechanically actuated cycled by control signals applied to a solenoid  28 . 
         [0022]    Air compressor  60  is loaded or “turned on” by connecting cavity  41  to the ambient environment (that is, applying zero gauge pressure). To unload or “turn-off” air compressor  60  cavity  41  is opened to the compressed air storage facility  54  through three way valve  26 . Provided that compressed air at sufficient pressure is in the compressed air storage facility/tank  54 , unloader piston  39  moves down under pressure which allows the intake valve  42  to operate. If for some reason three way valve  26  fails with the cavity  41  open to the compressed air storage facility  54  and air pressure is below minimum levels to operate the compressor unloader  38  the compressor unloader  38  will no longer hold the intake valve  42  open. 
         [0023]    The broad pressure range maintained in the compressed air storage facility  54  range from about 90 PSI (P_Min_Global) to about 150 PSI (P_Max_Global). P_Max_Global is a maximum limit on pressurization for the compressed air storage facility  54 . P_Min_Global is a minimum pressure selected to provide for meeting demands from vehicle pneumatic systems  46 . P_Max and P_Min for the compressed air storage facility  54  are, respectively, lower and higher than the global values and are termed here the normal operating maximum and operating minimum pressure limits. These values avoid excessive ON/OFF cycling of the air compressor  60  while recognizing that increasing air pressure in the air storage facility  54  results in increasing loading on the engine  30  as long as the compressor remains loaded. In broad terms, air pressure in the compressed air storage facility  54  is allowed to exceed P_Max only in response to opportunities to exploit engine  30  absorption of vehicle kinetic energy (engine braking). Slowing the vehicle is then used as a source of power to drive air compressor  60 . Air pressure is allowed to fall below P_Min under circumstances where engine operating variables indicate the engine  30  is operating under a particularly heavy load and is done to avoid diversion of power from the engine to drive the air compressor  60  under those circumstances until air pressure reaches the global minimum. This usually occurs when a vehicle is moving slowly and any braking demand is likely to impose a minimal drain on compressed air resources. 
         [0024]    The pressure signal from pressure sensor  57  is used as a basic feed-back signal by ECM  20  to maintain system pressure. ECM  20  utilizes additional engine related operating variables (engine speed (RPM), Throttle Position (TPS), Total Fuel (MF_TOT), Maximal Fuel Allowed for smoke limiting (MF_SLM)) to determine air compressor  60  ON/OFF state to implement finer control over pressure and to allow capture of energy otherwise lose during vehicle braking. In effect MF_TOT and MF_SLM function as the present or immediate power limits on engine  30  output at given engine speeds (RPM). Control of the compressor unloader  38  is implemented by application of an ON/OFF signal to solenoid  28  to position the three way valve  26  which in turn controls loading and unloading of air compressor  60 . 
         [0025]    The effect of engine operating variables on air compressor  60  loading may be seen in reference to the state diagram of  FIG. 4  and the chart of  FIG. 5 . Air compressor  60  loading and unloading (ON and OFF conditions) depend on engine state (ENG_STATE=1, ENG_STATE=2, ENG_STATE=3) and tank pressure. ENG_STATE=1 ( 80 ) corresponds to engine braking and occurs when engine speed (RPM) is above idle speed and throttle/accelerator position (TPS) is close to or at zero. Under these circumstances air compressor  60  may be kept loaded when air pressure exceeds P_MAX up to P_MAX_GLOBAL. This allows the air compressor to recapture energy during engine braking of the vehicle. Usually the air compressor  60  is unloaded above P_Max because operation at increasingly high pressures becomes an increasing load on engine  30 , but air pressure is allowed range up to P_MAX_GLOBAL as long as the engine  30  is not using fuel. Air compressor  60  may also shift from an unloaded to a loaded state during engine braking when air pressure is between P_MIN and P_MAX. State changes from ENG_STATE=3 ( 84 ) directly to ENG_STATE=1 ( 80 ) are not allowed in this embodiment. 
         [0026]    ENG_STATE=3 ( 84 ) corresponds to high power demands on the engine, such as occur under hard acceleration. ENG_STATE=3 ( 84 ) occurs when the current value of operating variable DLT_SLM_MF falls below a minimum limit. DLT_SLM_MF is the difference between MF_SLM and MF_TOT. When DLT_SML_MF falls below its minimum limit, DLT_MF_CMD_DISABLE, air pressure in the air storage facility  54  is allowed to drop all the way to the lower global limit, P_MIN_GLOBAL, without loading the air compressor  60 . However, if air pressure drops below P_MIN_GLOBAL air compressor  60  is loaded notwithstanding current engine operating conditions. In this way the compressor load on engine  30  is minimized during hard acceleration events or hill climbing. This can reduce the possibility of using boosted air during acceleration, improve engine acceleration and reduce smoke emissions. ENG_STATE=2 ( 82 ) is a steady state indicated for all engine  30  conditions not included in the other states. The compressor is unloaded and loaded as air pressure varies between P_MAX and P_MIN, respectively.