Came across this on an OZ biofuel web site, may give some of the reasons why its taking longer to remap this type of engine fitted with this device - I give you warning its heavy going !!
How the VW Particulate Filter Works
"The new diesel particle filter
The fundamental problem of particle filters suitable for long periods of use is the need to regenerate them. Although they extract almost all the soot particles from the exhaust gas flow, they become blocked after only a few hours of operation and then cause engine malfunctioning.
For this reason, most familiar particle filter systems are burned clean at intervals with the oxygen (O2) present in the exhaust gas, at exhaust temperatures above 500 °C as soon as the filter's acceptance limit is nearly exhausted. Such discontinuous systems as these, however, have the disadvantages of high thermal loads on the components and also the inability to avoid higher pollutant emissions during the regeneration phase. Furthermore, the severe load imposed by the filter leads to an undesirable increase in fuel consumption.
In view of these drawbacks, Volkswagen's engineers have developed a continuous-acting filter system that avoids them by imposing a load of only 20 to 30 percent on the filter in the most frequent operating ranges. This ingenious "Active CRT System" (CRT = Continuous Regeneration Trap) does not use oxygen in the customary way for regeneration but works with the nitrogen dioxide (NO2) occurring in the oxidizing catalytic converter. To ensure that the CRT system can indeed operate continuously, it needs a working temperature of more than 300 °C. This can either be generated externally, for example by electric heating, or by a mild form of post-injection requiring little energy expenditure.
In the lower load range, an accumulation of the filtered-out soot particles takes place up to a precisely defined threshold value representing 30 percent of total filter capacity. At this point the active CRT function commences. The oxidizing catalytic converter, which is located close to the engine, purifies the exhaust by breaking down the carbon monoxide (CO) and hydrocarbons (HC), so that the NO2 needed for soot oxidation can be generated at high efficiency in the second catalytic converter. If the load on the device drops below 20 percent of filter capacity, the active CRT function is shut down. The control parameters are the pressure differential before and after the filter and the temperature. Both of these can be determined reliably and accurately; this has a most beneficial influence on the high operating standard of the Active CRT System.
Since the diesel oil currently sold to the public still contains a high proportion of sulfur, the catalytic converters suffer from gradual sulfur contamination which would normally have an adverse effect on the CRT function and, unless external action were taken, would lead to a dramatic rise in the soot-particle burden on the filter. In order to ensure effective operation in these circumstances, the Active CRT System incorporates an 'emergency brake' which takes effect at an early stage. It causes fuel to be post-injected so that the exhaust gas temperature rises for a short time to more than 500 °C, a temperature at which the soot in the filter is burned off with oxygen (O2). At the same time, this considerable increase in temperature has the effect of cleaning the sulfur deposits from the catalytic converters. The general introduction of sulfur-free diesel fuel would enable this considerable increase in temperature, which has the drawback of increasing emissions for a short period, to be dispensed with and would at the same time significantly increase the efficiency of the Active CRT function.
Storage catalytic converter for oxides of nitrogen (NOx)
In order to achieve the standard of diesel-engine exhaust purity called for in the Euro-4 standard, even this almost complete elimination of soot particles by the 'Active CRT' method will be insufficient. Gaseous oxides of nitrogen (NOx) must also be lowered on larger vehicles by some form of after-treatment.
In principle, a storage catalytic converter for oxides of nitrogen can perform this task; such devices are familiar from gasoline direct-injection spark-ignition engines. The catalytic converter has a coating that absorbs the oxides of nitrogen (NOX) emitted by the engine and stores them in the same way that a sponge absorbs water. Since the capacity of this device is of course limited, it must itself be purified at intervals by enriching the mixture. The intake air supplied to the diesel engine - which normally runs with a high excess of air, in other words with an extremely lean mixture - has to be throttled and the engine run with post-injection. This process takes place every five to ten kilometers without the driver being aware of it, and ensures that the storage converter can remove between 50 and 70 percent of the oxides of nitrogen (NOX) contained in it during normal vehicle operating conditions.
However, the sulfur still present in the diesel oil offered for public sale at the moment would pollute the storage converter within a very short time: within only a few hours its effect would drop to a fraction of the original value. For this reason, the introduction of catalytic converters of this type is entirely dependent on sulfur-free diesel oil becoming available."
