GB2428602A - An emission control system for an engine - Google Patents

Description

An Emission Control System for an Engine This invention relates to the

reduction of exhaust emissions from an internal combustion engine and in particular to an improved emission control system for use with a lean burn engine.

It is well known to provide a lean burn internal combustion engine with an emission control system having an emission control device known as a lean NOx trap to store NOx produced when the engine is operating lean of stoichiometric. It is a problem with such lean NOx traps that they are sensitive to high temperatures and, when the engine is running rich of stoichiometric at high load, a lean NOx trap will deteriorate at a rapid rate due to the high temperatures produced and to increased sulphur poisoning.

It is an object of this invention to provide an emission control system for a lean burn engine that reduces the loss of lean NOx trap life due to running of the engine at high load.

According to the invention there is provided emission control system for a lean burn engine comprising a lean NOx trap arranged to receive exhaust gasses from the engine and an exhaust gas pressure actuated valve located upstream from the lean NOx trap wherein the exhaust gas pressure operated valve is operable to reduce the exposure of the lean NOx trap to the exhaust gases from the engine when the engine is operating at high load by permitting at least some of the exhaust gasses from the engine to flow through an alternative flow passage.

The exhaust gas pressure actuated valve may be a flap valve biased into a closed position by at least one spring and opened at least partially when the pressure of the exhaust gasses acting on the flap valve reaches a predetermined pressure level.

The predetermined pressure level may be set such that the engine will be operating rich of stoichiometric when the exhaust pressure reaches the predetermined pressure level.

The alternative flow passage may be a bypass passage which bypasses the lean NOx trap and, when the exhaust gas pressure valve is opened by the exhaust gas pressure, exhaust gasses may be able to flow through the bypass passage thereby reducing the flow of exhaust gasses through the lean NOx trap.

Alternatively, the system may further comprise an exhaust gas cooler located upstream from the lean NOx trap and, when the exhaust gas pressure valve is opened by the exhaust gas pressure, exhaust gasses may be able to flow through the alternative flow passage to the exhaust gas cooler thereby reducing the temperature of the exhaust gasses passing through the lean NOx trap.

Preferably, the system may further comprise a three way catalyst located upstream from the exhaust gas pressure actuated valve.

The invention will now be described by way of example with reference to the accompanying drawing of which:- Fig.l is a schematic representation of an engine and emission control system according to the invention; Fig.2 is a schematic diagram of part of a first embodiment of an emission control system according to the invention; and Fig.3 is a schematic diagram of part of a second embodiment of an emission control system according to the invention.

With reference to Fig.1 there is shown a lean burn engine 10 from which exhaust gasses flow to atmosphere via an exhaust outlet passage in the form of an exhaust system 13.

The exhaust gasses from the engine 10 flow firstly through a three way catalyst 11 and then through a lean NOx trap 12 before passing out from a tailpipe of the exhaust system 13 to atmosphere.

An exhaust gas pressure actuated valve 14 is located upstream from the lean NOx trap 12 and controls the entry to an alternative flow passage 15. The term exhaust gas pressure actuated valve as meant herein is a valve which is moved directly by the action of exhaust gas pressure there against without any other actuators or electronic control devices.

When the engine is running under low load and hence lean of stoichiometric, the exhaust gas pressure actuated valve 14 remains in a closed position. In the closed position the exhaust gas pressure actuated valve 14 closes off the entry to the alternative flow passage 15 and so all of the exhaust gasses flowing from the engine 10 must pass through the lean NOx trap 12. However, as the load on the engine 10 is increased to a high load state in which the engine 10 is operating rich of stoichiometric the exhaust gas pressure actuated valve 14 opens thereby allowing some of the exhaust gasses to bypass the lean NOx trap 12. It will be appreciated that the pressure drop across the lean NOx trap 12 will be considerably greater than the pressure drop across the exhaust gas pressure actuated valve 14 and the alternative flow passage 15. Therefore, as soon as the exhaust gas pressure actuated valve 14 is opened by more than a small amount, the majority of the exhaust gasses will flow through the alternative flow passage 15 due to the lower restriction to flow and the flow through the lean NOx trap will be minimal thereby reducing the damage caused to the lean NOx trap 12 by the very hot and rich of stoichiometric exhaust gasses.

When the load on the engine 10 decreases to a level where the engine 10 is likely to be operating at or close to stoichiometric the exhaust gas pressure actuated valve 14 will close thereby ensuring that all of the exhaust gasses once again flow through the lean NOx trap 12.

It will be appreciated that no significant loss in emission performance is incurred because the lean NOx trap 12 is only effective in reducing emissions when the engine is running lean of stoichiometric and the exhaust gas pressure actuated valve 14 is arranged so that it does not open until it can be guaranteed that the engine 10 is operating rich of stoichiometric. It will be appreciated that, when the engine 10 is running at or rich of stoichiometric, the exhaust emissions from the engine 10 are reduced by the three way catalyst 11.

With reference to Fig.2 there is shown in more detail part of the emission control system shown in Fig.1.

The lean NOx trap 12 is arranged to receive exhaust gasses from the engine 10 through the exhaust outlet passage 13 and an exhaust gas pressure actuated valve in the form of a spring biased flap valve 114 is located upstream from the lean NOx trap 12 so as to control entry to an alternative flow passage in the form of a bypass passage 115.

The flap valve 114 is biased into a closed position by at least one spring (not shown) and is opened, at least partially, when the pressure of the exhaust gasses acting on the flap valve 114 reaches a predetermined pressure level.

When the flap valve 114 is closed virtually all of the exhaust gasses pass through the lean NOx trap 12 and so there is substantially no exhaust gas flow through the bypass passage 115. However, it will be appreciated that there may be a very small flow through the bypass passage due to leakage past the flap valve 114.

