Abstract:
Preignition in an engine is prevented by identifying engine conditions that relate to autoignition. If one or more of the engine conditions exceed a threshold, a controller retards the spark in the engine. The controller detects whether autoignition occurs in the engine due to the retarded spark advance. If autoignition is detected, the controller corrects the one or more engine conditions by at least one of reducing a throttle opening, cutting of fuel to a cylinder, adjusting a camshaft phase, and storing a diagnostic code.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electronic engine management, and more particularly to preventing preignition with electronic engine management. 
     BACKGROUND OF THE INVENTION 
     A typical vehicle engine relies on internal combustion within engine cylinders to provide torque. A spark generated by a spark plug ignites a mixture of air and fuel within the cylinders to cause combustion. Spark timing and control of the air/fuel mixture determine the performance of the engine. 
     Autoignition may cause damage to the cylinders, pistons, and valves. Autoignition occurs when the air/fuel mixture combusts without spark. Ideally, the spark ignites the air/fuel mixture slightly before the piston reaches top dead center (TDC). However, when a combustion chamber or spark plug surface in a cylinder reach a sufficiently high temperature, auto ignition occurs. Preignition refers to autoignition that occurs before proper spark timing. 
     Causes of preignition include, but are not limited to, low coolant, coolant system deposits, combustion chamber deposits, improper sparkplug heat range, and spark plug electrode wear and failure. Additionally, the use of low octane fuel, lower than the control capability of the knock control system within the engine management system, may cause preignition. Poor fuel flow resulting in a lean air/fuel ratio may also lead to preignition. 
     An ion sensing system may detect preignition. The ion sensing system applies current to a sparkplug gap. If ions are present in the gap, a measurement of the current may provide an indication of conditions within the combustion chamber. Combustion efficiency and ignition faults may be derived from the current measurement. 
     SUMMARY OF THE INVENTION 
     A method for preventing preignition in an engine includes identifying at least one engine condition that relates to autoignition. The spark is retarded in the engine for a single cycle if the engine condition exceeds a threshold. It is then detected whether autoignition occurs in the engine during the retarded spark advance. The autoignition is eliminated by at least one of the following: Reducing a throttle opening, cutting off fuel to a cylinder, adjusting a camshaft phase, and storing a diagnostic code. 
     Another feature of the invention includes detecting autoignition by determining whether a cylinder misfire occurs in the engine due to retarding spark. If no cylinder misfire is detected, autoignition is diagnosed. Diagnosing autoignition further includes calculating a ratio of autoignition occurrences per number of engine cycles. The ratio is compared to a threshold. If the ratio exceeds the threshold, autoignition is diagnosed. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a block diagram of an engine control system; and 
         FIG. 2  is a flow diagram illustrating steps of a method for preventing preignition. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring to  FIG. 1 , an exemplary engine control system  8  is shown. A throttle  10  and a fuel system  12  determine the air and fuel delivered to an engine  14  through an intake  16 . An ignition system  18  ignites the air/fuel mixture in the engine  14 . Exhaust gas created by the ignition of the air/fuel mixture is expelled through an exhaust manifold  20 . A catalytic converter  22  receives the exhaust gas and reduces the emissions levels of the exhaust gas. 
     A controller  30  communicates with various components of the engine control system  8 , including but not limited to a throttle position sensor  32  (TPS), the fuel system  12 , the ignition system  18 , and an engine speed sensor  34  (RPM). The controller  30  receives a throttle position signal from the TPS  32  and uses the information to determine airflow into the engine  14 . The airflow data is then used to calculate fuel delivery from the fuel system  12  to the engine  14 . The controller  30  further communicates with the ignition system  18  to determine ignition spark timing. 
     The controller  30  may receive additional feedback from other components in the engine control system  8 , including but not limited to coolant temperature from a coolant temperature sensor  36  and engine speed from the engine speed sensor  34 . These and other variables may affect the overall performance and behavior of the engine control system  8 . The controller  30  utilizes data gathered from the various engine components to monitor and, in some cases, optimize engine performance. 
