Different particulate mass (PM) portable emission measurement
systems (PEMS) were evaluated in the lab with three heavy-duty
diesel engines which cover a wide range of particle emission
levels. For the two engines without Diesel Particulate Filters
(DPF) the proportional partial flow dilution systems SPC-472,
OBS-TRPM, and micro-PSS measured 15% lower PM than the full
dilution tunnel (CVS). The micro soot sensor (MSS), which measures
soot in real time, measured 35% lower. For the DPF-equipped engine,
where the emissions were in the order of 2 mg/kWh, the systems had
differences from the CVS higher than 50%.
For on-board testing a real-time sensor is necessary to convert
the gravimetric (filter)-based PM to second-by-second mass
emissions. The detection limit of the sensor, the particle property
it measures (e.g., number, surface area or mass, volatiles or
non-volatiles) and its calibration affect the estimated real-time
mass emissions. Thus, the choice of the real-time sensor might be
critical if the emissions are close to the certification (or
in-use) limit. Currently the emissions from DPF-equipped engines
are almost an order of magnitude below the laboratory PM
certification limit.
The particle number (PN) method (measurement of non-volatile
particles ≻23 nm with a condensation particle counter) had lower
detection limit than the emissions of the DPF engine and the
differences between the CVS and the partial flow dilution system
SPC were within ±15% for all engines. One advantage compared to
other real-time systems like DMM and MSS is that the number method
is directly comparable to the certification results since PN limits
will be introduced in the Euro VI heavy-duty regulation.