For most applications, the Tier 4 Final QSK engine and integrated SCR after-treatment offer the installation simplicity of a drop-in replacement for the current QSK engine and exhaust muffler, with a similar size and equivalent noise reduction. While achieving very low emissions levels, Cummins Tier 4 Final technology also reduces the overall cost of operation, with improved fuel efficiency, according to the company.
The new SCR clean exhaust system will be used on the next generation of 19- to 60-liter QSK engines across a broad 800- to 3,000-hp range. The SCR clean exhaust system will be scaled-up for QSK engines above 3,000 hp, including the new larger-displacement engine platform, with details due to be released in September.
“Our next-generation QSK engines represent a game-changing technology that leaps ahead to achieve Tier 4 Final with the benefit of installation simplicity and improved fuel efficiency,” said Mark Levett, vice president and general manager for Cummins’ high-horsepower business. “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. Our ability to leverage this preceding SCR experience and optimize for high-horsepower applications with the new SCR clean exhaust system enables Cummins to deliver the right emissions solution for Tier 4 Final.
“Our SCR technology has demonstrated outstanding durability in-service, equal to that of the engine platform. Using SCR after-treatment for high-horsepower applications means we are able to avoid any major change to the engine platform and retain the dependability and power performance of our current QSK engines—and this continuity is of key importance to our customers,” Levett said.
Cummins SCR clean exhaust system reduces oxides of nitrogen (NOx) emissions to 3.5 g/kW-hr for Tier 4 Final off-highway, representing a more than 40% reduction compared with the Tier 2 level. By focusing NOx reduction within the exhaust rather than during in-cylinder combustion, Cummins avoids the need to make any major change to the external engine platform. Using SCR also leaves a much higher performance threshold in-cylinder in order to focus on Particulate Matter (PM) reduction and improve fuel-efficiency. PM emissions are reduced by 80% to meet the 0.04 g/kW-hr level for Tier 4 Final. This is achieved by a clean-combustion formula using higher-pressure fuel injection and enhancements to the power cylinder design. This inherent capability to achieve higher fuel injection pressure is a key feature carried forward from the fuel system on today’s QSK engines. Highly efficient PM reduction in-cylinder eliminates the need for either a diesel oxidation catalyst or diesel particulate filter after-treatment in the exhaust stream.
Cummins believes the fuel savings achieved by the Tier 4 Final QSK engine will more than offset the cost of Diesel Exhaust Fluid (DEF) dosing required for the functioning of the SCR clean exhaust system. Depending on duty cycle and application, Cummins clean combustion for Tier 4 Final enables fuel-efficiency to be improved by 5% to 10% compared with Tier 2 engines, with DEF consumption to fuel held as low as 2% to 3%. The DEF is an aqueous solution of 32.5% high-purity urea and 67.5% deionized water, and requires no special handling.
For many applications, this results in a major reduction in overall operating costs, taking into account the cost of both diesel fuel and DEF. With the cost of bulk DEF lower than that of diesel fuel and anticipated to fall further as the off-highway market supply expands over the next few years, this brings an added cost advantage to using SCR after-treatment.
A series of advanced sensors will monitor the SCR system to match DEF dosing levels to engine load and duty cycle. Lower dosing also means the on-board DEF tank is easily sized to meet fuel tank capacity, or can be increased in size for extended refilling intervals to match specific equipment operation.
The SCR after-treatment operates as a passive, flow-through system and is capable of high NOx conversion with low backpressure, which helps with fuel economy. An innovative DEF decomposition process significantly reduces space claim and benefits overall equipment integration while ensuring that low NOx emissions levels are achieved. The SCR 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 the engine and SCR after-treatment operate as a single, integrated system.
The SCR modular configuration is scalable for the complete QSK engine line-up to ease integration. For applications below 1,500 hp, an SCR single-catalyst system is used in varying lengths sized to engine output. An SCR twin-catalyst system is used for applications above 1,500 hp, with each catalyst aligned with a cylinder bank and matched to engine power output. For most applications, the SCR system replaces the current muffler within an equivalent size envelope and provides similar sound suppression.