Manufacturers are busy rolling out solutions to meet Tier 4 Final (T4F) off-road requirements for diesel engine exhaust emissions.
The biggest change from the Tier 4 Interim engines will be the addition of a diesel oxidation catalyst (DOC) and selective catalytic reduction (SCR) technology. SCR has been successfully used in Europe for several years and was introduced tothe on-highway diesel engine market to meet the last round of on-road diesel engine emissions regulations in 2010.
The introduction of SCR technology does require the addition of another fluid – diesel exhaust fluid (DEF)—which is injected onto a catalyst to reduce emissions. But there are also many benefits, especially when paired up with the previous exhaust gas recirculation (EGR) technology. “Combining cooled EGR with SCR provides a very effective solution tobalance nitrogen oxides (NOx) reduction between the enginecombustion and exhaust aftertreatment,” says Kevan Browne, Cummins Inc., Columbus, IN. “This enables the optimum point of fuel efficiency and performance to be maintained in the ‘sweet spot’ for longer than an SCR-only approach can achieve. The engine EGR rate is rebalanced for the Tier 4 Final engines.”
The SCR systems were added to address the need to further reduce NOx emissions. “To meet T4F, it’s not possible without SCR,” says Hakan Sterner, Technical Directorfor Scania Engines, Södertälje, Sweden. “Since the system is available, it’s the best way to reduce fuel consumption, be less sensitive to sulfur and keep a good transient response. With high EGR rates, whichare required for Tier 4 Interim without SCR, it’s not possible to meet the emissions levels without a DPF.”
With SCR, DEF is consumed by injection into the DOC and the dosing rates can vary by the engine supplier. “The fuel cost is so much higher than the cost for DEF [that] it’s almost always a [savings] to reduce the fuel consumption by increasingthe DEF consumption,” says Sterner. “But there is a limit where increased NOx will give no reduced orvery limited reduction of fuel consumption. There are also other limitations which reducethe possibility to just increase the injection of DEF, for example the evaporation of DEF and efficiency in the catalytic converter.”
It is difficult to pinpoint the optimal SCR dosing rate. “This is a complicated formula that varies between engine platforms, the range of exhaust temperatures and the efficiency of the spray pattern,” says Browne. “The design goal is to achieve the best balance of fuel economy with DEF dosing rates, DEF tank sizing and required refilling intervals.”
There has been a lot of work to integrate the EGR and SCR technologies for optimum performance. “Until facing the NOx emissions levels required by T4F, John Deere achieved the mandated NOx levels with its cooled EGR technology,” says Doug Laudick, Manager of Product Planning at John Deere Power Systems,Waterloo, IA. “With the additional 80% NOx reduction required for T4F, we will combine our proven cooled EGR with an optimized SCR technology within the Integrated Emissions Control system to meet the more stringent emissions levels.”
Many of the initial concerns with SCR have been worked out as the technology was adopted for on-highway applications. “SCR is an appropriate technology building block for T4F,” says Laudick. “The DEF supply chain infrastructure is better developed.”
John Deere tracks total fluid economy, diesel fuel consumption plus DEF consumption,and has realized an improvement over Tier 4 Interim engines. “The total fluid economy (dieselfuel and DEF) with T4F engines is expected to meet or improve upon that of proven Tier 4 Interim engines with cooled EGR and exhaust filters operating on diesel fuel only,”says Laudick.
John Deere continues ‘building block’ approach
To meet increasingly stringent emissions regulations, John Deere has followed a building block approach in which technologies have been systematically adopted to meet each regulatory Tier. The Integrated Emissions Control system encompasses any combination of aftermarket and emissions-reduction components integrated in that building block approach.
Solutions depend upon engine size. “For engines 75 hp and above, the Integrated Emissions Control system will consist of a DOC,a DPF and a SCR system specifically designed to meet the demands of off-highway applications,”says Laudick. “The DOC/DPF exhaust filter reduces particulate matter while the cooled EGR and optimized SCR system reduce NOx to the regulatory levels of T4F. John Deere T4F engines below 75 hp will meet regulations using an Integrated Emissions Control system consisting of an exhaust filter without cooled EGR or SCR.”
The combination of cooled EGR technologyand the Integrated Emissions Control system will enable the engines to utilize minimal DEF. “DEF consumption with ourT4F engines will be 1 to 3% of diesel fuel consumption depending upon the application,”says Laudick. “Lower DEF consumption means DEF tanks can be smaller, impact on equipment applications is minimized, DEF filter service intervals can be extended, vehicles can achieve a longer interval between DEF tank refills, and operator involvement is reduced.”
Laudick adds, “Our building-block approachof utilizing cooled EGR, exhaust filter and SCR technologies has ensured that T4F engine performance will meet or exceed that of our Tier4 Interim engines. With low DEF dosing rates and a higher-pressure fuel delivery system, John Deere T4F engines will meet or improve upon the total fuel economy of our Tier 4 Interim engine models. Our enhanced electronic control unit monitors and controls the engine and the Integrated Emissions Control system components, providing superior fluid efficiency without compromising engine performance or machine productivity.”
Cummins Claims Tier 4 Final is a Relatively Small Step
