METHOD AND APPARATUS TO CONTROL EGR-RECIRCULATION FOR A COMBUSTION ENGINE

Abstract: An arrangement and a method for control of transfer of EGR gases in a turbocharged combustion engine (1), whereby exhaust gases are drawn off in a regulated manner via a draw-off valve (7) to drive an EGR pump (6) for creating an EGR transfer flow which is regulated by an EGR valve (10). Both the draw-off valve (7) and the EGR valve (10) are regulated by means of a single control valve (14), and the draw-off valve (7) responds to a control fluid pressure which is higher than the control fluid pressure to which the EGR valve (10) responds, causing the EGR valve (10) to open before the draw-off valve (7). The invention also relates to a diesel engine equiped with such an arrangement.

Description: Method and apparatus to control EGR-recirculation for a combustion engine FIELD OF THE INVENTION The present invention relates to a method and an arrangement for control of transfer in a combustion engine of EGR gases in accordance with the preamble to patent claims 1 and 5 respectively. The invention also relates to a diesel engine equipped with such an arrangement. STATE OF THE ART In a known diesel engine equipped with such an arrangement, two separate exhaust manifolds each serve the cylinders in a respective row of cylinders with a view to preventing exhaust pulses from an individual cylinder having negative effects on any other cylinder. This arrangement also increases the possibility of utilising the pulse energy in the exhaust lines of the respective cylinders. To this end, at least the exhaust gases from one exhaust manifold are led to a turbo-unit which is used for increasing the charge pressure in the inlet air to the engine. With the object of effecting recirculation of exhaust gases (EGR or exhaust gas recycling), a proportion of the exhaust gases from one exhaust manifold is drawn off for leading to the engine inlet line, after pressure increase in an EGR pump which is itself driven by exhaust gases drawn off from the other exhaust manifold. In general it may be found, and is well known, that recirculation of EGR gases makes it possible to reduce engine pollution levels considerably but that different operating situations require different EGR transfer levels. Unregulated feeding of the EGR pump is thus ruled out, partly because it is necessary to switch the EGR pump off on the occasion of large transients such as major acceleration or exhaust braking. The drive gas supply to the EGR pump is controlled by a draw-off valve. It is also necessary for the actual EGR supply to be controlled and at least regulated so that no "acid shortage" occurs in the engine, e.g. at high load. The EGR valve which regulates the EGR flow and is situated between the EGR pump and the engine inlet duct controls this flow. The EGR valve has also to be situated as close as possible to the inlet line with a view to achieving a rapid control system and thereby avoiding undesirable sluggishness in the system. Each of these two valves is regulated by means of a so-called control valve associated with it which typically utilises the tank pressure of the vehicle (at least in the case of a heavy-duty truck or bus), which is normally around 7-9 bar. This pressure is controlled via the two control valves and regulated by the engine's control system for feeding the respective valves with a control fluid pressure by which they are regulated to achieve EGR transfer control. The control valves are thus operated in accordance with electrical signals and the control system has to be accurately tuned, inter alia to ensure that the EGR valves opens before the draw-off valve. The reason for this is safety, since the EGR pump might otherwise sustain damage. Further regulation of the various valves makes it possible for different EGR quantities to be transferred depending on the operating state of the engine. The principle of this known kind of EGR control works acceptably but control is complicated and a resulting arrangement is relatively expensive. OBJECTS AND MOST IMPORTANT CHARACTERISTICS OF THE INVENTION One object of the invention is to provide a development of the previously known system and indicate a solution which eliminates or at least radically alleviates the abovementioned problems. This object is achieved with a method and an arrangement of the kind mentioned in the introduction by the features in the characterising part of patent claims 1 and 5 respectively. The resulting possibility of the two valves being controlled by a single control valve has a number of advantages for the system, namely easier regulation and easier programming of control systems, fewer components and an overall system which is less expensive but nevertheless reliable. The periods during which the valves are open may be separate or overlap, subject to ensuring that the EGR valve opens before the draw-off valve. The two valves may be of the same kind and are preferably spring-loaded pressure-controlled valves of a conventional kind, with the springs adapted so that the valves conform to the respective pressure range. Examples of such valves include pivoting butterfly valves or axially movable disc valves. The control valve is with advantage controlled electronically by the control system, and this is very simple to do for achieving the correct control of both the EGR valve and the draw-off valve. The principle of the invention is applicable to various types of engines in which EGR transfer is desired, but is particularly advantageous in engines of the abovementioned kind, i.e. diesel engines with two separate exhaust manifolds each serving its own group of cylinders. In such cases, exhaust gases are drawn off from a first exhaust manifold via the draw-off valve to drive the EGR pump, while exhaust gases from the second exhaust manifold are compressed in said EGR pump and thus become EGR gases. It is advantageous for the draw-off valve to be controlled with a pressure of about 2-6 bar for a conventional diesel engine in a heavy vehicle, preferably a control pressure of about 3-5 bar. Under the same conditions, the EGR valve is controlled with a pressure of about 1-3 bar and preferably a pressure of about 1.5-2.5 bar. