The increased use of EGR valves has led to a bigger problem with soot. So, in order to trap the soot, the government has mandated that all trucks built after Jan. 1, 2007, have a diesel particulate filter (DPF) and a catalytic converter.
In simple terms, the DPF is a huge screen that traps the soot coming from the engine. This filter looks like a rocket shell underneath newer trucks. The DPF has temperature sensors and pressure tubes used by the PCM to help in determining when the DPF screen is full. When the DPF needs cleaning it undergoes what is called “regeneration.”
Regeneration is done differently by the different manufacturers. The most common way is to inject diesel fuel into the exhaust stream in order to raise the temperature. When a certain temperature is reached, the soot will begin to burn out of the DPF that cleans the “screen.” Some manufacturers inject diesel into the exhaust through a separate nozzle in the exhaust pipe before the DPF. Others will pulse the injectors on the exhaust stroke, which will put diesel fuel into the exhaust system. The biggest drawback to this stage of the process is poor fuel economy. The fuel mileage can drop as much as 3 to 4 miles per gallon during regeneration.
Through the smog reduction learning curve of diesel engines, manufacturers have introduced other solutions into production. selective catalyst reduction (SCR) is something that has recently been released for the 2011 Ford trucks, which uses urea as the catalyst.
To explain in simple terms, urea is an odorless, colorless, non-toxic substance found in urine of mammals. Urea is often used in agriculture in a granular form as a fertilizer because it is rich in nitrogen. But the urea used in diesel exhaust systems is in liquid form. This liquid can now be purchased at most local parts stores under the name diesel exhaust fluid (DEF). This is made from a concentration of liquid urea with a percentage of deionized water. The truck has two tanks inside the fuel filler door; one is for the urea solution (DEF) and the other is for diesel fuel (the fuel tank has a green lid and the DEF has a blue lid).
If you’re thinking, “who cares” or “what is the point?” Well, let’s look a little deeper. When using urea, there is no need to use an EGR valve, which means that the engine will make more power without the diluted intake charge and cooled combustion temperatures. In return, there will be less particulate matter from the engine, so there is no need for a DPF.
However, the drawback now is that the engine makes NOx. So in order to combat the problem, urea is injected through a nozzle downstream in the exhaust system somewhere shortly after it exits the turbo. Remember from biology classes that this is a catalyst, so the process will separate the molecules. The urea has a reaction with the NOx gas. This reaction turns the NOx into ammonia. The ammonia enters the catalytic converter where it is separated into nitrogen and water.
If you think about it, SCR is the best choice for reducing emissions in diesel engines. Yes, there’s the added expense of the DEF, but in return you make more power and obtain better fuel economy. Manufacturers can now vary the engine’s injection timing to produce more power. With that, the engine will be less likely to produce soot, reducing the chances of having to replace the DPF (at a cost of around $2,000) if it becomes clogged and won’t regenerate.
Robert McDonald is owner of New South Diesel in Newton, NC, and specializes in high performance diesel and gasoline engines and cylinder heads for street, marine, dirt and drag racing. You can reach Bob at [email protected]