Patent Grant
7770385
This application claims priority under 35 U. S. C. §119 to United Kingdom Patent Application No. UK0327322.4 filed in the United Kingdom on Nov. 25, 2003.
The invention relates to an internal combustion engine exhaust system and particularly but not exclusively limited to an exhaust system for a diesel engine.
Due to increasingly stringent emissions regulations, engine manufacturers are faced with a requirement to reduce all forms of emissions. Particulate emissions from diesel engines are substantially higher than petrol engines and one way to reduce the level of particulate emitted, diesel engine exhaust systems may include a particulate filter trap which catches a proportion of the particulate emitted by the engine. Over time, the filter becomes clogged with the filtered particulates and it is necessary to regenerate the filter in order to prevent excessive back pressure building up in the exhaust system which can reduce the engine's power output and eventually lead to engine failure. One known method of regenerating the particulate filter is to use the NOx generated in the engine to regenerate the particulate filter. In those systems, either the filter substrate has a catalytic coating or a separate catalyst is installed so that passing NO2 over the soot-clogged filter under certain engine conditions will cause the particulates to be broken down and the filter to be cleaned. It is also known to provide a burner system, generally fuelled by diesel fuel which, when the filter becomes clogged, heats the filter substrate to burn off the particulates. Whilst the first system is a “passive” regeneration system which relies upon a catalytic action under certain engine conditions, the latter described system is an “active” system which can regenerate the filter regardless of engine operating conditions.
Another emission that is regulated by emission controls is NOx and one method of reducing NOx production is to provide an exhaust gas recirculation system in which a proportion of the exhaust gas flowing out of the engine is returned to the air intake. This has two effects. Firstly, the exhaust gas contains a high proportion of carbon dioxide and carbon monoxide which, for the purposes of combustion in the combustion chamber are inert gases. By displacing the oxygen inducted into the combustion chamber and replacing it with carbon dioxide and carbon monoxide, the rate of NOx formation is reduced. Also, a proportion of the heat energy created by the combustion is absorbed by the carbon dioxide in the exhaust stream due to the fact that carbon dioxide has a substantial heat absorption capacity and also due to the disassociation of carbon dioxide during combustion which also absorbs energy from the combustion process. Because of that energy absorption, the combustion pressure and temperature is reduced which also reduces the production of NOx. As stated above, catalytic regeneration systems for diesel particulate filters rely on the NOx emitted from the diesel engine to regenerate the filter and to prevent the filter from becoming clogged with particulates. Accordingly, the exhaust engine is faced with a conflict between reducing the level of NOx by exhaust gas recirculation which results in less NOx being available for regeneration of the filter which, in turn, results in the filter becoming clogged or allowing more NOx to be generated by the engine in order to regenerate the filter, with the deleterious effect of addition NOx production.
It is an object of the present invention to provide an improved internal combustion engine exhaust system.
According to one aspect of the invention there is provided an internal combustion engine exhaust system having an exhaust gas recirculation path, a particulate filter and a burner to effect regeneration of the particulate filter.
In that way, by applying the burner regeneration method, NOx is no longer required to regenerate the filter which means that a greater level of exhaust gas recirculation can be effected in order better to reduce the NOx produced by the engine. The present invention provides a system which allows for regeneration of the particular filter across all engine operating systems with no restriction on the level of NOx reduction available by the exhaust gas recirculation system.
In a preferred embodiment the internal combustion engine exhaust system includes a trigger mechanism arranged to trigger regeneration of the filter by activation of the burner. The trigger mechanism preferably senses the back pressure in the exhaust system. As the filter becomes clogged, the back pressure will increase and when the back pressure exceeds a predetermined limit, the trigger mechanism fires the burner to effect regeneration of the particulate filter. Alternatively, the trigger mechanism could be a timer that ignites the burner after a predetermined period of engine operation. Other alternative trigger mechanisms are envisaged including a sensor for determining the mass of particulates retained by the filter or sensors which determine particular engine operating characteristics which might give rise to an increased level of particulates in the filter, for example urban driving. The trigger mechanism may include a combination of the aforementioned sensors.
The internal combustion engine exhaust system preferably includes a control means, for example an electronic control unit, which controls both the burner operation and the exhaust gas recirculation.
The exhaust gas recirculation path preferably takes exhaust gas from the point in the exhaust system downstream of the particulate filter. The exhaust gas recirculation path preferably includes a cooling mechanism arranged in the exhaust gas recirculation path. The exhaust gas circulation is preferably effected by the provision of exhaust gas recirculation valve in the air intake path of the engine. Opening of the valve allows exhaust gases to be drawn along the exhaust gas recirculation path by means of positive pressure from behind in the exhaust system and the negative pressure effected by the venturi effect of air passing along the air intake. Recirculated exhaust gas is thus combined with air intake gas.
The cooling mechanism arranged in the exhaust gas recirculation path may be arranged to provide addition cooling during operation of the burner. In that way, the increase in exhaust gas temperature of exhaust gases exiting the filter during operation of the burner is compensated by the extra cooling effected by the cooling mechanism so that exhaust gas recirculation is not compromised by the elevated temperatures. Alternatively, the control means may be arranged to shut down the exhaust gas recirculation valve to prevent exhaust gas recirculation during operation of the burner. That prevents hot gases being recirculated into the engine.
