BACKGROUND OF THE INVENTION In connection with load-carrying vehicles powered by a diesel engine, there exists a general need for reducing, as much as possible, the emission of harmful pollutants from the engine exhaust gases. These emissions are primarily nitrogen oxide compounds (NOx), carbon monoxide (CO) and hydrocarbons (HC). In order to reduce these emissions, various measures can be taken. For example, it is previously known that the design of the combustion chamber in the engine cylinder and the process of injecting the engine fuel can be adjusted so as to minimise the emissions. In those cases where the engine is equipped with a turbocharger, the emission of NO, compounds may be reduced by cooling of the air fed into the engine (known as"intercooling").

In connection with petrol driven engines, the exhaust gases are normally purified by means of a catalytic converter located in the exhaust system. Because a diesel engine is operated with an air surplus, however, the three-way catalyst cannot be used for reduction of NOx compounds from a diesel engine.

Due to increasing environmental, requirements and expected future legislation demands, the need for reducing the emissions, primarily of NOx compounds, from diesel engines has grown ever stronger. In this context, it is previously known that the amount of NOx compounds from a diesel engine can be reduced by equipping it with something known as an EGR system (Exhaust Gas Recirculation), by means of which a certain amount of exhaust gases can be returned from the engine exhaust to its intake. The amount of NOx compounds generated in a diesel engine is principally exponentially proportional to the temperature inside the combustion

chamber, and, by using an EGR system, the local temperature during combustion can be lowered by dilution with exhaust gases (C02 and H2O). This in turn leads to less creation of NOx.

A diesel engine may be provided with an EGR system by connecting a separate line between the ordinary exhaust outlet of the engine and a point close to the fresh air intake of the engine. In this line, a controllable valve is arranged, this valve further being connected to a control unit. In dependence of the existing engine operating conditions, particularly regarding its rotational speed and its load, the control unit will determine the degree of opening of the valve, i. e. the amount of EGR gases to be recirculated to the engine air intake. A certain amount of EGR gases will then be fed from the engine exhaust side to its intake side, through the exhaust side pressure normally being higher than the intake side pressure, thus creating a natural "propulsion pressure"for the EGR gases.

In those cases where a diesel engine having an EGR system is utilised together with a turbocharger unit, a problem will be created by there being, for most of the operating points, a higher pressure after the turbocharger compressor (i. e. at the point of the engine intake manifold where the incoming fresh air is fed to the engine) than at the engine exhaust outlet. This in turn means that a recirculation of EGR gases will not be possible, as there will not be any natural propulsion pressure from the engine exhaust to its intake side. In this manner, no EGR gas flow can be injected into the engine.

In accordance with known art, this problem can be solved by providing the turbocharger with variable turbine geometry. One arrangement using this solution is shown in the patent document JP-08270454 A, disclosing a diesel engine having a turbocharger in turn comprising adjustable guide vanes, which, depending on the engine operating conditions, can be adjusted to a certain position by means of a control unit. In this way, a sufficiently high pressure can be created on the engine exhaust side, whereby a required amount of EGR gases can be recirculated to the intake side.

A substantial disadvantage of this known arrangement relates to the fact that it will entail an impaired gas exchange in the engine. Consequently, there is a need for-

engine arrangements providing a flow of EGR gases without impaired gas exchange in a diesel engine being equipped with a turbocharger.

SUMMARY OF THE INVENTION The object of the present invention is to provide an improved arrangement for a combustion engine, particularly a diesel engine being equipped with an EGR system and a turbocharger, providing an adequate propulsion pressure for the EGR gases, so as to achieve a reduction of the NOx emissions of the engine. This object is achieved by an arrangement, the characteristics of which will be defined in the ap- pended claim 1. This object is also achieved by a method, the characteristics of which will be defined in the appended claim 9.

