Diesel engine exhaust gas nitrogen oxide removal comprises exhaust gas oxygen content reduction, fuel injection into exhaust gas and controlled recycling of exhaust gas to intake manifold

摘要

Diesel engine exhaust gas nitrogen oxide (NOx) removal comprises a combination of exhaust gas oxygen content reduction, fuel injection into the exhaust gas and controlled recycling of exhaust gas to the intake manifold. A NOx removal process, for a diesel engine exhaust system (12,15) having a catalytic converter (17) operating in alternate NOx storage phases with lean mixture supply and regeneration phases with rich mixture supply, comprises fuel injection into the system to increase the hydrocarbon (HC) content and use of an upstream oxidation catalyst (16) for exhaust gas oxygen concentration reduction and CO concentration increase. Additionally, a certain volume of exhaust gas is recycled from the exhaust manifold (3) to the intake manifold (2), the quantity of gas recycled during the regeneration phases being greater than that of the fresh air supplied to the engine. Preferred Features: The engine fresh air supply (2) is interrupted during the regeneration phases. The recycled exhaust gas volume is determined from a table which restricts the exhaust gas soot content to an acceptable value. Fuel is injected from a fuel distributor (19) into the exhaust system upstream of the oxidation catalyst (16) and the injected amount is determined from a table which limits the additional fuel consumption to an acceptable value and ensures combustion stability of the engine. The air content of the fuel mixture is determined by measuring the exhaust gas oxygen content between the oxidation catalyst (16) and the catalytic converter (17) and the measurement is used to control the fresh air supply to the engine (1). During the storage phases, a table of engine speed and supplied fuel amount is used to calculate the NOx quantity at the catalytic converter inlet and thus the stored NOx quantity, taking into account the exhaust gas temperatures at the catalytic converter inlet and outlet. The regeneration phases are initiated when the calculated NOx quantity reaches a predetermined value and, at the end of each regeneration phase, the calculated residual NOx quantity in the catalytic converter (17) is used as the starting value for calculating the NOx quantity collected during a storage phase.