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Caterpillar Engines Use SCR Technology to Meet U.S. EPA Tier 4 Final and IMO III Emissions Standards

A rendering of Caterpillar’s 3516E U.S. EPA Tier 4 / IMO III capable engine along with the “U-Flow” selective catalytic reduction (SCR) system (“Z-Flow” also available).

A rendering of Caterpillar’s 3516E U.S. EPA Tier 4 / IMO III capable engine along with the “U-Flow” selective catalytic reduction (SCR) system (“Z-Flow” also available).

With U.S. Environmental Protection Agency (EPA) Tier 4 Final emissions standards a reality for dredges as of this year, engine manufacturers and dredge owners have different options to consider for meeting the standards. Engines with a power range of 2,682 to 4,962 hp (2,000-3,700 kW) were affected first by the new standards, but all new engines with maximum power 600 kW (804 hp) and greater must meet U.S. EPA Tier 4 Final emission standards by October 2017.

Caterpillar’s solution for dredging applications uses a selective catalytic reduction (SCR) system to reduce nitrogen oxides (NOx). Ryan Darnell of the product value engineering department at Caterpillar Inc. spoke at the WODCON XXI conference in Miami in June on “Legislation and Innovation in the Lifecycle Consideration of Dredging Industry Power Solutions.” IDR spoke with him afterwards about different engine options for dredges, why Caterpillar uses the SCR system and other engine design considerations.

Dredges operating in the U.S. generally have two options in the type of engines they can use – a non-road certified industrial engine or marine certified engine. The use of a smaller industrial engine could have benefits in some applications, Darnell said. If the work was an inland application on a lake, mining operation or small river, it might be more difficult to access Cat® Marine dealer support, which is typically located in coast-al regions. “It could possibly be easier for them to maintain parts inventory for an industrial engine,” Darnell said. Industrial engines also have a lower power density and lower initial purchase price, which might be suitable for smaller projects.

Under U.S. EPA Tier 2 and Tier 3 emission standards, Caterpillar industrial engines did not need SCR systems to meet the emission standards, but under Tier 4 Final emission standards, some industrial engines will use SCR as well as all marine certified engines supplied by Caterpillar.

“It’s the need to reduce nitrogen oxides that’s driving the need for SCR,” Darnell said. In diesel fuel combustion, NOx and fuel consumption are inversely proportional. In-cylinder enhancements such as increasing the pressure with which the fuel is delivered increases NOx, while in turn making a more efficient combustion. “There’s an optimum time to deliver the fuel into the cylinder and as you adjust that you can make the combustion less efficient by injecting fuel later and decreasing NOx, but increasing fuel consumption for the same power,” Darnell explained. Similarly, a diesel engine can produce a lot of NOx, but with very good fuel economy, and the SCR system will treat the excess NOx downstream of this more efficient combustion. 

Caterpillar’s SCR systems use a relatively small amount of Diesel Exhaust Fluid (DEF) to convert NOx emissions in the exhaust into nitrogen and water. DEF is a solution of urea dissolved in deionized water to produce a concentration that is about 1/3 urea and 2/3 deionized water.

Diesel engine customers have grown accustomed over the years to higher fuel bills as the emissions standards progressed through the tiers up to U.S. EPA Tier 3. However, with Caterpillar Tier 4 Final engines, fuel efficiency will improve with the use of its SCR system, which al-lows engine-out NOx production to be increased, resulting in decreased fuel consumption, Darnell said. DEF is considerably cheaper than diesel fuel, and those prices are expected to stay low and independent of oil prices for the foreseeable future.

SCR is not the only technology used to meet U.S. EPA Tier 4 Final emission standards, but Caterpillar believes it’s the best choice for dredging and other marine applications. Other technologies such as high pressure common rail fuel systems may also be used in the engine recipe to balance emissions compliance and fuel economy. A common rail fuel system delivers highly pressurized fuel to all the cylinders simultaneously and consistently. With common rail, fuel injection typically sees better atomization – the mixture of fuel particles and compressed air – which happens quicker under the higher fuel injection pressures. “The more mixing, the more efficient combustion and power you get,” Darnell said. “However, this also creates more NOx.” To be able to take advantage of the performance benefits made possible by the increased atomization, other knobs must be turned to counteract the increase in NOx, while still leveraging the fuel efficiency benefits of common rail. On the downside, common rail systems - under much higher system pressures – typically come with increased maintenance such as fuel cleanliness and filtration requirements. According to Darnell, the right emissions reduction technology solution depends on many factors, including engine platform, application, customer input and geographical considerations, to name a few. 

Other emissions reduction technologies such as exhaust gas recirculation (EGR) can reduce NOx. By cooling the exhaust coming out of the engine and mixing that back with fresh air intake, the system reduces the overall combustion temperature by recycling exhaust gas, which can reduce NOx. With higher pressure fuel systems that increase NOx for the sake of fuel efficiency gains, EGR systems can help to reduce the higher NOx. Darnell said it’s very common to see EGR used alongside of common rail fuel systems for that reason.

Diesel Particulate Filters (DPF) can be used to reduce particulate matter, a key regulated constituent of exhaust emissions. Particulate matter (PM) is categorized into two main components – soot and ash – where soot can be oxidized either actively or passively in DPF systems and ash must be cleaned out through service maintenance. An active system actively heats up exhaust gas before it enters the DPF system, whereas the passive system would rely on the regular heat and load cycling of the engine to oxidize the soot content of particulates passively. Essentially, a DPF traps additional particulate matter that’s carried through the exhaust stream, preventing it from being released into the atmosphere. The particulate matter is trapped in the unit until the soot portion is oxidized during regeneration and the ash content is manually removed.

SCR is the preferred path for NOx reduction in specific industries such as marine and dredging, as it produces lower fuel consumption and lower operating costs. Darnell said for high-speed marine engines, Caterpillar has developed a modular, highly flexible installation configuration for its SCR solution. A new dredge design may want to maximize engine room space where it can be tight, but in most cases, Darnell said, because the SCR system can be in different configurations, the engine can accommodate the SCR system in many current designs. For larger, medium speed engines, he said the SCR system can typically fit upright in the vessel’s exhaust stack.

Since most dredges are highly customized for specific applications, Darnell said each customer will need to assess any physical limitations, installation space, infrastructure availability and the specific dredging needs to find the best options and configurations for the next generation of U.S. EPA Tier 4 Final and International Maritime Organization (IMO) III engines on dredges.

At the 2016 International WorkBoat Show in New Orleans from November 30 to December 2, Caterpillar will have on display its U.S. EPA Tier 4 Final / IMO III 3500E engine with aftertreatment. 

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