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be explained in more detail on the basis of an embodiment with reference to the drawings, in which Fig. 1 depicts schematically an arrangement for EGR transfer according to the invention in a graphically suggested diesel engine. DESCRIPTION OF EMBODIMENTS Fig. 1 depicts an arrangement according to the invention in connection with a schematically suggested six-cylinder turbocharged diesel engine 1 intended to power a heavy-duty vehicle such as a truck or a bus. Exhaust gas ducts from the engine 1 lead respectively to a first exhaust manifold 2a and a second exhaust manifold 2b. The latter extend in a conventional manner to, and in order to drive, a turbocharger 3 which compresses fresh air from the atmosphere and passes it on via a charge air cooler 4 to the inlet side of the engine 1 via the ducts 12 and 13. Reference 6 denotes an EGR pump composed in the same manner as a turbocharger. It includes a turbine driven by exhaust gases which are drawn off from the first exhaust manifold 2a and reach the turbine via a line which incorporates a draw-off valve 7. EGR gases consisting of exhaust gases drawn off from the engine's second exhaust manifold 2b reach the compressor of the EGR pump via a duct 8 which also incorporates an EGR cooler 9 for cooling the exhaust gases led back. The compressed exhaust gases are led from the compressor of the EGR pump 6 via an EGR valve 10 to the engine's inlet air duct at a connection 11. The EGR valve 10 and the draw-off valve 7 are controlled in accordance with a fluid pressure in the lines 15,16 (suggested by broken lines) from a control valve 14 which is connected to a pressure source. In this example the pressure source consists of a compressed air reservoir 18 forming part of the vehicle's ordinary brake system. The control valve 14 is regulated electrically and is itself controlled by the engine's control system, suggested by reference 17. The regulation of the control valve 14 is arranged so that a control fluid pressure in the lines 15 and 16 within a lower pressure control range such as 1-2.5 bar causes the EGR valve 10 to open, while a control fluid pressure within a higher range such as 3-5 bar keeps both the valve 10 and the draw-off valve 7 open. The arrangement is regulated so that the draw-off valve 7 responds to (opens at) a control fluid pressure which is higher than the control fluid pressure to which the EGR valve 10 responds, resulting in the EGR valve 10 opening before the draw-off valve 7. A control sequence may be as follows: A 5 bar control fluid pressure in the lines 15,16 keeps both the EGR valve 10 and the draw-off valve 7 fully open. During operation, this means that exhaust gases drawn off are allowed to expand in the turbine portion of the EGR pump, while EGR gases are compressed in the compressor stage of the EGR pump for passing on to the engine's inlet line. In this operating situation, maximum EGR quantities will recirculate. Reducing the control fluid pressure, e.g. to 4 bar, causes the draw-off valve 7 to start closing, thus reducing the expansion of exhaust gases drawn off from the exhaust manifold 2a across the turbine of the EGR pump 6. The EGR valve 10 remains fully open. The reduced energy supplied to the EGR pump 6 causes a marked reduction in the EGR quantity supplied to the engine. Reducing the control fluid pressure below 2.5 bar causes the draw-off valve 7 to close completely so that no exhaust gases are supplied to the turbine of the pump 6 and hence no work is done in the turbine of the EGR pump. At the time when the pressure in the exhaust manifold is higher than the pressure in the inlet pipe, a certain proportion of the EGR gases will nevertheless spontaneously reach the engine's inlet pipe, which in practice means the creation of a so-called short-route system. Continued regulation of the EGR valve 10 may be applied to bring the EGR flow up from 0% to the maximum possible with this short-route system. All in all, the invention, utilising all the control facilities indicated above, makes it possible to adapt the EGR transfer within a large range of regulation with respect to EGR quantities transferred. Hence the EGR transfer can be adapted and optimised to cater for a long series of different operating situations. It should be noted that the draw-off valve 7 and the EGR valve 10 may be regulated approximately in the simplest situation in which the respective valves are in principle either completely closed or fully open. From this extreme situation, more sophisticated controls may be applied whereby in response to detected operating parameters such as engine speed, fuel consumption, torque extracted, vehicle speed etc. the degree of opening of the valves can be controlled sensitively in order to achieve as optimised EGR recirculation as possible.

