Abstract:
A method and apparatus for conducting dynamometric testing of an internal combustion engine at a test site under a simulated atmospheric pressure that differs substantially from an actual ambient atmospheric pressure existing at the test site. The internal combustion engine has an air inlet for supplying an intake airflow for combustion within the internal combustion engine and an exhaust outlet for exhausting an exhaust flow exiting from the internal combustion engine. The method includes the steps of subjecting the air inlet to the simulated atmospheric pressure, subjecting the exhaust outlet to the simulated atmospheric pressure and operating the internal combustion engine while both of the air inlet and the exhaust outlet are subjected to the simulated atmospheric pressure. The apparatus includes an exhaust pressure controller for maintaining the exhaust outlet of the internal combustion engine substantially equal to a determined exhaust pressure during operation of the internal combustion engine and an intake pressure controller for maintaining the air inlet of the internal combustion engine substantially equal to a determined intake pressure during operation of the internal combustion engine.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a method and apparatus used to test the operational performance (i.e., for dynamometric testing) of an internal combustion engine under various ambient atmospheric pressures, thereby simulating operation of the engine at various altitudes. More particularly, the invention pertains to a method and apparatus that allow dynamometric testing of an internal combustion engine at various ambient atmospheric pressures, without requiring that the entire engine be enclosed within a barometric chamber of controlled pressure. 
     BACKGROUND OF THE INVENTION 
     The following background information is provided to assist the reader in understanding the invention described and claimed herein. Accordingly, any terms used herein are not intended to be limited to any particular narrow interpretation, unless specifically so indicated. 
     The manufacturers of modern vehicles powered by an internal combustion engine subject the vehicles to various testing procedures. One such testing procedure that is typically performed is “dynamometric testing”, which involves running the engine under actual or simulated likely-to-be encountered conditions, while simultaneously testing and measuring various parameters. 
     In one sense, an internal combustion engine can be viewed as an air pump that also produces rotational power. Accordingly, the characteristics and performance of an internal combustion engine can be significantly altered by a change in the ambient atmospheric pressure at which the engine is operated. For example, according to Boyle&#39;s Law, air density varies directly with respect to atmospheric pressure and inversely with respect to atmospheric temperature, i.e., ρ(air density)=P/RT. Whereas a typical ambient atmospheric pressure at sea level is on the order of 100 kPa (i.e., kilopascals), a typical ambient atmospheric pressure in the location of Denver, Colo., U.S.A. is typically on the order of 80 kPa, or about 20% less that at sea level. With other factors remaining equal (e.g., ambient temperature and humidity), this results in an engine “at altitude” (e.g., in Denver) receiving a 20% less charge of oxygen with each intake stroke, given the same engine speed, throttle angle, EGR percentage, etc. Additionally, the internal combustion engines of most modern vehicles adjust, usually by software, the fuel delivery based on the ambient atmospheric pressure, sometimes directly measured, but usually estimated from other measured parameters. Accordingly, at altitude, the maximum power output of the engine can be significantly reduced. 
     Increasingly, the operation of a modern internal combustion engine vehicle is controlled by microprocessor software. Apart from the reduced maximum power output at altitude, there are a substantial number of factors in the vehicle&#39;s software that are influenced by the ambient atmospheric pressure. For all of these reasons, it has been customary for vehicle manufacturers to dynamometrically (i.e., operationally) test their engines under conditions of varying atmospheric pressure. One manner in which vehicles have been traditionally tested under reduced atmospheric pressures is to actually operate the vehicles at altitude, e.g., in Denver, up Pike&#39;s Peak, etc. For more preliminary testing, manufacturers have also used so-called “dynamometric chambers”. Such dynamometric chambers are closed barometric cells in which a lower than ambient pressure can be maintained. The engine is dynamometrically tested (run under various operating loads, conditions, etc.) within the chamber. 
     However, such dynamometric chambers can be expensive to build, operate and maintain. Since a rather large pressure differential must be maintained across the boundaries of the pressure cell, a dynamometric chamber is similar to a diving bell, requiring substantial and expensive structural support. 
     OBJECTIVES OF THE INVENTION 
     Accordingly, one objective of the present invention is the provision of a method and apparatus for the dynamometric testing of an internal combustion engine under varying ambient atmospheric pressures without requiring the building, operation or maintenance of a cumbersome and expensive barometric cell. 
     Another objective of the invention is the provision of a method and apparatus for the dynamometric testing of an internal combustion engine under varying ambient atmospheric pressures that is relatively inexpensive in construction and reliable in operation. 
