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The key technology developments reported at SAE conferences in 1999 that pertain to gasoline vehicle emission control are summarized in this report. Covered are integrated solutions, catalysts and substrates, fuel sulfur tolerance and effects, particulate emissions, direct injection spark ignition engine emissions, canning methods, and evaporative emissions.

The stringent emissions legislation has necessitated advances in the catalytic converter system comprising the substrate, washcoat technology, catalyst formulation and packaging design. These advances are focused on reducing light-off emissions at lower temperature or shorter time, increasing FTP efficiency, reducing back pressure and meeting the mechanical and thermal durability requirements over 100,000 vehicle miles. This paper reviews the role of cordierite ceramic substrate and how its design can help meet the stringent emissions legislation. In particular, it compares the effect of cell geometry and size on performance parameters like geometric surface area, open frontal area, hydraulic diameter, thermal mass, heat transfer factor, mechanical integrity factor and thermal integrity factor - all of which have a bearing on emissions, back pressure and durability. The properties of advanced cell configurations like hexagon are compared with those of standard square cell.

Thin wall ceramic catalysts offer improved performance by way of faster light-off, lower back pressure and higher FTP efficiency than standard ceramic catalysts. These advantages are attributed to their lower thermal mass, larger open frontal area and higher geometric surface area. This paper will focus on their physical durability, notably their thermal shock resistance. The critical physical properties which influence thermal shock resistance - namely modulus of rupture, elastic modulus and coefficient of thermal expansion - will be examined over a wide range of operating temperatures for both standard (400/6.5) and thin wall catalyst supports (600/4.3 and 400/4.5) with stable high temperature washcoat systems. These data help evaluate the thermal shock capability of each system via computation of thermal shock parameter. The validity of such computations is tested against the thermal shock data from oven test.

The emission and flow restriction characteristics of three different ceramic substrates with varying wall thickness and cell density (400 cpsi/6.5 mil, 600/4.3, and 600/3.5) are compared. These 106mm diameter substrates were catalyzed with similar amounts of washcoat and fabricated into catalytic converters having a total volume of 2.0 liters. A Pd/Rh catalyst technology was applied at a concentration of 6.65 g/l and a ratio of 20/1. Three sets of converters (two of each type) were aged for 100 hours on an engine dynamometer stand. After aging, the FTP performance of these converters were evaluated on an auto-driver FTP stand using a 2.4L, four-cylinder prototype engine and on a 2.4L, four-cylinder prototype vehicle. A third set of unaged converters was used for cold flow restriction measurements and vehicle acceleration tests.

The stringent emissions regulations, notably for cold start, have led to design modifications in each of the converter components, notably the catalyst support. With the faster light–off requirement, the catalyst support must have a lower thermal mass so as to reach the 50% conversion temperature as quickly as possible. Simultaneously, for higher warmed–up efficiency, the catalyst support must offer higher geometric surface area. Similarly, for improved fuel economy and for preserving engine power, the catalyst support must exert lower back pressure. Indeed, these three performance requirements might be met by certain thin wall ceramic substrates, including 400/4.5 and 600/4.3, which have 22% lower thermal mass, 25% higher geometric surface area and 8% larger open frontal area than the standard 400/6.5 substrate. Testing by automakers and international laboratories on engine dynamometers has verified the above advantages of thin wall substrates.

A completely passive cold-start emissions control system, without any secondary air source, was developed to reduce cold start hydrocarbon (HC) emissions. The Air-Less Adsorber (ALA) system has a first catalyst, an adsorber, and a second catalyst. The system is designed to adsorb a large fraction of hydrocarbons (HC) during cold start, followed by optimized heating of the second catalyst before adsorber HC desorption. During the HC desorption cycle, the engine is running in closed-loop control near stochiometric air/fuel ratio. There is enough oxygen to oxidize the desorbed HC over the second catalyst. The ALA system was evaluated using the FTP test on a 3.8 liter V6 vehicle. The ALA system reduced up to 38% of cold start HC emissions beyond the catalyst-only baseline. The system is truly passive.

An In-line hydrocarbon (HC) adsorber system was developed to reduce cold start HC emissions. The system comprises a first catalyst, adsorber unit, and a second catalyst for oxidation of desorbed HC. During cold start, exhaust gas is directed to the hydrocarbon adsorber using a fluidic flow diverter unit without any mechanical moving parts in the exhaust system. After the first catalyst lights off, the diverter is shut off and the major portion of the exhaust gas then flows directly to the second catalyst without heating the adsorber unit. After the second catalyst reaches light-off temperature additional air was added to oxidize the desorbed HC. The system attributes: NMHC emissions in ULEV range Straight line axial flow Reliable design Limited back pressure penalty The system was tested on a 3.8L U.S. vehicle.