Operation is as previously described, when the pressure acting on the flap valve 114 exceeds a predetermined pressure the force produced will overcome the spring force holding the flap valve 114 closed and so the flap valve 114 will open thereby allowing exhaust gasses to flow through the bypass passage 115 and significantly reduce the flow of exhaust gasses through the lean NOx trap 12.

The predetermined pressure level at which the flap valve 114 opens is set such that the engine 10 will be operating rich of stoichiometric when the exhaust pressure reaches this predetermined pressure level. This is done by carrying out test bed experimental work to establish relationships between exhaust gas pressure and exhaust gas composition. These relationships are then used to determine the spring loading required to maintain the flap valve 114 closed until it is certain that the engine 10 is running rich of stoichiometric.

With reference to Fig.3 there is shown part of an emission control system according to the invention which differs from those previously described in that an exhaust gas cooler 220 is positioned upstream from a lean NOx trap 212 in order to selectively cool the exhaust gasses flowing to the lean NOx trap 212.

Exhaust gasses from the engine 10 as previously described flow from the engine 10 through an exhaust outlet passage 213 through a three way catalyst (not shown) to an exhaust gas pressure actuated valve in the form of a flap valve 214 positioned at an entrance to an alternative flow passage 215 upstream from the lean NOx trap 212.

The flap valve 214 controls the flow of exhaust gasses through the exhaust gas cooler 220 which is supplied with coolant from a main cooling system (not shown) of the engine 10. The flap valve 214 is biased to a closed position, as shown on Fig.3, by one or more springs (not shown) . That is to say, the flap valve 214 is spring biased closed and is opened by exhaust gas pressure acting thereupon.

When the flap valve 214 is closed all of the exhaust gasses from the engine 10 flow through the exhaust outlet passage 213 directly to the lean NOx trap 212 and then out to atmosphere.

When the flap valve 214 is opened by the pressure of the exhaust gasses acting thereupon a proportion of the exhaust gasses flow through the alternative flow passage 215 to the exhaust gas cooler 220 where they are cooled before flowing on to the lean NOx trap 212. These cooled exhaust gasses mix with any un-cooled exhaust gasses that have not passed through the exhaust gas cooler 220 before passing through the lean NOx trap 212 and so the temperature of the exhaust gasses flowing through the lean NOx trap 212 is reduced. The reduction in the temperature of the exhaust gasses flowing through the lean NOx trap 212 reduces the thermal ageing of the lean NOx trap 212 that would otherwise occur due to the interaction of very hot exhaust gasses with the lean NOx trap 212.

It will be appreciated by those skilled in the art that the temperature of the exhaust gasses from an engine increase considerably as the load upon the engine is increased and so by using an exhaust gas cooler at high engine loads the effect of thermal ageing of a lean NOx trap can be significantly reduced. At high engine loads the pressure in the exhaust outlet passage is also increased and so by using this pressure to open a valve controlling the flow through the exhaust gas cooler increased lean NOx trap life can be obtained with minimal additional complexity and without requiring complex control systems or actuators.

It will be appreciated that, because the flap valve 214 will be closed when the engine 10 is operating at low load, the performance of the lean NOx trap 212 will not be compromised and, in particular, the time taken to reach light-off when the engine 10 is started from cold will not be adversely affected.

Because the lean NOx trap 212 is never bypassed, it is possible to arrange for the flap valve 214 to open at any convenient pressure and not only when the engine 10 is operating at or rich of stoichiometric.

Although the invention has been described in relation to the use of an exhaust gas pressure actuated valve in the form of a simple spring biased flap valve it will be appreciated that other forms of direct pressure operated valves could be used.

It will be appreciated by those skilled in the art that although the invention has been described by way of example with reference to one or more embodiments it is not limited to the disclosed embodiments and that modifications to the disclosed embodiments or alternative embodiments could be constructed without departing from the scope of the invention.

Claims (7)

1. Claims 1. An emission control system for a lean burn engine comprising a

   lean NOx trap arranged to receive exhaust gasses from the engine and an exhaust gas pressure actuated valve located upstream from the lean NOx trap wherein the exhaust gas pressure operated valve is operable to reduce the exposure of the lean NOx trap to the exhaust gases from the engine when the engine is operating at high load by permitting at least some of the exhaust gasses from the engine to flow through an alternative flow passage.
2. 2. A system as claimed in claim 1 wherein the exhaust gas pressure actuated valve is a flap valve biased into a closed position by at least one spring and opened at least partially when the pressure of the exhaust gasses acting on the flap valve reaches a predetermined pressure level.
3. 3. A system as claimed in claim 2 wherein the predetermined pressure level is set such that the engine will be operating rich of stoichiometric when the exhaust pressure reaches the predetermined pressure level.
4. 4. A system as claimed in any of claims 1 to 3 wherein the alternative flow passage is a bypass passage which bypasses the lean NOx trap and, when the exhaust gas pressure valve is opened by the exhaust gas pressure, exhaust gasses are able to flow through the bypass passage thereby reducing the flow of exhaust gasses through the lean NOx trap.
5. 5. A system as claimed in any of claims 1 to 3 wherein the system further comprises an exhaust gas cooler located upstream from the lean NOx trap and, when the exhaust gas pressure valve is opened by the exhaust gas pressure, exhaust gasses are able to flow through the alternative flow passage to the exhaust gas cooler thereby reducing the temperature of the exhaust gasses passing through the lean NOx trap.
6. 6. A system as claimed in any of claims 1 to 5 wherein the system further comprises a three way catalyst located upstream from the exhaust gas pressure actuated valve.
7. 7. An emission control system for a lean burn engine substantially as described herein with reference to the accompanying drawing.