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U.S. federal vehicle emission standards effective in 2007 require tight control of NOx and hydrocarbon emissions. For light-duty vehicles, the current standard of Tier 2 Bin 5 is about 0.07 g/mi NOx and 0.09 g/mi NMOG (non-methane organic gases) at 120,000 mi. However, the proposed future standard is 0.03 g/mi for NMOG + NOx (~SULEV30) at 150,000 mi. There is a significant improvement needed in catalyst system efficiencies for diesel vehicles to achieve the future standard, mainly during cold start. In this study, a less than 6000 lbs diesel truck equipped with an advanced urea Selective Catalytic Reduction (SCR) system was used to pursue lower tailpipe emissions with an emphasis on vehicle calibration and catalyst package. The calibration was tuned by optimizing exhaust gas recirculation (EGR) fuel injection and cold start strategy to generate desirable engine-out emissions balanced with reasonable temperatures.

Aqueous urea solutions are being considered for use on light-duty diesel vehicles to reduce engine-out NOx to 2007 Tier II Bin 5 levels (0.07 g/mi NOx). A 32.5 wt% urea solution freezes at approximately 12°F (-11°C). This is a challenge for use in colder climates. Propylene glycol is a safe and effective freeze point depressant, providing protection down to 0°F (-18°C). A solution of aqueous urea and propylene glycol was used during a series of transient dynamometer experiments performed with a 1.2 L diesel engine. A detrimental effect on NOx conversion was observed, and tailpipe emissions of HC dramatically increased. Results were confirmed in a laboratory flow reactor, along with effects of other proposed additives such as methanol and ethanol.

Diesel vehicles have significant advantages over their gasoline counterparts including a more efficient engine, higher fuel economy, and lower emissions of HC, CO, and CO2. However, NOx control is more difficult on a diesel because of the high O2 concentration in the exhaust, making conventional three-way catalysts ineffective. The most promising technology for continuous NOx reduction onboard diesel vehicles is Selective Catalytic Reduction (SCR) using aqueous urea. Recent work with urea SCR has involved aftertreatment for the 1.2L DIATA common-rail diesel engine. This engine was used in Ford's hybrid-electric vehicle, the Prodigy, which was developed under the PNGV (Partnership for a New Generation of Vehicles) program. An emission control system consisting of a diesel particulate filter followed by an underbody SCR system was used successfully to meet ULEV emission standards (0.2 g/mi NOx, 0.04 g/mi particulate matter (PM)).

The need for gasoline particulate filter (GPF) technology is expected to grow with increasingly tight particle emissions standards being implemented in US, EU, China and elsewhere. Derived from the successful experience with diesel particulate filters (DPF), GPFs adopted the characteristic alternately plugged honeycomb structure that provides a large area of porous cordierite wall for filtering particles with minimal additional backpressure. However, unlike DPFs, continuous soot regeneration in GPFs makes it difficult to grow and sustain the soot cake on the filter wall that gives DPFs their high filtration efficiency. Therefore, filtration performance of low mileage GPFs relies heavily on the porous structure of filter media, which depends on both the substrate and the applied washcoat. In this work, a blank, two fresh washcoated filters and two washcoated filters with 3000 km mileage accumulation were characterized to compare their filtration performance.

Selective Catalytic Reduction (SCR) of NOx with aqueous urea and a Catalyzed Diesel Particulate Filter (CDPF) has been considered as one of the emission control systems for diesel vehicles required to meet Federal Tier 2 and California LEVII emission standards. At Ford Motor Company, a DOC-SCR-CDPF system containing a copper / zeolite SCR catalyst was aged to 120k mi on the engine dynamometer using an aging cycle that mimicked both city and highway driving modes. A total of 643 CDPF regenerations occurred during the aging that raised the SCR catalyst to a temperature of up to 650°C on a regular basis. A series of lab analyses including activity tests, ammonia thermal desorption, BET surface area, XRF, XRD, and EPMA was conducted on cores taken from the 120k mi engine aged SCR catalyst brick. The lab post-mortem characterizations revealed the changes of catalyst properties, and the deterioration profile of the SCR catalyst brick after undergoing real aging conditions.

The increasing use of gasoline direct injection (GDI) engines coupled with the implementation of new particulate matter (PM) and particle number (PN) emissions regulations requires new emissions control strategies. Gasoline particulate filters (GPFs) present one approach to reduce particle emissions. Although primarily composed of combustible material which may be removed through oxidation, particle also contains incombustible components or ash. Over the service life of the filter the accumulation of ash causes an increase in exhaust backpressure, and limits the useful life of the GPF. This study utilized an accelerated aging system to generate elevated ash levels by injecting lubricant oil with the gasoline fuel into a burner system. GPFs were aged to a series of levels representing filter life up to 150,000 miles (240,000 km). The impact of ash on the filter pressure drop and on its sensitivity to soot accumulation was investigated at specific ash levels.

The 2007 emission standards for both light-duty and heavy-duty diesel vehicles remain a challenge. A level of about 90% NOx conversion is required to meet the standards. Technologies that have the most potential to achieve very high NOx conversion at low temperatures of diesel exhaust are lean NOx traps (LNTs) and Selective Catalytic Reduction (SCR) of NOx using aqueous urea, typically known as Urea SCR. The LNT has the advantage of requiring no new infrastructure, and does not pose any new customer compliance issues. However, Urea SCR has high and durable NOx conversion in a wider temperature window, a lower equivalent fuel penalty, and lower system cost. On a technical basis, Urea SCR has the best chance of meeting the 2007 NOx targets. This paper reviews the results of some demonstration programs for both light-and heavy-duty applications.

Selective catalytic reduction (SCR) is a promising technology for diesel aftertreatment to meet NOx emissions targets in several countries. In established markets such as the US and Europe, zeolite SCR systems are expected to be used due to their ability to survive the exhaust gas temperatures seen in an active diesel particulate filter regeneration. In emerging markets where the fuel sulfur level may be as high as 2000 parts per million, zeolite SCR catalysts may have durability issues. In these markets, low sulfur fuel is needed overall to meet emissions standards and to avoid high sulfate emissions, but the aftertreatment system must be durable to high sulfur levels because there is a risk of exposure to high sulfur fuel. Also, emissions standards may be met without a DPF in some applications, so that the exhaust system would not see temperatures of 600°C or higher.

An advanced diesel aftertreatment system utilizing Selective Catalytic Reduction (SCR) with urea for lean nitrogen oxides (NOx) control was tested on a 2.7L V6 Land Rover vehicle to demonstrate the capability of achieving Tier 2 Bin 5 and lower emission standards for light-duty trucks. SCR washcoat was applied to a diesel particulate filter (DPF) to perform NOx and particulate reduction simultaneously. Advanced SCR systems employed both traditional SCR catalysts and SCR-coated filters (SCRF) to improve the NOx reduction efficiency. The engine-out NOx level was adjusted by modifying the EGR (Exhaust Gas Recirculation) calibration. Cold start NOx performance was improved by SCR warm-up strategy and urea over injection. This study showed the advanced SCR system could tolerate higher NH₃ storage in the SCR catalyst, resulting in overall higher NOx conversion on the FTP-75 test cycle.