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The ELPI particle size instrument measures the number of particles in 12 size ranges using a series of impaction stages. To convert the measurement of number to mass, the instrument assumes that all the particles are spheres and are of a constant density, defined by the user, but normally around 1000kg/m3. Both of these assumptions are incorrect for all size ranges and the resultant mass emissions for PM10 usually do not agree with standard filter paper measurements. This paper presents a review of the current situation of the knowledge on converting particle number into mass, using the ELPI or other particle size instruments. Andersen Impactors were used for gravimetric determinations of the mass in the sizes above 400nm, in order to compare their resullts with ELPI number measurements. Gravimetric determination of mass using the ELPI was also attempted. The sampling time with both instruments was two hours to collect enough mass to weigh in each size range.

This work is part of a larger programme to investigate the storage at low power conditions and release at high power conditions in real diesel engine exhaust systems. The initial particle storage in the oxidation catalyst, followed by a release of particles a few minutes later, is explored, and the associated particle size distribution changes determined. A Ford 1.8L IDI Diesel Engine, Turbocharged and Intercooled (TCIC), and equipped with Exhaust Gas Recirculation (EGR), was used under high speed and high power conditions, both during cold start. The commercial close-coupled diesel oxidation catalyst had an associated hydrocarbon adsorber for cold start hydrocarbon control. The tests were carried out using a step cold start to a fixed low power output, typical of city driving. The ELPI particle size analyser was used together with constant temperature gravimetric filter based mass samples upstream and downstream of the catalyst.

Previous work on the transient storage of particulate in diesel exhaust systems (SAE 2000-01-0508) was carried out mainly at conditions where storage in the exhaust would dominate the process. The test involved a preconditioning of the engine the previous day with about 4 hours of engine idling. This ensured that the exhaust had a significant level of deposits. The following day low power cold starts were investigated and the movement of particulate between the two silencer boxes was determined as well as the net blow out of deposited particulate. Continuing deposition if particulate was also shown even in the presence of net blow out. The present work extends this previous work to higher power blow out conditions. Also investigated was the preconditioning of the engine at high power the previous day so that the exhaust was in a much cleaner condition. The previous tests were then repeated.

The accumulation and release of particulate matter within the exhaust system of a modern light-duty diesel engine was studied during a step cold start to three steady state load conditions, idle, 10 and 15kW. Changes in particulate mass concentration through the various components of the exhaust system were dependent on the previous short-term history of the engine operation, and on the engine speed reached during cold start. Particulate matter was deposited within the oxidation catalyst when cold and in the downstream exhaust silencer at idle and low power conditions. Particulate matter was released from the first silencer at all three test conditions and this release exceeded the deposition in the downstream silencer and resulted in a net exhaust system particulate blowout, at all three test conditions. This was a large fraction of the tailpipe emissions at all three test conditions.

Diesel exhaust particle size distribution and total number concentration were measured at different positions along the exhaust system of a practical light-duty passenger car diesel engine. Continuous particle size measurements during the diesel cold start were made in 12 particle size ranges using the ELPI particle size analyser. Three engine speeds were studied using a step cold start procedure to the set load and speed condition. The exhaust system had an oxidation catalyst with hydrocarbon absorber and two silencers. Particle size distributions were determined upstream and downstream of the catalyst and the two silencers. There were considerable variations in the particle number and size distribution after the cold start. The catalyst was shown to act as a store for fine particles and there were further particle losses across the two silencers.