When sizing the filter, the aim is to be able to collect the DPM for the required time period between regenerations, without the pressure drop exceeding acceptable limits. Pressure drop across a particle filter increases as the DPM load increases, and in fact a fully loaded filter may have up to ten times the pressure drop of a completely clean one. As a rule of thumb, filter manufacturers quote the amount of DPM their filter can hold in terms of mass per unit volume of the filter. About eight grams per litre is typical. If the DPM emissions from an engine are known, and so is the time interval between regenerations, a calculation can be made to size the filter. For example, assume an engine produces 100 grams of DPM per hour, and the filter will be regenerated every 30 minutes. The DPM produced in 30 minutes will be 50 grams. The filter can hold 8 grams per litre, therefore a filter with a volume of at least 6.25 litres is required (50 divided by 8). It can be seen that accurately calculating the maximum intervals between regenerations is crucial.
If a passive regeneration technique is being relied on, regeneration will only take place when a certain minimum temperature is present in the exhaust. This temperature may only occur when the engine is working under load, for example buses and trucks tend to regenerate their particle filters when going up hills. In practice, in the case of engines experiencing transient conditions, the situation is made more complicated by the fact that regenerations are usually only partial. The following graph illustrates this.
In the case above, the maximum DPM loading on the filter is 50 grams, so assuming that the filter can hold 8 grams per litre, a 6.25 litre filter should be adequate. A graph such as this can be produced by data logging exhaust temperature during a typical duty cycle.
