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It amazes me that so many people here have the attitude that if it can be done on gasoline engine you should be able to do the same on a diesel.
There are substantial differences in the way they operate and this causes a different set of problems. For one the exhaust temperatures of a diesel under typical operation are vastly lower than that of a gas engine. This is partly due to the excess air that diesels run with but its also due to the fact that the diesel cycle allows more energy to be extracted by the piston. This lower exhaust temperature makes it hard for DPFs and catalysts to function.
Excessive back pressure increases PM emissions the same way that excessive EGR does because it doesn't allow for proper clearance of combustion gasses in the cylinder before the next intake charge (Note: A turbo doesn't create this problem because the back pressure it creates forces more fresh air into the cylinder on the intake stroke negating the effect ). This effect causes a DPF filter to plug up more because it has to deal with even more PM. On the 80's California emissions Mercedes with DPFs this often accelerated to a run away condition. This run away plugging led to cracked heads, destroyed turbos, and in some cases a car that wouldn't run at all. On a gasoline engine this isn't an issue since any fuel brought into the cylinder comes with a stochiometric amount of air to burn it.
The main reductions in emissions to the modern diesel engines has been due to the growth of computing power to allow engineers to actually simulate engines. The complex interaction and combustion of fuel mixing with air in a dynamically challenging chamber is much more difficult than simulation of a uniform mixture combusting like in a gas engine. My lap top has more computing power than the computing power of Mercedes Benz's computers put together when my car was built. As such a modern diesel engine will emit far less pollution with no pollution controls than my 1985 california emissions engine with all of its controls.
I fear that as long as a diesel engine maintains better mileage than a gas engine the EPA and CARB will see that as justification to impose stricter emissions controls on diesels regardless of whether it makes any difference in the big picture of things or not. Most of the diesel emissions today are made by vehicles that are 20 years or older. Due to the longevity of diesel engines these vehicles last longer than gas powered ones so it just takes longer for accidents and wear and tear to take them out of the picture. Regardless of how clean you make the new ones its not going to make a big dent in diesel pollution for a while.
Should old diesels be banned? IMHO no! The gov't said these vehicles were good enough when they were sold and the gov't should stick to its word. Maybe manufacturers should be able to sell a diesel with no emissions controls for every old polluting diesel they take off the streets. That is a far better and possibly cheaper option than adding a DPF to modern vehicles since you can pick up an old diesel for less than $3000.
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