Abstract:
The present invention provides a system and method for controlling the retard limit of an engine having one or more cylinders in which combustion occurs. A sensor is employed to detect the ionization of the combustion process in the cylinders. The ionization is related to a retard limit feedback, and the sensor further transmits a signal associated with the retard limit feedback. A controller relates the retard limit feedback signal to a metric, monitors the stochastic behavior of the metric, and adjusts the ignition timing of the engine in response to the stochastic behavior to operate the engine below a retard limit target.

Description:
BACKGROUND 
     The present invention generally relates to a system that manages the retard limit in internal combustion engines. 
     Internal combustion engines are designed to maximize power while meeting exhaust emission requirements and minimizing fuel consumption. In a conventional spark-ignition (“SI”) engine, combustion is initiated at the spark plug by an electrical discharge. Recent advances in the powertrain electronic controls (“PCM”) make it possible to employ online spark adjustment to optimize the engine operation in terms of power, fuel economy, and emissions. However, the range of ignition timing is often limited by knock in the advance direction and by combustion instability (i.e., partial burn and misfire) in the retard direction. Since the feasible ignition timing range depends on the engine operating conditions, it is desirable to extract online in-cylinder combustion information to optimally adjust the operational boundaries of the engine. 
     SUMMARY 
     In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides a system and method for controlling the retard limit of an engine having one or more cylinders in which combustion occurs. A sensor is employed to detect the ionization of the combustion process in the cylinders. The ionization is related to a retard limit feedback, and the sensor further transmits a signal associated with the retard limit feedback. A controller relates the retard limit feedback signal to a metric, monitors the stochastic behavior of the metric, and adjusts the ignition timing of the engine in response to the stochastic behavior to operate the engine below a retard limit target. The stochastic behavior may include the mean, standard deviation, and the stochastic distribution of the retard limit feedback. 
     The controller may include one or more feed back loops. For example, an adaptive feedback loop may be employed to adjust the retard limit target in response to changing operating conditions. The controller may include a stochastic feedback loop that maintains the mean value of the retard limit at a level to ensure that the retard limit feedback does not exceed the retard limit target. Additionally, an instant correction feedback loop may be employed to avoid engine misfire when the stochastic feedback loop fails to keep the retard limit feedback below the retard limit target. 
     Although these limits can be imposed in the form of open loop tables through intensive calibrations, among other advantages, the use of an ionization signal as a feedback signal enables optimizing the limits online, such that calibration efforts are minimized. 
     Further features and advantages of this invention will become readily apparent from the following description and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a system to control the retard limit in an IC engine in accordance with the invention; 
         FIG. 2  is a plot of the ionization behavior in a cylinder; 
         FIG. 3A  is a plot of the stochastic behavior of the ionization peak; 
         FIG. 3B  is a plot of the stochastic behavior of the ionization distribution; and 
         FIG. 4  is a block diagram of a controller of the system of  FIG. 1  in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now  FIG. 1 , a retard limit management system  10 , embodying the principles of the present invention, utilizes signals associated with ionization that occur during the in-cylinder combustion process of an engine to maintain the desired combustion quality and prevent the occurrence of misfire under all operating conditions. In this way, the system  10  smoothens the combustion process and minimizes emissions from the engine. 
     The system  10  implements a stochastic control approach for ignition retard limit management of an internal combustion (IC) SI engine. The system  10  processes an in-cylinder ionization signal to derive a metric for combustion quality and closeness of combustion to partial burn/misfire limit, which is, in turn, used to provide a limiting value for the baseline ignition timing in the retard direction. For normal operations, this assures that the combustion variability is kept within an acceptable range. During start-up operations, the retard limit management further implements a rapid catalyst light-off strategy by maximally delaying the combustion as long as misfire and partial-burn are avoided. This improved start-up strategy reduces cold-start hydrocarbon emissions by reducing the time required to increase the catalysts temperature to its light-off level. The closed loop nature of the system  10  provides maximum usage of the possible ignition timing range in the retard direction in any given operating conditions. 
     More specifically, during normal operating conditions, if the baseline ignition strategy (that is, the baseline timing to achieve MBT for the best fuel economy) tends to push the timing to a level where the combustion variability is not acceptable, the retard limit management system  10  limits the ignition in the retard direction. Since this limit is continually adjusted by monitoring the combustion process via the in-cylinder ionization signals, different criteria can still be weighted-in and optimized online in determining the final timing instead of using a one-fits-all limit value or map. 
     Further, during engine warm-up, the retard limit management system  10  seeks the maximum retard possible while assuring that misfire is avoided with the objective of increasing the catalyst temperature rapidly. Delaying the combustion through high values of ignition retard can shorten the time that the catalyst reaches its light-off temperature; therefore, the closed loop control of the air-fuel-ratio becomes effective much sooner and starts reducing tail-pipe emissions. However, if the retard is too high, hydrocarbon emissions become excessive because of due to incomplete combustion, as well as misfire. An open loop retard calibration needs to provide enough margins to avoid misfire at all conditions and therefore is inherently conservative. On the other hand, the closed loop strategy employed in the system  10  alleviates this conservatism by further pushing the timing in the retard direction if things are favorable. In this way, the catalyst light-off time is minimized and tight air-fuel-ratio control can be activated earlier to control tail-pipe emissions most effectively. 
     The architecture of stochastic retard limit management system is shown in  FIG. 1 . As its primary components, the system  10  includes a signal conditioning module  12 , a knock intensity calculation module  14 , a stochastic knock control limit module  16 , an ignition control strategy module  18 , and an ignition control signal general module  20 , all associated with a powertrain control module (“PCM”)  21 , and a set of ionization detection ignition coils  22  associated with respective cylinders. 
     Each ionization detection ignition coil  22  provides a single ionization output signal  23 , and the signals from all the cylinders are fed into the signal conditioning module  12  where the signals are merged into one or two signals with an analog switch that is based upon combustion event information of the current cylinder. The conditioned signals are then sampled in the retard limit metric calculation module  14  (nominally, at one crank degrees resolution) and processed to determine the ionization first peak location and ionization percentage distribution for each combustion event. This information is sent to the stochastic retard limit control module  16 , which determines the appropriate retard limit. The retard limit information is then passed to the ignition control strategy module  18  which instructs the ignition control signal generation module  20  to provide a dwell control input command signal  25  to the cylinders. 
     As shown in  FIG. 2 , the first ion peak location is defined as the crank angle at which the chemical-ionization part of the ion signal takes its maximum value. The ionization signal is also cumulatively integrated over a user-specified window, and the crank angle at which the ion integral reaches a calibratable percentage of the total area is determined. This parameter is referred to as the ionization percentage distribution. 
     The normalized ion integral is also shown in  FIG. 2 . It is further possible to use the square integral in this computation instead. The bias level of ionization signal is subtracted in these calculations. Note that, 100% distribution location is ideally reached when the ionization signal completely dies out. A percentage close to 100 is used to approximately locate the crank location after which the ionization signal strength (combustion activity) is minimal. The relative crank location of this parameter can be interpreted to monitor how the combustion evolves at each stroke (expansion and exhaust). 
       FIG. 3  shows the stochastic properties of the introduced ionization retard limit parameters. Experimentally, it has been observed that these parameters show variations. In fact, the cycle-to-cycle event variation in the combustion process results in a feedback signal that is similar to the output of a random process. The mean value of each parameter increases as the timing is moved in the retard direction, demonstrating good controllability and consistent control gradient. This invention proposes to use not only the raw derived retard limit parameters but also their stochastic properties (mean, variance, as well as the evolution of its stochastic distribution) for closed loop control. 
     Referring also to  FIG. 4 , there is shown details of a stochastic closed-loop knock borderline limit controller  60  in which the control module  16 , the ignition control strategy module  18 , and the ignition control signal generation module  20  are implemented. The primary inputs to the controller  60  include:
         a) A engine speed/load input  62  associated with the current engine operational speed and load.   b) A desired retard criteria  64  which defines the retard limit target. For normal operations, the maximum allowed combustion variation (COVariance of Indicated Mean Effective Pressure IMEP) is input as a calibration parameter for block  64 . For start-up conditions, a less conservative limit target is selected since heating up the catalyst as fast as possible while making sure that no misfire occurs has more weight over the combustion quality.   c) A retard limit feedback signal  66  which is either one or the composite of the derived retard limit parameters from the ionization signal. The feedback signal  66  is a scalar feedback parameter updated at each firing event.   d) A desired ignition timing input  68  which is the signal from the ignition control strategy module  18  based upon the engine MBT (“Maximum Brake Torque”) ignition timing for the best fuel economy.       

