Abstract:
A method for reducing the magnitude of a knock retard angle in a vehicle having a quantity of fuel and a system for determining the quantity of fuel. The method determines a previous fuel quantity and a present fuel quantity, determines from said present fuel quantity and said previous fuel quantity whether the quantity of fuel has increased; and decreases the magnitude of the knock retard angle upon determining the quantity of fuel has increased.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a method of determining spark delivery angle in an internal combustion engine, and more particularly to such a method wherein the quality of fuel being delivered to the engine is subject to change when fuel is replenished. 
     BACKGROUND OF THE INVENTION 
     Modern spark ignited (SI) internal combustion engines are often equipped with a knock sensor for detecting spark knock. Upon detection of knock, which may be caused by poor quality fuel, an engine control system retards the angle of spark delivery to the engine. The amount of retard, which may be referred to as knock retard, may then be decremented after a predetermined amount of time. If the variable, such as poor quality fuel, which caused the spark knock has passed through the engine or otherwise dissipated, then the engine will continue to operate without knocking at the advanced spark angle. 
     While such a system provides protection from spark knock, the engine may operate with knock retard for an undesirable amount of time, even when the variable, such as poor quality gas, has long passed through the engine. 
     SUMMARY OF THE INVENTION 
     To address the described shortcoming in the art, a method is provided for reducing the magnitude of a knock retard angle in a vehicle having a quantity of fuel and a system for determining the quantity of fuel. The method determines a previous fuel quantity and a present fuel quantity, determines from said present fuel quantity and said previous fuel quantity whether the quantity of fuel has increased; and decreases the magnitude of the knock retard angle upon determining the quantity of fuel has increased. 
     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  depicts crankshaft angles during the compression and power strokes of an SI internal combustion engine, and 
         FIG. 2  depicts a method for adjusting knock retard. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Turning to  FIG. 1 , an end view of a crankshaft  12  in a spark ignited internal combustion engine (SIIC) is depicted. Such engines are well known in the art. An outer circle  16  represents 360 degrees of crankshaft  12  rotation in the clockwise direction. The top of outer circle  16  is marked TDC to indicate Top Dead Center of the compression stroke of a given cylinder as is known in the art. Similarly, the bottom of outer circle  10  is marked BDC to indicate Bottom Dead Center of the given cylinder&#39;s power stroke as is known in the art. 
     A line segment indicates a base spark angle  20 , which represents the angle of crankshaft  12  rotation at which a spark would be delivered, absent knock retard angle  14 , to the engine cylinder which is approaching TDC  22 . The total spark angle is represented by line segment  18 , and is calculated by subtracting the knock retard angle  14  from the base spark angle  20 . The base spark angle and total spark angle are measured with respect to TDC  22 , and are expressed as a number of degrees Before Top Dead Center (BTDC). 
     The knock retard angle  14  varies over time. During operation of the engine, the knock retard angle is increased by an engine control unit when it detects engine knock via a knock sensing arrangement as is know in the art. Fuel octane is an engine operating variable that can cause engine knock, and therefore it is known for an engine controller to increase the knock retard angle upon detecting knock, and decreasing the knock retard angle after a predetermined amount of time. In the instant invention, the knock retard angle is decreased after the fuel supply is replenished, thereby reducing the possibility of reducing the knock retard angle while poor quality fuel is being supplied to the engine. 
     Turning now to  FIG. 2 , the method of decreasing the knock retard angle is shown. In a preferred embodiment, the method is executed by an engine control unit having access to a fuel level signal and a non-volatile memory. The fuel level signal is representative of the quantity of fuel in the storage tank supplying the engine. The non-volatile memory stores the fuel level information while the engine is shut off. 
     The method may be executed periodically, such as once per minute, or several minutes, allowing it to determine whether fresh fuel has been added to the tank while the engine is running. Alternatively, the method may be executed once each time the engine is run, with the single execution taking place earlier, rather than later, in the engine run cycle. The run cycle includes the period when the engine controller has been turned on, but the engine has not yet started. 
     The method begins at block  30  and proceeds to block  32  where it reads the present level of fuel in the storage tank. The method then proceeds to decision block  34  and determines whether the fuel level has increased since the previous determination of the fuel level. In a preferred embodiment, the fuel level must increase by more than a predetermined amount to reduce the effects of fuel sloshing and other measurement errors inherent in the control unit and fuel level signal. If the method is executed periodically, then the previous determination would have occurred during the previous execution of the method, such as one minute prior to the present execution. In the alternate embodiment, where the method is execute once per run cycle, then the previous determination of fuel level would have taken place during the previous run cycle and stored in the non-volatile memory of the controller. If the method determines that the fuel level has not increased, then the method proceeds to block  38  and terminates without reducing the magnitude of knock retard. 
     Returning to decision block  34 , if the method determines that the fuel level has increased then it proceeds to block  36  and reduces the magnitude of knock retard by a predetermined amount. The knock retard is reduced under a presumption that the quality of the added fuel may be of better quality (e.g. higher octane) than the fuel which was in the tank prior to the storage tank being refilled. The knock retard may be reduced by a predetermined amount in a simple embodiment; however the knock retard angle should not be reduced to less than zero. Reducing the knock retard angle  14  to less than zero would cause the total spark angle  18  to be greater than the base spark angle  20 , thereby undesirably causing the engine to knock once again. 
     In alternative embodiments, which are not mutually exclusive, the method, at block  36 , may reduce the knock retard by a percentage. The method may also delay reducing the knock retard for a predetermined amount of time, which would allow the new fuel blend in the fuel tank to propagate to the engine. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.