Abstract:
A fluid compression system includes a gaseous fluid compressor having a fluid inlet and a compressed fluid discharge, a prime mover coupled to the gaseous fluid compressor in driving relation, and a demand sensor positioned to measure a compressor demand and output a demand signal. An engine control unit is coupled to the prime mover and is operable to measure prime mover speed and prime mover power output. The engine control unit sets a variable maximum rated speed that maintains the engine power below a predetermined limit. A compressor controller unit is arranged to receive the demand signal and vary an operating speed based in part upon the demand signal. The operating speed is limited to the maximum rated speed set by the engine control unit.

Description:
BACKGROUND 
       [0001]    The present invention relates to fluid compressors. More specifically, the invention relates to control algorithms for air compressors. 
         [0002]    For positive displacement compressors, capacity (volumetric flow rate) is roughly proportional to an input speed provided by a prime mover. Power output required to maintain volumetric flow rate is dependent upon mass flow rate (volumetric flow rate multiplied by the air density). When ambient pressure decreases, such as when altitude increases, the density of the air decreases and the mass flow rate decreases. Since the amount of energy required by the air compressor is dependent upon mass flow rate, the amount of power used by the compressor element at a uniform speed decreases with decreased air density. 
       SUMMARY 
       [0003]    In one embodiment, the invention provides a fluid compression system that includes a gaseous fluid compressor having a fluid inlet and a compressed fluid discharge, a prime mover coupled to the gaseous fluid compressor in driving relation, and a demand sensor positioned to measure a compressor demand and output a demand signal. An engine control unit is coupled to the prime mover and is operable to measure prime mover speed and prime mover power output. The engine control unit sets a variable maximum rated speed that maintains the engine power below a predetermined limit. A compressor controller unit is arranged to receive the demand signal and vary an operating speed based in part upon the demand signal. The operating speed is limited to the maximum rated speed set by the engine control unit. 
         [0004]    In another embodiment the invention provides an air compression system that includes an air compressor including an ambient air inlet and a compressed air discharge, an engine coupled to the air compressor and operable to drive the air compressor to produce a flow of compressed air, and a receiver tank coupled to the compressed air discharge to receive the flow of compressed air and to maintain a volume of pressurized air. A demand sensor is positioned to measure a pressure of the receiver tank and is operable to generate a first signal related to the pressure of the receiver tank. An engine control unit is coupled to the engine to operate the engine at a desired operating speed in response to a control signal and is operable to measure engine speed and engine power to calculate a maximum engine speed at which the engine power is at a preset maximum value. A compressor control unit operable to generate the control signal to vary the desired operating speed in response to the first signal, the desired operating speed being limited to the maximum engine speed calculated by the engine control unit. 
         [0005]    In yet another embodiment, the invention provides a method of controlling a gas compressor system. The method includes setting a maximum engine speed at a factory preset maximum engine speed. The factory preset maximum engine speed corresponds to a maximum desired engine power output at a first atmospheric pressure. The method also includes coupling the engine to a compressor such that the engine operates at an operating speed to drive the compressor at a speed that is proportional to the operating speed, delivering a flow of compressed gas in response to operation of the compressor; and sensing an actual power output of the engine at the operating speed. The method further includes changing the maximum engine speed to a new maximum engine speed based on the sensed actual engine power output at the operating speed to maintain the engine output at or below the maximum desired engine output, sensing a compressed gas demand, varying the operating speed of the engine in response to the sensed compressed gas demand, and limiting the operating speed to the new maximum engine speed. 
         [0006]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0007]      FIG. 1  is a block diagram of an air compressor and compressor control system according to one aspect of the invention. 
           [0008]      FIG. 2  is a graph of compressor speed versus power at several ambient pressure levels. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawing. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
         [0010]      FIG. 1  illustrates an air compressor system  10  that includes a prime mover  14 , a compressor unit  18 , a receiver tank  22 , and a compressor control system  26 . The compressor unit  18  has an inlet  28  and a discharge  30 . An external load  32 , such as an air-operated tool, receives compressed air from the receiver tank  22 . 
         [0011]    The prime mover  14  provides motive force (e.g., torque) for driving the compressor unit  18 . The illustrated prime mover  14  may be a diesel internal combustion engine. The prime mover  14  may also be a gasoline internal combustion engine, a gas turbine, an electric motor, or other suitable prime mover. 