Claims: PATENT CLAIMS 1. Method for control of transfer of EGR gases in a turbocharged combustion engine (1), whereby exhaust gases are drawn off in a regulated manner via a draw-off valve (7) to drive an EGR pump (6) which compresses EGR gases to create an EGR transfer flow, and the EGR transfer flow between the pump (6) and an inlet line (13) to the engine (1) is regulated by an EGR valve (10), and the valves (7,10) are regulated by control fluid supply to the respective valves (7,10), characterised in that both the draw-off valve (7) and the EGR valve (10) are regulated by means of a single control valve (14), and the draw-off valve (7) responds to a control fluid pressure which is higher than the control fluid pressure to which the EGR valve (10) responds, causing the EGR valve (10) to open before the draw-off valve (7). 2. Method according to claim 1, whereby the engine (1) is of the kind which has two separate exhaust manifolds (2a,2b) which each serve a respective group of cylinders, characterised in that exhaust gases from one exhaust manifold (2a) are drawn off via the draw-off valve (7) and used for driving the EGR pump, and that exhaust gases from the second exhaust manifold (10) are drawn off to become EGR gases for leading back to the engine inlet. 3. Method according to claim 1 or 2, characterised in that the draw-off valve (7) is controlled so as to open at a control fluid pressure of about 2-6 bar and more preferably about 3-5 bar. 4. Method according to claim 1, 2 or 3, characterised in that the EGR valve (10) is controlled so as to open at a control fluid pressure of about 1-3 bar and more preferably about 1.5-2.5 bar. 5. Arrangement for control of transfer of EGR gases in a turbocharged combustion engine (1), including a draw-off valve (7) for regulating the drawing off of exhaust gases to drive an EGR pump (6) which is intended to compress EGR gases for creating an EGR transfer flow, while an EGR valve (10) is intended to regulate the EGR transfer flow between the pump (6) and an inlet line (13) to the engine (1), and devices are arranged to regulate the valves by control fluid supply to the respective valves, characterised in that a single control valve (14) is arranged to regulate both the draw-off valve (7) and the EGR valve (10), and the draw-off valve (7) is arranged to respond to a control fluid pressure which is higher than the control fluid pressure to which the EGR valve (10) is arranged to respond, causing the EGR valve (10) to open before the draw-off valve (7). 6. Method according to claim 5, whereby the engine (1) is of the kind which has two separate exhaust manifolds (2a,2b) which each serve a respective group of cylinders, characterised in that a duct is arranged for drawing off exhaust gases from one exhaust manifold (2a) via the draw-off valve (7) to the EGR pump (6) in order to drive the latter, and that a second duct is arranged for drawing off exhaust gases from the second exhaust manifold (2b), which gases are intended to become EGR gases for leading back to the engine inlet. 7. Arrangement according to claim 5 or 6, characterised in that the draw-off valve (7) is arranged to open at a control fluid pressure at about 2-6 bar and most preferably about 3-5 bar. 8. Arrangement according to claim 5, 6 or 7, characterised in that the EGR valve (10) is arranged to open at a control fluid pressure of about 1-3 bar and most preferably about 1.5-2.5 bar. 9. Diesel engine including an arrangement according to any one of claims 5-8.

Inventors: LOENNQVIST TOM, LOENNQVIST, TOM
Applicants: LOENNQVIST TOM, SCANIA CV ABP, LOENNQVIST, TOM, SCANIA CV AKTIEBOLAG (PUBL)
Application year: 2000
Main classification: F02M25
IPC codes: F02B37, F02D21, F02M25