Patent Publication Number: US-6671614-B2

Title: System and method for engine data trending and analysis

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 09/840,719 filed on Apr. 23, 2001, now U.S. Pat. No. 6,339,742. This application is a continuation of commonly owned U.S. pat. app. which is a continuation of Ser. No. 09/420,496 Filed Oct. 19, 1999, now U.S. Pat. No. 6,220,223 which is a continuation of U.S. pat. app. Ser. No. 09/050,685 filed Mar. 30, 1998, now U.S. Pat. No. 6,026,784 which is a continuation of U.S. pat. app. Ser. No. 08/737,486 filed Mar. 12, 1997, now U.S. Pat. No. 5,732,676, which is a continuation-in-part under §371 of PCT/US95/06052 filed Mar. 15, 1995 based on U.S. pat. app. Ser. No. 08/243,103 filed May 16, 1994, now U.S. Pat. No 5,477,827. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a method and system for controlling an internal combustion engine. 
     BACKGROUND ART 
     In the control of engines, the conventional practice utilizes electronic control units having volatile and nonvolatile memory, input and output driver circuitry, and a processor capable of executing a stored instruction set, to control the various functions of the engine and its associated systems. A particular electronic control unit communicates with numerous sensors, actuators, and other electronic control units necessary to control various functions, which may include various aspects of fuel delivery, transmission control, or myriad others. 
     Early complex systems and subsystems which performed critical functions required separate control units which could promptly respond to dynamic vehicle situations and initiate appropriate actions. For example, a vehicle may have employed a brake controller, a cruise control module, a cooling fan controller, an engine controller, and a transmission controller, such that each vehicle system or subsystem had its own stand-alone controller. These controllers were either electronic control units or electronic circuits which may have had little or no communication among themselves or with a master controller. Thus, the vehicle was operated as a distributed control system, which often made it difficult to optimize overall vehicle performance by coordinating control of the various systems and subsystems. 
     As control systems became more sophisticated, the various distributed controllers were connected to communicate status information and coordinate actions. However, inter-controller communication delays were often unacceptable for critical control tasks, thus requiring independent processors or circuitry for those tasks. This expanded the overall capabilities of the control system and was often necessary to meet increasing consumer demands as well as more stringent emission control standards. 
     To meet these stricter standards, it has been necessary to expand the capabilities of the engine control system to more accurately control the engine operation. The complexity of the resulting control systems has often resulted in difficulty in the manufacturing, assembling, and servicing of vehicles. Manufacturers have attempted to decrease part proliferation, while increasing the accuracy of control, by combining increasingly more control functions into a single controller. 
     Advancements in microprocessor technology have facilitated the evolution of engine control systems. These systems began by implementing relatively simple control functions with mechanical apparatus, and progressed to more involved control schemes with dedicated controllers, before having matured as complex control strategies realized by a comprehensive engine controller. Many engine control systems found in the prior art address only a single subsystem control strategy and fail to capitalize on the advantages afforded by these microprocessor advancements. Another difficulty encountered by traditional, distributed engine control systems is the inability to protect the engine or engine components from system failures. Certain engine components, operated under extreme operating conditions, may fail. 
     The desire to provide application specific vehicles at a competitive price has led to the availability of a number of customer options which may include some of the systems already noted, such as vehicle speed control, engine speed control, or engine torque control. This in turn has led to a large number of possible subsystem combinations, thus increasing the costs associated with manufacturing and assembly as well as the cost of field service due to the large number of spare components which must be manufactured and stored. 
     It is desirable to have an electronic control unit capable of integrating the control of various engine functions and associated vehicle systems, thus eliminating inter-controller communication delays and harmonizing engine control with other vehicle subsystems. An additional benefit accrues from replacing independent stand-alone controllers with a comprehensive controller, to reduce part proliferation in the vehicle manufacturing, assembly, and service environments, leading to an associated reduction in the cost of these functions. 
     It is also desirable in optimizing overall vehicle performance, to have an electronic control unit which coordinates control of the engine with control of the transmission for smoother, more efficient shifting of the transmission. It is desirable to provide for throttle logic, and to provide for cylinder balancing to determine relative power contribution from each cylinder. 
     Due to increasing cost of fuel, it is further desirable to provide a controller which encourages certain driving techniques which enhance fuel economy. For example, it is desirable to provide an incentive to limit engine idling while the vehicle is stationary to reduce average noise levels and to reduce fuel consumption. It is further desirable to encourage the use of cruise control to minimize transmission shifting and increase overall fuel economy whenever possible. 
     It is also desirable to provide a controller which can control the engine in a manner which protects engine components during extreme operating conditions. For example, if a turbocharged vehicle is operated at high altitudes, the turbocharger will spin faster than a similar turbocharger operated at lower altitudes, and can be damaged. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an integrated engine controller capable of performing a balance test for determining cylinder contributions. 
     It is a further object of the present invention to provide an integrated engine controller capable of controlling fuel delivery to maximize fuel economy via driver incentives, and capable of protecting engine components in the presence of certain operating conditions. 
     It is an additional object of the present invention to provide an integrated engine controller capable of performing throttle logic, and capable of controlling the transmission in conjunction with the engine. 
     Another object of the present invention is to provide an engine controller which limits engine idling time while the vehicle is stationary to reduce unnecessary fuel consumption and noise. 
     Yet another object of the present invention is to provide an engine controller which limits engine idling based on ambient air temperature so as to permit engine idling under conditions justifying use of vehicle heating or cooling systems. 
     A further object of the present invention is to provide an engine controller which estimates ambient air temperature so that an additional temperature sensor is not required. 
     Still further, it is an object of the present invention to provide an integrated engine controller capable of determining service intervals and performing trend analyses. 
     In carrying out the above objects and other objects and features of the present invention, a system and method for collecting operating data from a multi-cylinder internal combustion engine having an engine control module include monitoring engine operating conditions to determine when to store selected engine operating data and periodically storing engine operating data in the engine control module in response to the step of monitoring. 
     The above objects and other objects, features, and advantages of the present invention will be readily appreciated by one of ordinary skill in the art from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an integrated control system for an internal combustion engine in accordance with the present invention; 
     FIG. 2 is a flow chart detailing the steps for cylinder balance testing according to the present invention; 
     FIG. 3 is a flow chart detailing the steps for fuel economy speed limit addition according to the present invention; 
     FIG. 4 is a graphical representation of the high altitude torque reduction as a function of barometric pressure and engine speed according to the present invention; 
     FIG. 5 is a flow chart detailing the throttle logic according to the present invention; 
     FIG. 6 is a flow chart detailing the gear ratio torque limit strategy of the present invention; 
     FIG. 7 is a flow chart illustrating a system and method for engine idle shutdown based on ambient air temperature according to the present invention; 
     FIG. 8 is a graphical representation of a system and method for ambient air temperature estimation according to the present invention; and 
     FIG. 9 is a graphical representation of absolute torque versus engine speed for use with the air temperature torque limit according to the present invention. 
    
