The Veil is Lifted on Tier 4 Final Solutions

Manufacturers are busy rolling out solutions to meet Tier 4 Final off-road requirements for diesel engine exhaust emissions, and the technologies look very similar to those already in use in the on-road market.

The biggest change from current Tier 4 Interim engines will be the addition of selective catalytic reduction (SCR) technology. SCR technology 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 emissions regulations in 2010.

SCR requires the addition of diesel exhaust fluid (DEF), which is injected into a catalyst to reduce exhaust emissions. While this means yet another fluid to handle, there are many benefits, especially when paired with the previous exhaust gas recirculation (EGR) technology.

“Combining cooled EGR with SCR provides a very effective solution to balance NOx reduction between the engine combustion and exhaust aftertreatment,” 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.”

A Careful Balance

SCR systems were added to address the need to further reduce NOx emissions.

“It’s not possible to meet Tier 4 Final without SCR,” states Hakan Sterner, technical director for Scania Engines. “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 diesel particulate filter (DPF) [on engines above 174 hp].”

With SCR, DEF is consumed by injection into the decomposition pipe in the exhaust stream; 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 DEF dosing rate for the SCR. “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 done to integrate the EGR and SCR technologies for optimum performance. “Until facing the NOx emissions levels required by Tier 4 Final, John Deere achieved the mandated NOx levels with its cooled EGR technology,” says Doug Laudick, manager of product planning, John Deere Power Systems. “With the additional 80% NOx reduction required for Tier 4 Final, we will combine our proven cooled EGR with an optimized SCR technology within the Integrated Emissions Control system.”

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 Tier 4 Final, now that it is more mature for off-highway applications,” says Laudick. “The DEF supply chain infrastructure is better developed, and John Deere has optimized the performance of our SCR system.”

The company tracks total fluid economy as diesel fuel consumption plus DEF consumption, and has realized an improvement over Tier 4 Interim engines. “The total fluid economy with Tier 4 Final engines is expected to meet or improve upon that of our proven Tier 4 Interim engines with cooled EGR and exhaust filters operating on diesel fuel only,” says Laudick.

Ditching the DPF

Some companies have proven that it’s possible to meet Tier 4 Final without use of a DPF.

For instance, Kohler Engines started with a “clean sheet” design and, while its engines still use a diesel oxidation catalyst (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 have a two-stage cooled EGR which is electronically controlled by the ECU. Turbocharging is also necessary and it has to be intercooled. This has an added benefit. It improves power density.”

The engine block design itself is 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,” Wilke points out. “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.”

Scania’s scalable approach

Scania 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 aftertreatment management system. The DOC also helps us reduce particles for Tier 4 Final.”

Scania plans to use a common system to emissions solutions throughout its engine range.

“The system we have for Tier 4 Interim is our new engine platform (five, six and eight 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 into consideration.”

He 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 catalysts. It’s important that we can meet any duty cycle with the same engine since it’s sometimes difficult to predict the use.”

The introduction of the DOC will affect the engines’ cost. “Since one more system is added, there will be a price increase,” says Sterner. However, 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 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. “To achieve compliance, we developed the Integrated Emissions Control system, a solution that optimizes engine performance, operating efficiency and reliability,” says Laudick.

The system encompasses any combination of aftermarket and emissions-reduction components. Solutions depend upon engine size.

“For engines 75 hp and above, the Integrated Emissions Control system will consist of a DOC, DPF and 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 Tier 4 Final. John Deere Tier 4 Final 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 the Integrated Emissions Control system will enable the engines to utilize minimal DEF. “DEF consumption with our Tier 4 Final 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.”

He adds, “Our building-block approach of utilizing cooled EGR, exhaust filter and SCR technologies has ensured that Tier 4 Final engine performance will meet or exceed that of our Tier 4 Interim engines.” Low DEF dosing rates and a higher-pressure fuel delivery system will enable the engines to meet or improve upon total fuel economy. “Our enhanced ECU monitors and controls the engine and the Integrated Emissions Control system components, providing superior fluid efficiency without compromising engine performance or machine productivity.”

Cummins Expects an Easier Transition

For Cummins, the transition to Tier 4 Final should be a relatively smooth one.

“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,” says Browne. “Essentially, it is about incorporating SCR within the exhaust aftertreatment system, with no major change required to engine systems.”

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. This is combined with a high-efficiency DEF spray pattern ahead of the catalyst in the decomposition pipe,” Browne explains.

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 improve overall engine power density and productivity.”

Caterpillar builds on ACERT

For the next emissions tier, Caterpillar will continue to build on its ACERT technology, which uses a modular approach to provide the right technologies to match the application.

ACERT technology blends an 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.

The Clean Emissions Module 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. In some cases, it includes an SCR system.

Under most operating conditions, the engine exhaust is hot enough to oxidize soot via a passive regeneration process. However, if conditions call for supplemental regeneration, the Cat Regeneration System is designed to work transparently, operating automatically without any interaction from the operator and regenerating when conditions are optimal.

One Question Left Unanswered

Based on the comments from engine manufacturers, there will be no degradation in engine performance with Tier 4 Final solutions. Fuel economy will be similar or improve, depending on the particular engine make and model.

That said, you may now have another fluid (DEF) to purchase and maintain. In addition, while many engine suppliers were unwilling to share information on pricing at this time, it takes more resources to design and develop the additional components required. Time will tell how much of the cost may be passed on to off-road equipment manufacturers and, ultimately, the end users.