BACKGROUND OF THE INVENTION 
     The invention relates generally to control systems for internal combustion engines, and more particularly, concerns a powertrain output monitor for electronic throttle control-equipped vehicles. 
     Present powertrain output monitor techniques typically compute an estimate of engine output and compare that value to the requested engine output. Such methods typically take the form of resolving one or more engine operating parameters and comparing the estimated versus requested output value. Such operating parameters can include: engine output torque, engine output power, wheel torque, wheel power, and wheel acceleration. The requested output is typically a function of driver demand as measured by the accelerator pedal position, combined with internally automated demands such as idle speed control and catalyst heating. 
     Due to the complex nature of determining estimated and requested engine output as a function of one or more engine operating characteristics and driver inputs, diagnostics based upon such monitoring techniques are inherently complex. Therefore, there exists a need for a simplified method of monitoring the powertrain control system. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the invention to provide an improved method of monitoring the powertrain output. It is also an object to provide a simplified powertrain output monitor as compared to present output monitoring technologies. 
     According to the present invention, the foregoing and other objects and advantages are attained by a method of monitoring the powertrain controller for an internal combustion engine responsive to an accelerator pedal input, the engine having a throttle responsive to a throttle position command. The method comprises the steps of determining the engine speed, determining the accelerator pedal position, and generating a desired throttle position value. If the commanded throttle position is greater than the desired throttle position value, the commanded throttle position is set equal to the desired throttle position value. 
     In one aspect of the invention, the desired throttle position value is generated as a function of the accelerator pedal position. In another aspect, the desired throttle position value is generated as a function of the accelerator pedal position and engine speed. If the commanded throttle position is greater than the desired throttle position value, the commanded throttle position is limited to the desired value, or other powertrain control action is taken. Such action can include retarding or eliminating the spark timing, reducing the fuel quantity injected per engine stroke, or the fuel delivery rate. 
     An advantage of the present invention is that little or no field calibration is required. Another advantage is that few inputs are necessary, thus, the main control element interface is simplified. 
     Other advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of this invention, reference should now be had to the embodiments illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention. In the drawings: 
     FIG. 1 is a graph of throttle position versus accelerator pedal positions. 
     FIG. 2 is a graph of throttle position versus engine speed for various accelerator pedal positions. 
     FIG. 3 is a schematic diagram of an internal combustion engine and associated control system according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning first to FIG. 1, there is shown a graph of the throttle position versus accelerator pedal position for an engine operating in steady state. As can be seen with reference to region  10  which corresponds to an accelerator pedal position of zero (i.e., the operator&#39;s foot is off-pedal), the throttle position should be near its minimum. In other words, it is undesirable to deliver a large quantity of air when in the foot-off-pedal condition since, in engines operating at stoichiometry or rich of stoichiometry, airflow correlates to power and cylinder air mass correlates to torque. Accordingly, the throttle position at foot-off-pedal, preferably is no greater than the maximum throttle position required for the highest idle torque desired. This corresponds to the region  10  in FIG.  1 . 
     Similarly, when the accelerator pedal is equal to 1.0, i.e., fully deflected, the throttle position should be limited to fully open and no further. This is represented by region  12  in FIG.  1 . 
     Also, when the pedal position transitions from 0.0 (foot-off-pedal) to an intermediate position such as 0.2, the throttle position preferably should not immediately be commanded to its maximum open value. Thus, the region  14  correlates to a desired pedal-to-torque gain for the particular engine under consideration. 
     FIG. 2 shows graphically a similar relationship for the throttle position as it relates to engine speed and accelerator pedal position. When the pedal position (PP) is 0.0 (foot-off-pedal), and the engine speed is above 1600 RPM, the throttle position is minimized as shown as point  20  of FIG.  2 . Similarly, when the pedal position is only slightly deflected (PP=0.08) and the engine is operating approximately 2000 RPM, the throttle position is minimized as shown at point  21 . The difference between the minimum throttle position value for the various pedal positions, i.e., between points  20 ,  21  and  22 , allows for engine braking modulation. In this example, the more the pedal deflects, the less engine braking desired. Similarly, the regions  23 ,  24  and  25  correspond to the maximum throttle position for near-idle conditions. At a certain pedal position such as PP=1.0, the throttle position clip or detection threshold is preferably maximized in accordance with engine speed to allow for the greatest amount of torque. This is shown in FIG. 2 as line  28 . Line  26  represents the throttle position as it relates to a medium pedal deflection value. 
     From the foregoing graphs illustrated in FIGS. 1 and 2, a relationship can be seen between accelerator pedal position and throttle position from which an effective engine output monitoring scheme can be created. 
