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Aftertreatment system design involves multiple tradeoffs between engine performance, fuel economy, regulatory emission levels, packaging, and cost. Selection of the best design solution (or “architecture”) is often based on an assumption that inherent catalyst activity is unaffected by location within the system. However, this study acknowledges that catalyst activity can be significantly impacted by location in the system as a result of varying thermal exposure, and this in turn can impact the selection of an optimum system architecture. Vehicle experiments with catalysts aged over a range of mild to moderate to severe thermal conditions that accurately reflect select locations on a vehicle were conducted on a chassis dynamometer. The vehicle test data indicated CO and NOx could be minimized with a catalyst placed in an intermediate location.

The differences in hydrocarbons (HCs) emitted by gasoline (E0) and ethanol (EtOH) blend fuels from flex-fuel capable engines can lead to differences in the performance of aftertreatment devices. Vehicle emission results have shown either better performance on E0 compared to E85 or vice versa, dependent on the vehicle calibration. In order to separate the impact of the vehicle and the catalyst, a laboratory study was conducted to evaluate performance on a pulse-flame (pulsator) reactor and compare reactivity towards E0 and E85 (85% EtOH-15% E0) exhaust. The catalysts evaluated were substrate-only, washcoat-only and fully formulated catalysts that had been aged either on a pulsator reactor or dynamometer engine. Catalyst performance was evaluated with light-off tests utilizing both slow and fast temperature ramp rates.

Robust on-board diagnosis of emission catalyst performance requires the development of artificially damaged "threshold" catalysts that accurately mimic the performance of damaged catalysts in customer use. The threshold catalysts are used by emissions calibrators to determine fore-aft exhaust oxygen sensor responses that indicate catalyst failure. Rather than rely on traditional trial-and-error processes to generate threshold catalysts, we have used a DFSS (Design For Six-Sigma) approach that explores, at a research level, the relationship between oxygen storage capacity (OSC) of the catalyst (i.e., the fundamental property dictating the response of the aft oxygen sensor) and key process input variables: high-temperature exposure, phosphorus poisoning, and catalyst "recovery."

Phosphorus is known to reduce effectiveness of the three-way catalysts (TWC) commonly used by automotive OEMs. This phenomenon is referred to as catalyst deactivation. The process occurs as zinc dialkyldithiophosphate (ZDDP) decomposes in an engine creating many phosphorus species, which eventually interact with the active sites of exhaust catalysts. This phosphorous comes from both oil consumption and volatilization. Novel low-volatility ZDDP is designed in such a way that the amounts of volatile phosphorus species are significantly reduced while their antiwear and antioxidant performances are maintained. A recent field trial conducted in New York City taxi cabs provided two sets of “aged” catalysts that had been exposed to GF-4-type formulations. The trial compared fluids formulated with conventional and low-volatility ZDDPs. Results of field test examination were reported in an earlier paper (1).

Catalyst poisoning from engine oil additives is a complicated process that depends in part on the pathway by which the oil is consumed in the engine. Engine studies were conducted to assess the relative impact of three major modes of oil consumption - through the PCV system, past the piston rings, and through the valve guides. Minimal phosphorus poisoning was observed with oil consumed through the PCV system and piston rings, whereas oil consumed through the intake valve guides demonstrated severe catalyst poisoning. The former produces effects characteristic of complete combustion of the ZDDP additive previously shown to produce relatively innocuous washcoat overlayers of porous zinc phosphate. In contrast, the latter produces effects characteristic of incomplete combustion (i.e., spray of oil additive into the exhaust and, most notably a washcoat pore-plugging effect accompanied by a marked decrease in washcoat surface area.

Using local emissions measurements immediately downstream of a close-coupled catalyst, spatial variations in emissions have been analysed for close-coupled catalysts with different ageing histories. Comparison of the radial emissions profiles between a uniformly-aged (oven-aged) catalyst and two vehicle-aged parts suggests that the vehicle-aged parts have substantial variations in catalyst damage across the radius of the catalyst. The radial variations in damage were confirmed by bench reactor and post-mortem studies. The radial catalyst damage profiles inferred from engine-based evaluations of vehicle aged catalysts show broad correlation with high flow areas identified by CFD predictions and high temperature regions as measured during engine tests.

