Manufacturers are busy rolling out solutions to meet Final Tier 4 off-road requirements for diesel engine exhaust emissions, and the technologies look very familiar to those already in use in the on-road diesel engine market.
The biggest change from the Interim Tier 4 engines will be the addition of a diesel oxidation catalyst (DOC) and selective catalytic reduction (SCR) technology. Selective catalytic reduction (SCR) has been successfully used in Europe for several years and was introduced to the 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 recirulation (EGR) technology. “Combining cooled EGR with SCR provides a very effective solution to balance NOx reduction between the engine combustion and exhaust aftertreatment,” says says Kevan Browne, Cummins. “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 Tier 4 Final, it’s not possible without SCR,” says Hakan Sterner, Technical Director for Scania Engines. “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, which are required for Tier 4 Interim without SCR, it’s not possible to meet the emissions levels without a DPF.”
With SCR a diesel exhaust fluid 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, so it’s almost always a save to reduce the fuel consumption by increasing the DEF consumption,” says Sterner. “But there is a limit where increased NOx will give no reduced or very limited reduction of fuel consumption. There are also other limitations which reduce the 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 Final Tier 4, John Deere achieved the mandated NOx levels with its cooled EGR technology,” says Doug Laudick, manager of product planning at John Deere Power Systems. “With the additional 80% NOx reduction required for Final Tier 4, we will combine our proven cooled EGR with an optimized SCR technology within the Integrated Emissions Control system to meet the more stringent Final Tier 4 NOx 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 Final Tier 4, now that it is more mature for off-highway applications,” says Laudick. “The diesel exhaust fluid (DEF) supply chain infrastructure is better developed, and John Deere has optimized the performance of our SCR system.”
John Deere tracks total fluid economy, diesel fuel consumption plus DEF consumption, and has realized an improvement over Interim Tier 4 engines. “The total fluid economy (diesel fuel and DEF) with Final Tier 4 engines is expected to meet or improve upon that of our proven Interim Tier 4 engines with cooled EGR and exhaust filters operating on diesel fuel only,” says Laudick.
No DPF possible
Some companies have proven that it is possible to meet Tier 4 Final without use of a DPF. For instance, Kohler Engines started with a ‘clean sheet’ design and while the engine still uses a DOC, the DPF has been eliminated. “We developed engines clean enough to meet the emissions without a DPF,” says Jeff Wilke, Kohler Engines. This required a combination of technologies. “High-pressure common rail fuel injection was a big contributor. We went to a four-valve head and we centrally located the fuel injector in that four-valve head. A lot of work was done on the injector in terms of the spray pattern and the nozzle. It is a direct injection engine.”
An electronic control unit (ECU) drives the fuel injection system. “We did a lot of work on the mapping of that ECU so that we have the right fuel ratio at all times to get the best fuel burn,” says Wilke. “We do a cooled EGR which is electronically controlled by the ECU. Turbocharging is also necessary and it has to be intercooled.”
The engine block design itself is also a big contributor to eliminating the DPF. “A lot of work was done to reduce oil consumption because oil is definitely a contributor to particulate matter,” explains Wilke. “The block design helps the cylinders stay in the proper shape. We have a web structure that helps with the structure of the crankcase as well. It was multiple newer technologies that were brought together to get an engine that would run clean enough that it didn’t require a DPF.”
Sania has also been able to meet Tier 4 Final without the use of a DPF. “Low smoke has been a strategic decision for Scania for many years,” says Sterner. “This has forced us to continuously improve our engine. Several parts are important for this. Our common rail system where we can have up to 2,400 bar injection pressure, our single cylinder concept where we can put a lot of development in one piston bowl and our completely own developed engine and after treatment management system. The DOC also helps us reduce particles for Tier 4 Final.”
Scania’s scalable approach
Scania will use a common system to meet emissions throughout its engine range. “The system we have for Tier 4 Interim is our new engine platform – 5-, 6- and 8-cylinder – with common rail (XPI) and SCR,” says Sterner. “For Tier 4 Final we will add some EGR to reduce the NOx before the SCR system. This is to ensure that we have the required emission margin, including deterioration, to meet the emissions in all conditions taking in use compliance (IUC) in consideration.”
Sterner adds, “To improve the efficiency in the SCR catalyst we will also add a DOC. The system will be equal on all our engines, but there will be different sizes of the catalysts. It’s important that we can meet any duty cycle with the same engine since it’s sometimes difficult to predict the use.”
While there is a price for the technology, fuel consumption should remain about the same. “Since one more system is added, there will be a price increase,” says Sterner. Differences in fuel consumption will be minimal. “In some cases lower and in some cases a bit higher, but very small changes. This depends on how we use EGR in combination with the variable vane geometry turbocharger (VGT) and SCR system.
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. “To achieve compliance, we developed the Integrated Emissions Control system – a solution that optimizes engine performance, operating efficiency and reliability,” says Laudick.
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 Final Tier 4. John Deere Final Tier 4 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 technology and Integrated Emissions Control system will enable the engines to utilize minimal DEF. “DEF consumption with our Final Tier 4 engines will be 1 to 3 percent 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 approach of utilizing cooled EGR, exhaust filter and SCR technologies has ensured that Final Tier 4 engine performance will meet or exceed that of our Interim Tier 4 engines. With low DEF dosing rates and a higher-pressure fuel delivery system, John Deere Final Tier 4 engines will meet or improve upon the total fuel economy of our Interim Tier 4 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 relatively small step
The major change for Cummins will be the addition of SCR for Tier 4 Final. “The technology change for Cummins to move from Tier 4 Interim to Tier 4 Final 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 aftertreatment system – with no major change required to engine systems. In terms of incremental emissions technology and installation cost, the change to achieve Tier 4 Final will therefore be lower than from moving from Tier 3 to Tier 4 Interim.”
The same Cummins aftertreatment 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 copper zeolite-based catalyst. “The copper-based catalyst utilized for SCR enables a higher conversion of NOx at a broader exhaust temperature range when combined with a high-efficiency DEF spray pattern ahead of the catalyst in the decomposition pipe,” says Browne.
The result will be similar performance with better fuel efficiency. “We expect Cummins Tier 4 Final 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 Tier 4 Final 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 Tier 4 Final Caterpillar will continue to build on its 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 extremely 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 aftertreatment 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 System is designed to work transparently, operating automatically without any interaction needed from the operator and regenerating when conditions are optimal.
So, as you can see the manufacturers are bust rolling out Tier 4 Final solutions. It appears that there will be no degradation in performance. Fuel economy will be similar or improve depending of your particular make and model of engine and you may now have another fluid to maintain, DEF. In addition, many were not willing to share information on pricing at this time, but it does take additional resources to design and develop the additional components required.