To ensure overall optimisation of heavy duty engine performance (with the respect of NOx&PM future European and US emissions standards), the use of a high efficiency NOx after-treatment system such as a NOx trap appears to be necessary. But running in rich conditions, even for a short time, leads to a large increase of particulate emissions so that a particulate filter is required.
A first investigation with a NOx-trap only has been carried out to evaluate and optimise the storage, destorage and reduction phases from the NOx conversion efficiency and fuel penalty trade-off. The equivalence ratio level, the fuel penalty and the temperature level of the NOx-trap have been shown as a key parameter.
Respective DPF and LNA locations have been studied. The configuration with the NOx-trap upstream provides the best NOx / fuel penalty trade-off since it allows NOx slip reduction and does not disturb the rich pulses. On the other way, for low NOx target, the requirement of high NOx-trap regeneration frequency increases soot production so that forced CDPF regeneration are required. The implementation of the CDPF upstream can eliminate this drawback up to lower NOx target without any CDPF loading. Furthermore, setting the CDPF upstream prevents the soot from flowing through the NOx-trap. With this configuration including an oxidation catalyst downstream of the NOx-trap, a fuel penalty of about 1.8% is observed to reduce NOx from 5 to 2 g/kW.h.
The sulphur contained in the fuel gradually deactivates the NOx trap, making high temperature periodic desulphation necessary. The results obtained show that the nitrate storage and regeneration durations have to be adjusted to balance the decreasing of NOx storage capacity. Various conditions using an alternation of lean and rich running conditions have been tested to purge the sulfates. In these conditions, and with a low sulphur fuel (∼ 5 ppm), the fuel penalty due to sulphur treatment is about 0.2 % and the optimal sulphur regeneration period should be about 10000 km.