Emission Control Systems
Diesel Particulate Filters
There are several types of particulate filters on the market and being used in Europe. They haven't been instituted in the U.S. yet, mainly because the diesel fleet in the U.S. is a fraction of the size it is in Europe, where over half the fleet is diesel powered. But they're coming, so we'll talk about them briefly. Basically, most designs use a ceramic matrix somewhat like the familiar gasoline-engine catalytic converter. They catch the particulate mass in the matrix and then burn it off to regenerate (or unclog) the filter.
Oxidizing catalytic converters
These are chemical reactors that convert a portion of the chemical emissions mix-unburnt hydrocarbons and carbon monoxide-from the combustion of fuel into different chemicals that are deemed benign.
Selective catalytic reduction (SCR)
This technology neutralizes nitrogen oxide emissions by chemical reaction. Basically, the systems use urea dissolved in water then injected into the exhaust stream. The urea changes to carbon dioxide and ammonia. The ammonia then reduces the nitrogen oxides in the selective catalytic converter, producing water and nitrogen. This is the technology, or some variant of it, in DaimlerChrysler's Bluetec clean diesel technology.
Ultra Low Sulfur Diesel fuel
Removing sulfur from diesel fuel reduces acid rain by allowing catalysts to work without clogging. Using particular filters is dependent on low sulfur fuels as well.
Injection/engine management
By controlling injection pressure, timing, and fuel quantity, we can control nitrogen oxides and particulates. This is one of the main benefits along with great power and efficiency of the modern common rail direct-injection diesels. We've discussed most of the issues regarding soot formation and injecting fuel to burn most effectively with little left over to heat the exhaust system. For example, retarding injection timing is known to reduce nitrogen oxide formation but also increases EGT and reduces fuel mileage. It can also increase the amount of unburnt hydrocarbons in the exhaust gas, requiring more aftertreatment with catalysts.
Exhaust gas recirculation
EGR is one of the more useful techniques for reducing nitrogen oxide emissions. Diesel exhaust consists mainly of carbon monoxide, nitrogen oxides, and good ol' H2O. Recirculating a portion of the exhaust gas dilutes the intake charge mixture and reduces the O2 concentration.
There are three ways EGR is thought to reduce nitrogen oxide formation. It seems that the dilution of the intake charge increases ignition delay, and, therefore, has the same effect as retarding the injection timing. It also seems that EGR-diluted air mixture reduces the flame temperature, which reduces combustion chamber temperature. It's also thought that the addition of the inert EGR gas in the intake increases the heat capacity (specific heat) of the non-reacting elements present during combustion. The increased heat capacity has the effect of lowering the peak combustion temperature.
Tuning the amount of EGR centers on balancing the trade-off between reducing nitrogen oxides and increasing soot, carbon monoxide, and hydrocarbons. We know a lot about the effects of EGR in diesels, so it's simply a matter of designing the sensors and actuators that can actively and accurately control EGR percentages in relation to engine load conditions. After about 50 percent load, EGR particulate emissions increase significantly and other measures, such as a particulate trap, need to be employed.
Diesel Emissions That Are Regulated
These chemicals are limited to a specified gram per mile output. We didn't put the values because it's just dizzying and ridiculous to list them. If you want to find out more, see the CARB or EPA Web sites. But be warned, your eyes will just glaze over. It's so Byzantine that even the factories with their armies of lawyers are paying billions of dollars in fines for non-compliance.
A final observation on this subject. It's interesting that Tier 2 monitored emissions switches from unburned hydrocarbons to NMOGs and HCHO, which are emission elements from ethanol in gasoline.