     Yet another objective of the present invention is the provision of a method and apparatus for the dynamometric testing of an internal combustion engine under varying ambient atmospheric pressures which is, on the whole, safer than previously used barometric chambers, since pressures are controlled only across the inlet and output interfaces of the engine system, as opposed to over the entire surface of a dynamometric chamber. Therefore, the overall pressure-induced forces acting on the control surfaces are considerably reduced. 
     In addition to the objectives and advantages listed above, various other objectives and advantages of the invention will become more readily apparent to persons skilled in the relevant art from a reading of the detailed description section of this document. The other objectives and advantages will become particularly apparent when the detailed description is considered along with the drawings and claims presented herein. 
     SUMMARY OF THE INVENTION 
     The foregoing objectives and advantages are attained by the various embodiments of the invention summarized below. 
     In one aspect, the invention generally features a method for conducting dynamometric testing of an internal combustion engine at a test site under a simulated atmospheric pressure that differs substantially from an actual ambient atmospheric pressure existing at the test site. The internal combustion engine has an air inlet for supplying an intake airflow for combustion within the internal combustion engine and an exhaust outlet for exhausting an exhaust flow exiting from the internal combustion engine. The method includes the steps of subjecting the air inlet to the simulated atmospheric pressure, subjecting the exhaust outlet to the simulated atmospheric pressure and operating the internal combustion engine while both of the air inlet and the exhaust outlet are subjected to the simulated atmospheric pressure. 
     In another aspect, the invention generally features an altitude simulator for dynamometer testing for conducting dynamometric testing of an internal combustion engine at a test site under a simulated atmospheric pressure that differs substantially from an actual ambient atmospheric pressure existing at the test site. The internal combustion engine has an air inlet for supplying an intake airflow for combustion within the internal combustion engine and an exhaust outlet for exhausting an exhaust flow exiting from the internal combustion engine. The altitude simulator for dynamometer testing includes an exhaust pressure controller for maintaining the exhaust outlet of the internal combustion engine substantially equal to a determined exhaust pressure during operation of the internal combustion engine and an intake pressure controller for maintaining the air inlet of the internal combustion engine substantially equal to a determined intake pressure during operation of the internal combustion engine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an overall diagrammatic view of an altitude simulator for dynamometer testing constructed according to the invention. 
     FIG. 2 is a diagrammatic view of an exhaust pressure controller unit of the altitude simulator for dynamometer testing of FIG.  1 . 
     FIG. 3 is a diagrammatic view of an air intake pressure controller of the altitude simulator for dynamometer testing of FIG.  1 . 
     FIG. 4 is a diagrammatic view of an air conditioning unit used in the air intake pressure controller of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIG. 1, an altitude simulator for dynamometer testing, constructed according to the present invention, is generally indicated by reference numeral  10 . The altitude simulator  10  is, in FIG. 1, shown connected to an internal combustion engine system  12 , which includes an internal combustion engine  14 , an air intake system  16  for supplying the engine system  12  with a flow of air for supporting the combustion of fuel therein, and an exhaust system  18  for exhausting the products of combustion therefrom. Commonly, the air intake system  16  will include such components as an air cleaner/filter, a carburetion system, an intake manifold, etc. Typically, the exhaust system  18  will include such components as an exhaust manifold, a catalytic converter unit, and various exhaust pipes and connections terminating in a tailpipe  20 . 
     In contrast to the previously utilized dynamometric chamber approach to altitude simulation, the present invention recognizes that, since the only parts of an internal combustion engine that are affected by barometric pressure are the air inlet and the exhaust outlet of the engine, only these relatively small volumes need be pressure controlled in order to subject the operational characteristics of the engine as a whole to changes in ambient pressure (thereby simulating changes in altitude). 
     The altitude simulator  10  itself generally includes a exhaust pressure controller  22  and an intake pressure controller  24 , each of which is shown in more detail in FIGS. 2 and 3, respectively, and described more fully below. 
     In FIG. 1, the intake pressure controller  24  is shown as being connected upstream of the air intake system  16 , and the exhaust pressure controller  22  is shown as being connected downstream of the tailpipe  20 . It will be understood by those of ordinary skill in the art that the precise points at which the air intake pressure controller  24  and the exhaust pressure controller  22  are connected to the engine system  12  are not of primary concern. The primary consideration is rather that the air inlet through which intake air is introduced into the engine system  12  and the exhaust outlet and through which exhaust gases are exhausted from the engine system  12  be maintained at the desired pressure. 
