The new Ford 6.7-liter Power Stroke V-8 turbocharged diesel engines will employ an after treatment system to help comply with 2010 federal regulations to reduce nitrogen oxide levels in diesel emissions by more than 80 percent compared with the previous standards. The Ford after treatment system is a three-stage process; a key component is the use of Diesel Exhaust Fluid (DEF).
Injection of DEF to reduce NOx is a proven technology that’s been used throughout the automotive industry. Unlike other solutions used to control NOx, the DEF system allows the diesel engine to run at its optimum range in terms of fuel mixture. Some systems require the engine to run richer – which can be harmful to diesel engines – in order to control the NOx.
Step #1. Cleaning and Heating – The first step in cleaning the diesel exhaust occurs when the exhaust gasses enters the Diesel Oxidation Catalyst (DOC). The role of the DOC is twofold. #1. it converts and oxidizes hydrocarbons into water and carbon dioxide. This conversion happens at about 250 degrees Celsius. #2.1. The DOC is used to provide and promote heat, using specific engine management strategies, into the exhaust system. Through appropriate thermal management, this heat increases the conversion efficiency of the downstream subsystem(s) in reducing emissions.
Step #2. Knocking Out the NOx – The next step in the process is what’s known as Selective Catalytic Reduction (SCR). In this process, the NOx in the exhaust stream is converted into water and inert nitrogen, which is present in the atmosphere and harmless. Before the exhaust gas enters the SCR chamber, it is dosed with DEF, an aqueous solution that is approximately 67.5 percent water and 32.5 percent pure urea.
When heated, the DEF splits into ammonia and carbon dioxide. These molecules are atomized and vaporized, then enter a mixer that resembles a corkscrew. This twist mixer evenly distributes the ammonia within the exhaust flow. The ammonia enters the SCR module, which contains a catalyzed substrate (The material or substance on which an enzyme acts.), and through chemical reactions combines and converts the NOx and ammonia into the harmless inert nitrogen and water. Dosing occurs between 200 and 500 degrees Celsius.
Step #3. Scrubbing Away the Soot – The final part of the cleansing system for the diesel exhaust gas involves the Diesel Particulate Filter (DPF). The DPF traps any remaining soot, which is then periodically burned away, known as regenerating, when sensors detect the trap is full. The regeneration process sees temperatures in excess of 600 degrees Celsius to burn away soot.
Quieter, more refined diesel sound for improved NVH performance
Customers of the 6.7-liter Power Stroke turbocharged diesel engine will notice a quieter, more refined sound. Improvements to the combustion system, structural integrity of the compacted graphite iron block and the single turbocharger mounted to the engine block account for many of the NVH improvements.
Specific design upgrades were made to both the piston and the piston bowl to optimize the combustion process, which features a two-stage combustion event instead of a single-injection event, causing harsh, sudden and loud combustion. Instead, a starter or pilot injection of fuel begins the combustion process before the main injection.
The result is smoother combustion and a more refined sound for the customer. When at idle, two pilot injection events are used to make the firing process even smoother and aid in quietness. The “ticking” of the high-speed injectors also is quieted by specially designed covers on the engine.
Mounting the turbocharger from the center housing directly to the block provided several advantages as well in terms of NVH.
Using one turbocharger, instead of two operating in series or sequentially, helped solve some NVH challenges as well.
Other improvements include the addition of two resonators in the intake system as well as a third resonator near the air cleaner.
The new diesel engine, which will be built at Chihuahua Engine Plant, is the perfect complement to the 2011 Ford Super Duty, delivering both capability and reliability.
Developing the new 6.7-liter Power Stroke V-8 turbocharged diesel engine is a big deal, after all the engineering and testing, they are confident this engine will ensure the new Super Duty continues its leadership in capability, reliability and productivity.
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