Conference report: Emission sessions at SAE 2012 Congress
1 May 2012
The SAE Congress was held this year on April 24-26 in Detroit. It was the third year the Congress used the new condensed format, with a shortened, three-day schedule, 20 minute presentations, admission fees for members and scaled-down exhibition. The number of technical papers, however, appears to have remained similar to the previous year, with some 1200 papers in total, in all topics. In addition to technical presentations, the conference featured a number of open forums, keynote talks and “chats with experts”.
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While this year’s Congress, with a motto ’GetConnected’ and a grand opening speech by Jeff Immelt—the Chairman of General Electric— seemed to gravitate toward electric mobility, a number of fine presentations were given on engine emission technologies. Emission sessions were opened by a technical keynote by Tim Johnson of Corning [Paper #2012-01-0368] who presented his traditional review of recent developments in emission regulations and technology. Important new emission regulations include the California LEV III standards, to be followed by a federal Tier 3 regulation (as the Tier 3 fuel provisions would put upward pressure on gasoline prices, a formal proposal is not expected before the US presidential elections). In addition to diesel aftertreatment, this year’s review also covered gasoline engine technology, with developments where gasoline engines reach high bmep levels that could once be achieved only by diesels.
An interesting example of an advanced gasoline combustion concept—the gasoline direct-injection compression-ignition (GDCI) engine—was presented by Mark Sellnau of Delphi [2012-01-0384]. The GDCI is being developed for full-time operation over the entire speed-load map. It is a low temperature combustion system that uses a high compression ratio with multiple late injections (MLI), intake boost and moderate EGR for high efficiency, low NOx and low PM emissions. The relatively long ignition delay and high volatility of gasoline (RON 91) combined with an advanced injection system and variable valve actuation provides controlled mixture stratification for low combustion noise. Some preliminary results were presented from a one cylinder research engine. For IMEP from 2 to 18 bar, engine-out NOx and PM emissions were below 0.2 g/kWh and 0.1 FSN, respectively, indicating that NOx/PM aftertreatment could be reduced or eliminated. A modeled 1.8 L GDCI engine for vehicle simulations showed large regions with fuel consumption below 190 g/kWh. In a mid-size passenger car, this could deliver a combined fuel economy improvement of about 50%.
NOx Reduction. The demand for highly efficient SCR systems has been driven by future NOx emission standards as well as by CO2 and fuel economy regulations. The decreasing urea-to-fuel cost ratio creates a further incentive to operate engines at increased NOx levels while using SCR to meet emission standards. Emissions of nitrous oxide from SCR systems must be minimized, as N2O—a potent greenhouse gas— can offset the CO2 benefit realized through the use of SCR. There is also a need to improve low temperature performance of SCR systems in order to ensure compliance in light-duty vehicles and to reduce off- cycle NOx emissions in heavy-duty engines.
Ford reported [2012-01-0371] on the development of an SCR-based emission system to meet the LEV III SULEV30 emission standard (NMOG+NOx = 0.030 g/mi) in a light-duty truck. To achieve this standard, the SCR system efficiency during cold start must be significantly improved. System optimization was discussed that involved the EGR, fuel injection and cold start strategies. The emission system included a DOC, a Cu/Z SCR catalyst, an SCR-coated, high porosity SiC DPF, and an ammonia slip catalyst.
System developers from Donaldson shared their experience from the development of an SCR system for a Tier 4 final nonroad application [2012-01-1087]. The required NOx conversion efficiency was 95% over a temperature window of 220-550°C. Special attention was devoted to the system design to ensure mixing of urea with the exhaust—in order to maximize conversion efficiency and to avoid the formation of solid deposits—while maintaining compact size of the package. In one design, a mixing tube was positioned in the center of the DOC. In another, tangential baffles were used after the DOC. Vehicle integration was illustrated using the example of a 262 kW Massey Fergusson tractor. Another noteworthy study, dealing specifically with mitigation of solid deposits in SCR systems, was presented by Cummins [2012-01-1287].
Diesel Particulate Filters. Several papers were presented on the development of new and improved particulate filter materials. Japan’s Sumitomo Chemical discussed their SC-AT aluminum titanate particulate filter material with innovative, hexagonal call configuration [2012-01-0838]. The cell structure has three smaller inlet channels per one outlet channel, for large ash holding capacity. This hexagonal cell structure was also reported to have a pressure drop advantage. Kubota Materials reported on the development of a silicon nitride (Si3N4) DPF material [2012-01-0849]. While not a new idea—Si3N4 filters were presented about 10 years ago by Asahi Glass (a Mitsubishi company)— the material continues to attract attention due to its advantageous micro-structure and compatibility with catalyst coating. The material showed low pressure drop with washcoat loads from 20 to as much as 60 g/L. Si3N4 filters can be extruded in one body (not segmented).
One of the methods to lower filter pressure drop (and to reduce the pressure drop hysteresis) is to provide a thin layer/membrane of smaller pore size material at the surface of the inlet channels. Such membrane limits the penetration of soot into the wall porosity and promotes early formation of filtration cake in the inlet channels. Using a 13 micron thick surface layer with 1.2 micron pore size, Ibiden reported [2012-01-0842] an 18% decrease in pressure drop and very high, 97% initial filtration efficiency. Claus-Dieter Vogt of NGK discussed the application of a surface membrane on a high porosity substrate [2012-01-0843], designed for SCR-on-DPF applications. A 14-35% pressure drop advantage, depending on engine operating conditions, was reported due to the membrane. The membrane also improved the particle number (PN) filtration efficiency by 90%, providing a safety margin over the standard high porosity material, which reduces PN emissions to levels comparable to the EU PN limit. When the filter was coated with an SCR catalyst, the PN efficiency improvement was 66%.
MIT researchers applied a technique called focused ion beam (FIB) milling to study interactions between the soot/ash/DPF interfaces [2012-01-0836]. The FIB method, used in semi-conductors, allows to directionally mill away material from the soot, ash and substrate layers on a nm scale using a beam of high momentum Ga+ ions. Coupled with high resolution scanning electron microscope (SEM) and dispersive x-ray analysis (EDX), the technique provided a new insight and impressive visualizations (SEM still pictures and movies) of ash and soot particles, the ash-soot, soot-DPF and ash- DPF interfaces, pore structure of the respective layers, and the extent of intra-layer mixing. Among many observations, it was found that once ash is accumulated, soot does not permeate into the ash layer. Primary ash particles were found to be porous, with large pores inside the particle and smaller pores toward the surface. The same team presented another paper [2012-01-1093] discussing the effects of high temperature, transient engine events on ash packing and DPF pressure drop.
Conference web site: www.sae.org/congress