On Reduction of Nitrogen Oxides in Heavy-Duty Engines : with NOx Model Feedback, PPC and SNCR

Abstract: European emission legislation has in recent years put a strict limit on NOx emissionsfrom heavy-duty truck engines. EGR and selective catalytic reduction are the two mostcommon technologies applied in a modern heavy-duty diesel engine to fulfill the legislation.The scope of this thesis is to explore other ways of reducing NOx emissions emittedby a heavy-duty engine. Several technology options are studied with this particular taskin mind. First, a NOx model was developed and implemented for fast execution on afield-programmable gate array (FPGA). Model validation showed that a good fit to datacould be achieved with the NOx modeling approach suggested in this thesis. The modelwas then used for split fuel injection control strategy in a Scania D13 to reduce NOxformation during combustion by defining a threshold value. In order to obtain controllabilitywith this type of strategy, the net indicated load had to be above 20 bar with afuel injection pressure of 700 bar. Still, very little could be done to reduce the formationof nitrogen oxides with this strategy.If exhaust temperatures are above 700 ◦C it is possible to obtain thermal reduction(SNCR) when an ammonia based reductant is added to the exhaust gases. The cylinderhead of the Scania D13 was modified to inject AUS 32 directly into the combustioncylinder, using the FPGA NOx model as input. However, in most cases only a 15 %reduction was achieved with SNCR. The SNCR was then tested in a novel split-cycleengine concept where a large tank is used between the combustion cylinder and expander.The gas temperature in the tank is high enough for SNCR to occur. In this case, both experimentsand simulations showed high reduction of 40 % to 50 % when the normalizedstoichiometric ration (NSR) is 1. The experiments were run on a modified Volvo HD13single-cylinder engine. For NSR = 2 and 3, above 85 % reduction of NOx was obtained inboth simulations and experiments. A problem with the strategy was that large amountsof NH3 would be released into the air. The results suggest that an SCR and ammonia slipcatalyst would still be needed in the final design of the double compression expansionengine.Lastly, partially premixed combustion (PPC) was studied in a multi-cylinder VGT VolvoHD13 engine with respect to soot-NOx trade-off using PRF70 fuel and a novel pistondesign. A brake specific NOx emission of approximately 0.4 g/kWh could be achievedat 1000 Nm with 0.07 g/kWh soot.