According to another aspect of the invention there is provided a method of operating an internal combustion engine exhaust system comprising the steps of providing a particulate filter, operating the particulate filter to filter particulates from the exhaust gas stream, recirculating exhaust gas into the engine air intake and periodically regenerating the particulate filter by means of a burner.
The step of periodically regenerating the particular filter preferably comprises effecting regeneration in response to a trigger, the trigger including one or more of the back pressure in the exhaust, the time elapsed since the last regeneration, other engine parameters indicative of particular engine operating conditions.
The step of recirculating the exhaust gas preferably includes cooling the recirculated exhaust gas. The method preferably includes the step of providing additional cooling to the recirculated exhaust gas on operation of the burner to regenerate the filter.
An internal combustion engine exhaust system in accordance with the invention will now be described in detail by way of example and with reference to the accompanying drawings in which:
The exhaust system 10 comprises an exhaust manifold (not shown) which takes exhaust gas from the engine 12 and passes it to an exhaust pipe 14, via the exhaust side of a turbo charger 16. Downstream of the turbo charger 16, the pipe 14 passes into a particulate filter assembly 18. Downstream of the particulate filter 18, the exhaust gases pass into a second pipe 20. Part way along second pipe 20, there is an exhaust gas recirculation pipe 22 in fluid communication with the pipe 20. The pipe 20 extends beyond the junction with the exhaust gas recirculation pipe 22 to the remainder of the exhaust system, which is conventional and need not be described herein. The exhaust gas recirculation pipe 22 has a cooling mechanism 24 which allows gases passing along pipe 22 to be cooled. The end of the pipe 22 that is spaced from the exhaust pipe 20, has a junction with the air inlet pipe 26 of the diesel engine 12. The junction of the exhaust gas recirculation pipe 22 and the air inlet pipe 26 is controlled by a valve 27 which its open condition allows exhaust gases to flow from the pipe 20, from the pipe 22, through the cooler 24 and into the air pipe 26 and when closed prevents such flow.
The filter assembly 18 comprises a chamber 28 with exhaust inlet 30 and the exhaust outlet 32. The chamber 28 includes a filter element 34 arranged in such a way that exhaust gases entering the chamber 28 through the inlet 30 must pass through the filter 34 to reach the outlet 32. The chamber 18 also includes a burner arrangement 36.
The burner arrangement 36 receives fuel from a fuel storage, for example the fuel tank of a vehicle, via a fuel line 38. Combustion air is provided via an airline 40. An ignition device 42 is provided to effect ignition of the burner when necessary. An electronic control unit 44 or an array of electronic control units is provided to control the engine 12, the exhaust gas recirculation valve 27 and the burner assembly 36.
An exhaust gas back pressure sensor 46 senses the back pressure immediately upstream of the particulate filter 18 and passes that data to the ECU 44. Three temperature sensors T1, T2 and T3 pass sensed temperature data back to the ECU 44. T1 senses the burner temperature, T2 senses the temperature upstream of the filter and T3 senses the temperature downstream of the filter. The ECU 44 also receives data from the engine, for example engine speed and boost pressure are sensed and transmitted to the ECU.
In use, the engine 12 is operated in the normal way and exhaust gases leaving the engine 12 pass through the turbo charger 16 and into exhaust line 14. The exhaust gases which carry particulates are passed to particulate filter 18 where the particulates are filtered out from the exhaust gas stream. Downstream of the particulate filter 18, the exhaust gas passes into exhaust pipe 20 and then on to the remainder of the exhaust system. Where necessary, the ECU 44 controls the valve 27 at the junction of the air inlet pipe 26 and the exhaust gas recirculation pipe 22 to open. A combination of the positive pressure behind exhaust gas in the line 22 and negative pressure ahead from the venturi effect of the air passing through the air intake pipe 26, exhaust gases are drawn along pipe 22 through the cooling mechanism 24 and into the air inlet stream 26. In that way, air and exhaust gases are mixed together in the air inlet stream before passing through the inlet side of the turbo charger 16 into the combustion chambers of the engine. As described above, the presence of the exhaust gases in the air inlet stream reduces the proportion of oxygen in the combustion chamber which substantially reduces NOx. Also, NOx tends to be produced when the engine is running at high temperatures and the increased proportion of carbon dioxide in the inlet gas stream absorbs more energy from the combustion for a lower increase in temperature. Accordingly, the exhaust gas stream emerging from the combustion chambers of the engine 12 is at a lower temperature than would occur if operating in clean air.
Over time, the filter 34 becomes clogged with the filtered out particulates. As that occurs, the force required to push the exhaust gases through the filter 34 increases which increases the back pressure in the exhaust pipe 14 immediately upstream of the filter. If the back pressure exceeds a predetermined limit, the electronic control unit 44 initiates operation of burner 36. Fuel is supplied along the line 38 and air is supplied along the line 40. The fuel/air mixture is mixed in the burner head and the fuel/air mixture is ignited by means of the ignition device 42. When the burner ignites, the filter temperature is elevated which causes burning off of the clogging filter particles so as to clear the filter.
Accordingly, it is possible to operate the exhaust system 10 at high levels of exhaust gas recirculation to reduce the level of NOx output whilst avoiding the compromise requirement of particulate filters that require NOx regeneration of the filter itself.
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