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With emissions standards increasing diesel engines are having to become more efficient and have less pollutants in their exhaust. As a result engineers have come up with two separate systems to make the US2010 emissions criteria. One system is Exhaust Gas Recirculation (EGR) and the other system is Selective Catalytic Reduction (SCR). Both systems are in the exhaust system of diesel engines to promote efficiency. Currently light duty truck must have NOx emissions less than .07 g/mile and the US2010 the proposed NOx emissions must be less than .03 g/mile. In recent years; the United States, Europe, and Japan have become more stringent and are extending the emissions control regulations from on road vehicles to include; locomotive, marine, stationary generator applications and farm vehicles.

Advantages and Disadvantages of EGR
EGR is the process of recirculating the exhaust gasses from the exhaust manifold of a diesel engine into a EGR valve which is timed with the intake valves to allow some exhaust back into the cylinder for compression and the power stroke. Another way of achieving an EGR like effect is to have cam overlap so both the intake and exhaust valves remain open simultaneously for a period of time. By recirculating the exhaust gasses less fuel is consumed on the power stroke thereby avoiding engine knock allowing the engine to run on a much leaner fuel to air ratio. This leaner mixture results in the engine having better fuel economy as well as less gaseous emissions. However with this system there is more particulate exhaust which requires a Particulate Matter(PM) filters in the exhaust.

Advantages and Disadvantages of SCR
SCR is the process of using a chemical reaction to reduce NOx pollution. This process is done by injecting a water and urea mix, known as Diesel Exhaust Fluid (DEF), into the exhaust of a diesel engine to react chemically with the exhaust gasses to reduce the nitrogen oxide gasses in the exhaust into; nitrogen, water, and small amounts of carbon dioxide. SCR has the ability to reduce 90% of the NOx in the exhaust system. SCR systems do not require the use of a PM filter however when the SCR and PM filters are combined the engine is 3-5 percent more fuel efficient. One disadvantage of having the SCR system is the need to refill the DEF tank which varies based on the miles driven, load factors and the hours used. Another disadvantage is that the SCR system is not as efficient at higher revolutions per minute. Therefore when the engine speed increases the efficiency of the SCR system decreases. SCR is being optimized to have higher efficiency with broader temperatures, be more durable and more precise in its application

Applications of SCR

 * Heavy Duty Truck Diesels
 * Medium Duty Truck Diesels
 * Light Duty Truck Diesels
 * Economic Passenger Vehicle Diesels
 * Marine Vessels including
 * Cargo Vessels
 * Ferries
 * And Tugboats

EGR and SCR comparison
When compared to SCR, EGR requires a pressure differential across the exhaust manifold and intake manifold. This requires the use of a Variable Geometry Turbocharger which has inlet guide vanes on the turbine to build exhaust backpressure in the exhaust manifold causing the exhaust gasses to flow from the exhaust manifold to the intake manifold. EGR is a far more complicated and primitive system that requires external piping and a valve that would require more maintenance. In new farm tractor diesel engines John Deere is implementing a 9 liter inline 6 diesel that has compound turbochargers the first on the exhaust manifold being variable geometry and contains the EGR system and the second being a fixed geometry turbocharger. The recirculated exhaust gasses and the compressed air from the turbochargers have separate coolers and then the air merges together before entering the intake manifold. Also in the exhaust system of the same John Deere diesel there is an SCR system to reduce the particulate matter along with a PM filter and another oxidation catalyst. All these systems are controlled by a central engine control unit for the optimized minimum of pollutants released in the exhaust gasses.