     There are three feedback loops associated with the controller  60 , in particular,
         a) An adaptive seeking feedback loop  70  which provides a two-fold purpose: 1) reducing the calibration conservativeness of operating the engine at its “TRUE” retard limit target, and 2) improving robustness of the retard limit controller  60  when the engine is operating at different environments. This is accomplished by using an error signal between the initial desired retard limit  64  and a stochastic criterion, referred to as a distribution maximum value, which is obtained by finding the maximum value of a given distribution (e.g., 90%). For instance, if the given distribution is 90%, 10% of the data in the stochastic calculation buffer is above the calculated maximum value, and the distribution maximum value is the maximal of the remainder of the 90% of the data. The adaptive seeking algorithm  92  described below reduces the desired retard limit when the distribution maximum value is greater than the retard limit; otherwise, it increases the desired retard limit value.   b) A stochastic feedback loop  72  which keeps the mean of retard limit feedback  66  at a certain level. The final retard limit target is obtained by further reducing the initial retard limit target by an offset value generated by the stochastic information calculation block  82  and the confidence level offset block  94  described below. The calculation of the offset value is either based upon the variance of the retard limit feedback or its stochastic distribution. When using the variance, with a given confidence level (e.g., 93.3% of the retard limit feedback is below the desired retard limit), the offset value is based upon the assumed normal distribution function (e.g., for 93.3%, the offset value is three times the variance); and when using the distribution function directly, the offset value is found by locating the given confidence level over the distribution function.   c) An instant correction feedback loop  74  which serves to avoid engine misfire when the stochastic feedback control loop  72  does not prevent the engine from moving beyond its retard limit. Note that this control feedback loop feeds to an integration portion of a PI controller  100  described below.       