    
     BEST MODE(S) FOR CARRYING OUT THE INVENTION 
     Referring now to FIG. 1, there is shown an electronic control module (ECM)  20  in communication with typical engine components, shown generally by reference numeral  22 , and a user-interface  34 . ECM  20  includes a microprocessor  24  having volatile random-access memory (RAM)  26 , and nonvolatile read-only memory (ROM)  28 . Of course, ECM  20  may contain other types of memory instead of, or in addition to, RAM  26  and ROM  28 , such as flash EPROM or EEPROM memories, as is well known in the art. 
     ROM  28 , or other nonvolatile memory, may contain instructions, which are executed to perform various control and information functions, as well as data tables, which contain calibration values and parameters characterizing normal engine operation. Microprocessor  24  imparts control signals to, and receives signals from, input and output (I/O) drivers  32 . The I/O drivers  32  are in communication with the engine components  22  and serve to protect the controller from hostile electrical impulses while providing the signals and power necessary for engine control according to the present invention. The ECM components detailed above are interconnected by data, address and control buses. It should be noted that there are a variety of other possible control schemes which include various combinations of microprocessors and electric or electronic circuits which could perform the same function. 
     With continuing reference to FIG. 1, engine components  22  include a plurality of electronic unit injectors (EUI)  40 , each associated with a particular engine cylinder; and a plurality of sensors  42  for indicating various engine operating conditions, such as coolant temperature, ambient air temperature, intake manifold air temperature, inlet air temperature, engine oil temperature, fuel temperature, innercooler temperature, throttle position, intake manifold pressure, fuel pressure, oil pressure, coolant pressure, cylinder position, and cylinder sequencing, to name a few. Engine components  22  also include actuators  44  which may include solenoids, variable valves, indicator lights, motors, and/or generators. It should be appreciated that ECM  20  may also be in communication with other vehicle components and microprocessors which control associated vehicle systems, such as the brakes, the transmission, a vehicle management system or a fleet management radio transponder. 
     The user-interface, or data-hub,  34  is used to store user-selected monitoring parameters and associated values for those parameters, and to determine service intervals and perform trend analyses. User selected parameters may include adjustable limits, such as desired engine oil life. Engine historical information may include diagnostic information which is used to assist personnel performing routine maintenance, or troubleshooting malfunctions, as well as engine and vehicle operation data, which may be analyzed to evaluate vehicle operator performance in addition to vehicle performance. The user-interface  34  also preferably performs component lifing and trend analyses, as described in greater detail below. It should be appreciated that although FIG. 1 illustrates the user-interface as being external to the ECM  20 , certain operations performed by the user-interface could, of course, also be performed by the ECM  20 . 
     Electronic control module  20  executes software to implement the various features of the present invention. In describing these features, equations will be provided and reference will be made to variables utilized by the ECM in executing the software. It should be noted that equation variables shown in lower-case italics are calibration variables, whereas equation variables shown in SMALL CAPS represent function variables, whose values vary and are based on, for example, operating conditions such as intake manifold pressure or engine speed. 
     Referring now to FIG. 2, there is shown a flow chart detailing the steps for an injector balance test according to the present invention. In the preferred embodiment, the ECM  20  attempts to balance the injectors  40  so that power delivery is approximately equal, by performing an acceleration test to determine the relative power from each injector. At step  50 , the ECM determines whether the acceleration balance test is to be started. In the preferred embodiment, the test is activated if an injector balance message is received by the ECM  20  from the user-interface  34  and either the vehicle speed is zero or the vehicle speed sensor (VSS) is not configured and the balance flag (BALENB) is set. The BALENB flag is set during calibration. The acceleration balance test can be terminated in a number of ways. Preferably, the test is terminated when the test is complete, or when ECM  20  receives a clear message from the user-interface  34 . Additionally, the test is terminated when the ignition is turned to the “off” position, or if the vehicle speed is non-zero. 
     With continuing reference to FIG. 2, once the balance test is activated at step  50 , a test sequence is entered at step  52  which includes an initializing step during which the idle speed is set to the value of BALSTR, a variable representing the balance start ramp RPM (the engine RPM at which the test starts). BALSTR preferably has a range of 0-2500 RPM and defaults to 1000 RPM. Additionally, requested torque is set to zero and the test cylinder variable (CYL TST ) is set to a value of ‘1’ at step  52 . 
     At step  54 , the engine is allowed to stabilize prior to commencement of the actual test. During the stabilizing period, the test cylinder is cut out of the fueling scheme. Control flow proceeds to step  56  when the stabilizing time exceeds or is equal to the settling time variable (BALTIM), which has a range of 0-30 seconds and a default value of 2 seconds. 
     As shown in FIG. 2, once the engine has stabilized, at step  56  the acceleration RPM with the test cylinder cut out is measured, starting with the first cylinder cut out. To do this, the fueling control signal, which could be a pulse width signal or derived from torque, for the remaining injectors, is set to the value of BALFPW, and the beginning of injection (BOI) time for the remaining injectors is set to BALBOI, wherein BALFPW is the balance pulse width variable, which has a range of 0°-30° and a default value of 13° (of crankshaft rotation), and wherein BALBOI is the balance beginning of fuel injection variable, which has a range of 0°-30° and a default value of 10°. 
     Control flow preferably proceeds to step  58  of FIG. 2 when the number of actual engine revolutions since the pulse width and injection time were set, equals or exceeds BALREV, the variable representing the balance end ramp revolutions, which has a range of 0-100 revolutions with a 1 revolution resolution and a default value of 8 revolutions. The value of BALREV is set to ensure the occurrence of a certain number of firings for the test. As is known, in a four stroke 8-cylinder engine, four cylinders fire for each crankshaft revolution, whereas all eight cylinders are fired each revolution in a two stroke 8-cylinder engine. With BALREV and BALFPW set to the indicated defaults, an increase in engine speed of approximately 500 RPM will be realized during the test. At this point (revs≧BALREV), the engine speed is stored in memory for the cut out injector. 
     At step  58  of FIG. 2, the ECM determines whether or not an additional injector needs to be tested or whether the injector balance test is complete. If the value of CYL TST  exceeds or equals the number of engine cylinders (ZNCYLH), no additional injectors need to be tested and control flow skips to step  62 , wherein adjustment factors are determined. If, however, additional injectors are to be tested, the value of CYL TST  is incremented at step  60 , and control flow returns to step  54  so that steps  56  and  58  can be repeated for each injector to be tested. 
     With continuing reference to FIG. 2, at step  62  adjustment factors are calculated after all the injectors to be tested (i.e. cut out) have been tested. In a preferred embodiment, this entails first determining an average RPM (RPM AVG ) for the tested injectors according to:          RPM   AVG     =       ∑     (   RPM   )       ZNCYLH                     
     wherein Σ(RPM) is the sum of all RPMs measured at the conclusion of step  56 . Next, an adjustment RPM ratio (RPM ADJX ) is determined for each cylinder according to:          RPM   ADJX     =       RPM   x       RPM   AVG                       
     for x=1 to 8 for an eight cylinder engine, wherein RPM x  is the RPM measured at the conclusion of step  56  for the particular cylinder. The adjustment factors (FCT ADJX ), which will ultimately modify the final injector pulse width, are determined according to: 
     
