Powertrain & Energy - January 25, 2012 - 6

tech Report
alyst), will result in a degradation in engine transient performance. This is due to the heat sink effect of the aftertreatment system components, particularly the particulate filter. Pre-turbine aftertreatment is therefore best suited for engines that operate in a largely steady-state manner, or that feature very gradual load changes. Non-road, large-bore applications are therefore optimal candidates for pre-turbine aftertreatment. By virtue of its placement, PTCs would need to be smaller in cross-section to not suffer a significant reduction in space velocity, which would be detrimental to the effectiveness of the catalyst. This effect is dominated by the increasing mass flow rate associated with the increasing power. The thermodynamic state of the gas in the downstream position is not changed significantly enough to alter this direct relationship between mass flow rate and velocity over the engine speed range. This could be somewhat different depending on the combustion system and air-fuel calibration; however, the general increasing trend can be expected for most applications. Catalyst designs must therefore be capable of effective operation over a range of flow rates and hence velocities. This requires some degree of compromise so that acceptable performance is attained across the operating range, with no adverse consequences at either end of the range. In the case of the pre-turbine application, the pressure of the gas increases in proportion to the mass flow. This in turn causes a densification of gas that is also proportional to the mass flow. The net effect is a constant gas velocity, because the mass flow and density rise in conjunction with each other. This constant velocity behavior represents a prime opportunity for optimization of the catalyst design. There is essentially a single design point, due to the insensitivity of velocity to mass flow. The target velocity for the pre-turbine PM-Metalit was in the 6-8 m/s (20-26 ft/s) range. This velocity resulted from a catalyst diameter of 230 mm (9 in) for pre-turbine system. The post-turbine system featured a diameter of 298 mm (11.7 in), with a target velocity range of 9-14 m/s (29-46 ft/s). The pre-turbine system had the added benefit of being designed for lower-space velocities due to its more or less constant velocity operation, which allows careful customization of the substrate for lower velocity (and hence pressure drop) operation. The wider operating bandwidth of the post-turbine system requires overall higher velocities for adequate performance across this wider range. All the results correspond to an unloaded PMMetalit. Due to physics of the expansion ratio choking present with post-turbine systems, a loaded particulate filter will aggravate the difference seen between a pre- and post-turbine system. This is because the pressure drop of the aftertreatment affects engine backpressure additively for a pre-turbine system, but multiplicatively for a post-turbine system. Analysis has shown the potential for a 40% reduction in catalyst volume with a simultaneous reduction in fuel consumption of between 0.5 and 1% with a pre-turbine system as compared to a comparably functioning post-turbine system. The pre-turbine aftertreatment system can also be configured to minimize package size and cost with equivalent fuel consumption to a comparably functioning post-turbine system. In this scenario, a reduction of catalyst volume of up to 70% is possible. Key aspects of a pre-turbine aftertreatment system that are crucial to its success are durability and robustness given the harsher thermodynamic environment upstream of the turbine, and the lack of any significant transient operation. It is therefore recommended that a robust metallic catalyst be selected for pre-turbine application, and that pre-turbine aftertreatment should only be considered for applications that are characterized by largely steadystate operation. Information for this article is based on SAE technical paper 2011-01-0299 by Mark N. Subramaniam, Chris Hayes, and Dean Tomazic, FEV Inc., and Markus Downey and Claus Bruestle, Emitec Inc.

January 25, 2012

SAE Powertrain & Energy

Powertrain & Energy - January 25, 2012

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