Catalyst placement for Tier 4 aftertreatment

  • 07-Dec-2011 04:04 EST
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Examples of pre-turbine catalysts (PTCs), as well as the implementation into a turbine housing.

Aftertreatment is a well-accepted enabler for reduction of pollutant emissions beyond the exhaust manifold. Unfortunately, the reality of aftertreatment is that it aggravates exhaust system restriction, and often involves the use of additional reductants such as diesel exhaust fluid (aqueous urea), or fuel, to promote catalytic effects. A direct consequence is an inevitable increase in fuel consumption. Powertrain engineers must minimize this increase in fuel consumption, as circumventing it is simply not possible, say researchers from FEV and Emitec.

Aftertreatment component size also affects fuel consumption because of the added weight, making it beneficial to minimize the overall size of the system. Smaller, more compact systems also typically exhibit better thermal characteristics, a key determinant for catalyst efficiency. From a commercial standpoint, equally performing, but more modestly sized aftertreatment components offer a clear opportunity for cost reduction.

The notion of a pre-turbine catalyst (PTC) is by no means new. There have been numerous attempts to incorporate at least part of the aftertreatment system in the pre-turbo position.

The motivation was to utilize a compact, precursor catalyst in very close proximity to the exhaust port, enabling rapid light off, and efficient conversion of pollutant emissions. A second, more generously sized underfloor catalyst would then complete the conversion of any remaining pollutants prior to the exhaust stream exiting the tailpipe.

Such a split configuration was sufficient to reduce the overall size of the aftertreatment, due to the increased efficiency offered by the PTC. A PTC can be configured for application in the cylinder head, within the exhaust manifold, and even as part of the turbine housing.

PTC usage provides significantly higher catalyst inlet temperatures, which allows more rapid and efficient oxidation of exhaust pollutants. The higher temperature regime is particularly beneficial during cold starting and at lower engine loads, where exhaust gas enthalpy is lower due to leaner air/fuel ratio operation. Extensive use of exhaust gas recirculation in modern engines further increases the overall gas-to-fuel ratio during part load operation, making catalyst functionality even more challenging than before.

Another benefit of PTC usage is a significant reduction in catalyst volume over the post-turbine configuration. This is due to higher gas density prior to expansion through the turbine, allowing the same substrate space velocity to be attained with a smaller cross sectional area. The packaging benefits alone can be quite attractive, particularly if space is a commodity in the application of concern.

Despite the clear benefits associated with PTC use, the concept is not without its challenges. The pre-turbine location is a much harsher thermodynamic environment, compared to the post-turbine location. A PTC needs to be robust and durable against higher temperature and pressure gradients, as well as pulsation effects associated with engine firing order.

Secondly, it should be noted that any substantial application of aftertreatment in the pre-turbine positions (i.e., anything beyond a very modest pre-catalyst), 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 PM-Metalit. 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 steady state 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.

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