HJS Fahrzeugtechnik has developed a City Filter based on its sintered metal filter (SMF^®) with a special view to complying with the demands placed on retrofit filters. It can be set to a specific retention rate and combines the advantages of a high retention rate with outstanding operational reliability. As a result of its specific design, the City Filter can also be operated under unfavourable conditions without the need for complex regeneration management. An upstream diesel oxidation catalytic converter is used to aid regeneration. The City Filter is also catalytically activated.

The City Filter has an integrated particulate mass control tube, developed by HJS, which is fitted in its longitudinal axis and means that the collection rate for soot particles and the filter's regeneration behaviour can be adjusted to a variable anti-clogging device and to whatever level of reduction in particulates is required. The smaller the hydraulic cross-section the higher the filter's particulate collection rate.

Seen under ecological considerations it would initially seem surprising that HJS does not make use of the Jetfilter's full PM collection capacity in its City Filter. The design of the new City Filter is driven by the necessity to make significant cuts in system costs. Reducing the filter's collection efficiency - in a way that still complies with current emission requirements - makes for considerably lower system costs, since a regeneration management system involving complex technology is not needed. In the best case, the original oxidation catalytic converter provided by the car manufacturer can still be used. The City Filter complements it, and in addition its catalytic coating means that it cleans itself as the vehicle is driven.

Current trends in the field of limit values for soot particles and the planned introduction of the EURO 5 standard mean that limit values are set to become even more stringent. With the City Filter it is possible to respond to this situation, depending on the current demands, by modifying the flexible hydraulic cross-section of the particulate mass control tube. The filter also has the potential to be upgraded with a regeneration unit for soot oxidation. This flexibility allows today's limit values and legal requirements to be met while keeping system costs low. At the same time, the City Filter can be upgraded at reasonable cost to meet the demands of tomorrow.

A further advantage of the City Filter's design, apart from the integrated particulate mass control tube, is that the filter regeneration process does not always have to run to completion since its flexible hydraulic cross-section makes it impossible for it to become fully clogged. This virtually rules out the possibility of overloaded filters causing inadmissibly high backpressure.