SPARK IGNITION INTERNAL COMBUSTION ENGINEField of the inventionThe present invention relates to a spark ignition internal combustion engine having an exhaust gas after-treatment system that includes a three-way catalyst and an NOx trap, wherein the NOx trap stores NOx gases when the exhaust gases contain excess oxygen and is regenerated by releasing the NOx gases for reduction by the catalytic converter to N2 when the exhaust gases contain an excess of hydrocarbons (HC) and/or carbon monoxide (CO) in the absence of oxygen, the engine being operated with an overall lean air/fuel ratio during which the NOx gases contained in the exhaust gases are stored in the trap, and having an NOx trap regenerating means for supplying a reducing atmosphere to the NOx trap at regular intervals.
Background of the inventionA spark ignition engine as set out above is already known.
The NOx trap may be integral with the catalytic converter or it may be formed separately and positioned downstream of the main three-way catalyst. The expression "main" three-way catalyst is used herein because the trap itself also contains a three-way catalyst to react the NOx that is released in the presence of a reducing exhaust atmosphere.
To regenerate the NOx trap, the current procedure that is adopted is to modify the mixture strength applied to the engine to produce a strongly reducing atmosphere in the trap. For a short period of time, less than a second, a very rich mixture (about 0.7 relative air/fuel ratio) is fed to the combustion chambers. This major jump in mixture strength is required to compensate for the fact that some of the excess HC and CO will be neutralised by the three-way catalyst before they reach the trap.
Such a major step change in the air/fuel ratio to the engine is undesirable because it produces a perceptible jerk in the output torque and because it perturbs the fuel equilibrium in the intake ports, which complicates engine calibration, especially during transients. There is also a risk of damaging the catalyst by overheating if excess fuel should be stored in the catalyst and react with the excess air present when a lean mixture is resumed. There are also the risks of fouling of the spark plug and the combustion chamber and increasing fuel consumption that result from running with an excessively rich mixture at regular, albeit short, intervals.
Object of the inventionThe present invention therefore seeks to enable the NOx trap to be periodically regenerated without having to enrich the mixture strength supplied to the combustion chambers excessively.
Summary of the inventionAccording to the present invention, there is provided a spark ignition internal combustion engine having an exhaust gas after-treatment system that includes a three-way catalyst and an NOx trap, wherein the NOx trap stores NOx gases when the exhaust gases contain excess oxygen and is regenerated by releasing the NOx gases for reduction by the catalytic converter to N2 when the exhaust gases contain an excess of hydrocarbons (HC) and/or carbon monoxide (CO) in the absence of oxygen, the engine being operated with an overall lean air/fuel ratio during which the NOx gases contained in the exhaust gases are stored in the trap, and having an NOx trap regenerating means for supplying a reducing atmosphere to the NOx trap at regular intervals, characterised in that the NOx trap regenerating means comprise means for setting the overall engine air/fuel ratio tostoichiometry to prevent excess oxygen in the exhaustgases, andmeans for briefly injecting fuel vapour into the exhaustgases directly upstream of the NOX trap.
Setting the engine air/fuel ratio from lean to stoichiometry may preferably be effected by shutting off a bypass air dilution stream while supplying a stoichiometric core mixture stream to the engine.
Injecting fuel vapour into the exhaust gases may preferably be carried out by a pump drawing fuel vapour either directly from the top of a fuel tank or from a canister vapour purge system.
Brief description of the drawingsThe invention will now be described further, by way of example, with reference to the accompanying drawing, which is a block diagram of a spark ignition internal combustion engine of the present invention.
Detailed description of the preferred embodimentThe single figure shows a spark ignition engine 10 having an intake manifold 11 and an exhaust manifold 13. A fuel and air metering system 12 draws fuel from the bottom of a fuel tank 14 and meters the fuel into the intake air and thereby sets the desired air mass and air/fuel ratio. The exhaust gases flow through the exhaust manifold 13 to a catalytic converter 18 and an NOX trap 20 before being discharged by way of a silencer (not shown) and the remainder of the exhaust system to the ambient atmosphere.
As so far described, the engine is entirely conventional and for this reason does not require more detailed explanation.
The catalytic converter 18, which contains a three-way catalyst, and the NOX trap 20, which also contains a threeway catalyst, may either be two separate components, as illustrated, or the NOX trap 20 may be integrated into the same substrate as the three-way catalyst of the main converter 18.
In use, the engine is run with a lean mixture for most of the time to achieve good fuel economy. When the engine operates with such a lean mixture, the three-way catalyst acts only as an oxidation catalyst and, in the presence of excess oxygen, the NOX gases are not reduced. Instead, they pass into the trap 20 where they are stored by being chemically adsorbed. If this is continued indefinitely, the trap would eventually reach saturation and at this point the NOX gases would pass untreated into the atmosphere.
To prevent this, before the trap 20 reaches saturation, it is necessary to regenerate the trap by passing a reducing atmosphere through it. If a reducing atmosphere containing no oxygen and an adequate mass of hydrocarbons and/or carbon monoxide is present in the trap for less than a second, typically 700 ms, the NOX gases will be released and will react in the presence of the catalyst in the trap 20 with the hydrocarbons or carbon monoxide to revert to nitrogen gas that can safely be discharged to atmosphere.
In the prior art, this supply of reducing gases was created by overfuelling the engine and relying on the incomplete combustion products reaching the trap 20.
In the present invention, the reducing atmosphere required to regenerate the trap 20 is not derived from the combustion gases but is injected through a pipe 26 into the exhaust stream immediately upstream of the trap 20. Of course, if the engine is still run with a lean mixture while reducing gases are injected into the exhaust stream, they would react with the excess oxygen in the exhaust gases before reaching the trap 20. For this reason, the fuel and air metering system 12 does need to modify the engine air to fuel ratio temporarily, but not to achieve a rich mixture, only a stoichiometric mixture.
Modifying the air to fuel ratio temporarily from lean to stoichiometry may conveniently be achieved by supplying the intake air to the engine in two parallel streams, the two streams being regulated to be in a fixed ratio with one another by separate throttles ganged together and operated in unison. Fuel is metered stoichiometrically only in dependence upon one of the streams. In this way, when air flows through both streams, the overall mixture is lean but when the second stream is suddenly obstructed, the mixture instantly reverts to stoichiometry without any adjustment to the rate of fuel flow. In this way, the equilibrium of the fuel wetting the intake ports need not be disturbed and there is no sudden change in the engine output torque, which is dictated by the quantity of fuel burnt rather than the air mass supplied to the combustion chambers.Because the excess fuel required at the trap 20 never passes through the combustion chambers, any risk of damage to the engine 10 or the catalytic converter 18 on account of overfuelling is averted.
While stoichiometric gases are present in the exhaust system, the trap 20 is regenerated by supplying fuel containing vapour into the exhaust gases immediately upstream of the trap 20. The fuel-rich vapours are supplied by a pump 22 that is connected to the fuel tank 14 above the level of the stored fuel by a pipe 30. The space at the top of the tank is connected to ambient atmosphere through a vapour canister 16 that absorbs fuel vapours to prevent them from being released to the atmosphere through normal evaporation. When gases are drawn from the top of the fuel tank 14 by the pump 22, air will be drawn through the vapour canister 16 so that the latter is purged in the process.
The pump 22 therefore derives its fuel vapour both from the tank 14 and from the vapours absorbed in the canister 16.