Technical field

[001 ] The invention relates to an internal combustion engine comprising an inlet gas conduit system, and an exhaust gas system; a first turbo- charger unit, a turbine part thereof arranged in connection with the exhaust gas system and a compressor part thereof arranged in connection with the inlet gas conduit system to pressurize the oxygen containing combustion gas by means of the energy of the exhaust gas of the engine, a second turbocharger unit, a turbine part thereof is arranged in connection with the exhaust gas system the inlet of which turbine part is coupled parallel with the turbine part of the first turbocharger unit and a compressor part the inlet of which is arranged in connection with the exhaust gas system and the outlet of which arranged in connection with the inlet gas conduit system via an exhaust gas recirculation conduit system, the compressor part being arranged to pressurize recirculated portion of the exhaust gas by means of the energy of exhaust gas of the engine. [002] Invention relates to method of operating an internal combustion engine in which combustion air is introduced to the engine through an inlet gas conduit and the air is supercharged by a first turbocharger unit by making use of energy of the exhaust gases of the engine arranged to flow in and exhaust gas system, and a controllable amount of exhaust gas is recir- culated to the inlet gas conduit system and back to the combustion process of the engine, in which method the recirculation of the exhaust gas is assisted by means of a second turbocharger unit, and in which method the exhaust gas of the engine is divided into two partial streams the first partial stream is led to a turbine part of the first turbocharger unit and the second partial stream is led to a turbine part of the second turbocharger unit. Background art

[003] Turbochargers are well-known for supplying air to the intake of an internal combustion engine at pressures above ambient pressure.

[004] Generally, a turbocharger comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel produces compressed air to the intake manifold of the engine, thereby increasing engine power. In order to facilitate control of the operation of the turbo charger, the turbine may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size or function of the inlet passageway can be varied to control the gas introduction into the turbine so that the power output of the turbine can be varied to suite varying engine demands. [005] During the operation of an internal combustion engine nitrogen oxides (NOx) are formed. NOx is produced during the combustion process in an engine due to high temperature in the presence of oxygen and nitrogen. In order to meet the requirements of exhaust emissions exhaust gas recirculation (EGR) systems may be used, in which a portion of the engine's ex- haust gas is recirculated back to the combustion chambers of the engine. This is typically achieved by directing an amount of the exhaust gas from the exhaust manifold to the inlet manifold of the engine. This is commonly called as external recirculation. The recirculated exhaust gas lowers the peak temperature produced during combustion. As NOx production in- creases with increased peak temperature, recirculation of exhaust gas reduces the amount of undesirable NOx formed. Turbochargers may form part of the EGR system.

[006] For example in publication US 2010/122530A1 there is disclosed such a system. The EGR system for an engine with a turbocharger com- prises a second turbocharger which operates in parallel with the main turbocharger. The second turbocharger, herein referred to as the EGR turbocharger, has a turbine part, which is powered by a portion of the engine exhaust. The compressor part of the turbocharger is arranged to feed a por- tion of the engine exhaust gas, after pressurising the exhaust gas, to the inlet manifold of the engine. As such, the turbine part of the EGR turbocharger drives the EGR turbocharger's compressor part so that the EGR turbocharger feeds a portion of engine exhaust gas to the engine intake.

[007] A problem relating to exhaust gas recirculation by means of an EGR turbocharger is that the EGR turbocharger utilizes the very same source of energy which is utilized in the main turbocharger unit of the engine and thus controlling its operation has an effect also on the engine's main turbocharger.

[008] It is an object of the invention to provide a turbo charged internal combustion piston engine comprising an exhaust gas recycle turbocharger which provides advantageous operation of overall charging system.