The arrangement according to the invention is intended for a combustion engine having at least one cylinder, an intake for the provision of air, an exhaust outlet for discharging exhaust gases, a further line for recirculation of exhaust gases from said outlet to said intake for reduction of harmful emissions from the engine (in the form of CO and NOX and HC compounds), a controllable valve arranged in said further line and a turbocharger unit comprising a first means for absorbing energy from the exhaust gases and a means for compressing air to said intake. The invention is characterised in that it comprises a second means for absorbing energy from the exhaust gases, arranged downstream of said first means, for building a pressure in said outlet which surmounts the pressure in said intake.

According to a preferred embodiment of the invention, said engine can be equipped with an EGR system and a system for further extraction of energy from the exhaust gases and feedback of that energy to the engine crankshaft. In this way it is ensured that the required propulsion pressure is built on the engine exhaust side without deterioration of the engine efficiency, as an impaired gas exchange will be compensated for by the extraction of further energy from the exhaust gases and feeding it back to the crankshaft.

Preferred embodiments of the invention are described in the accompanying dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in greater detail below, with reference to a preferred embodiment example and the enclosed drawing, which, in schematical form, illustrates an arrangement according to the present invention.

PREFERRED EMBODIMENT Fig. 1 schematically illustrates an arrangement according to the present invention, which can be utilised particularly for a combustion engine of the diesel type.

According to a preferred embodiment, the diesel engine 1 is intended for use in a load-carrying vehicle, and comprises six cylinders 2. The invention is however not limited to any specific number of cylinders or any specific cylinder configuration.

In a way as such previously known, the engine 1 is equipped with an intake manifold 3, to which air is fed from the atmosphere via an intake conduit 4. The input air is then divided between the various cylinders 2. Furthermore, fuel is supplied to the cylinders 2 by a corresponding number of fuel injection devices 5 that are each connected to a central control unit 6 via an electrical connection 7. The control unit 6, which is preferably computer based, is in a known manner operative to control each fuel injection device 5 so as to supply, in each instant, an appropriate fuel/air mixture to the engine 1.

During operation of the engine 1, the control unit 6 is operative to control the respective injection device 5 in such a way that the fuel/air mixture supplied to the engine 1 will be adapted, in each instant, to the current operating conditions. The fuel supply is hereby obtained in a generally known manner, i. e. in dependence of a multitude of parameters representative of the operating conditions of the engine 1 and the vehicle in question. For example, the control can be performed depending on the present throttle position and the rotational speed and load of the engine 1.

Each cylinder 2 is provided with an exhaust outlet 8. Together, the exhaust outlets 8 converge into an exhaust manifold 9, continuing into an exhaust pipe 10. This exhaust pipe runs via a turbocharger unit 11, which as such is substantially conventional. Thus, the turbocharger unit 11 comprises a means for absorbing energy from the exhaust gases in the form of a turbine 12, which is arranged in the exhaust pipe 10 and is being rotated by the exhaust gases flowing through the- exhaust pipe 10. As an alternative to the embodiment shown in the figure, which is

designed in such a way that the exhaust outlets 8 join into a single exhaust pipe 10 (known as a"single inlet"), the exhaust outlets may be grouped into two groups, making the exhaust duct consist of two pipes leading to the turbine 12 (known as a "twin inlet").

The turbine 12 is arranged on a shaft 13, on which a compressor 14 is likewise arranged. The energy absorbed from the exhaust flow by the turbine 12 is in this way transferred to the compressor 14, which functions to compress the in-flowing air to the intake conduit 4 of the engine 1. In this manner, an increased fuel amount may be fed to the engine 1, whereby its power output can be increased.

The engine 1 is further equipped with an arrangement for recirculation of a certain amount of exhaust gases to the intake side of the engine 1. According to what was discussed in the introduction, an EGR system ("Exhaust Gas Recirculation) is as such previously known. According to the embodiment, a further line in the form of an EGR line 15 is therefore connected to the exhaust pipe 9, at a point upstream of the turbine 12. The EGR line 15 debouches in the intake conduit 4, at a point upstream of the intake manifold 3 of the engine 1. Along the EGR line 15, a controllable valve 16 is arranged, connected to the control unit 6 via a further connection 17.