     Referring now to FIG. 2, the exhaust pressure controller  22  includes an exhaust pump  26 , which is connected to the exhaust outlet of the engine system  12 . In the presently preferred embodiment, as shown in FIGS. 1 and 2, the exhaust pump  26  is connected to the tailpipe  20 , which functions as the exhaust outlet of the engine system  12 . The exhaust pump  26  is chosen to have a capacity (i.e., in cubic feet per minute, etc.) sufficient to produce an absolute pressure at the exhaust outlet (e.g., the tailpipe  20 ) of the engine system  12  which is at least as low as, and preferably lower than, the absolute pressure at which the dynamometric testing is to be carried out, during such time as the engine system  12  is running, and over the entire range of operation of the engine system  12 . For example, as noted above, an absolute pressure of 80 kPa is typically used for dynamometric testing to represent a common absolute ambient atmospheric pressure likely to be encountered in Denver, Colo. Assuming that the actual atmospheric pressure at the testing site is on the order of 100 kPa, then the capacity of the exhaust pump  26  must be sufficient to pull at least a negative pressure of 20 kPa, in order to reach the desired 80 kPa representative of the Denver, Colo. ambient atmospheric pressure. Additionally, the exhaust pump  26  must be of sufficient capacity to maintain this negative pressure differential (e.g., 20 kPa) throughout the entire operating range over which the engine system  12  is tested. For example, maintaining a given negative pressure differential (e.g., 20 kPa) will require a greater flow capacity for exhaust pump  26  if the tested operating range of the engine system  12  is to include operation at full throttle (i.e., with the throttle wide open) as opposed to operation only at lower engine speeds and light loads. 
     In actual operational tests, a diesel particulate exhaust pump has been employed for the exhaust pump  26 . Such a diesel particulate exhaust pump is used in the testing of diesel engines and, since it is utilized to maintain the particulate matter produced by diesel engines (e.g., soot) airborne, a diesel particulate exhaust pump has a substantially high flow capacity and the ability to pull a substantial negative pressure. It is estimated, for example, that such a diesel particulate pump was able to pull a negative pressure of 25 kPa over the entire dynamometrically tested range of a typical 4-cylinder passenger vehicle engine. 
     The exhaust pump  26  is connected to exhaust the effluent from the exhaust outlet of the internal combustion engine system  12  (i.e., the tailpipe  20 ) and therefore, in effect, creates a barometric control surface  28  at the terminus of the exhaust outlet of the internal combustion engine system  12  (i.e., at the outlet end of the tailpipe  20 ). In the present invention, the barometric control surface  28  extends only over the exhaust outlet of the internal combustion engine  12 . This is in contrast to the prior art approaches, wherein a barometric control surface extending over the entire engine had to be established and maintained. Since the exhaust pump  26 , as explained above, will pull a higher than desired negative pressure throughout the range of testing, air at the ambient atmospheric pressure is admitted from the test site to raise the pressure at the barometric control surface  28  (i.e., the outlet of the tailpipe  20 ) to the desired simulated atmospheric pressure. To this end, the exhaust pressure controller  22  includes an inlet valve  30  for admitting air at ambient atmospheric pressure from the test site to a point which is preferably located substantially adjacent the barometric control surface  28 . 
     The opening and closing of the inlet valve  30  is controlled by a Feedback and Feed Forward Controller  32 , which is preferably provided in the form of a numerical processor, such as, for example, a microprocessor. The Feedback and Feed Forward Controller  32  is provided with a variable input of the “Commanded Tailpipe Pressure” and also receives a data signal indicative of the absolute pressure existing at the barometric control surface, namely, P Tailpipe  which is generated by an absolute pressure sensor  34  positioned preferably to read the pressure at a point substantially adjacent the barometric control surface  28 . If P Tailpipe  is less than the Commanded Tailpipe Pressure, the Feedback and Feed Forward Controller  32  controls the inlet valve  30  so as to admit ambient air from the test site to a point substantially adjacent the barometric control surface  28  and thereby raise the pressure at the barometric control surface  28  to the desired level. 
     The Feedback and Feed Forward Controller  32  controls the inlet valve  30  through a valve actuator  36 . Preferably, the valve actuator  36  includes a failsafe driver  38 . Such valve actuators incorporating a failsafe driver are available commercially and are well known to those of ordinary skill in the art in the field of the invention. The failsafe driver  38  may be built into the valve actuator  36  but can be a standalone device. The failsafe driver  38  is, in fact, a controller for the valve actuator  36 . The failsafe driver  38  can be programmed to act as a simple (or local) feedback controller for the valve actuator  36  and can also accommodate external commands to drive the valve actuator  36  through this local controller. The failsafe driver  38  can produce a 2-10 mA signal that is typical of such industrial applications. The 2-10 mA signal commands the valve actuator  36 , which in turn positions the larger intake valve  30 . 
