Particulate Emissions Legislation

For many years the PM mass emissions from diesel engines have been restricted, compelling manufacturers of diesel engines to adopt improved injections systems, more sophisticated engine controls and calibrations and after-treatment options such as Diesel Particulate Filters. Various techniques have been used to monitor legislated PM emissions, ranging from weighing filter papers (the legislated technique) to online optical absorption. The steadily falling PM limits have pressed against the feasibility of making mass measurements with filters- factors such as humidity and filter buoyancy become increasingly significant.

Mass measurement through filter paper analysis is an inherently offline technique- it can not offer development engineers and calibrators insights into engine operation. An online measurement technique is essential for transient calibration, which is increasingly necessary when developing for transient drive cycles such as NEDC, FTP75, FTP transient cycle or the Non-Road Transient Cycle (NRTC).

The proposed particle number limit (N/km) of Euro 6 (measured through particle counting according to the PMP specifications) adds another parameter for which the engine should be optimized, and many engine developers have adopted Diesel Particulate Filters (DPF) as a means of compliance.

DPF Measurements

Since a DPF may reduce the particle mass emitted from the tailpipe by a factor of 100, the previous gravimetric filter paper technique is no longer feasible. A DPF will have an effect on the engine behaviour, meaning that engine calibration should be performed with the DPF fitted. This means that tailpipe measurements are no longer suitable for monitoring engine-out emissions and feeding back to improve the calibration.

Gasoline Direct Injection

Increasing focus on fuel economy has encouraged widespread adoption of Direct Injection Gasoline (GDI) (also known as Direct Injection Spark Ignition DISI) engines, often coupled with turbo-charging. GDI brings difficulties in mixture preparation- the reduced time available for evaporation means that such engines may have higher particle emissions than port fuel injected engines, especially at cold start and during transients. The prospect of particle number emissions restrictions applying to direct injection gasoline engines in Euro 6 means that these engines may also require significant calibration effort to meet particle legislation. Approaches such as Gasoline Particulate Filters are amongst the tools available, although improved calibration may be able to reduce the emission levels sufficiently.

Legislated Measurement Technique

The proposed PMP method for measuring particle number uses a Condensation Particle Counter, typically with a T10-90% time response of several seconds. A sample conditioning system to remove volatile particles is required. Commercially available systems include the MEXA-1000 SPCS from Horiba and the AVL Particle Counter 489 from AVL.

While the technique allows accurate measurement of steady state particle concentrations, it is less suited to accurately locating the brief but significant spikes in particle number and mass output that may occur during engine transients.

See application note for an example of these spikes. With a faster time response, the DMS instruments allow exact location and correct resolution of these events, feeding forward to improved calibration and benefits on the legislated cycles.

Cambustion DMS Series

DMS500 sampling from a vehicle exhaust
Cambustion's DMS500 addresses the needs of engine and after-treatment engineers and developers in both diesel and gasoline areas. The DMS500 uses electrical mobility measurements to measure particle size/number spectra at 10Hz, with response times as low 200ms T10-90%. This data is processed in real time to give outputs corresponding to total particle mass, and total particle number corresponding to that measured by the proposed PMP method for Euro 6. Software configurable analogue inputs and outputs allow easy interfacing, and the instrument may be controlled via the AK protocol for easy test cell integration.

Integrated Dilution and Sampling System

DSA sampling from pre- and post-DPF

The DMS500 can also be supplied with fully integrated dilution and sampling system, allowing sampling directly from the engine exhaust. This offers the ability to look upstream of after-treatment such as DPFs, to evaluate the engine performance. The Dual Sampling Accessory may be used to switch the sample location between two points- for example pre- and post-DPF. This allows study of DPF and engine performance.