SCR Vs. EGR

When the EPA changes its emissions standards in 2010, there will be two to meet them. The first is SCR, which stands for Selective Catalytic Reduction. This process reduces the output of nitrogen oxides by adding a urea solution to the exhaust. The high temperature triggers a chemical reaction that changes nitrogen oxides to harmless byproducts. The second is Cooled EGR, or Exhaust Gas Recirculation. In this engine, some exhaust is re-circulated into the engine air intake and mixes with fresh air. This lowers the amount of oxygen in the combustion tank, which in turn, lowers the temperature and the nitrogen oxide output. The exhaust can then be cooled to reduce emissions even further.

Both engines can reduce emissions by up to 90%. The differences between them come down to cost. SCR uses a smaller, lighter engine. It requires an added tank for urea, but improves both thermal efficiency and fuel economy. Major considerations will include replacing urea. If it runs out, there will be a 40% reduction in torque output. The cost effectiveness of this engine rests on the price stability of the urea solution. Cooled EGR requires a larger and heavier engine, but has no additive or tank costs and the user wouldn’t need to replenish urea to keep the engine from powering down. The engine would require a larger radiator and fan to lower heat and its fuel cost is higher than that of SCR. Both engines meet the new standards and both have pros and cons.