Retrofitting TRU-Diesel Engines with DPF-Systems Using FBC and Intake Throttling for Active Regeneration

Transport Refrigeration Units (TRU) powered by small diesel engines emit high PM and cause locally high PM levels. The concomitant health risks spurred efforts to devise a cost-effective curtailment of these emissions. Diesel particulate filters (DPF) of ceramic honeycomb construction very efficiently trap PM emissions, even ultrafines in the lung penetrating size range of below 300 nm. A fuel borne catalyst (FBC) can facilitate trap regeneration, by lowering the exhaust temperature requirements, but cannot alone guarantee reliable regeneration under all operating conditions of the TRU. A Swiss development team together with industrial partners therefore developed a fully automatic active regeneration system for the California Air Resources Board. It uses FBC-strategy, incorporates a fast acting intake air throttle valve, which when closing raises the exhaust gas temperature by > 250 °C and when opened immediately after again provides high oxygen.content to the heated filter thus decoupling the availability of temperature and oxygen for a controlled regeneration of the DPF. The electronic control unit (ECU) monitors back-pressure, exhaust temperature, oxygen content and regeneration time and includes self-adapting elements. This paper describes development and prototype testing of a TRU unit powered by a 26 kW typical diesel engine. Particle emissions were curtailed by > 99% (number of solid soot particles below 300 nm), EC-mass by 97%, PM by 86%, HC and NO2 on average by about 60%. This system is expected to be cost-effective even for retrofitting such small engines and might be applicable for other off-road retrofits also. Demonstration testing on a pilot fleet will follow.