       
           FCT   ADJX =(RPM ADJX −1)*( ZNCYLH −1)* BALGAN   
       
     
     wherein BALGAN is the balance gain which has a range of 0-2 with a default value of 0.8. A temporary adjustment factor (FCT TMPX ) is then determined according to: 
       FCT   TMPX =min( BALLIM,  max(− BALLIM , ( FCT   ADJ(X−1)   +FCT   ADJX ))) 
     wherein BALLIM represents a balance limit having a range of 0-1 with a 0.01 resolution and a default value of 0.07. Thus, to determine the temporary adjustment factor, the ECM first identifies the maximum of −BALLIM and the sum of FCT ADJ(X−1)  and FCT ADJX  (wherein FCT ADJ(X−1)  is the previous adjustment factor for that cylinder). The ECM then compares that maximum to the value of BALLIM, and takes the minimum of those quantities as the temporary adjustment factor. 
     At step  64 , the adjustment factors are updated, and the results are output to the user. In the preferred embodiment, updating the adjustment factors includes normalizing the calculated adjustment factors. Normalization of the adjustment factors is accomplished utilizing a normalizing factor, determined according to: 
     
       
           FCT   NRMX =Π(1 +FCT   TMPX ) 
       
     
     Thus, FCT NRMX  obtained by taking the product of all (1+FCT TMPX ) quantities. The fuel injector pulsewidth adjustment factor for each cylinder can then be determined according to:          FCT   x     =         (     1   +     FCT   TMPX       )       FCT   NRMX       -   1                     
     Once determined, at step  64 , all of the adjustment factors for the cut-out cylinders (i.e. all values of FCT X ) are stored in memory and then multiplied by the final injector pulsewidth: 
     
       
         
           BALFPW=PWMULT*INCFAC 
           X 
           *FCT 
           X 
           *SPW 
         
       
     
     wherein PWMULT is an engine-specific horsepower adjustment constant stored in non-volatile memory, INCFAC X  is the injector fuel flow rate, and SPW is the base fuel pulse width determined by the ECM based on operating conditions and known principles of ignition and combustion. Although not specifically shown in FIG. 2, the methodology could be performed using multiple iterations of the steps. 
     It is known that certain fuel injectors, sometimes termed high fuel or hot injectors, generate more power than other injectors. With the balance test of the present invention, the effects of this variation can be minimized by compensating the fuel delivery of each fuel injector. For example, if the engine has a high output injector, when that injector is cut out, the associated rise in engine speed resulting from the balance test will be less than the rise in engine speed occurring when the high output injector is fueled and a nominal injector is cut out. Adjustment factors compensate for a high output injector by reducing the fuel delivered to that injector. By compensating the fuel injectors, the present invention operates to reduce engine wear, reduce the amount of engine smoke and particulates generated, and provide smoother power delivery. It should be appreciated that the injector specific information described above may be retrieved and/or compared in the user interface  34  or in the ECM  20  for engine diagnosis and tracking purposes. 
     Referring now to FIG. 3, there is shown a flow chart detailing the steps for a fuel economy speed limit adder according to the present invention. The goal of this aspect of the present invention is to provide an incentive to the vehicle operator to behave in a way generally consistent with the goals of the fleet managers to maximize fuel economy. Typically, drivers strive to increase vehicle speed. In light of this fact, increasing the speed available to the driver as fuel economy increases as a result of for example, minimization of idle time, selection of the optimum transmission gear, maintaining a steady throttle, or reducing the use of engine-driven accessory loads, provides a powerful incentive for the driver to behave as desired. 
     The strategy to provide the speed-based incentive utilizes numerous variables. In the preferred embodiment, calibration values for these variables are as follows: 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                   
                   
                 MINIMUM 
                 INITIAL 
               
               
                 NAME 
                 RANGE 
                 RESOLUTION 
                 VALUE 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 THRESH MPG   
                 2-20 
                 MPG 
                 0.1 
                 MPG 
                 7 
                 MPG 
               
               
                 MAX MPH MPG   
                 0-10 
                 MPH 
                 1 
                 MPH 
                 5 
                 MPH 
               
               
                 MPH MPG GN   
                 0-20 
                 MPH/MPG 
                 0.1 
                 MPH/MPG 
                 5 
                 MPH/MPG 
               
               
                 MPG FILT CON   
                 0-0.01 
                   
                 0.00002 
                   
                 0.00055 
               
               
                 MPG LIM 
                 0-10 
                 MPG 
                 0.1 
                 MPG 
                 3 
                 MPG 
               
            
           
           
               
               
               
               
            
               
                 MPG TYPE 
                 Switch 
                 0 = trip avg., 
                 1 = filtered 
               
               
                   
               
            
           
         
       
     
     Fuel economy, measured in terms of miles per gallon (MPG), can be either the trip average fuel economy (MPG_TYPE=0) or a filtered fuel economy (MPG_TYPE=1). In a preferred embodiment, filtered MPG (FIL MPG ) is utilized and is calculated at step  70  using a standard 1 st  order lag calculation limited between calibration limits according to: 
     
