The Case for SCR

Tier 4Tier 4 mandates are a hot topic of conversation everywhere from the halls of equipment dealers right out to the jobs where you work every day. There’s a lot of information to digest, but it’s important to know that, based on equipment type and style, there are certain Tier 4 solutions that make the most sense for each type of machine. In this article, we’ll take a look at Tier 4 solutions for wheel loaders and explain why selective catalytic reduction (SCR) is the optimal solution for this type of equipment.

Tier 4 Final Options
The need to consider off-road engine emissions standards began years ago when the U.S. Environmental Protection Agency (EPA) called for the reduction of nitrogen oxides (NOx) and particulate matter (PM), as well as carbon monoxide and hydrocarbons. The EPA’s phased-in approach to the standards culminates with Tier 4 Final regulations.

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Most construction equipment manufacturers have focused on two options for Tier 4 Final solutions on heavy equipment — both of which involve an exhaust treatment system. One is SCR. The second is Cooled Exhaust Gas Recirculation (CEGR). Both reduce PM and NOx emissions in keeping with the EPA mandates. NOx is produced when the engine is at its hottest. PM is created when not all fuel is burned during combustion, usually when the engine is cooler.

Using SCR technology allows the engine to create less PM and treats exhaust gases with diesel exhaust fluid (DEF) to eliminate pollutants. An engine that uses an SCR system is tuned for maximum efficiency. The higher combustion temperatures involved reduce PM levels but increase NOx levels. To address high NOx levels, the after-treatment-only system creates a chemical reaction by adding DEF. This transforms the NOx into nitrogen and water, which occur naturally in the atmosphere.

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With a CEGR system, exhaust gases are cooled, blended with fresh air and then returned to the engine. The process lowers combustion temperatures and dramatically reduces NOx. The cooler combustion temperature results in a higher PM level, which is then reduced through the use of an after-treatment diesel particulate filter (DPF).

There are advantages to both SCR and CEGR, yet the benefits of one solution can outweigh the other based on the application of the machine — and how it affects overall productivity and profitability.

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Application-Specific Demands
A wheel loader spends most of its time loading trucks or stockpiling materials. The machine can also be equipped with a number of attachments that make it an excellent housekeeping tool for quickly and efficiently moving materials from point A to point B.

A wheel loader, by its nature, produces inconsistent engine loads. In a common loading Y-cycle, for example, the operator accelerates just after putting the wheel loader into forward toward the pile and then decelerates just before reaching the pile. The operator then throttles up again to penetrate the pile and throttles back to complete the bucket fill. Then, the operator puts the machine in reverse and accelerates backward before decelerating prior to making a full stop. At that point, the operator adjusts the bucket height, puts the machine in forward and accelerates to reach the truck, and then decelerates again as it reaches it. Again, the operator throttles up to lift the load to the desired height and empty the bucket. Passes are kept short in order to conserve fuel and save time. The loader may then idle as the operator waits for the next truck to come into the loading zone.

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Tier 4Suited for the Job
The tasks performed and machine operating characteristics help illustrate why SCR technology is well suited for wheel loaders.

Based on the work performed, the wheel loader must generate power at varied engine outputs and speeds. Because SCR is an after-treatment system and works separately from the engine, it lets the machine maintain horsepower and doesn’t sacrifice breakout force. It can actually improve overall performance because the engine breathes more freely than a machine that uses CEGR technology. The technology also doesn’t require the use of regeneration to burn off accumulated particulate for faster throttle response time, which takes on added importance in a heavy loading operation.

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Another factor that comes into play is combustion temperatures. CEGR technology is highly effective in certain applications, but the technology is best suited when conditions call for consistent combustion temperatures (as found in excavators). Steady temperatures allow a portion of the exhaust gases to be cooled relatively easily, blended with fresh air and circulated back into the combustion chamber to lower combustion temperature, which in turn, reduces NOx. Given the operating characteristics and varied engine loads, a wheel loader typically does not generate the consistent combustion temperatures required for an effective CEGR solution.

Wheel loader idling is yet another important consideration as it lowers combustion temperatures. The lower temperatures ultimately result in higher PM levels. The higher levels can be readily addressed with the DPF on a CEGR system, but that ultimately leads to more frequent regeneration of the DPF. More frequent regeneration can also inhibit production and increase maintenance costs. SCR technology eliminates regeneration and the associated downtime and hassle.

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Lowered Long-Term Costs
Many decision-makers also appreciate the design simplicity of an SCR system, which doesn’t require a DPF for lower long-term maintenance costs. An SCR system only requires periodically filling the DEF tank and does not involve any special fuel. A warning light signals when the DEF is running low. Refilling the DEF tank can be done as part of regular maintenance when checking fluid levels or when refueling. Filters in the DEF circuit are usually easy to access and have a maintenance schedule equal to engine oil. Additionally, SCR-equipped wheel loaders will continue using standard oils without any concern for switching to low-ash oils or other more expensive formulations. DEF, which is only needed in small amounts, is also easy to find. SCR engines do not require any special fuel.

SCR Makes Sense
As the industry transitions from Tier 4 Interim to Tier 4 Final, we expect to see more wheel loader manufacturers transition to SCR as the ideal solution for these machines. Based on the workload and machine engineering, it makes the most sense. Preconceptions about SCR and the hassles of adding DEF will fade away as regeneration becomes a thing of the past, fuel efficiency improves and wheel loaders perform the way they are supposed to — with strength and without hesitation.

Brad Stemper is a Solutions Marketing Manager for Case Construction Equipment.