Disclosure of the Invention [009] Objects of the invention are substantially met by an internal combustion engine comprising an inlet gas conduit system, and an exhaust gas system, a first turbocharger unit, a turbine part thereof arranged in connection with the exhaust gas system and a compressor part thereof arranged in connection with the inlet gas conduit system to pressurize the oxygen con- taining combustion gas by means of the energy of the exhaust gas of the engine, a second turbocharger unit, a turbine part thereof is arranged in connection with the exhaust gas system the inlet of which turbine part is coupled parallel with the turbine part of the first turbocharger unit and a compressor part the inlet of which is arranged in connection with the ex- haust gas system and the outlet of which arranged in connection with the inlet gas conduit system via an exhaust gas recirculation conduit system, the compressor part being arranged to pressurize recirculated portion of the exhaust gas by means of the energy of exhaust gas of the engine. It is characteristic to the invention that the inlet of the turbine part of the second turbocharger unit is connected to the exhaust gas system through a first control valve.

[0010] This result in an effect that the flow rate of the partial stream of the exhaust gas to the second turbocharger may be controlled. Particularly the flow of exhaust gas to the first turbocharger may be maximized by throttling the first control valve temporarily.

[001 1 ] According to an embodiment of the invention the exhaust gas recirculation conduit system comprises a control circuit leading from upstream side of the compressor part to downstream side of the compressor part of the second turbocharger unit, which control circuit is provided with a second control valve.

[0012] By means of the control circuit it is possible to minimize the power demand of the compressor part of the second turbocharger unit particularly when the first control valve is throttled down.

[0013] According to an embodiment of the invention the exhaust gas recir- culation conduit system comprises a valve arranged between an outlet the compressor part of the second turbocharger unit and the inlet gas conduit system. By means of the valve the recirculation may be controlled or totally shut off.

[0014] According to an embodiment of the invention the exhaust gas recirculation conduit system comprises a first gas cooler unit arranged upstream the compressor part. [0015] According to an embodiment of the invention the control circuit is provided with a second gas cooler unit. By means of the second gas cooler unit the temperature gas flowing in the control circuit may be controlled.

[0016] According to an embodiment of the invention the control system of the engine is arranged to throttle down the first control valve during engine's load increase.

[0017] According to an embodiment of the invention the first turbocharger unit is provided with a waste-gate and that the engine is provided with a control system which is arranged to throttle down the first control valve dur- ing engine's load increase and when the waste-gate is closed.

[0018] According to an embodiment of the invention the turbine part of first turbocharger unit and the turbine part of the second turbocharger unit are arranged parallel in to the exhaust gas system.

[0019] Objects of the invention are also met by method of operating an in- ternal combustion engine in which combustion air is introduced to the engine through an inlet gas conduit and the air is supercharged by a first turbocharger unit by making use of energy of the exhaust gases of the engine arranged to flow in and exhaust gas system, and a controllable amount of exhaust gas is recirculated to the inlet gas conduit system and back to the combustion process of the engine, in which method the recirculation of the exhaust gas is assisted by means of a second turbocharger unit, and in which method the exhaust gas of the engine is divided into two partial streams the first partial stream is led to a turbine part of the first turbocharger unit and the second partial stream is led to a turbine part of the se- cond turbocharger unit. It is characteristic to the invention that the operation of the second turbocharger unit is controlled by controlling the flow rate of the second partial stream by means of controlling a throttling effect of a first control valve arranged between the inlet of the turbine part of the second turbocharger unit and the exhaust gas system. [0020] According to an embodiment of the invention power of the first turbocharger unit is temporarily increased by temporarily throttling the flow rate of the second partial stream to the turbine part of the second turbocharger unit. [0021 ] According to an embodiment of the invention operation of the compressor part of the second turbocharger is controlled by routing a controlled amount of the compressed gas back to the inlet side of the compressor part via a control circuit leading from upstream side of the compressor part to downstream side of the compressor part. [0022] According to an embodiment of the invention the flow rate of the second partial stream to the turbine part of the second turbocharger unit is controlled to maintain the rotational speed of the second turbocharger at preset level. The recirculation of the exhaust gas is temporarily shut off during the throttling of the flow rate of the second partial stream. [0023] The invention specifically advantageous in connection with transient operational phases where power output of the engine is increased.

