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Evaluation of Non-Contiguous PM Measurements with a Resistive Particulate Matter Sensor
Abstract:
The resistive particulate matter sensor (PMS) is rapidly becoming ubiquitous on diesel vehicles as a means to diagnose particulate filter (DPF) leaks. By design the device provides an integrated measure of the amount of PM to which it has been exposed during a defined measurement period within a drive cycle. The state of the art resistive PMS has a large deadband before any valid output related to the accumulated PM is realized. As a result, most DPF monitors that use the PMS consider its output only as an indicator that a threshold quantity of PM has amassed rather than a real-time measure of concentration. This measurement paradigm has the unfortunate side effect that as the PM OBD threshold decreases, or the PMS is used on a vehicle with a larger exhaust volume flow, a longer measurement is required to reach the same PM sensor output. Longer PMS measurement times lead to long particulate filter monitoring durations that may reduce filter monitor completion frequency.
This work investigates a way to improve the completion frequency of a filter monitor by allowing PMS measurements to be interrupted and later resumed, for example from one drive cycle to the next. The experimental results presented show the effect of stopping a PMS measurement and resuming it after delays of various lengths and initiated at various levels of PM sensor soot loading. In most cases the measurement variability increased by 50% or more and the overall sensitivity for the same cumulative conditions decreased by as much as 20%. Observations of the PM dendrite behavior before and after an interruption in the sensor electrode bias voltage using an optical access PMS are included to provide some explanation for the increased variability.