Testing was performed on Corning's Generation 3 Electrically Heated Catalyst (EHC) to determine product reliability and durability. A number of functional measurements was performed before and after all electrical, thermal/mechanical and environmental tests. EHCs were also successfully tested on vehicles for 100,000 miles. The results of all tests were favorable and indicated that the new design meets or exceeds requirements.

New test methods were developed to characterize the high temperature durability of intumescent mats that are used to mount ceramic catalyst supports in stainless steel cans. The key attribute of these tests is the use of an electric resistance heating method to maintain a temperature gradient through the thickness of the mat when a cyclic or constant shear stress is applied to the mat interface. These tests are simple to perform and do not require expensive equipment or highly skilled operators. Using these new test methods, the durability of ceramic preconverters mounted with 4070 gm/m2 intumescent mat was studied. The results of these tests indicate that a preconverter package with 4070 gm/m2 intumescent mat can perform satisfactorily in the close-coupled application where temperatures exceed 900°C. The mat performance can be quantified in terms of applied stress and test temperature by utilizing the experimental methods described in the present study.

The 290,000 vehicle-mile durability requirement for diesel/natural gas oxidation catalysts calls for robust packaging systems which ensure a positive mounting pressure on the ceramic flow-through converter under all operating conditions. New data for substrate/washcoat interaction, intumescent mat performance in dry and wet states, and high temperature strength and oxidation resistance of stainless steels, and canning techniques insensitive to tolerance stack-up are reviewed which help optimize packaging durability. Factors contributing to robustness of converter components are identified and methods to quantify their impact on design optimization are described. CERAMIC FLOW-THROUGH catalysts for diesel exhaust aftertreatment have met with much success since their introduction in 1993.

A new adsorber concept has been tested. A zeolite adsorber with a central hole is mounted below the first catalyst, with a second catalyst downstream. During the cold start, when the adsorber is cool and the HC concentration high, HCs are adsorbed from the gas fraction passing through the channels. The small fraction of exhaust gas passing through the hole impinges directly on and heats the second catalyst. The rationale is to design the hole to maximize the second catalyst heating rate, minimize desorption during heat-up and simultaneously keep the HCs which pass through the hole at an acceptably low level. FTP test results on the 3.8 L engine give 0.081 g/mi NMHC with no hole (same as base line) and decrease to 0.056 g/mi NMHC with the hole. This concept exhibits NMHC performance in the LEV range.

A by-pass zeolite adsorber system consisting of a first catalyst, a by-pass loop containing the zeolite adsorbers followed by a downstream second catalyst was FTP tested using a U.S. vehicle equipped with a 3.8 L, V6 engine. The system exhibited ULEV emissions performance with hydrocarbon adsorption and regeneration (desorption and oxidation) within the FTP cycle and required only a single diversion valve within the exhaust line. Adsorption takes place during the initial 70 seconds of the FTP cycle. The adsorbers were regenerated with the exhaust gas plus injected air.

Motorcycle exhaust emission standards throughout the world are becoming more stringent. Emission control systems utilizing the catalytic converter are already in production in Taiwan for 2-stroke engine motorcycles. Catalysts designed for 2-stroke engines encounter a more severe exhaust environment than do those designed for 4-stroke engines. The two aspects of increased severity are the higher temperatures and higher stresses due to engine vibrations. Precious metal catalysts have been designed to operate in the thermal environment of 2-stroke engines and such catalysts have been successfully applied to both metal and ceramic substrates. However, until now, only the metal substrate catalysts have been utilized in motorcycle application. Ceramic based catalysts have not been considered because the mounting material that holds the catalyst substrate in place did not have enough durability to withstand the thermal/vibrational forces encountered in 2-stroke engine exhaust.

Low mass, extruded electrically heated catalysts (EHC) followed directly by a light-off and main converter reduced cold start non-methane hydrocarbons (NMHC) by greater than 80 percent. These reductions were demonstrated on several vehicle applications operating over the Light Duty Federal Test Procedure (FTP). To achieve this level of reduction, the design of the EHC cascade system, power level and heating time must be appropriately established. This paper discusses the impact of these design parameters on cold-start emissions reduction. From the test results, a generic empirical model was developed to predict EHC system conversion efficiency as a function of EHC power, heating time, and inlet exhaust temperature to the EHC.

Ceramic preconverters have become a viable strategy to meet the California LEV and ULEV standards. To minimize cold start emissions the preconverter must light-off quickly and be catalytically efficient. In addition, it must also survive the more severe thermomechanical requirements posed by its close proximity to the engine. The viability of the ceramic preconverter system to meet both emissions and durability requirements has also been reported recently(1,2). This paper further investigates the impact preconverter design parameters such as cell density, composition, volume, and catalyst technology have on emissions and pressure drop. In addition, different preconverter/main converter configurations in conjunction with electrically heated catalyst systems are evaluated. The results demonstrate that ceramic preconverters substantially reduce cold start emissions. Their effectiveness depends on preconverter design and volume, catalyst technology, and the system configuration.