New emissions standards put the squeeze on particle measurement

As many countries have begun tightening their standards for diesel emissions, automakers and suppliers have begun looking for new diesel-emissions test equipment and test methods. Certification test equipment has to measure lower levels of contaminants required by government tests, while the equipment used in development must give manufacturers more insight into what's going on in their engines.

At this year's SAE World Congress (March 3–7, 2003, Detroit), papers and products provided insight as to where diesel emissions measurement is headed. One paper, "The Choice of Instrument (ELPI and/or SMPS) for Diesel Soot Particulate Measurements" (Ref. 1) evaluates the accuracy of two different types of instruments: electrical low-pressure impactor (ELPI) and scanning mobility particle sizer (SMPS) instruments. Both are used for measuring diesel particulate emissions (see "How ELPI and SMPS instruments work," p. A7).

The test setup for this research included a small diesel engine that produced a diesel soot aerosol, an aerosol-sampling system, and an ELPI instrument to measure particle sizes. While this engine was small compared to commercial engines, the researchers felt their results would apply to measuring the particulate matter generated by larger engines as well. They performed a series of experiments to ensure that each part of the setup was stable and operated reliably.

From their experiments, the researchers concluded that the engine was a repeatable source of diesel soot, but that the aerosol-sampling system and the ELPI instrument had deficiencies. The main deficiency with the sampling system was that the soot quickly clogged the ejector nozzle inside the diluter. To remedy this problem, the researchers recommend cleaning the nozzle after five hours of operation.

Instruments that use an ELPI first charge the emitted particles then separate the particles by mass into 12 bins. By measuring the charge on the bins with an electrometer, the intrument determines how many particles are in each bin.

To reduce the effects of particle bounce, the researchers advise the use of oil-soaked, sintered impactors instead of standard ELPI impactors. In their tests, the sintered impactors eliminated errors caused by particle bounce.

Another problem with the ELPI instrument is impactor overloading, which occurs when a small mound of particles builds up on an impactor and affects the airflow pattern around it. Oil-soaked impactors eliminate this deficiency, too. The researchers theorize that the oil wets the particles, which are then evenly distributed on the impactor surface.

The researchers also assessed the performance of an SMPS instrument in this application and compared it to the performance of the ELPI instrument. This comparison was not straightforward because the two instruments measure different characteristics. The ELPI measures the aerodynamic diameter of particles, while the SMPS instrument measures the mobility diameter. The aerodynamic and mobility diameters differ because the particles are not perfect spheres, but rather fractal-like agglomerates.

The researchers learned that both the ELPI and SMPS measurements were affected by this characteristic. The ELPI instrument tended to count more particles than were actually present because some of the diesel soot particles carry more charge than an ideal particle with the same aerodynamic diameter would carry. Thus, the ELPI instrument will measure more charge and calculate a higher particle count.

The SMPS instrument doesn't have this problem because it uses a particle counter that directly counts the number of particles in a sample. The SMPS does, however, have accuracy problems because there are particles with a mobility diameter greater than 1000 nm present. These particles are likely to have multiple charges and therefore get through the differential mobility analyzer (DMA) chamber in the SMPS instrument and be counted by the particle counter. While SMPS instruments have an impactor that is supposed to prevent particles larger than 1000 nm from entering the DMA, this impactor is not 100% effective for non-ideal particles, such as diesel soot.

While ELPI and SMPS instruments may be the most popular methods of measuring particle size, other methods were topics of discussions at SAE 2003. Two researchers from the Instituto Motori (Naples, Italy) and one from Shiraz University in Iran proposed a method of measuring both NOx and particulate emissions using ultraviolet light spectroscopy (Ref. 2). This method is interesting because it measures both NOx and particulate size and concentrations simultaneously, but it is just an experimental technique and is probably a long way from being commercially available.

One technology that is currently available is the differential mobility spectrometer. An instrument using this technology was introduced at the show by Cambustion (Cambridge, UK). In this instrument, a dilute sample of diesel exhaust is bombarded with ions, which electrically charge the particles in the exhaust. These charged particles then flow into a measurement chamber with a high-voltage electrode in the center, surrounded by a column of clean air.

The electric field deflects the charged particles toward a series of electrodes connected to electrometer amplifiers. The smallest particles impact the electrodes nearest to the inlet, while the heavier ones land on electrodes further down the chamber. Cambustion claims that with this technology, engine manufacturers can acquire data in real time as the instrument can complete a spectrum measurement every 200 ms.

One thing about diesel emissions is certain—regulatory agencies are going to continue to lower the amount of pollutants that diesel engines are allowed to emit. To stay in the game, makers of diesel engines and their suppliers will have to adopt sophisticated measurement techniques.