Soot (PM) Sensors

Mansour Masoudi, Alexander G. Sappok

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• Classification of Sensors
• Sensors for DPF Soot Mass Estimation
• DPF Leakage and OBD Sensors

Classification of Sensors

Various types of soot sensors, also known as particulate matter or PM sensors, are used for the control and diagnostics of emission systems utilizing diesel particulate filters (DPF). Soot sensors have been developed for two main types of applications:

• Estimation of the amount (mass) of the soot accumulated in a diesel particulate filter, in order to utilize accurate DPF regeneration strategies.
• DPF failure detection which may result in excess PM emissions, to trigger an OBD fault signal.

An accurate estimate of a DPF soot mass allows devising a proper regeneration strategy (how often, when to start or stop a regeneration), while inaccurate estimates result in unsuitable regeneration timing. If the soot mass is over-estimated, too-frequent (excessive) regenerations take place, resulting in unnecessary fuel consumption penalty and rapid system wear-out, amongst other adverse effects. Conversely, under-estimating a DPF soot mass may cause excessive regeneration exotherms inside a DPF, inducing rapid aging, washcoat loss or DPF deterioration, or even a total DPF failure.

The other area of soot sensor application has been driven by advances in OBD regulations, especially those adopted by the California ARB/US EPA, as well as by the EU OBD requirements. These OBD regulations demand more diagnostic measures of the emission system, such as monitoring the DPF filtration efficiency and tailpipe PM emissions. While US emission standards are expressed in terms of particle mass, European regulations (Euro 5 & Euro VI) additionally include particle number (PN) limits applicable to diesel and GDI vehicles. Therefore, it may be desirable that sensors for EU applications be also capable of PN emission monitoring.

Various technology approaches have been employed to devise soot sensors for the above applications—DPF soot load estimation, DPF failure monitoring and PN emission monitoring. Based on the type of the sensing technology, soot sensors may be divided into four main types:

• Differential pressure (∆P)—The DPF soot load is estimated from the increase in the filter pressure drop.
• Radio frequency (RF)—The DPF load is calculated based on the absorption of a microwave signal by soot accumulated in the filter.
• Accumulating electrode—A DPF failure is detected by measuring a change of electrical properties of an electrode due to a time-dependent soot deposition. Resistance is the electrical property most commonly used, and the corresponding devices are often called resistive electrode sensors or simply resistive sensors.
• Electric charge—Particle charging methods are used to determine real time particle concentrations in the exhaust gas.

The above soot sensor technologies are summarized in Table 1. Sensors listed in the table (as well as those discussed in the following sections) include technologies at different stages of development—from R&D through proof-of-concept and validation to series production, as well as technologies that have been aborted and were never commercialized.

This article focuses on soot sensors used in engine emission control systems—installed permanently on-board of a vehicle or otherwise in the exhaust system of an internal combustion engine. There is an overlap between on-board sensors and laboratory or portable emission analyzers. The same technology (for example, particle charging methods) can be used in on-board sensors and in laboratory instruments. Compared to laboratory equipment, on-board sensors are significantly smaller, but also less accurate. A research consortium by the SwRI that evaluated six soot sensors for OBD applications found that the accuracy was ±60% [3239]—quite an encouraging result, considering that the sensors were of the size of a spark plug and most of them were still pre-production prototypes.

A number of laboratory grade instruments for the measurement of various properties of particle emissions are discussed in the paper on in-situ methods of PM measurement.

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