Brief Description of Drawings

[0024] In the following, the invention will be described with reference to the accompanying exemplary, schematic drawing, in which Figure 1 illustrates an internal combustion engine according to an embodiment of the invention. Detailed Description of Drawing

[0025] Figure 1 depicts schematically an internal combustion engine 1 according to an embodiment of the invention. The engine comprises a body 2 in which several cylinders 4 are arranged with in-line arrangement. The engine further comprises an inlet gas conduit system 6 coupled to an inlet channel 8 of each cylinder 4 of the engine 1 . The inlet gas conduit system is arranged for conveying inlet gas, typically air, to the combustion chambers of the engine. The engine comprises also an exhaust gas system 10 and an exhaust gas recirculation conduit 12 system connecting the exhaust gas conduit system 10 with the inlet gas conduit system 6.

[0026] In the embodiment of figure 1 the inlet gas conduit system 6 comprises firstly a combustion gas manifold 14 through which the oxygen con- taining gas needed for combustion process may be delivered to each of the cylinders of the engine. Typically the combustion gas is the air but it should be noted that the operation of the engine according to the invention may be practised by means of the any desired oxygen containing gas. Each of the cylinders 4 or the engine is provided with an inlet channel 24, which con- nects the combustion gas manifold to the cylinders. The combustion gas manifold 14 connected to an outlet 16 of a compressor part 18 of a turbo- charger unit 20, which is called here as the first turbocharger unit. There is also advantageously a combustion gas cooler 22 arranged downstream the compressor part and upstream the combustion gas manifold 14. In figure 1 the turbocharger unit is depicted by a one-stage system, but it is clear that the turbocharger unit i.e. the turbine part and/or the compressor part, may comprise several stages.

[0027] The exhaust gas system 10 comprises an exhaust manifold 26 from which an exhaust conduit 28 extends to an inlet 30 of a turbine part 32 of the turbocharger unit 20. The turbine part 32 and the compressor part 18 of the turbocharger are coupled with each other in a manner known as such to operate the compressor part by means of the turbine part. The turbine part is here provided with a waste-gate 32'.

[0028] The exhaust gas recirculation conduit system 12 connects the ex- haust gas conduit 10 system with the inlet gas conduit system 6 so that a portion of the exhaust gas flow of the engine may be recirculated back to the engine. The exhaust gas recirculation conduit system 12 comprises an exhaust gas recirculation manifold 34 which is connected separately to each inlet channel 8 of the cylinders of the engine by means of a branch conduits 36 arranged to extend from the exhaust gas recirculation manifold to an individual inlet channel 8. This way according to the invention the recirculated exhaust gas and fresh combustion gas are mixed at the earliest in the inlet channel 8 so that the recycled exhaust gas stream is divided into sub-streams, each of which is introduced to separate inlet channels 8.

[0029] Thus in the inlet gas conduit system 6 according to an embodiment of the invention an inlet channel for a cylinder comprises a first connection to an inlet gas conduit system 6 of the engine and a second connection to an exhaust gas systeml O. The inlet channel 8 is further provided with a third connection to the exhaust gas recirculation conduit system 12.

[0030] The branch conduits are according to an embodiment provided with controllable inlets. The branch conduits are according to another embodiment provided with fixed geometry inlets.

[0031 ] There is an on/off valve 38 arranged to the exhaust gas recirculation conduit system 12 in order to shut down the recirculation by closing the valve when so desired. [0032] According to the invention the engine comprises a second turbo- charger unit 40 arranged to the exhaust gas recirculation conduit system 12 in order to pressurize recirculated portion of the exhaust gas by means of the energy of exhaust gas of the engine. This way the pressure of the recy- cled portion of the exhaust gas is at least at a level of the pressure of the oxygen containing combustion gas in the inlet channels 8. In the operation of the invention combustion air is introduced to the engine through an inlet gas conduit and the air is supercharged by a first turbocharger unit by mak- ing use of energy of the exhaust gases of the engine arranged to flow in and exhaust gas system and a controllable amount of exhaust gas is recirculated to the inlet gas conduit system and back to the combustion process of the engine. The recirculation of the exhaust gas is assisted by means of the second turbocharger unit.