     Referring now to FIG. 3, there is shown a schematic diagram of an internal combustion engine  40  and associated powertrain control module  42  as well as an operator interface  68  in accordance with one embodiment of the present invention. 
     The engine  40  includes a plurality of combustion chambers  41  each having an associated intake  43  and exhaust  44  operated by respective valves  45 ,  46 . Combustion occurs as a result of the intake of air and fuel from the intake manifold  47  and fuel injector  48  respectively, compression by the piston  49  and ignition by the spark plug  50 . Combustion gases travel through the exhaust manifold  44  to the downstream catalytic converter and are emitted out of the tailpipe. A portion of the exhaust gases may also be recirculated back through the intake manifold  47  to the engine cylinders  41 . 
     The airflow through the intake manifold  47  is controlled by a throttle comprising a throttle plate  51  and throttle actuator  52 . A throttle position sensor  53  measures the actual throttle position. Mass airflow sensor  54  measures the amount of air flowing into the engine  40 . An engine speed sensor  54  provides value indicative of the rotational speed of the engine  40 . 
     The powertrain control module (PCM)  42  receives as inputs the throttle position signal, the mass airflow signal, the engine speed signal, and the driver demand inputs. In response, the PCM  42  controls the spark timing of the spark plugs  50 , the pulse width of fuel injectors  48  and the position of the throttle  51  by way of the throttle actuator  52 . All of these inputs and outputs are controlled by the main microcontroller  60 . The main microcontroller  60  controls the throttle position by outputting a throttle position command to the throttle plate position controller  62  to drive the throttle actuator  52  to the desired position. 
     The PCM  42  includes an electronic throttle control (ETC) monitor  64  which communicates with the main microcontroller  60  and throttle plate position controller  62 . The ETC monitor  64  includes a microprocessor  65  and associated memory separate from the microprocessor in the main microcontroller  60 . The ETC monitor  64  receives as inputs the engine speed signal from engine speed sensor  54 , and the driver demand signal  66  which represents, among other things, the accelerator pedal position  70 . As will be described in further detail below, the ETC monitor  64  monitors the commanded throttle position. 
     Although the ETC monitor  64  is shown separate from the main microcontroller  60 , it is to be understood that it could also be wholly or partially integrated with the main microcontroller  60 . Similarly, the ETC monitor  64  could be wholly or partially integrated into the throttle position controller  62 . 
     The PCM  42  also receives as an input driver demand signals  66 . The driver demand signals can include such things as accelerator pedal position  70 , ignition switch position, steering input, brake sensor, transmission position input, as well as inputs from the vehicle speed control system. 
     In operation, the ETC monitor  64  monitors the accelerator pedal position and engine speed separate from the main microcontroller  60  which executes the primary engine control. In this case, the function of the ETC monitor  64  is to detect throttle position commands as defined by regions  15  and  29  discussed above with respect to FIGS. 1 and 2. 
     From the inputs of engine speed and accelerator pedal position (PP), the ETC monitor generates a desired throttle position value. The desired throttle position value corresponds to the graphs of FIGS. 1 and 2. Accordingly, a first throttle position value is determined as a function of pedal position as shown in FIG. 1. A second throttle position value is then determined as shown in FIG. 2 for the measured pedal position and engine speed. The desired throttle position value is then clipped to the lesser of the first and second values. 
     If the commanded throttle position is greater than the desired value, action may be taken to limit the powertrain output. The action can take the form of limiting the commanded throttle position to the desired throttle position value or other powertrain control action can be taken. Powertrain control action can include retarding or eliminating the spark timing of the spark plugs  50 , eliminating the signal transmitted to the fuel injectors  48 , removing power to the throttle actuator  52  causing a throttle plate  51  to go to a partially open state, and/or varying the amount of exhaust gas recirculation. 
     Additionally, if the desired throttle position value is exceeded, an indicator can be illuminated on the instrument panel of the vehicle to alert the operator. 
     From the foregoing, it will be seen that there has been brought to the art a new and improved powertrain control monitor. While the invention has been described in connection with one or more embodiments, it will be understood that the invention is not limited to those embodiments. On the contrary, the invention covers all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the appended claims.
A powertrain control method for an internal combustion engine responsive to an accelerator pedal input, the engine having a throttle responsive to a commanded throttle position. The method comprises the steps of determining the engine speed, determining the accelerator pedal position, and generating a desired throttle position value as a function of at least the accelerator pedal position and engine speed. If the commanded throttle position is greater than the desired throttle position value, the commanded throttle position is limited to the desired value.