According to what will be described in greater detail below, the control unit 6 is operative, in dependence of the present operating conditions, to adjust the valve 16 to a closed, open or partially open position. Depending on the position of the valve 16, a corresponding amount of exhaust gases will thus be recirculated to the intake manifold 3 via the EGR line. Through the recirculation of these EGR gases to the intake manifold 3, a temperature reduction during the combustion is achieved in the respective cylinder 2, whereby the NOx generation in the cylinder 2 is reduced.

The NOx generation in the respective cylinder 2 is temperature-dependent and for this reason it is desirable to lower, as far as possible, the temperature of the gases (i. e. including air and recirculated EGR gases) coming in to the engine 1. For this reason, the EGR line 15 is provided with a cooler 18 functioning to cool the EGR gases recirculated to the intake manifold 3. To this end the cooler 18 includes a loop 19 through which a suitable coolant is circulated. Preferably, this coolant is the ordi- nary coolant for the engine 1, but air may also be used for this cooling. By means of this cooler 18, the EGR gases can be cooled, which further contributes to reducing the generated amount of NOx compounds.

The intake conduit 4 is equipped with another cooler 21, (known as an"intercooler"), which is used for cooling the compressed air supplied by the compressor 14. This also contributes to a reduction of the amount of NOx compounds generated in the engine 1. The second cooler 20 is preferably arranged for cooling by air, which is schematically indicated at the reference figure 21.

According to the invention, a second turbine 22 is used to absorb energy from the exhaust gases. The exhaust gases leaving the engine 1 and being passed through the first turbine 12 are thus also fed through the second turbine 22, which is then brought to rotate. For this purpose, the second turbine 22 is rotatably arranged on a further shaft 23. After having transferred part of their energy to the second turbine 22, the exhaust gases are conducted out to the atmosphere, preferably by way of a silencer (not shown).

Furthermore, the second turbine 22 is connected to the output crankshaft 24 of the engine 1, via a power transmission 25, not shown in any detail, which in the figure is schematically indicated by a dashed line. The power transmission 25 is preferably of a mechanical type including a gear transmission connecting the shaft 23 to the crankshaft 24. The power transmission is moreover provided with a gear reduction for conversion of the rotational speed of the second turbine 22 to a rotational speed suitable for the crankshaft 24. In this manner, power is transferred between the second turbine 22 and the crankshaft 24, i. e. a certain amount of energy in the combustion gases can be regained from the exhaust flow and be used as additional power to the crankshaft 24.

A system for an engine having a turbocharger unit arranged for extraction of a certain amount of energy from the exhaust gases and feeding this energy back to the engine crankshaft is normally called a"turbo compound"system. By using, according to the invention, the two turbines 12,22 that are arranged in series, a pressure is achieved, during operation of the engine 1, on the exhaust side of the engine 1, which is higher than the pressure on its intake side. In this way, a sufficient propulsion pressure is obtained for recirculation of EGR gases to the intake manifold 3, without deterioration of the efficiency of the engine 1. Hereby, part of the energy contained in the exhaust gases is utilised as additional power to the crankshaft 24 instead of being conducted out into the atmosphere and thus being lost.

According to what has been explained above, the invention includes a controllable valve 16, in connection with the control unit 6. Depending on the current operating conditions of the engine 1 and the vehicle in general, the valve 16 is used in such a manner that a certain amount of exhaust gases from the respective exhaust outlets 8 of the engine 1 are passed back to the intake manifold 3. According to the preferred embodiment, the valve 16 consists of an electronically controlled valve, which, by means of the control unit 6, can be controlled continuously between an open and a closed position. Through a certain setting of the aperture area of the valve 16 in the EGR line 15, a correspondingly sized flow of EGR gases to the intake manifold 3 is obtained.

For control of the valve 16, the control unit 6 is arranged for determination of the rotational speed and load (torque) of the engine, and for calculation, depending on these parameters, of the desirable amount of EGR gases to be recirculated to the intake manifold. This amount of EGR gases is preferably determined in the control unit 6 by using a stored table giving the required amount of EGR gases as a function of the speed and load. Depending on the calculated value of the amount of EGR gases, the valve 16 is adjusted to a corresponding position through a signal from the control unit 6.