[0033] The second turbocharger unit 40 comprises a turbine part 42 which is operated by a second partial stream of exhaust gas of the engine while the first partial stream is lead to the first turbocharger unit 20. Thus, the exhaust gas of the engine is divided into two partial streams the first partial stream of which is led to a turbine part of the first turbocharger unit and the second partial stream is led to a turbine part of the second turbocharger unit The exhaust gas conduit 28 is provided with a branch conduit 28' which connects the exhaust manifold 26 to the turbine part 42 of the second turbocharger unit, i.e. to the inlet thereof. The second partial stream of ex- haust gas of the engine which has passed through the turbine part 42 is returned back to the downstream side of the turbine part 32 of the first turbocharger unit 20. It is also possible to lead the partial stream of exhaust gas away through a dedicated exit conduit. There is a first control valve 48 arranged for controlling the flow rate of second partial stream of exhaust gas through the turbine part 42 of the second turbocharger unit 40. The valve 48 is here arranged to the branch conduit 28'. Controlling is performed by adjusting a throttling effect of the first control valve 48. This allows precise control of the operation of the second turbocharger unit and also precise control of the recirculated exhaust gas. The valve may be of any kind of valve apparent to a skilled person in the art, which may provide a throttling effect to the gas flow through the turbine part 42. [0034] The compressor part 44 of the second turbocharger unit 40 is connected also to the exhaust manifold 26 of the engine to receive exhaust gas of the engine. An outlet of the at least one compressor part 44 is connected to the inlet gas conduit system 6 by means of the exhaust gas recir- culation conduit system 12. The inlet of the compressor part is in connection with the exhaust manifold 26 engine. The exhaust gas recirculation conduit system 12 may comprise a gas cleaning device 50, such as hot gas particulate material filter. Additionally according to an embodiment of the invention the exhaust gas recirculation conduit system 12 comprises a first gas cooler unit 52 arranged in the figure 1 downstream the gas cleaning device but prior to the compressor part 44 in the gas flow direction. The directions of gas flow in the various conduits are shown by arrows in the figure. [0035] Further, according to an embodiment of the invention the exhaust gas recirculation conduit system 12 comprises a control circuit 56 leading from upstream side of the compressor part 44 to downstream side of the compressor part 44 of the second turbocharger unit. In this embodiment the control circuit 56 is connected to the upstream side of the first gas cooler unit 52. There is a second control valve 46 arranged to the control circuit for controlling the portion of the gas flow arranged to circulate in the control circuit. In other words, the control circuit 56 forms a recirculation line for the compressor part 44. By means of the control circuit the compressor part of the second turbocharger together with controlling the operation of the first control valve 48 the rotational speed of the second turbocharger may be maintained at desired level also in circumstances where recirculation of the exhaust gas is minimal or totally shut off. When the control circuit is open the compressor part may be rotated at desired speed with minimum energy. And, when the rotational speed is maintained, normal operational circum- stances may be reached quickly when desired or needed. This is particularly advantageous in transitional situations. [0036] According to an embodiment of the invention the engine is operated in transient situations so that power of the first turbocharger unit 20 is temporarily increased by temporarily throttling down the flow rate of the second partial stream to the turbine part of the second turbocharger unit 40. Thus maximum exhaust gas flow rate and pressure may be delivered to the turbine part of the first turbocharger.

[0037] According to another embodiment, power of the first turbocharger unit 20 is temporarily increased by temporarily closing the valve 38, so that no recirculation of the exhaust gas will take place. Simultaneously the flow rate of the second partial stream to the turbine part of the second turbocharger unit 40 is throttled down by the valve 48. Further the valve 46 may be opened to open the connection between the upstream side of the com- pressor part 44 and downstream side of the compressor part 44. Thus a controlled amount of the compressed gas is recirculated back to the inlet side of the compressor part via a control circuit 56. The first valve 48 is controlled to maintain the rotational speed of the second turbocharger at a preset level while the valve 38 is closed. This way the second turbocharger is readily available when needed.

[0038] The exhaust gas recirculation conduit system 12 comprises further a second gas cooler unit 54 between the compressor part 44 and the exhaust gas recirculation manifold 34.

[0039] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the invention, as defined in the appended claims. For example the invention may be well applied to two stroke or four stroke engines oper- ating with various cycles, such as the otto- or diesel cycle. The engine configuration may also vary from in-line engine to e.g. V engine. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.