Engine ignition systems may include a spark plug for delivering an electric current to a combustion chamber of a spark-ignited engine to ignite an air-fuel mixture and initiate combustion. Based on engine operating conditions, spark plug fouling can occur wherein a firing tip of the spark plug insulator becomes coated with a foreign substance, such as fuel, oil, or soot. Once fouled, the spark plug may be unable to provide adequate voltage to trigger cylinder combustion for all engine cycles until the spark plug is sufficiently cleaned or changed. As an example, the spark plug may be cleaned by burning off the accumulated soot from the spark plug tip.
Spark plug fouling and pre-ignition caused by hot fouled spark plugs is also a significant issue in areas with poor fuel quality control. Fuel additives such as methycyclopentadienyl manganese tricarbonyl (MMT), lead or ferrocene may build up electrically conductive and thermally insulating deposits on the spark plug ceramic. Such build up may cause misfires or pre-ignition (PI). Due to the potential severity of misfires or PI at high speed and load in boosted engines, vehicle manufacturers may recommend very short spark plug change intervals.
The inventors herein have recognized that the cause of spark plug fouling may determine the control action to be taken. For example, spark plug fouling due to soot accumulation may not necessitate spark plug changes as frequently as may be required for spark plug fouling due to fuel additives. Likewise, spark plug fouling due to soot may be less prone to pre-ignition while spark plug fouling due to fuel additives may require additional pre-ignition mitigating (or pre-empting) control actions. Further still, the presence of fuel additives can accelerate exhaust catalyst and exhaust air-fuel ratio sensor degradation.
Thus in one example, the different causes of spark plug fouling may be better detected and differentiated by a method for an engine comprising: for each engine cylinder, differentiating spark plug fouling due to soot accumulation from spark plug fouling due to fuel additive accumulation based on a fraction of combustion cycles, over one or more given drive cycles, having an ignition current switching time that is higher than a threshold duration. In this way, spark plug fouling from fuel additives may be better addressed.
As an example, an engine ignition system may include a spark plug coupled to each engine cylinder, an ignition coil for initiating ignition at the spark plug, and a control wire for adjusting a control current of the ignition coil. An engine controller may output a dwell command to the control wire for initiating dwell of the ignition coil. In response to the dwell command, a current of the control wire may rise. The controller may then monitor a duration elapsed since the dwell command until the control wire current falls below a predetermined value, herein also referred to as the switching time. The expected switching time may be based on the dwell command. If the actual switching time is more than the expected time for a threshold number of combustion cycles during a given drive cycle (e.g., a fraction of combustion or engine cycles), spark plug fouling due to fuel additives may be suspected. Accordingly, a diagnostic code indicating spark plug change may be set and various mitigating actions to pre-empt spark plug fouling induced pre-ignition may be performed.
Alternatively, if the fraction of engine cycles for which the switching time is above a threshold duration is not greater than a threshold percentage, the spark plugs may be fouled due to soot accumulation. Accordingly, a diagnostic code for cleaning the spark plug may be set and, to burn off the soot from the spark plug, an engine speed-load may be raised, and/or spark timing may be advanced, so as to raise a tip temperature of the spark plug above a threshold temperature for a duration. If following the raising of tip temperature, the switching time remains above the expected switching time, spark plug fouling due to fuel additive accumulation may be determined.
In this way, by identifying spark plug fouling due to fuel additives, and differentiating it from spark plug fouling due to soot accumulation, spark plug fouling induced pre-ignition may be reduced and timely mitigated. In addition, exhaust catalyst and air-fuel ratio sensor degradation may be timely identified and addressed. By providing spark plug change recommendations based on evidence of malfunction or degradation, rather than a predetermined period of time or amount of vehicle usage, spark plug change recommendations may not be provided too soon, lowering overall vehicle operational costs for the driver. By diagnosing spark plug health, engine life is extended.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.