Comparison of Two Dilution and Conditioning Systems for Particle Number Measurements along the Exhaust After-Treatment System of an HD Diesel Engine

In heavy-duty engines, Euro VI legislation regulates the total particle number (PN) in the exhaust based on the particle measurement program (PMP) guidelines. By PMP directives, the exhaust sample is diluted and conditioned to contain non-volatile particles before measuring the PN. The fraction of non-volatile and volatile particles changes along the exhaust after-treatment system and could affect the total PN measured. Therefore, it is of interest to compare the performance of the dilution systems at different positions along the after-treatment system. For this purpose, a standard PMP compliant two-stage dilution system (DS1) with evaporation tube (ET) was compared with a close coupled two-stage ejector dilution system (DS2). In DS2, the non-volatile PN was measured with a dilution temperature of 350°C (same as the DS1 ET temperature) while the volatile PN was measured with a dilution temperature of 150°C. Experiments were carried out on a heavy-duty Euro VI engine equipped with an after-treatment system consisting of diesel oxidation catalyst (DOC), diesel particulate filter (DPF) and selective catalytic reduction (SCR) unit (with AdBlue injection) followed by ammonia slip catalyst. Sampling was made at four locations along the exhaust after-treatment system while varying the exhaust conditions namely temperature, flow rate and fuel injection pressure to vary the total PN concentration and the fraction of nucleation and accumulation mode particles from the engine. The total PN was measured using a condensation particle counter (CPC) and the particle number distribution using an Engine Exhaust Particle Sizer spectrometer (EEPS). An overall comparison shows that at higher fractions of nucleation mode particles, before the DPF, DS1 exhibited losses in nucleation mode particle in comparison with DS2. Whereas after the DPF, the loss was minimal. After the SCR, the nucleation of salt particles during excess AdBlue injection events was captured only by DS1. During motoring operation, emitting high volatile particle concentration, DS2's capacity does not seem to suffice to fully evaporate the volatile material as DS2 relies on the heat capacity of hot dilution air whereas DS1 uses an externally powered heater.