U.S. Environmental Protection Agency Tier 4 emissions standards for off-road vehicles — phased in from 2008 through 2015 — introduced substantial reductions of oxides of nitrogen (NOx) and particulate matter (PM). The highest level of regulations, referred to as Tier 4 Final for engines over 751 horsepower (560 kW) took effect Jan. 1, 2015. The regulations reduce particulate matter (PM) by 80% and oxides of nitrogen (NOx) by 45% compared with Tier 2 engines produced in 2006.
The technology paths to Tier 4 Final are numerous. Most engine manufacturers chose to utilize a selective catalytic reduction (SCR) solution to meet the latest set of stringent emissions requirements. Exhaust gas recirculation (EGR)-only solutions require a significantly larger cooling system, and put extreme stress on components while increasing fuel use and adding reliability concerns, particularly in high horsepower applications. The use of SCR allows for optimized combustion and lower fuel consumption with higher productivity.
For example, after evaluating all viable technology paths, Cummins Inc. determined that the only solution that would meet its customers’ needs for engine uptime and reliability is selective catalytic reduction (SCR). Designed with the same durability as the engine, Cummins SCR design for high horsepower industrial engines has enhanced protection against the vibration and shock loading encountered by off-road equipment.
Since 2006, over 1 million Cummins SCR systems are in operation. The Cummins system replaces most silencers, is fully integrated with the engine system controls and is protected against vibration and shock loads.
Designed by Cummins Emission Solutions, SCR is a completely passive flow-through system with control fully integrated with engine management.
Here’s how it works:
- Engine exhaust containing oxides of nitrogen (NOx) enters the SCR.
- The Cummins doser injects diesel exhaust fluid (DEF) into the decomposition chamber.
- DEF atomizes, and quickly decomposes in the hot exhaust gas. The result is ammonia (NH3) and water vapor.
- NH3 Ammonia and water vapor initiate a catalytic reaction, in which NOx are reduced to nitrogen gas and water.
- Slip catalysts prevent NH3 from slipping past the SCR catalyst. Flow-through substrate coated with oxidation catalyst reduces NH3 gas to below 25 ppm.
- Clean exhaust of nitrogen gas, carbon dioxide and water vapor is emitted.
- SCR control is driven by the electronic control module (ECM) so that the engine and SCR operate as a single, integrated system. More efficient dosing means less DEF. Catalyst cleaning is automatically initiated based on duty cycle.
The SCR’s integrated design significantly reduces the size of the catalyst, allowing the SCR to fit into the exhaust system in place of most mufflers or silencers.