END QUOTE
How the VW Particulate Filter Works
"The new diesel particle filter
The fundamental problem of particle filters suitable for long periods of use is the need to regenerate them. Although they extract almost all the soot particles from the exhaust gas flow, they become blocked after only a few hours of operation and then cause engine malfunctioning.
For this reason, most familiar particle filter systems are burned clean at intervals with the oxygen (O2) present in the exhaust gas, at exhaust temperatures above 500 °C as soon as the filter's acceptance limit is nearly exhausted. Such discontinuous systems as these, however, have the disadvantages of high thermal loads on the components and also the inability to avoid higher pollutant emissions during the regeneration phase. Furthermore, the severe load imposed by the filter leads to an undesirable increase in fuel consumption.
In view of these drawbacks, Volkswagen's engineers have developed a continuous-acting filter system that avoids them by imposing a load of only 20 to 30 percent on the filter in the most frequent operating ranges. This ingenious "Active CRT System" (CRT = Continuous Regeneration Trap) does not use oxygen in the customary way for regeneration but works with the nitrogen dioxide (NO2) occurring in the oxidizing catalytic converter. To ensure that the CRT system can indeed operate continuously, it needs a working temperature of more than 300 °C. This can either be generated externally, for example by electric heating, or by a mild form of post-injection requiring little energy expenditure.
In the lower load range, an accumulation of the filtered-out soot particles takes place up to a precisely defined threshold value representing 30 percent of total filter capacity. At this point the active CRT function commences. The oxidizing catalytic converter, which is located close to the engine, purifies the exhaust by breaking down the carbon monoxide (CO) and hydrocarbons (HC), so that the NO2 needed for soot oxidation can be generated at high efficiency in the second catalytic converter. If the load on the device drops below 20 percent of filter capacity, the active CRT function is shut down. The control parameters are the pressure differential before and after the filter and the temperature. Both of these can be determined reliably and accurately; this has a most beneficial influence on the high operating standard of the Active CRT System.
Since the diesel oil currently sold to the public still contains a high proportion of sulfur, the catalytic converters suffer from gradual sulfur contamination which would normally have an adverse effect on the CRT function and, unless external action were taken, would lead to a dramatic rise in the soot-particle burden on the filter. In order to ensure effective operation in these circumstances, the Active CRT System incorporates an 'emergency brake' which takes effect at an early stage. It causes fuel to be post-injected so that the exhaust gas temperature rises for a short time to more than 500 °C, a temperature at which the soot in the filter is burned off with oxygen (O2). At the same time, this considerable increase in temperature has the effect of cleaning the sulfur deposits from the catalytic converters. The general introduction of sulfur-free diesel fuel would enable this considerable increase in temperature, which has the drawback of increasing emissions for a short period, to be dispensed with and would at the same time significantly increase the efficiency of the Active CRT function.
Storage catalytic converter for oxides of nitrogen (NOx)
In order to achieve the standard of diesel-engine exhaust purity called for in the Euro-4 standard, even this almost complete elimination of soot particles by the 'Active CRT' method will be insufficient. Gaseous oxides of nitrogen (NOx) must also be lowered on larger vehicles by some form of after-treatment.
In principle, a storage catalytic converter for oxides of nitrogen can perform this task; such devices are familiar from gasoline direct-injection spark-ignition engines. The catalytic converter has a coating that absorbs the oxides of nitrogen (NOX) emitted by the engine and stores them in the same way that a sponge absorbs water. Since the capacity of this device is of course limited, it must itself be purified at intervals by enriching the mixture. The intake air supplied to the diesel engine - which normally runs with a high excess of air, in other words with an extremely lean mixture - has to be throttled and the engine run with post-injection. This process takes place every five to ten kilometers without the driver being aware of it, and ensures that the storage converter can remove between 50 and 70 percent of the oxides of nitrogen (NOX) contained in it during normal vehicle operating conditions.
However, the sulfur still present in the diesel oil offered for public sale at the moment would pollute the storage converter within a very short time: within only a few hours its effect would drop to a fraction of the original value. For this reason, the introduction of catalytic converters of this type is entirely dependent on sulfur-free diesel oil becoming available."
END QUOTE
