DOC Temperature Control for Low Temperature Operating Ranges with Post and Main Injection Actuation

SAE World Congress, 2013
Author(s):Stadlbauer S., Waschl H., Schilling A., Del Re L.
In a typical Diesel engine exhaust after treatment system consisting of a Diesel oxidation catalyst (DOC), a Diesel particulate filter (DPF) and a selective catalytic reduction (SCR) system the main purpose of the DOC, besides the oxidation of CO to CO2, is the oxidation of NO to NO2. The NO to NO2 conversion is an essential contribution for the downstream SCR system because the fast SCR reaction which provides the highest conversion rates of NOx to H2O and N2 works well only under roughly equal concentrations of NO and NO2. The typical amount of NO to NOx ratio produced by theengine is about 0.95, hence the DOC is necessary to decrease this coefficient close to 0.50. Due to the temperature dependency of the DOC reaction mechanism the oxidation of NO to NO2 takes only place sufficiently if the temperature of the DOC is higher than 200°C, which, however, cannot be reached during low engine speed and low load situations. As a consequence, under these circumstances the whole reduction system is ineffective and, moreover, the potentially high raw NOx emissions in this operating range may cause significant tailpipe emission values. Against this background this paper presents a strategy to raise the DOC temperature during lower temperature operating ranges in its most effective way by acting on the post or the main injection. The final temperature control approach is designed with respect to the additionally required fuel amount and the effect of the different injections on the total NOx emissions. Finally the control strategy was implemented on a test bench to evaluate the benefit of the DOC temperature increase and consequently the gain in SCR conversion efficiency against the fuel penalty. Of course, the additional consumption must be traded off against other system modifications, especially in the after treatment case.
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