In connection with recirculation of EGR gases, the recirculated amount has to be adapted conceming predetermined limit levels for soot and carbon monoxide released with the exhaust gases. As the EGR recirculation reduces the amount of air available at the engine intake, it must be ascertained that the amount of air is sufficient with regard to said limit levels.

By virtue of the valve 16 discussed above, which can be continuously adjusted to a desired position, a very large variation of the aperture area of the valve 16 is allowed between different extremes in the operating conditions of the engine 1. For the arrangement according to the embodiment, where energy is transferred from the second turbine 22 to the crankshaft 24, this large variation is required, as the pressure differential between the exhaust side and the intake side varies to a high degree as a function of the engine load. As a comparison it can be mentioned that this pressure differential varies considerably more than by previously known sys- tems comprising turbocharger units with variable turbine geometry. By means of the control according to the invention of the valve 16, a correct flow of EGR gases can

be ascertained, which is substantially independent of the operating conditions of the engine 1.

Besides the control of the valve 16 as a function of the speed and load of the engine 1, the control unit 6 may also be functional to control the valve 16 in dependence of other parameters. For example, transients can be taken into account to avoid unwanted puffs of smoke in the engine 1 exhausts.

Moreover, the control unit 6 is also functioning to allow control of the injection timing of the respective injection devices 5, substantially independently of the prevailing speed and load. For the control of the valve 16, the main objective is that the flow of EGR gas is adjusted for an optimum reduction of NOx emissions. Then the injection point in time required to achieve minimum fuel consumption is determined, however without exceeding the given NOx level. Hereby applies that the injection time is set to as early as possible, as this will lower, in the manner known, the fuel consumption of the engine 1. Furthermore applies that the larger the provided flow of EGR gases, the earlier lies the possible injection time for a given NOx level.

According to the preferred embodiment, each cylinder 2 of the engine 1 is provided with two intake valves (not shown), through which air and EGR gases are sucked in.

The reason being that the engine 1 according to the invention needs just as large an air/fuel relationship as an engine without EGR recirculation, in order to obtain a good combustion without large amounts of smoke exhausted from the engine 1. This in turn entails that a larger gas flow (i. e. the input fresh air together with the EGR gases) has to be sucked into the engine 1. In order not to impair the flow properties and increase the pressure drop across the engine 1 it is, by the invention, an advantage to provide the engine 1 with two intake valves per cylinder 2, e. g. two relatively small intake valves instead of one, relatively large, intake valve. It is also an advantage from a strength point of view to use two smaller valves instead of one large valve.

The invention is not limited to the embodiment described above, but can be varied within the scope of the appended patent claims. For example, the valve 16 may be of the type operating according to an on/off control principle, i. e. that can be set only to an open or a closed position.

As an alternative to the electronically controlled valve 16 described above, the valve may be comprised of a pneumatically controlled valve. In such a case, the control unit 6 may be arranged to activate an electrical solenoid valve (not shown) (known as a PWM valve), operative to allow air to pass from a source of compressed air (not shown) when activated. By means of this compressed air, the valve can be activated so as to assume a required position.

Instead of using a single controllable EGR valve 16, two or more such controllable valves may be used, according to an alternative embodiment. For example, the exhaust outlets 8 may be divided into two groups of each three outlets, whereby two pipes will lead to the turbine 12, instead of designing the exhaust outlets 8 to join into one single exhaust pipe 10 (compare the figure). If two such groups are used, exhaust gases are fed through each of these groups (from three cylinders each) to an exhaust pipe provided with an EGR valve. Further, the control unit is operative to control the two EGR valves independently of each other. To further enhance the function in case the two EGR valves are comprised of valves operated by on/off control, they may consist of one relatively small EGR valve and one relatively large EGR valve. In this way, three different conditions are defined, depending on if each valve is closed or open. In dependence of which valves are activated by the control unit, three different levels of EGR gases fed from the engine are obtained.