       
           FIL   MPG =min( MPG   —   LIM +THRESH MPG , max( THRESH   MPG   −MPG   —   LIM,  ( FIL   MPG     t−1     +MPG   —   FILT   CON *( INST   MPG   −FIL   MPG     t−1   )))) 
       
     
     wherein MPG_LIM represents the maximum permissible deviation from the fleet fuel economy goal, THRESH MPG  represents a threshold fuel economy, MPG_FILT CON  is a fuel economy filter constant, INST MPG  represents an instantaneous fuel economy, and FIL MPGt−1  represents the previously determined filtered fuel economy. This calculation is done frequently, such as once per second, to react in a timely manner to current driver behavior. 
     The MPGMPH term, which represents the speed adder, is calculated to increase allowable vehicle speed once the threshold fuel economy has been obtained, although it is limited to the maximum speed calibration value MAX_MPH MPG . More specifically, allowable vehicle speed is proportionally increased according to the amount by which the threshold fuel economy is exceeded. In a preferred embodiment, the MPGMPH term is determined at step  72  according to: 
     
       
           MPGMPH= min( MAX   —   MPH   MPG   , MPH   —   MPG   GN *max(0, ( INST   MPG   −THRESH   MPG ))) 
       
     
     wherein MAX_MPH MPG  represents the maximum amount the vehicle speed may be increased, and MPH_MPG GN  is the fuel economy gain the value of which may vary based on customer input. For the feature to function as intended, the calculated MPG value is preferably saved across ignition cycles, and initialized to zero in the case of the first power up or in the case of an error. If the VSS fails, the speed adder is set to zero. At step  74 , the ECM adjusts the speed maximums. More particularly, the cruise maximum MPH (CCMAXS) and the road speed limit maximum MPH (RSLMPH) are modified by the addition of MPGMPH to CCMAXS and RSLMPH when this feature is enabled. 
     According to another aspect of the present invention, fuel delivery to the engine is limited at certain altitudes. The goal of this aspect of the present invention is to prevent damage to the turbocharger from excessive speed or compressor surging at certain barometric pressures (such as those typically encountered at high altitudes), preferably by reducing power via a torque limit at those barometric pressures. 
     In a preferred embodiment, a limit torque (HATQ) is defined according to:              TPM   =                      (       RPM   MX     -   RPM     )     *     (     1   -     TRQ   MN       )           RPM   MX     -     RPM   MN         +                                PR   TQ     *     max        (     0   ,     (     BARO   -     PR   MN       )       )                               
     and 
     
       
           HATQ =min(1,  TRQ   MN +max(0,  TMP )) 
       
