Misfire detection using ion current integration and RPM adaptation

A method of detecting misfires in an ignition system of a combustion engine, comprising:

• repetitively comparing a stair step signal with a sensed ion current during an integration window;
• accumulating a count during times when the sensed ion current has a magnitude greater than the stair step signal;
• and
• comparing the accumulated count to a threshold to detect whether a misfire occurred.

A system for determining misfire of an ignition system in a combustion engine, the system comprising:

• an ignition coil for an ignition system, the ignition coil including a primary winding and a secondary winding;
• a spark plug connected to a high voltage terminal of the secondary winding;
• a battery connected to a low voltage terminal of the secondary winding;
• a feed-forward diode with sensing resistor network connected between the low voltage terminal and the battery providing an ion current signal;
• a controller configured to provide a dwell command on a control wire of an ignition system;
• and
• an ignition interface comprising:
• a state machine monitoring a dwell period between consecutive dwell commands and adjusting a rate of a variable clock signal according to the dwell period;
• a multi-bit counter clocked by the adjusted clock signal, the counter having a first set of bits used by the state machine to establish an integration period, and a second set of bits coupled to a resistive ladder generating a stair step signal having a plurality of cycles during the integration period;
• a comparator coupled to the resistor network to compare the ion current signal to the stair step signal;
• an ion current counter incremented during the integration period whenever the ion current signal is greater than the stair step signal, wherein the ion current counter is decremented during a subsequent dwell command from an accumulated count obtained at the end of the integration period;
• and
• a current sink connected between the control wire and ground to sink current during the dwell command, wherein a first current is sinked while the ion current counter is decrementing from the accumulated count until the ion current counter reaches zero and then a second current is sinked;
• wherein the controller detects a misfire in response to a time duration that the current sink is sinking the first current.