     In the present invention, the controller  30  communicates with components such as the TPS  32 , the engine speed sensor  34 , the fuel system  12 , and/or the ignition system  18 . If the controller  30  determines that conditions-such as engine speed, coolant temperature, and/or throttle position-are such that autoignition is likely, the controller  30  initiates a preignition prevention method according to the present invention to prevent autoignition. 
     Referring to  FIG. 2 , a preignition prevention method  100  for an electronic engine controller is shown. The preignition prevention method  100  attempts to detect autoignition and prevent subsequent autoignition before engine cylinder temperatures increase enough to cause preignition. The engine controller begins the preignition prevention method at step  110 . One or more engine conditions must be met for autoignition to occur. These engine conditions typically exist under high engine loads and make autoignition events more likely. 
     The engine conditions, which are calibratable, are determined at step  120 . For example in step  120 , control determines whether engine coolant is above a certain temperature. Other engine conditions affecting autoignition include, but are not limited to, engine speed, throttle position, air/fuel ratio, manifold pressure, cylinder airflow, and/or knock. The controller  30  may require that the conditions be met for a particular time period before proceeding with the sub-routine  100 . Additionally, all conditions do not necessarily have to be met. In one embodiment, all conditions must be met for the specified time period. In an alternative embodiment, only certain conditions must be met. For example, the controller  30  may assign a different weight to each condition and calculate an overall likelihood of autoignition from collected data. The controller  30  may also require M out of N conditions to be met. Still other variations will be apparent to those skilled in the art. 
     If the likelihood of autoignition conditions are not met, the preignition prevention method  100  terminates at step  160 . If the particular autoignition conditions are met, however, the sub-routine  100  proceeds to step  130  where the controller  30  retards spark. The amount of spark retard is calibrated to assist autoignition detection. The spark retard should be sufficient to reduce torque enough to be considered a cylinder misfire. Typically, the spark retard value is larger than a typical knock control system. A typical knock control system retards spark a minimal amount, such as five degrees, to decrease the probability of continued knock and to minimize torque reduction. However, spark retard should not be calibrated too high, which may cause significant combustion in the exhaust system. Combustion in the exhaust system may lead to damage and other performance issues. An example of a spark retard for the present invention is twenty degrees. Testing may be performed to determine the optimal spark retard calibration. For example, a spark angle between zero and ten degrees after top dead center (ATDC) for a fraction of total cylinder cycles at the high load condition may be adequate for most engines to test for autoignition. 
     The controller  30  determines if autoignition was detected after spark retard for the corresponding cylinder in step  140 . The controller  30  checks for a misfire during the cylinder&#39;s cycle. Misfire may be detected by RPM variation analysis techniques. Alternatively, misfire may be detected by ion sense methods, cylinder pressure sensing, or any other suitable method. If a misfire is not detected, the controller  30  may assume that autoignition occurred sufficiently before TDC so that torque was not reduced significantly as compared to torque variance associated with a steady state spark advance. The absence of a misfire indicates that the intra-cylinder environment has sufficient energy to cause preignition. A filter may be used to increase confidence in this determination. The filter may calculate a ratio of autoignition counts per cycle and compare the calculation to a calibratable threshold. It the threshold is met or exceeded, autoignition is detected and the sub-routine  100  proceeds to step  150 . Otherwise, the preignition prevention method  100  terminates at step  160 . 
     Alternatively, the controller  30  may detect autoignition in step  140  by monitoring ion current through a spark gap. This ion current is monitored when the spark is removed due to the spark retard. Variances in ion current under these conditions may indicate the likelihood of autoignition and preignition. 
     In step  160 , the controller  30  may take corrective or preventative measures to remedy autoignition conditions. Specifically, the controller  30  may take measures to remedy any engine conditions diagnosed in step  120 . These measures may include, but are not limited to, reducing throttle opening, cutting off fuel, and adjusting camshaft phase. Additionally, the controller  30  may store a diagnostic code in controller memory, warn a vehicle driver, and/or activate an autoignition indicator. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.