     
     wherein RPM MX  represents the RPM for maximum barometric pressure compensation which has a range of 0-2500 with a default value of 1800 RPM, RPM is the engine speed, TRQ MN  represents the minimum torque after compensation which has a range of 0-100 and a default of 100%, RPM MN  represents the minimum RPM for compensation which has a range of 0-2500 and a default of 1100 RPM, PR TQ  represents the pressure gain for compensation which has a range of 0-2% per kPa and a default value of 1% per kPa, and PR MN  represents the minimum pressure for compensation which has a range of 0-120 and a default value of 50 kPa. The calculations are performed, and the torque limit is imposed, when the conditions for torque limiting exist, such as if the barometric pressure is below that which provides a torque that is greater than or equal to the current requested torque. It should be noted that the variables are calibrated such that RPM MX &gt;RPM MN . 
     FIG. 4 is a graphical representation of the torque reduction (i.e. percent of maximum engine output torque) as a function of altitude and engine speed. The solid lines indicate typical requested torque limit, and lines of constant power. Limited torques are shown for barometric pressures of 0 kPa, 50 kPa, 72.86 kPa, 81.12 kPa and 100 kPa. As shown, the torque varies inversely with the barometric pressure (BARO) so that more torque reduction (less torque) is provided at lower barometric pressures typically encountered at high altitudes. More particularly, the graph illustrates various torque reductions for a particular engine application, with TRQ MN =57%, RPM MN =1050 RPM, RPM MX =2100 RPM, PR TQ =0.0080, and PR MN =50 kPa. Once determined, the limit torque may be imposed as a global engine torque limit as described above. 
     As previously noted, the user interface, or data hub,  34  is used to store user calibration parameters, fleet management information, and retrieve engine historical information logged as a result of diagnostic or malfunction codes. The data hub  34  preferably stores this information in sets referred to herein as pages, although various other methods of storage are possible, such as ASCII files. The first page relates to vehicle information. In the preferred embodiment, the vehicle information page includes total vehicle distance, total fuel used, total engine hours and engine serial number. All values are maintained from the first use of the engine, and can not be reset. 
     The electronic control module  20  can sample various sensors at regular intervals under controlled conditions, or it can sample continuously. For instance, the coolant temperature may be sampled continuously when the engine is expected to have reached operating temperature. Studying the trend data over a period of time gives an indication of the condition of the element being measured or may give an indirect indication of the condition of other elements of the engine. For example, a decreasing average for coolant temperature may indicate a malfunctioning thermostat, whereas an increasing average temperature may indicate a clogged radiator. 
     Accordingly, the electronic control module  20  cooperates with the data hub  34  to maintain a plurality of trend pages. Generally, trend information can be reset, but the preferred practice is to allow the trends to run continuously over the life of the engine. In this manner, a predetermined number of most recent samples, for example 100, are always available for trend analyses. The values represented by the letters “aaa”, “bbb”, and so on, can be user-specified to customize the trend information, and the value within parentheses after the letters indicates the default value. Of course, the default values may be hard-coded into the system, removing that aspect of customer input. Lastly, it should be noted that all trend pages may be reset. Resetting will clear all of the trend sample points stored on a particular page. 
     The first trend page maintained relates to oil pressure. An oil pressure trend sample is monitored during every aaa(20) engine hours. The average oil pressure during the time interval of the sample period where the RPM exceeded bbb(1600) but was less than ccc(1800), and the oil temperature exceeded ddd(180° F.) but was less than eee(220° F.) is taken as the sample. The starting engine hours at which time the sample period began is also stored. Generally, the oil pressure trend is monitored for an unusual change in pressure, such as a large drop in pressure. Such a drop in pressure may be indicative of mechanical problems, a regulation problem, fuel dilution, or a low quantity of oil. 
     The data hub also maintains a turbo boost pressure trend page. In the preferred embodiment, a turbo boost pressure trend sample is determined based on every aaa(20) engine hours. The average turbo boost pressure during the time interval of the sample period where the RPM was greater than bbb(1400) and less than ccc(1600) and the powertrain demand is greater than or equal to ddd(96%) and less than or equal to eee(100%) is taken as the sample. The starting point from which the sample had begun is also stored in the form of engine hours. The turbo boost trend operates to monitor the fuel and air system. Generally, an air leak manifests itself as a reduction in the turbo boost. Similarly, a fuel restriction (e.g. clogged fuel filter) manifests itself as a reduction in the turbo boost. 
     As a means of monitoring the electrical system, the data hub  34  maintains a battery voltage trend page. A battery voltage trend sample is taken every aaa(20) engine hours for a period of bbb(60) minutes. The average battery voltage for all of the time in the sample period where the rpm was greater than ccc(1600) and less than ddd(1800) is taken as the sample. The starting point of the sample period is also stored. Generally, an unusual increase in battery voltage may be indicative of a voltage regulator failure, whereas an unusual decrease in battery voltage may be indicative of a broken alternator belt. 
     The data hub also maintains a fuel economy trend page. A fuel economy trend takes a sample every aaa(20) engine hours and records the average fuel economy between the last sample and the current sample. The starting point of the sample period is also stored. 
     As a means of monitoring the oil cooling system, the data hub  34  maintains a maximum oil temperature trend page. A maximum oil temperature sample is determined over every aaa(20) engine hours. The maximum oil temperature reached between the time the last sample was taken, and the current sample, is recorded on the page. The starting point of the sample period is also stored as cumulative engine operating hours. An oil temperature measurement which indicates a rise in maximum temperature may be indicative of engine mechanical problems. 
     Additionally, a coolant temperature sample is taken every aaa(20) engine hours. The maximum coolant temperature reached between the time the last sample was taken, and the current sample, is recorded. The starting engine hours point from which the sample was taken is also stored. A coolant temperature measurement which indicates a rise in maximum temperature may be indicative of a plugged radiator, a malfunctioning thermostat, or cooling fan anomalies. 
     The data hub also maintains a maximum RPM trend page. An RPM sample is taken every aaa(20) engine hours and the maximum RPM reached between the time the last sample was taken, and the current sample, is recorded on the page. The starting point of the sample period is stored in the form of cumulative engine operating hours. Through the maintenance of maximum RPM trend pages, driver comparisons are possible. 
     As another means of making driver comparisons, the data hub also maintains maximum vehicle speed trend pages. A speed sample is taken every aaa(20) engine hours. It records the maximum speed reached between the time the last sample was taken and the current sample, in addition to starting point of the sample period in the form of cumulative engine operating hours. 
     Additionally, the data hub  34  maintains minimum throttle position sensor voltage trend pages. It should be appreciated that various types of throttle position sensors, such as a ratiometric sensor, may be used. A throttle position voltage sample is taken every aaa(20) engine hours. The minimum throttle position voltage reached between the time the last sample was taken, and the current sample, is recorded, in addition to the starting point of the sample period in the form of cumulative engine hours. With this information, throttle position sensor wear can be monitored. Generally, the output of the throttle position sensor decreases in value as the sensor wears. 
     In addition to the trend pages discussed in greater detail above, the data hub  34  is capable of performing lifting of vehicle components, such as engine components. In a preferred embodiment, up to 10 components can be monitored independently. During the setup of the electronic control module  20  information, the names of the 10 components to be monitored can be specified by an operator, i.e. oil filter, coolant, oil, and the like. Each of the components can be monitored by one or more of the following mechanisms: vehicle distance traveled, engine hours used, calendar time, total engine revolutions, total fuel burned, or idle time. The lifting information is maintained on a service interval page, which includes the value set for the service life and the total usage currently incurred, for each of the monitored engine components. The lifting information also preferably includes percent of life left, as well as the expected replacement/service date based on vehicle usage rates. The following components, to name a few, can be selected as standard for component lifing: oil filter, oil, air filter, fuel filter, and coolant. Each of the individual components for which the service interval is being calculated may be reset, so that its accumulated use is set to zero. 
     The event log page provides a coarse indication of the usage of the engine. In the event log, the state of the engine will be logged for the ninety-six (96) quarter hour intervals in a day. The event log will keep this information for a predetermined number of (the most recent) days, such as five (5). The following information is preferably available: start date and time, end date and time, number of entries, and entry (indicating engine on, off, idle, and cruise) for the 480 quarter hour entries. If the ECM clock is not properly set, or the power has been removed, this page may give inappropriate results. 
     In addition to the features described above, the electronic control module  20  also performs throttle control logic. More specifically, the electronic control module  20  determines a throttle position offset to ensure that the throttle position sensor (TPS) value is zero when the throttle is fully released, and to ensure that the value is forced to zero in error conditions as a safety precaution. An additional mechanism, if configured, operates to prevent the engine from being accelerated when a vehicle door is open. 
     In determining the throttle position offset, the electronic control module  20  utilizes an impulse filtered offset, a smoothed offset, and a computed offset. Generally, the impulse filtered offset is obtained by selecting the middle or median value of a group (such as three) of samples. Thus, the effect is to drop the highest and lowest samples of a group of samples. The smoothed offset is obtained utilizing a first order lag filter. The computed offset is obtained as described below. 
     With reference now to FIG. 5, at the start of an ignition cycle, the impulse filtered offset, the smoothed offset, and the computed offset are initialized to the maximum physically possible raw value (e.g. 1023). If the A/D converter device associated with the TPS is in a conversion fail condition, or if a digital input is configured as a dual drive EFPA switch and the debounced state of that input changes indicating a change in the active TPS (step  80 ), then at step  82  the impulse filtered offset, the smoothed offset, and the computed offset are held at the maximum physically possible raw value. 
     As shown in FIG. 5, if a sensor fail low or sensor fail high fault is detected, or if one of the digital inputs to the ECM  20  is configured to be a door switch, and the debounced state of that input represents a low external input indicating an open vehicle door (step  84 ), the impulse filtered offset, the smoothed offset, and the computer offset are held at the maximum physically possible raw value at step  86 . 
     Otherwise, at each conversion, the raw value of the TPS output is passed through the impulse noise filter at step  88 , which, as described above, will keep the most recent three raw values and pass the arithmetic middle value (median). The result, which is the impulse filtered offset (IFO), is then filtered with a first order lag filter to obtain the smoothed offset as follows. 
     With continuing reference to FIG. 5, if the impulse filtered offset is greater than the previously determined smoothed offset (step  90 ), the equation (step  92 ) is: 
     
       
           SO   T   =SO   T−1   +TPINFC ( IFO−SO   T−1 ) 
       
     
     wherein SO T  represents the new smoothed offset, SO T−1  represents the old smoothed offset, TPINFC represents the throttle position sensor offset increasing filter constant which has a range of 0-1, and a default value of 0.01. 
     Otherwise, at step  94  the smoothed offset is determined as follows: 
     