     The controller  60  includes the following components:
         1) A retard limit map  80  that includes a lookup table with “engine speed” and “engine load” as inputs. The output from the map  80  is the “retard limit target” that is the product of the lookup table output and the desired retard criteria  64 . The map  80  enables the control system  10  to be calibrated for different retard comfort levels at different operating points and conditions.   2) A stochastic information calculation block  82  calculates four key variables used for the three feedback loops  70 ,  72 , and  74  and receives the retard limit feedback as an input. The data is buffered for stochastic analysis with different data lengths. The four outputs are defined as follows:
           i) A “distribution max value” defined as the maximum value over a given distribution, where the data is arranged from low to high. For example, if the given distribution is 90 percent, 10 percent of the data will be above the calculated distribution max value, and the distribution max value is the maximal of the remainder of the 90 percent of the data.   ii) A “long standard deviation” defined as the finite standard deviation based upon the actual retard limit feedback  66  with a specific data size, where the data length is relatively longer than the shorter data length defined below in iv).   iii) A “long mean” defined as the calculated mean using the same data length of the long standard deviation   iv) A “short mean” defined as the calculated mean using a relatively shorter data length than the longer data length.   
           3) An adaptive seeking algorithm 92 which utilizes the error between the “retard limit target” from the map  80  and the “distribution max value” to generate the adaptive seeking output by integrating the error over the engine combustion event with a given gain. This adaptation is performed on a much smaller time scale as compared to the control loop. The purpose of the algorithm is to reduce the conservativeness of the stochastic feedback loop  72 .   4) A confidence level offset  94  which calculates a confidence offset value to be subtracted from the “retard limit target” after it is corrected by the adaptive seeking algorithm 92. The final target value is the target from the mean regulation loop. The offset value can be determined by: 1) defining the offset value as a given value multiplied by the “long standard deviation”; or 2) making the offset value as a function of the engine speed and load  62 .   5) An instant correction map  96  which calculates an instant correction signal to be fed in to the integration portion  106  of the PI controller  100  described below. When the error between the “retard limit target” and the “retard limit feedback” is greater than zero, the output is zero; that is, no correction is required. And when the error is less than zero, the error is fed into a one-dimensional lookup table and the output is set as a negative instant correction signal for the integration portion  106  of the PI controller  100 .   6) A dynamic feedforward  98  which includes a two-dimensional lookup table using engine speed and load  62  as inputs and a dynamic controller using the lookup table output as input. The lookup table generates an open-loop borderline retard limit signal as a function of engine speed and load  62 , and the dynamic controller consists of a first order controller with a time constant that is a function of engine load rate change.       

     7) A PI controller  100  with three primary components: a) a feedforward control  102 ; b) a proportional control  104 ; and c) an integration control  106 . The feedforward control  102  takes the output of the dynamic feedforward block  98  as an input and adds it to the output of the PI controller  100 . The input to the proportional control  104  is the error between the “long mean” variable of the stochastic information calculation block  82  and the “retard limit target” signal. This further adjusted by subtracting the offset value obtained from the confidence level offset block  94 . The proportional control  104  further multiplies the input with a proportional gain, and adds the result to the PI controller  100  output. Input to the integration control  106  includes both input from the instant correction loop block  74  and the mean error between the “short mean” from the stochastic information calculation block  82  and the “retard limit target” from the retard limit map  80  signal, with the error being further adjusted by subtracting the offset value obtained from the confidence level offset block  94 . Note that using a relative “short mean” error for the integration improves the response time.
         8) A saturation management  110  which provides an average ignition-timing signal. If the PI controller  100  output is more retarded than the desired ignition timing  68 , the output becomes the desired ignition timing  68 ; otherwise, the output is the output from the PI controller  100 .   9) An individual cylinder retard limit  112  which generates an offset vector based upon the retard limit feedback  66 . Since the ignition retard limit is different for each individual cylinder, this block  112  calculates the offset value for each individual cylinder so that the global retard limit, generated by the PI controller  100 , can be corrected afterwards.   10) An individual cylinder compensation  114  which adds the offset value to the average ignition retard limit generated by the PI controller  100 .       

     In sum, the system  10  utilizes in-cylinder ionization signals to determine a metric for ignition retard limit in terms of the first ionization peak location and the ionization signal distribution. The stochastic properties (e.g., the mean, variance and probability distribution functions) of the peak ionization location and the ionization percentage distribution are correlated to a combustion quality criterion (e.g., the COVariance of Indicated Mean Effective Pressure IMEP) and the combustion type (e.g., the normal, late but complete, partial-burn or incomplete and misfire). A stochastic closed loop controller manages the ignition in the retard direction to maintain good combustion quality at normal operating conditions and applies an aggressive controlled retard limit search during a short time window (e.g., less than 60 seconds) at warm-up. 
     As a person skilled in the art will readily appreciate, the above description is meant as an illustration of an implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.