Misfire Algorithm Processing (2017)

The F650-750 6.7L uses a misfire monitor that operates only at idle. The Misfire Monitor divides two rotations of the crankshaft into 16 half-segments, each 45 degrees of crankshaft rotation. The crankshaft speed shows increases due to combustion of fuel in the cylinder followed by decreases due to friction and other forces between cylinder firing events. The location of the half-segments is chosen such that for each cylinder one half-segment contains the majority of the higher crankshaft speed values (the "high" half-segment) and the other half-segment the majority of the lower crankshaft speed values (the "low" half-segment). The range of crankshaft speed within each half-segment is averaged. The sum of the eight low half-segment speeds is subtracted from the sum of the eight high half-segment speeds and the result divided by eight to get an average increase in speed due to combustion. The Misfire Monitor then calculates the difference between the high and low half-segments for a specific cylinder combustion event and increments a misfire counter for the firing cylinder if this value is less than 20% of the average increase in speed due to combustion described above.

The Misfire Monitor collects blocks of data consisting of 20 crankshaft rotations. Upon achieving the correct entry conditions for the Misfire Monitor as described below, the first block of 20 rotations is discarded to ensure stable idle operation. All subsequent blocks of data are counted unless vehicle conditions change such that the entry conditions are no longer satisfied. In this case, any data in the current partial block are discarded, along with the data from the block immediately prior, as stable idle cannot be ensured for these data. The Misfire Monitor completes once 50 valid blocks (1000 crankshaft revolutions) have been collected, and a fault is reported if a cylinder shows 350 or more misfire events (out of 500 possible combustion events) in this time.

Certain engine operating parameters are monitored to ensure misfire operates in a region that yields accurate misfire results. The table below outlines the entry conditions required for executing the misfire monitor algorithm.

The 3.2L diesel uses a similar algorithm to the one described above except that there are fewer cylinders.

The F250-F550 6.7L engine uses a misfire monitor that operates across much of the engine speed and torque range of the vehicle. The misfire monitor evaluates crankshaft angular acceleration in terms of cylinder "segments" representing the arc in which each cylinder fires. Each cylinder segment is 90° in length (720° / 8 cylinders = 90°). The monitor compares angular acceleration of the crankshaft from one cylinder event to the next. For various powertrain configurations and transmissions statuses, there are threshold maps populated of the minimum segment-to-segment response that indicates a misfire event. These maps are populated from real misfire conditions throughout an engine map. Once a threshold is reached, it is flagged a misfire event and counted. An interval is 4 complete segments of 1, 000 crankshaft revolutions. If the summation of misfires reaches 5% of the total number of combustion events in any 4 complete segment interval, a fault is then set for misfire. In the case of cold starting there is a special "no glow" function. This function evaluates the glow lamp status. In the event that an end operator does not allow for sufficient time glow, the monitor is temporarily disabled. This is to ensure no misfire detection when the engine is unstable from a non-complete or no glow. Once the proper thresholds are met after a no glow, the misfire monitor is reinstated to its normal operation.