       
           SO   T   =SO   T−1   +TPDEFC ( IFO−SO   T−1 ) 
       
     
     wherein TPDEFC represents the throttle position sensor offset decreasing filter constant which has a range of 0-1, and a default value of 0.2. 
     If the ECM  20  receives, as a digital input, a throttle idle switch output and the debounced state of that input represents a low external input to the ECM indicating a closed throttle idle validation switch (step  96 ), then the computed offset is obtained at step  98  as follows: 
     
       
           CO   T =max( CO   T−1 , max( SO   T   +TPOHIS,TPO MIN)) 
       
     
     wherein CO T  represents the new computed offset, CO T−1  represents the old computed offset, SO T  represents the smoothed offset, TPOHIS represents the throttle position sensor offset hysteresis which has a range of 0-250, and TPOMIN represents the throttle position sensor minimum offset which has a range of 0-250. Thus, the ECM  20  first compares the sum of the current smoothed offset and the TPS offset hysteresis and the TPS minimum offset and takes the maximum of the two. The ECM then compares that quantity with the previous computed throttle offset and takes the maximum of those two as the new throttle offset. 
     Otherwise, the computed offset is determined at step  100  according to: 
     
       
           CO   T =max(min( CO   T−1   , SO   T   +TPOHIS ), TPOMIN ) 
       
     
     Thus, the ECM first compares the previous computed offset to the sum of the current smoothed offset and the TPS offset hysteresis and takes the minimum. Next, the ECM  20  compares that quantity to the TPS minimum offset and takes the maximum of that comparison. 
     In addition to the features described above, the present invention includes a gear ratio torque limit embodied in a final torque determination by the electronic control module  20 . The gear ratio torque limit strategy will first be provided, followed by an explanation of the variables, terms and the like used therein. It should be appreciated that although the present discussion focuses on a low gears torque limit, the strategy is equally applicable to other gear ratios with appropriate modifications. 
     With reference now to FIG. 6, generally, the low gears torque limit strategy limits engine torque based on engine speed (ES) and vehicle speed (VS) in an effort to protect the transmission from damage. As shown, the engine and vehicle speeds are measured at step  110 . More particularly, at step  112 , a virtual gear ratio (VGR) is determined. VGR is defined as the ratio of engine speed and vehicle speed (VGR=ES/VS). At step  114 , VGR is compared to a predetermined value, such as a low gear torque limit threshold (trlrat). Based on that comparison, engine torque may be limited. It should be appreciated that there could be predetermined values associated with a variety of gear ratios—rather than a single threshold. 
     With continuing reference to FIG. 6, in the preferred embodiment, at step  116 , the ECM  20  determines if the VGR has not been below the threshold plus/minus some hysteresis (trlhys) since it was last above trlrat. Generally, if VGR is decreasing, VGR is compared to the quantity (trlrat−trlhys), whereas VGR is compared to the quantity (trlrat+trlhys) if VGR is increasing. The use of hysteresis has known benefits. Based on the comparison at step  116 , engine torque is limited to the value of the low gear torque limit (trllim) at step  118 , which is calibratable. The result is that when the transmission is in a low gear (i.e. high engine speed relative to vehicle speed), the engine torque is limited. In this manner, a lighter duty transmission, with its attendant cost savings, can be utilized. 
     In the preferred embodiment, if the engine is being shutdown, due for example to the existence of a stop engine condition, final torque is set to zero. The determination of final torque (FTQ) varies based on numerous considerations described below. If the electronic control module is a master controller (rather than a slave controller), and a slave-to-master message has been read from a communications link (such as an SAE J1939 link) during the current ignition cycle, then final torque (FTQ) is determined according to: 
     
       
           FTQ =min( RDTQ   CTL   , RDTQ   MSS   , SCTQ ) 
       
     
     wherein RDTQ CTL  is the rampdown torque determined by the ECM, RDTQ MSS  is the rampdown torque from the most recent message received over the link, and SCTQ is the smoke control torque. 
     Otherwise, if the vehicle speed sensor is enabled, low gear torque limit is enabled, external engine synchronization is not set, and a vehicle speed sensor failure fault is detected, FTQ is: 
     
       
           FTQ =min( RDTQ, trllim, SCTQ ) 
       
     
     wherein RDTQ is rampdown torque, trllim is the low gear torque limit value having a range of 0-100% and a coarse resolution of 0.5%, and SCTQ is smoke control torque. Thus, the minimum of the three quantities is utilized as final torque. 
     Otherwise, if the vehicle speed sensor is enabled, low gear torque limit is enabled, external engine synchronization is not set, and the vehicle speed is less than the cruise control minimum speed to set the cruise, FTQ is determined according to: 
     
       
           FTQ =min( RDTQ, trllim, SCTQ ) 
       
     
     Otherwise, if the vehicle speed sensor is enabled, low gear torque limit is enabled, external engine synchronization is not set, and VGR has not been below trlrat−trlhys since it was last above trlrat, then FTQ is determined according to: 
     
       
           FTQ =min( RDTQ, trllim, SCTQ ) 
       
     
     wherein trlrat is the low gear torque limit VGR threshold having a range of 0-300 and a default of 0.01 RPM/MPH, and trlhys is the low gear torque limit VGR hysteresis having a range of 0-300 and a default of 0.01 RPM/MPH. Otherwise, FTQ is determined according to: 
     
       
           FTQ =min( RDTQ, SCTQ ) 
       
     
     Rampdown torque (RDTQ) is determined based on a stop engine limit torque, an over temperature limit torque, and a marine limit torque, according to: 
     
       
           RDTQ =(min  SETQ, OTTQ, MLTQ ) 
       
     
     wherein SETQ is the stop engine limit torque, OTTQ is the over temperature limit torque, and MLTQ is the marine limit torque. 
     Generally, stop engine torque limiting occurs when a stop engine condition exists, such as low oil pressure. In the preferred embodiment, SETQ is determined according to: 
     