         [0012]    The compressor unit  18  receives torque from an output shaft  34  of the prime mover  14 . In some embodiments, the prime mover  14  and compressor unit  18  may be on a common shaft. In other embodiments, there may be a reduction arrangement between the prime mover and compressor to increase or decrease their relative speeds, as required. 
         [0013]    In the illustrated example, the compressor unit  18  is a positive displacement type, as opposed to a dynamic-type. Positive displacement air compressors work by filling an air chamber with air and then reducing the chamber&#39;s volume. Examples of positive displacement compressors include reciprocating piston, rotary screw, rotary vane, and scroll compressors. The illustrated compressor unit  18  is a rotary screw compressor. 
         [0014]    The compressor unit  18  is designed to deliver a volumetric flow rate (e.g., 1000 cubic feet per minute) of compressed gas (e.g., air) at an elevated pressure (e.g., 350 pounds per square inch). The compressed gas is discharged to the receiver tank  22 , where the compressed gas remains available for use by the external load  32 . In some constructions, filters, oil-separators, moisture separators and the like are disposed between the compressor unit and the external load. 
         [0015]    The compressor control system  26  includes an engine control unit (ECU)  38  and a compressor control unit (CCU)  42 , and may include an ambient pressure sensor  46 , a receiver tank pressure sensor  50 , and an engine speed sensor  52 . Although these components of the compressor control system  26  are illustrated as being separate, it should be appreciated that two or more of the components may be combined. For example, in some embodiments the CCU  42  and ECU  38  may be integrated into a single combined controller. In some embodiments, the ambient pressure sensor may be integrated into an ECU or CCU housing or board. In still other embodiments, the ECU may monitor engine speed without a separate speed sensor, such as by monitoring the ignition signals. 
         [0016]    The receiver tank pressure sensor  50  detects pressure within the receiver tank  22  and generates a first signal related to the receiver tank pressure. The first signal is received by the CCU  42  and provides an indication of when compressor speed (and therefore engine speed) should be increased to meet demand. In other embodiments, this function could be served with a load pressure sensor at the external load  32 , a discharge pressure sensor at the compressor discharge  30 , or using other control schemes that assure demand is met. It should be noted that in systems that do not employ a tank, the tank pressure sensor  50  may be replaced by another sensor that senses and indicates demand and allows the CCU  42  to adjust the compressor output to meet that demand. 
         [0017]    The CCU  42  monitors compressor-related parameters, including receiver tank pressure as indicated by the receiver tank pressure sensor  50 . The ECU  38  monitors engine related parameters such as engine speed, engine torque, and engine power and receives commands from the CCU  42  to run at certain speeds under certain conditions. ECU  38  and CCU  42  communicate via a controller area network (CAN) communication protocol, or other protocol as may be appropriate. 
         [0018]    The ECU  38  controls and adjusts performance of the engine (air/fuel mixture, power output, speed, etc.) to meet the demands placed on it. However, the ECU  38  is programmed or configured with limits based upon, for example, engine mechanical limitations. Maximum rated speed and maximum rated power output are examples of such limits. The ECU  38  may be configured to prevent the prime mover  14  from exceeding 95% of a maximum rated power output, for example. In some embodiments, the prime mover may incorporate a mechanical governor to prevent the prime mover from exceeding these limits. 
         [0019]    In one mode of operation, the compressor unit  18  may deliver a desired volumetric capacity (e.g., 1000 cubic feet per minute (CFM)) of compressed air at a pressure (e.g., 350 pounds per square inch (PSI)) at a first operating speed (e.g., 1800 RPM) and at a standard atmospheric pressure. The prime mover  14  (e.g. a diesel engine) has a maximum rated power level which is greater than the power level required at the first operating speed of the compressor. In this first operating mode, the speed of the prime mover  14  (and thereby compressor speed) is controlled by commands from the CCU  42 . 
