Tier 4 emissions legislation is emerging as a clear precursor for widespread adoption of exhaust aftertreatment in offhighway applications. Large bore engine manufacturers are faced with the significant challenge of packaging a multitude of catalyst technologies in essentially the same design envelope as their pre-Tier 4 manifestations, while contending with the fuel consumption consequences of the increased back pressure, as well as the incremental cost and weight associated with the aftertreatment equipment. This paper discusses the use of robust metallic catalysts upstream of the exhaust gas turbine, as an effective means to reduce catalyst volume and hence the weight and cost of the entire aftertreatment package. The primarily steady-state operation of many large bore engine applications reduces the complication of overcoming pre-turbine catalyst thermal inertia under transient operation. Upstream placement of the catalyst packages also offers potential for reducing the overall fuel consumption penalty (associated with the use of aftertreatment) in comparison to the conventional post-turbine placement. This softening of the fuel consumption penalty can be attributed to better light-off and performance of catalyst substrates, as well as a reduction in the impact of aftertreatment pressure drop on engine pumping work. The investigation involved numerical simulation of pre-turbo application of a diesel oxidation catalyst (DOC), partial-flow diesel particulate filter (DPF), and selective catalytic reduction (SCR) catalyst on a 30-35L class diesel engine. The effect of this placement over traditional downstream placement in terms of fuel consumption, package size, weight and cost was examined. The investigation revealed that the inherently higher gas density in the pre-turbine location allows a dramatic reduction in catalyst volume of up to 70%. The fuel consumption penalty associated with the addition of aftertreatment can also be reduced by approximately 1% with upstream placement of the catalyst packages.
For the first time in the locomotive industry, an advanced exhaust aftertreatment system for a locomotive application was successfully demonstrated to reduce nitrogen oxides from 6.46 g/kW·hr to 1.21g/kWhr to meet the needs of local NOx reduction requirements for non-attainment areas. Five 2,240 kW (3,005 horsepower) PR30C line-haul repowered Progress Rail locomotives were equipped with diesel oxidation catalyst and selective catalytic reduction technologies to accumulate more than 27,000 hours in total in revenue service. Full emissions performance including carbon monoxide, hydrocarbons, nitrogen oxides and particulate matter was conducted at Southwest Research Institute on a regular basis to measure the change of emissions performance for two selected locomotives. The emissions performance of the aftertreatment system did not show any degradation during 3,000 hours operation. After 3,000 hours operation, 0.13 g/kW·hr carbon monoxide (89–91% reduction), 0.027 g/ kW·hr hydrocarbons (91% reduction), 1.08–1.21 g/ kW·hr nitrogen oxides (81–83% reduction) and 0.05–0.08 g/ kW·hr particulate matter (38–58% reduction) were measured on the line-haul cycle. The baseline emissions levels of the engine are within Tier 2 EPA locomotive limits. The newly developed close loop control software successfully controlled targeted nitrogen oxides reduction with minimum ammonia slip during the locomotive emission cycle tests.
With the impending implementation of the Tier 4 emissions standards in the non-road and locomotive sectors, exhaust gas aftertreatment systems will be needed on applications that previously did not require it. Based on the fact that the displacement of these engines is very large, the aftertreatment systems will also be relatively large, heavy and expensive. Additionally, even in these large engine applications, packaging space and systems cost is at a premium. Placing a robust metal aftertreatment system up-stream of the turbo-charger offers an elegant solution to these issues. The higher temperatures and faster temperature rises before the turbine yield faster light-off and better emissions performance. The higher gas density allows the total size of the aftertreatment system in the pre-turbine position to be substantially smaller for a given conversion efficiency, leading to a remarkable packaging and cost benefit of up to 64%. Additionally, by placing the flow-restriction of an aftertreatment system upstream of the turbine, a fuel consumption benefit in the pre-turbine position can be realized as pumping losses of the engine are reduced. The largely steady-state operation of these large engines negates the heat sink effect of the pre-turbine catalytic converters in transient operating conditions. This paper will investigate the benefits of placing an oxidation catalytic converter and partial-flow particulate filter up-stream of the turbo-charger on the fuel consumption of a stationary engine in the 30–35L class by simulation with GT-Power. Different locations for the aftertreatment package as well as optimized sizing for the different locations are investigated to identify the optimum solution for the engine. In addition to the fuel consumption benefits, the cost and weight advantage of the smaller pre-turbine system is emphasized. This view of both the technical and commercial side of the applications, demonstrates a clear advantage for the pre-turbine arrangement of the metal emissions reduction components on large bore engines.
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