Soot (PM) Sensors

Mansour Masoudi, Alexander G. Sappok

This is a preview of the paper, limited to some initial content. Full access requires DieselNet subscription.
Please log in to view the complete version of this paper.

Abstract: Various types of soot or particulate matter sensors have been developed to estimate the amount of soot in a diesel particulate filter or to detect excessive PM emissions downstream of a DPF in case of a filter failure. The estimation of soot mass in the filter largely relies on a differential pressure measurement, but other methods such as radio frequency (RF) sensors have also been developed. Sensors for DPF fault determination include accumulating type sensors using a resistive electrode, as well as electric charge based devices.

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:

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:

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.

Table 1
Classification of soot sensors based on technology and application
Soot sensor technologyApplicationExample developers
DPF soot mass estimateOBD
DPF failure monitoringPN monitoring
Delta-P (differential pressure)Bosch, Delphi, Continental, Sensata, EngineSens, others
Radio frequency (RF)GE, Amphenol Corporation, CTS (formerly Filter Sensing Technologies)
Accumulating electrodeBosch, Stoneridge, Continental, Delphi, Electricfil, Denso, NGK, Heraeus
Electric charge aPegasor, NGK-NTK, Emisense, Continental, Honywell
a Pegasor, NGK-NTK

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.