emissions
QSK engines prepared for EPA Tier 4 Final
An advanced selective catalytic reduction system for treating exhaust gases aids both efficiency and regulatory compliance of Cummins engines
by Doug Woodyard
combination of clean combustion and a new selective catalytic reduction (SCR) system for treating the exhaust gases will be exploited by Cummins QSK high speed engines to meet EPA Tier 4 Final off-highway 2015 emission limits. The measures also promise to reduce overall operating costs, with fuel efficiency improved by 5-10 per cent, depending on the duty cycle.
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Simple ‘drop-in’ replacement of existing QSK engines and exhaust silencers by Tier 4 Final models will be facilitated for most applications by an integrated SCR after- treatment system of similar size and noise reduction performance. The new SCR clean exhaust system will be used on the next generation of 19-litre to 60-litre QSK engines covering a 597-2,237kW power range. In addition, the system will be scaled up to serve Cummins QSK engines with higher outputs, including the new V16-cylinder/95- litre QSK95 design, which delivers up to almost 3,000kW. A V20-cylinder/120-litre model is planned for power outputs exceeding 3,728kW. “No other engine company has the depth of experience with SCR technology as Cummins, dating back to 2006 in Europe and now with
almost 100,000 EPA 2010 on-highway engines operating with SCR in North America,” reported Mark Levett, Cummins vice president and general manager, high-horsepower business, last May. Its SCR technology had demonstrated ‘outstanding durability in service’, he added. Cummins’ SCR clean exhaust system is said to reduce NOx emissions to 3.5 g/kWh for Tier 4 Final off-highway, representing a 40 per cent- plus reduction over the Tier 2 level. Focusing NOx reduction within the exhaust rather than during in-cylinder combustion avoided the need to make any major change to the external engine platform. SCR also leaves a much higher in-cylinder threshold for concentrating on particulate matter (PM) reduction and fuel efficiency improvement.
PM emissions are reduced by 80 per cent to meet the 0.04 g/kWh level for Tier 4 Final, a reduction achieved by a clean-combustion formula exploiting higher pressure fuel injection and power cylinder design enhancements. High efficiency PM reduction inside the cylinder obviates the need for either a diesel oxidation catalyst or diesel particulate filter after-treatment in the exhaust stream.
The fuel savings achieved by Tier 4 Final QSK engines reportedly more than offset the cost of diesel exhaust fluid (DEF) dosing required for the SCR system to function. Depending on the duty cycle and application of the engine, Cummins clean combustion for Tier 4 Final enables fuel efficiency to be improved by 5-10 per cent compared with Tier 2 engines, with DEF consumption to fuel held to as low as 2-3 per cent. With the cost of bulk DEF lower than that
of diesel fuel and anticipated to fall further as the off-highway market supply expands in the next few years, Cummins notes, this promises an added cost advantage from using SCR after-treatment. A major reduction in overall operating costs is possible, taking into account the cost of both fuel and DEF.
Low DEF consumption is secured by a series
of advanced sensors monitoring SCR system operation, matching DEF dosing levels to engine load and duty cycle. Lower dosing also means that the onboard DEF tank is easily sized to meet fuel tank capacity or can be increased in capacity for extended refilling intervals. The SCR system is described as a robust, fully integrated after-treatment package specifically developed by Cummins Emissions Solutions for high power applications. Operating as a fully passive flow-through system, it is capable of high NOx conversion with low backpressure, enabling enhanced fuel economy.
An innovative DEF decomposition process significantly reduces space requirements and benefits overall equipment integration while ensuring that low NOx emission levels are achieved. The system also features an integral DEF dosing injector with a high-efficiency spray pattern. SCR logic control
is driven by the
QSK electronic engine management system, upgraded with extra processing power for Tier 4 Final so that engine and after-treatment operate as a single integrated system. A modular configuration allows the SCR
system to be scalable for the complete QSK engine programme,
easing integration and
assembly. For applications below 1,119kW, an SCR single-catalyst system is used in varying
Cummins own-design turbochargers help curb emissions
96 I Marine Propulsion I February/March 2012
Higher pressure fuel injection contributes to cleaner combustion
A robust, fully integrated exhaust after- treatment system is capable of high NOx emissions reduction
www.mpropulsion.com
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