         [0020]    As air demand from the external load  32  (e.g., a pneumatic driven power tool) increases, receiver tank pressure (as indicated by the receiver tank pressure sensor  50 ) decreases and the CCU  42  instructs the ECU  38  to increase engine speed, up to the maximum operating power limit of the prime mover (e.g., 95% of rated maximum power). When the maximum power level is reached, no additional flow can be delivered. However, if the ambient pressure is below the ambient pressure used to rate the prime mover (e.g., the altitude is higher), the power output of the prime mover  14  is actually lower than the rated power at that speed. In other words, the engine is actually not operating at its rated power output capacity due to the lower density of the air. 
         [0021]    In prior compressor systems, the effects of changing altitude were not considered. Thus, the ECU prevented operation of the compressor above a speed that, at standard atmospheric pressure required the rated power of the prime mover  14 , even when the CCU indicated that the demand was not being met. In the preceding example, the ECU was programmed to inhibit engine operation beyond 95% power. The ECU achieved this by limiting the speed of the prime mover. However, at the higher altitude, the prime mover might actually be operating at only 85%. Thus, as altitude increases, the prime mover  14  operates at a decreasing percentage of its maximum power output limit and capacity of the compressor system  10  is unused. 
         [0022]    The present invention provides the ability to compensate for altitude changes to the air compressor system  10 . Rather than limiting the speed of the prime mover based on a maximum speed rating that corresponds to a maximum power level at standard atmospheric conditions, the system measures the actual prime mover power level and adjusts the maximum rated prime mover speed to maintain the maximum power level at some predetermined value such as 95% of rated output. For example, the CCU  42  may be configured to increase the maximum operating speed until the power output (as determined by the ECU) is again 95% of the engine&#39;s maximum rated power output. Recalling that power is roughly proportional to speed for constant discharge pressures, the increase in power output from 85% to 95% results in an approximately 12% increase in airflow. Similarly, prime mover speed would increase within acceptable limits (e.g., from 1800 RPM to approximately 2016 RPM, with a maximum rated speed of 2100 RPM or greater). 
         [0023]    An additional controller or the existing ECU  38  may read the power setting of the engine and command the ECU  38  to vary the speed to load the engine to a preset horsepower level. In another embodiment, the CCU  42  could be programmed so that absolute horsepower limits can be selected instead of a percentage of available power. An absolute upper engine speed could be programmed to prevent damage to the engine if the engine speed were to be too high (e.g., greater than 2100 RPM). A mode setting may be used to the select an ambient pressure compensated or uncompensated mode. 
         [0024]      FIG. 2  illustrates graphically an example of the pressure compensated operating mode.  FIG. 2  is a graph of compressor speed versus power, for three different ambient pressures P 1 , P 2  and P 3 , where P 1 &gt;P 2 &gt;P 3 . As ambient pressure decreases from P 1  to P 2  to P 3 , the maximum operating speed increases, without the compressor exceeding 95% of the maximum power output and without exceeding the maximum rated speed of the prime mover  14  or the compressor  18 . 
         [0025]    The foregoing example provides for ambient pressure or altitude compensation without the need to measure the actual ambient pressure. In other constructions, an ambient pressure sensor is provided and the data from that sensor is used to compensate the prime mover operation. 
         [0026]    The ambient pressure sensor  46  may be described as an altitude sensor, as ambient pressure generally decreases as altitude increases. However, it should be appreciated that ambient pressure at a given location and altitude also varies depending upon current atmospheric conditions, such as low or high pressure weather systems. The ambient pressure sensor  46  detects an ambient pressure and generates an ambient pressure signal related to the ambient pressure. In the illustrated embodiment, the ambient pressure signal is received by the CCU  42 . In other embodiments, the signal from the ambient pressure sensor  46  may be received by the ECU  38 . 
         [0027]    For a diesel engine or other prime mover in combination with a positive displacement compressor, power is roughly proportional to speed for constant discharge pressures at constant ambient pressure. Because speed and power are related, the maximum rated power output may be based upon a fixed RPM limit for a nominal or standard ambient atmospheric condition. Alternatively, the maximum rated power may be based upon a sliding scale RPM limit that varies with ambient atmospheric pressure. A database or algorithm for determining an RPM limit at an ambient atmospheric pressure can be provided. 
         [0028]    Thus, the invention provides, among other things, a new fluid compressor and method of operating a fluid compressor. Various features and advantages of the invention are set forth in the following claims.