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
The major change for Cummins will be the additionof SCR for T4F. “The technology change for Cummins to move from Tier 4 Interim to T4F is significantly less than that required to move from Tier 3 to Tier 4 Interim,” notes Browne. “Essentially, it is about incorporating SCR within the exhaust after treatment system – with no major change required to engine systems. In terms of incremental emissions technology and installation cost, the change to achieve T4F will therefore be lower than from moving from Tier 3 to Tier 4 Interim.”
The same Cummins after treatment system will be common from 75 to 400 hp, with modular scaling for engine output (not duty cycle). “Various configurations will also be available to enable greater equipment installation flexibility,” says Browne. The system is fully passive and flow-through without the need for active regeneration or any ash cleaning. “It consists of a DOC, combined with SCR,” says Browne.“ Cummins has been utilizing these systems for many years in on-highway applications. We believe this offers the simplest and most effective solution to achieving near-zero emissions.” Cummins has developed next generation SCR technology with a copperzeolite-based catalyst. “The copper-based catalyst utilized for SCR enables a higher conversion of NOx at a broader exhaust temperature range when combined witha high-efficiency DEF spray pattern aheadof the catalyst in the decomposition pipe,”says Browne.
The result will be similar performance with better fuel efficiency. “We expect Cummins T4F engines to retain—and exceed—all of the performance enhancements achieved for Tier 4 Interim engines,” says Browne. “Engine fuel efficiency will be better for T4F than that achieved for Tier 4 Interim. Our field test work is showing that this improvement will exceed the cost of using DEF to lower overall operating costs. For some engine platforms, peak horsepower and peak torque availability has also increased to increase overall engine power density and productivity.”
Caterpillar builds on ACERT For T4F
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Caterpillar will continue to build onits ACERT technology. ACERT uses a building block approach to provide the right technologies to match the application. ACERT technology blends an innovative intake-air management system, using optimized turbocharging, with electronically controlled fuel injection that precisely shapes each combustion cycle through multiple-injection fuel delivery. This meticulous control of combustion parameters yields clean combustion, steady power in all operating conditions and fuel economy.
The Cat NOx-Reduction System continually diverts a small volume of the engine exhaust gases to the combustion chamber. This process reduces cylinder temperatures and lowers NOx formation.
A Cat Clean Emissions Module (CEM) is a compactly packaged after treatment unit that includes a DOC, a DPF to remove particulate matter from the exhaust stream and a Cat Regeneration System that removes soot from the DPF. And in some cases an SCR system. Under most operating conditions the engine exhaust is hot enough to oxidize soot through a passive regeneration process. However, if conditions are such that supplemental regeneration is needed, the Cat Regeneration Systemis designed to work transparently, operating automatically without any interaction needed from the operator and regenerating when conditions are optimal.