       
         SET Q =max( setmin, ST*STPTST ) 
       
     
     wherein setmin is the stop engine minimum torque, ST is the saved torque—the value of final torque FTQ at the time the first stop engine condition occurred, and STPTST is the stop engine throttle scaling time, which has a range of 0-100 and a coarse resolution of 0.5% 
     Thus, to determine the stop engine torque limit, the ECM  20  compares the values of the stop engine minimum torque, and the quantity of the saved torque and the stop engine scaling time, and sets SETQ as the maximum of the compared values. If no stop engine condition exists, SETQ is 100% of the available engine torque or the final torque (FTQ). 
     As shown above, the rampdown torque is also based on an over temperature limit torque. Generally, over temperature torque limiting occurs when at least one over temperature condition exists. Typical over temperature conditions include, but are not necessarily limited to, excessive cylinder head temperatures, coolant temperatures, oil temperatures and transmission temperatures. If over temperature protection is enabled, in the preferred embodiment, OTTQ is determined according to: 
     
       
           OTTQ =max( setmin, ST*DIWTST ) 
       
     
     wherein ST is saved torque—the value of filtered torque (STQ) at the time the over temperature condition began, and DIWTST is the warning throttle scaling table value, the value of which varies based on the magnitude of the over temperature. In one embodiment, DIWTST assumes a value between 0 and 100. 
     Marine limit torque is also utilized in determining the rampdown torque. Preferably, the marine limit torque is determined according to:        MLTQ   =     lmt   +     {         tqtret   -     TMR   TL         tqtret   -   tqtrst       *     (     MAXTQ   -   lmt     )       }                       
     and 
     
       
           lmt=DTQLMT ( ENG RPM)(1 +TQADV ( ARN,ENGRPM )) 
       
     
     wherein tqtret is the maximum torque reduction end time, TMR TL  is the torque limiting timer the value of which represents the time since the torque limit was exceeded, tqtrst is the maximum torque reduction start time variable, MAXTQ is the maximum torque of the engine, DTQLMT is the digital torque limiting table value which is based on the engine speed (ENGRPM), TQADV is the torque adjustment table value which is based on the engine rating and the engine speed, and ARN is the active rating number. If the TMR TL  is less than the value of the maximum torque reduction start time variable (tqtrst), then the marine limit torque is 100% of the final torque. 
     As shown above, the final torque FTQ is determined with a comparison to SCTQ, the start and smoke control torque. If the engine is in the start mode of operation, SCTQ is determined according to: 
     
       
         
           SCTQ=RTQI+SMDTQ 
         
       
     
     wherein RTQI represents the driver requested torque and SMDTQ is the starting torque, the value of which varies based on oil temperature and engine speed. In the preferred embodiment, the engine is in the start mode of operation if the engine speed is within a predetermined speed window. More specifically, the engine is in a start mode if the engine speed has not been above the quantity smiddl+ISPD (wherein smiddl represents a predetermined delta speed above the engine idle speed which must be exceeded to exit the start mode of engine operation, and ISPD represents the idle speed) since the engine speed was last below smback (which represents the engine speed to reenter the start mode). 
     Otherwise, if governor torque (GOTQ) exceeds the smoke limit torque (SLTQ), the engine is in a smoke control mode of operation, and SCTQ=SLTQ, which is determined utilizing the smoke limit torque (SCTORQ) function, the value of which varies based on engine rating (ARN), SCBST (smoke control boost pressure), and engine speed (ENGRPM): 
     
       
           SLTQ=SCTORQ ( ARN, SCBST, ENGRPM ) 
       
     
     Otherwise, SCTQ=requested torque. 
     In a preferred embodiment, the ECM  20  also implements improved fan control logic. A detailed discussion of fan control is provided in U.S. patent application Ser. No. 08/113,424, filed on Aug. 27, 1993, titled “Method for Engine Control” and assigned to the assignee of the present application, the specification of which is hereby expressly incorporated by reference in its entirety. More specifically, this feature adds the capability of providing fan operation based on the operational state of the transmission retarder, the coolant temperature, manifold air temperature, or the air inlet temperature. In a preferred embodiment, the system and method of the present invention impose a minimum engine output torque requirement prior to operating the cooling fan due to a high air temperature indication. 
     According to the present invention, the ECM  20  includes a digital input function providing for fan operation when the transmission retarder has been activated for a period of time and the coolant temperature has exceeded a particular temperature. In this way, ECM  20  automatically energizes the fan to assist in the cooling of the engine to anticipate heat which will be absorbed by the coolant due to the operation of the transmission retarder. Prior art systems often rely primarily on the coolant temperature to activate the cooling fan. By anticipating the rise in coolant temperature, the present invention provides improved control of the engine operating temperature which has a number of attendant advantages as discussed below. 
     Still further, in a preferred embodiment, the ECM  20  includes an air temperature torque limit. If any part of the fan system were to fail, or if a vehicle operator forgets to remove a winter radiator cover prior to driving in warm ambient temperatures, the compressed air from the turbocharger will not be cooled sufficiently prior to delivery to the cylinders. As a result, combustion temperatures rise, as does friction, wearing the cylinder walls and piston prematurely. Although other configurations are possible, this feature is preferably configured such that torque is reduced if the air inlet temperature is greater than ATNTMP and engine speed is greater than ATNRPM to a maximum reduction of ATQMIN at ATXRPM. Setting ATNRPM to an RPM greater than the maximum transmission override RPM will disable this feature. 
     More specifically, whenever the air (compressed or uncompressed) inlet temperature rises above ATNTMP and the engine speed exceeds ATNRPM, an absolute torque limit will be computed. The torque of the engine will not exceed this limit. The air temperature torque limit is preferably configured to approximate constant horsepower for a given temperature which provides predictable engine behavior and appropriate engine protection. If the air temperature sensor fails high, the air temperature will ramp down or up to the default air temperature. The input filter constant AIIFC determines the ramp rate which preferably spans several seconds so that abrupt torque limit jumps do not occur. 
     The air temperature torque limit is determined according to:        A   =     max        (     0   ,       ATXTMP   -   AIT       ATXTMP   -   ATNTMP         )               B   =     max        (     0   ,     1   -       ENGRPM   -   ATNRPM       ATXRPM   -   ATNRPM           )                       C=A* ( ATQMAX−ATQMIN )+ B* (1 −ATQMIN )+ ATQMIN   
     
