Patent GB-1416382-A (GB)
Title: Removal of pollutants from exhaust gas of internal combustion engines
IPC: F01N13/02
Filed: 1973-01-11
Abstract: 1416382 Treating I.C. engine exhaust gas EXXON RESEARCH & ENG CO 11 Jan 1973 [12 Jan 1972] 1510/73 Heading B1W A method for removing pollutants from the exhaust gas of an I.C. engine in which fuel is burned with air at a substantially less than stoichiometric air-fuel ratio during cold engine operation and at a greater but still substoichiometric air-fuel ratio during warm engine operation and in which the exhaust gas is contacted under wet reducing conditions with a nitrogen oxide reduction catalyst in a first conversion zone and is then treated in a second conversion zone with at least sufficient added secondary air to oxidise carbon monoxide and hydrocarbons and maintain net oxidising conditions in the zone, is characterised in that a quantity of air not greater than stoichiometric is supplied to the first conversion zone during cold engine operation, the supply being shut off when the average air-fuel ratio of the mixture supplied to the engine reaches substantially its normal warm-engine value, operation then being continued without substantial addition of external air to the first conversion zone. A V- eight engine 10 draws combustion air through a filter 11, past a choke valve 13 and through a filter 11, past a choke valve 13 and through a carburettor 15, the flow rate being controlled by a throttle valve 18. A catalytic exhaust gas purifying device is provided for each bank of cylinders, only one being shown. Cooling water flows through a feed line 21 and an outlet line 22. Exhaust gas from a manifold 23 flows to a first catalytic reactor 24 in which nitrogen oxides are reduced and a second reactor 26 which is preferably catalytic and in which carbon monoxide and hydrocarbons are oxidised. An engine-driven pump 30 supplies air to the first reactor through a solenoid actuated valve 36 and supplies air to the second reactor through a check valve 29. The solenoid 37 is energised through a temperature sensitive device 38. In modifications the device 38 may be situated in the second reactor or may be replaced by a switch actuated by a cam which moves with the choke valve 13. Operation. When the engine is started from cold, it is supplied with a very rich mixture having an air-fuel ratio between 10 and 12, so that the exhaust gas contains a considerable quantity of carbon monoxide and hydrocarbons. An amount of air, not greater than stoichiometric and preferably less is supplied through the line 35. This air oxidises part of  the reducing constituents of the exhaust gas thus causing heating of both catalysts and also provides a mildly reducing atmosphere favourable to the conversion of most of the nitrogen oxide to nitrogen rather than ammonia. As the engine warms up, the ratio of air to fuel is gradually increased until the "warm-engine" value is reached i.e. a slightly sub-stoichiometric air-fuel ratio. The valve 36 is then closed by the device 38 and only the second reactor continues to be supplied with air. Preferably the valve 36 has a variable orifice so that the air flow can be reduced progressively. Catalysts. The reduction catalyst is preferably a nickel-copper alloy containing 40-95% by wt. of nickel, which may be supported on a stainless steel base or a carrier e.g. alumina having a surface area less than 5 sq. m/gm. Other catalysts Which may be used are nickel or manganese chromite or copper chromite. The oxidation catalysts is preferably platinum on a cordierite honeycomb carrier. This catalyst may be divided into a small portion near the reactor inlet and a large portion downstream thereof, the small portion heating rapidly to its active temperature. The small portion may also be electrically heated.