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.