       
           ATTQ=MIN (1,  MAX ( ATQMIN, C )) 
       
     
     wherein AIT is the air temperature (air inlet or manifold air temperature), ATNRPM is the engine speed at or below which ATTQ is 100% for any air temperature, ATQMAX is the absolute torque limit when the engine rpm is greater than or equal to ATXRPM and the air temperature is less than or equal to ATNRMP, ATQMIN is the minimum absolute torque limit (ATTQ equals this value when speed is greater than or equal to ATXRPM and the air inlet temperature is greater than or equal to ATXTMP), ATTQ is an absolute torque limit and is a factor in determining the rampdown torque (the final torque (FTQ) can not exceed this limit), ATXRPM is the engine speed at which ATTQ=ATQMAX when air temperature≦ATNTMP and engine speed is at or above which maximum torque reduction occurs, specifically ATTQ=ATQMIN when air temperature≧ATXTMP, and ENGRPM is the engine RPM averaged over 90° for 8- and 16-cylinder engines, 180° degrees for 4-cylinder engines, and 120° for others. 
     Referring now to FIG. 7, a flow chart illustrating an idle shutdown feature between ambient temperature limits according to the present invention is shown. Similar to the previously described features, this feature is implemented by ECM  20 . This feature provides for selective engine shutdown after a predetermined (or possibly adaptive) time period during which predetermined conditions are met. Since idling at low idle speeds produces sulfuric acid which deteriorates oil quality and may attack bearings, rings, valve stems, and other engine surfaces, this feature limits the period of time which an operator can allow the engine to idle. Furthermore, this feature helps to improve the overall fuel economy of the vehicle while reducing noise and emissions. 
     Block  130  of FIG. 7 performs various initialization functions such as determining whether an ambient air temperature sensor has been installed and is working properly, whether the parking brake is set, and the engine is idling, i.e. the accelerator pedal is not depressed and the high idle function is not active. The high idle function is provided to facilitate a vehicle warm-up cycle while avoiding the disadvantages associated with a low idle as described above. If an ambient air temperature sensor is not installed or is installed but not configured, the system will estimate the ambient air temperature as illustrated and described in detail with reference to FIG.  8 . Similarly, if an ambient air temperature sensor is installed and a short circuit to ground is detected, the ambient air temperature will be estimated as described below. If an ambient air temperature sensor is installed and a short circuit to the vehicle battery is detected, the ambient air temperature is set to a value of 70° F., a fault is recorded, and the check engine light is illuminated. This prevents the idle shutdown from being overridden as long as 70° F. falls within the range established by the lower calibration limit (LL) and upper calibration limit (UL) as explained below. The idle shutdown feature can be disabled by setting the upper calibration limit (UL) to a value less than the lower calibration limit (LL). 
     Block  132  of FIG. 7 determines the ambient air temperature. This may be accomplished directly by monitoring the appropriate sensor, or may be an estimate as described in detail below. Block  134  monitors the idle timer to allow idling for a predetermined period of time, preferably five (5) minutes, before warning the operator of an impending engine shutdown at block  136 . This warning may be any appropriate signal such as a buzzer, light, or the like. In a preferred embodiment, the check-engine light flashes for about ninety (90) seconds prior to engine shutdown. 
     During the warning period, the operator may override the shutdown, as indicated by block  138 , provided operator override is enabled as determined by a calibration variable which may be set via user interface  34 . Preferably, an override request is indicated by momentarily depressing the accelerator pedal. If operator override is not allowed as determined by the calibration variable, or the override is not requested by depressing the accelerator pedal (or a similar indication), the engine will be shutdown as indicated by block  150 . If an operator override is enabled and detected by block  138 , then control is passed to block  140 . 
     Since it is desirable to allow extended idle periods under certain ambient temperature conditions, such as those which may justify operation of the vehicle heating or cooling system, block  140  determines whether the current ambient air temperature (AATMP) is within the range determined by the values of LL and UL. In a preferred embodiment, LL has a value corresponding to about 40° F. and UL has a value corresponding to about 80° F. If the value of AATMP is within the limits determined by the values of LL and UL, or the value of AATMP exceeds a high limit (HL) value, as determined by block  140 , control passes to block  144 . Otherwise, block  142  resets a delay timer which results in an unlimited idling time while the appropriate operating conditions are satisfied. The value of HL, which is preferably about 176° F., is utilized to detect an attempt to defeat the sensor by placing the sensor on a relatively hot surface. Thus, if the ambient air temperature determined by a temperature sensor exceeds this value, then the operator override is disabled resulting in engine shutdown as indicated by block  150 . 
     If the ambient air temperature is being estimated as described above and illustrated by block  144 , the delay timer is set (if it is not already running) at block  146 . Preferably, the delay timer is set to about twenty (20) minutes. Block  148  then determines whether the delay timer has expired. The delay timer provides a sufficient settling time for various parameters used for the ambient air estimation function as described below. Once the timer initiated at block  142  or  146  expires, the engine will be shut down as indicated at block  150 , i.e. only one override is allowed per ignition cycle. Similarly, if an ambient air temperature sensor is installed and functioning properly, settling time is not required and the engine is shutdown after the predetermined idling period elapses. 
     Referring now to FIG. 8, a graphical representation of ambient air temperature estimation is shown. In a preferred embodiment, ambient air temperature is estimated using information commonly available from standard engine sensors according to:        temp   =       T   air     -     max        {     0   ,           T   oil     -     T   air         RPM     K   E         *     T     a   off           }       +     Fan   off                       
     and 
     
       
           T   AATP   =T   AATP   +K   F *(temp− T   AATP ) 
       
     
     where T air  represents air temperature  166  which is the value of the manifold air temperature if a manifold air temperature sensor has been configured, as represented by block  160 . Otherwise, air temperature  166  is equal to the air inlet temperature (AIT)  162 , described above, as represented by element  164 . Block  170  represents the engine oil temperature (T oil ) as determined by an associated sensor. Block  172  represents engine RPM which is scaled by the value of K E  (6 in a preferred embodiment), as represented by block  174 . Block  180  represents the ambient air offset factor (T aoff ) and block  188  represents a cooling fan offset (Fan off ). The “max” function, represented by block  182  selects the greater of the values within braces and delineated by a comma, i.e. the “max” function returns a value of zero for otherwise negative values. The ambient air temperature (T AATP ), represented by block  196 , is equal to its previous value added to a scaled difference value as determined by scaling factor K F , represented by block  194 . Engine speed is included in the estimate since, as engine speed increases, the air temperature has less time to be heated by the engine. Similarly, the cooling fan offset is included when the cooling fan is on since this also reduces the air temperature. 
     FIG. 9 provides a graphical representation of absolute torque versus engine speed which illustrates the relationship between engine speed, air temperature and torque limit in a vehicle system such as in a preferred embodiment of the present invention. 
     It is understood, of course, that while the forms of the invention herein shown and described constitute the preferred embodiments of the invention, they are not intended to illustrate all possible forms thereof. It will also be understood that the words used are descriptive rather than limiting, and that various changes may be made without departing from the spirit and scope of the invention disclosed.