     The Feedback and Feed Forward Controller  32  may optionally be furnished with additional variable input signals, including “Engine Mass Rate Out”, “P Ambient ” and “T Ambient ”. As is well understood in the field, the inclusion of these additional variables allows the engine intake mass rate to be calculated. By using this additional information, the feed forward section of the Feedback and Feed Forward Controller  32  can be made more effective. This is most useful for dynamic testing, where the speed and the loading conditions of the engine system  12  are changing during testing. In steady state testing, use of these additional variables is not critical. Use of the additional variables Engine Mass Rate Out, P Ambient  and T Ambient  allows the control command to be instantly changed when the operating conditions of the engine system  12  change, using feed forward on these variables, together with a dynamic physical model of the system at hand. This is in contrast to using just feedback and having to wait for control errors to arise before the control command is changed. This feed forward approach is important in non-linear applications and in applications that can vary over a wide range of such non-linearities, which is the case of an engine&#39;s air intake over the span of normal operating conditions. 
     Referring now most particularly to FIG. 3, the intake pressure controller  24  includes an air conditioning unit  40 , described more fully below in connection with FIG. 4, which supplies a sufficient flow of conditioned air at a predetermined pressure, humidity and temperature to exceed what the engine system  12  might consume at the upper limit of dynamometric testing. In order to reduce the pressure at which this conditioned air is supplied to the engine system  12 , a choke valve  42  is positioned downstream of the air conditioning unit  40  and upstream of the air inlet of the engine system  12 . Whereas, in the exhaust pressure controller  22 , the exhaust pump  26  is employed to draw down the pressure to below the ambient pressure existing at the test site, here, the engine system  12  itself acts as an air pump. The choke valve  42  positioned between the air conditioning unit  40  and the air inlet of the engine system  12  provides a resistance against which the engine system  12  can produce the required pressure drop. 
     The degree of closure of the choke valve  42  is controlled by a first pressure controller  44 , which is also preferably provided in the form of a numerical processor (e.g., a microprocessor). The first pressure controller  44  is provided with a Desired Intake Pressure variable and a signal generated by a pressure transducer  46 . The pressure transducer  46  is preferably mounted just upstream of the air inlet of the engine system  12  (e.g., just ahead of the air cleaner thereof). The first pressure controller  44  manipulates the choke valve  42  (through an actuator  48  associated with the choke valve  42 ) so as to maintain the pressure registered by the pressure transducer  46  within acceptable limits of the Desired Intake Pressure. 
     Another barometric control surface  49  is established at the intake system  16  of the engine system  12 . Again, in contrast to the prior art approach, the barometric control surface  49  extends only over the air inlet of the engine system  12  and not over the entire extent of the engine system  12 . 
     The effectiveness of the choke valve  42  is increased if the pressure drop across the choke valve  42  is maintained within a certain range. If the pressure differential across the choke valve  42  is too small, the choke valve  42  becomes, to some degree, ineffective. If, on the other hand, the pressure differential across the choke valve  42  is too great, the choke valve  42  becomes too sensitive, in that small changes in the configuration of the choke valve  42  produce very large changes in downstream pressure. Accordingly, in order to maintain the pressure just upstream of the choke valve  42  within this desired range, the intake pressure controller  24  is preferably provided with a pressure dump mechanism  50 , which generally includes a dump valve  52 , an actuator  54  for actuating the dump valve  52 , a second pressure controller  56  and a differential pressure transducer  58 , which is mounted across the choke valve  42 . The second pressure controller  56  receives a signal from the differential pressure transducer  58  indicating the pressure existing across the choke valve  42  and signals the actuator  54  to actuate the dump valve  52  so as to maintain this pressure within a range wherein, preferably, the effectiveness of the choke valve  42  will be maximized. Provision of the pressure dump mechanism  50  additionally prevents the ductwork connecting the air conditioning unit  40  to the remainder of the system from being exposed to excessive pressure forces. 
     Referring primarily now to FIG. 4, the air conditioning unit  40 , shown there in more detail, generally includes a dryer stage  60  for dehumidifying the air intake into the unit, a conditioned air supply stage  62  for adjusting the temperature and humidity of the air and a final electronic pressure control stage  64 . Air conditioning units such as are shown in FIGS. 3 and 4 and which are used in the practice of the present invention are available as off the shelf units from commercial vendors, as is well understood by the average artisans in the field to which the present invention pertains. 
     The Feedback and Feed Forward Controller  32  of FIG.  2  and the first and second pressure controllers  44  and  54 , respectively, of FIG. 3 may be all be implemented within a single microprocessor configuration, or the different controllers can be each implemented within a separate microprocessor, as desired. 
     While the present invention has been disclosed by way of a description of a particularly preferred embodiment or a number of particularly preferred embodiments, it will be readily apparent to those of ordinary skill in the art that various substitutions of equivalents can be effected without departing from either the spirit or scope of the invention as set forth in the appended claims.