Emission studies and component analyses were carried out on Clear-fueled and MMT®-fueled 100,000 mile Escort vehicles from the Alliance study [SAE 2002-01-2894]. Previously reported analyses of these vehicles indicated that all differences in emission system performance could be attributed, with a 90% confidence level, to the engine cylinder head, spark plugs, oxygen sensors, and catalysts [SAE 2004-01-1084]. These parts from the Clear and MMT®-fueled vehicles were further analyzed to determine the root causes of the differences in emission system performance. The intake/exhaust valves, fuel injectors, and EGR valves from the cylinder heads were tested, individually and in groups, for differences in vehicle emission performance. Deposits from the exhaust valves of the MMT®-fueled vehicle were characterized by X-ray diffraction (XRD) and energy-dispersive X-ray spectrometry (EDX), and shown to resemble Mn3O4 with partial substitution of Zn2+ for Mn2+.

Emission studies were carried out on clear-fueled and MMT®-fueled 100,000-mile Escort vehicles from the Alliance study [SAE 2002-01-2894]. Alliance testing had revealed substantially higher emissions from the MMT-fueled vehicle, and the present study involved swapping the engine cylinder heads, spark plugs, oxygen sensors, and catalysts between the two vehicles to identify the specific components responsible for the emissions increase. Within 90% confidence limits, all of the emissions differences between the MMT- and Clear-vehicles could be accounted for by the selected components. NMHC emission increases were primarily attributed to the effects of the MMT cylinder head and spark plugs on both engine-out and tailpipe emissions. CO emission increases were largely traced to the MMT cylinder head and its effect on tailpipe emissions. NOx emission increases were linked to the MMT catalyst.

The relationship between engine oil formulations and catalyst performance was investigated by comparatively testing five engine oils. In addition to one baseline production oil with a calcium plus magnesium detergent system, the remaining four oils were specifically formulated with different additive combinations including: one worst case with no detergent and production level zinc dialkyldithiophosphate (ZDTP), one with calcium-only detergent and two best cases with zero phosphorus. Emissions performance, phosphorus loss from the engine oil, phosphorus-capture on the catalyst and engine wear were evaluated after accumulating 100,000 miles of taxi service in twenty vehicles. The intent of this comparative study was to identify relative trends.

Advances in fuel control strategy, emission system architecture, and catalyst technology have led to dramatic decreases in exhaust emissions in recent years. To continue this trend, especially at high mileages, the impact of engine oil derived contaminants will need to be minimized. In this study, the deactivating effects of oil-derived contaminants on advanced catalyst technologies was assessed using an oxalic acid washing technique to remove phosphorus and other oil-derived contaminants from fleet-aged automotive three-way exhaust catalysts. Acid washing removed most of the phosphorus on the catalyst (chief poison associated with decomposition of the engine oil antiwear additive ZDDP) without significantly affecting other catalyst properties. Catalysts from eight high-mileage vehicles were analyzed, representing four vehicle families.

Vehicle evaluations and model calculations were conducted on a vacuum-insulated catalytic converter (VICC). This converter uses vacuum and a eutectic PCM (phase-change material) to prolong the temperature cool-down time and hence, may keep the converter above catalyst light-off between starts. Tailpipe emissions from a 1992 Tier 0 5.2L van were evaluated after 3hr, 12hr, and 24hr soak periods. After a 12hr soak the HC emissions were reduced by about 55% over the baseline HC emissions; after a 24hr soak the device did not exhibit any benefit in light-off compared to a conventional converter. Cool-down characteristics of this VICC indicated that the catalyst mid-bed temperature was about 180°C after 24hrs. Model calculations of the temperature warm-up were conducted on a VICC converter. Different warm-up profiles within the converter were predicted depending on the initial temperature of the device.