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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 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.

The emission capability of an exhaust system tuned for improved engine performance from an in-line four-cylinder engine has been investigated. The exhaust system comprises two close-coupled catalysts; each located in separate exhaust streams and has been termed the 4-2 close-coupled catalysts (CCC) -1 system. It has been shown that, given equivalent total catalyst volume, this system configuration results in compromised high exhaust flow rate emissions performance compared with a single catalyst (4-1semi-CCC) system. This emissions performance deficit has been attributed to the effect of engine frequency flow pulsations, which result in relatively high peak space velocities in the 4-2CCC-1 system despite the mean space velocity being consistent. Engine-based AFR Bias Sweep tests suggest that hydrocarbon emissions are most strongly affected by this phenomenon. At lower exhaust flow rates, the difference in performance between the two systems is negligible.

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.

This paper charts the development of three three-way catalyst (TWC) and four lean-NOx trap (LNT) formulations in four vehicle systems over a four-year period. All LNTs were installed in an underbody location behind a close-coupled TWC on vehicles equipped with either port fuel injection (PFI) or direct injection spark ignition (DISI) engines. In addition to the standard regulatory European drive cycles, a series of steady-state tests were conducted to determine changes in LNT NOx efficiency with increasing NOx storage, and changes in the levels of individual nitrogen-containing exhaust components. Each vehicle system was subjected to a durability cycle up to an equivalent of 80,000 km. The early LNT formulations on systems ‘1’ and ‘2’ suffered from inadequate thermal durability with system efficiencies for NOx deteriorating to ≤ 55% after vehicle aging under lean operating drive cycle conditions (from ≥ 80% when fresh).

This paper describes a series of vehicle emission tests on a port-fuel injected lean-burn engine, to determine the preferred aftertreatment configuration yielding the most efficient regeneration of a lean-NOx trap (LNT). Three configurations were tested: (A) single starter three-way catalyst (TWC) upstream of an underfloor LNT; (B) bifurcated system with short downpipes comprising parallel TWCs upstream of a single underfloor LNT (Y-pipe configuration); and (C) bifurcated system with extended downpipes. System ‘A’ exhibits satisfactory LNT regeneration behaviour, and is within the European Stage III limits after accelerated aging. Results for system ‘B’, with identical TWC and LNT formulations as the single system, show that this LNT cannot be adequately regenerated under standard purge conditions; even with a fresh trap. In this non-optimized bifurcated system, the AFR profile entering the LNT during the rich purge deviates markedly from that requested by the calibration.

A method for improving three-way catalyst (TWC) performance by superimposing a low frequency lean air-to-fuel ratio (AFR)bias perturbation onto the standard AFR oscillations is described. This observation of Catalyst Regeneration (CatRegen) has been attributed to a reactivation of poisoned precious metal sites on the catalyst surface. Preliminary tests under steady-state conditions show that there is a gradual reduction in TWC activity for NOx after a lean-rich transition, suggesting a temporary poisoning of the active precious metal sites on the TWC under rich conditions. This deactivation can be prevented by periodically exposing the catalyst to lean exhaust gas; which has led to the development of the CatRegen system.

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.