Filtered EGR – a step towards an improved NOX/soot trade-off for DPF regeneration

Abstract

Exhaust gas recirculation (EGR) is currently widely used in commercial diesel engines to provide an effective solution in reducing the levels of nitrogen oxide (NO X ) emissions. However, this currently comes at the expense of an exponential increase in particulate matter (PM) emissions resulting directly from the dilution effect (i.e. reduction in oxygen availability), as well as a further penalty arising from the recirculation of the exhaust emissions such as soot and hydrocarbons. In our earlier work it was observed that filtered EGR (FEGR) was able to play a significant role in controlling the soot recirculation penalty and thus improve the overall NO X /soot trade-off. In order to further our understanding of the effect of recirculated exhaust gases and in particular recirculated soot and hydrocarbon (HC), comparisons were made between standard EGR, FEGR and pure nitrogen (N 2 ), a direct cleaner replacement of the exhaust gas. When implementing FEGR, a diesel oxidation catalyst (DOC) and diesel particulate filter (DPF) were introduced into the exhaust to not only filter the soot particulates but reduce the recirculation of HC which can play a role in particulate surface growth. It was observed that the recirculated HC species and soot particles (especially at high load and EGR ratios) play a role in promoting the production and growth of further particles within the combustion chamber. Similarly, by comparing at the same O 2 intake concentration as that of FEGR and introducing N 2 as the EGR replacement gas, it was possible to correlate the increase in engineout mass of soot with EGR to the recirculation of soot particles, HC species as well as the presence of H 2 O and CO 2 .

Study of particulate matter and gaseous emissions in gasoline direct injection engine using on-board exhaust gas fuel reforming

“…The increase in the in-cylinder temperature assists in HCs and PM precursor oxidation [38]. Furthermore, hydrogen inhibits soot nucleation [44] and promotes soot and soot precursors oxidation through the OH radical [45,46]. On the other hand, the high recirculation of CO, in the case of REGR, reduces the oxygen and OH concentration available for soot oxidation to CO 2 [46].…”

“…On the other hand, the high recirculation of CO, in the case of REGR, reduces the oxygen and OH concentration available for soot oxidation to CO 2 [46]. In addition, the recirculation of particles within the EGR loop could increase engine output PM emissions [44,45].…”

Effects of diluent gases on sooting transition process in ethylene counterflow diffusion flames

“…Several researchers have studied soot generation characteristics in actual equipment, providing a strong basis for reducing soot formation. 11,12 To reduce the emission of soot particles, the effects of various diluent gas additives on soot emission in hydrocarbon ames have attracted more and more attention. Helium (He), nitrogen (N 2 ), and carbon dioxide (CO 2 ) are currently commonly used diluents.…”

Simultaneous control of NOx, soot and fuel economy of a diesel engine with dual-loop EGR and VNT using economic MPC

“…Dual-loop EGR supplements the conventional high pressure (HP) EGR with the incorporation of a low pressure (LP) route which comes from downstream of the diesel particulate filter (DPF) and joins the air intake upstream of the compressor. The filtered LP EGR further reduces Soot formation at higher engine loads [6]. It also results in more appropriately cooled and homogeneous mixture of exhaust and fresh air whilst preserving higher pressure differential for the VNT, compared to the HP EGR.…”