A method is proposed for designing a turbine of a turbocharger of an engine system having an exhaust gas recirculation (EGR) path. At at least one engine speed, such as the peak torque speed, a pressure drop along the EGR path is determined, and then the turbine design is selected such that the asymmetry factor is according to a function of the pressure drop. The invention further proposes turbines in which the asymmetry factor has a specific relationship to the pressure drop along the EGR path, and engines incorporating such turbines.

WE CLAIMS

1. A method of designing a turbine of a turbocharger of an engine system, the engine system comprising:

an internal combustion engine comprising at least one cylinder defining a respective bore, within which a piston is arranged to reciprocate, the cylinder having a gas inlet, a gas outlet and a fuel inlet;

the internal combustion engine having an inlet and an outlet including an EGR manifold and a Lambda manifold;

an exhaust gas recirculation system comprising an exhaust gas recirculation path arranged to pass at least a portion of the gas exhaust from the engine outlet back to the engine inlet;

a turbocharger comprising a compressor and a turbine;

the turbine comprising a housing, the housing defining a turbine outlet, a turbine chamber between the at least one turbine inlet and the turbine outlet, an annular inlet passageway arranged around the turbine chamber, an EGR volute having a first turbine inlet for receiving exhaust gas from the engine exhaust manifold, and a Lambda volute having a second turbine inlet for receiving exhaust gas from the Lambda manifold,

the EGR volute and Lambda volute communicating with the inlet passageway around the radially outer portion of the inlet passageway and defining respective flowpaths from the respective turbine inlets to the inlet passageway, and

the turbine further comprising a turbine wheel rotatably mounted within the turbine chamber for rotation about an axis such that it is rotated by gas exhaust from the engine outlet passing from the turbine inlets to the turbine outlet;

the compressor comprising a housing, the housing defining a compressor inlet, in gas communication with an air source, a compressor outlet in gas communication with the engine inlet, a chamber between the compressor inlet and the compressor outlet and an impeller wheel rotatably mounted within the chamber for rotation about an axis such that rotation of the impeller wheel compresses air from the compressor inlet and passes the compressed air to the compressor outlet;

the turbine wheel being coupled to the impeller wheel such that the rotation of the turbine wheel drivably rotates the impeller wheel;

wherein the method comprises the steps of:

(i) for at least one engine speed determining an EGR loop pressure drop (x) along the recirculation path;

(ii) determining an asymmetry ratio representing the ratio of mass flow parameters along the flowpaths, from an expression substantially of the form:

y=a In (x) + b (A)

where the parameter y is the asymmetry ratio multiplied by a pressure ratio Z of the ambient pressure and the turbine outlet pressure, and a and b are real valued constants;

(iii) selecting a turbine design with a ratio of the mass flow parameters at the at least one engine speed according to the determined asymmetry ratio.

2. A method according to claim 1 in which the expression is of the form:

y = -0.1311n (x) + 0.8523. (B)

3. A method according to claim 1 or claim 2 in which the step of selecting the turbine design comprises selecting the areas of respective critical areas at which the EGR volute and the Lambda volute join the inlet passageway.

4. A method according to claim 3 which further comprises the step of manufacturing a turbine having first and second inlets with the selected critical areas of the first and second turbine inlets.

5. A method according to any preceding claim, in which the engine system further comprises at least one control valve, the at least one control valve comprising:

(i) a balance value for controlling a flow of exhaust gas between the EGR volute and the Lambda volute, and a control mechanism for the balance valve, and

(ii) a wastegate valve for controlling a diverted flow of gas from the Lambda volute to the output of the turbine avoiding the turbine wheel;

the step of selecting a turbine design comprising selecting a control relationship for the at least one control valve, according to which the asymmetry ratio varies with engine speed according to Eqn. (A).

6. A method according to any preceding claim which further comprises the step of manufacturing an engine system according to the selected turbine design.

7. A method according to claim 6 when dependent on claim 5, in which the engine system is manufactured including a control valve control system for controlling the at least one control valve according to the selected control relationship with the engine speed.

8. A method according to any preceding claim in which the at least one engine speed includes the peak torque speed for the engine system.

9. A method according to any preceding claim in which the at least one engine speed includes the rated power speed for the engine system.

10. A method according to any preceding claim in which the at least one engine speed includes one or more engine speeds which are a first engine speed Π|0 plus a respective proportion X of the difference between the first engine speed nto and a second engine speed nhi, where X is selected from the group comprising 15%, 25%, 50% and 75%, and nto and nhi are respectively the lowest engine speed for which a first predetermined power value is achievable by the engine system, and a highest engine speed for which a second predetermined power value is achievable by the engine system.

11 . A method according to claim 10 in which the first predetermined power value is 50% of the maximum power achievable by the engine system, and the second predetermined power value is 70% of the maximum power achievable by the engine system.

12. An engine system comprising:

an internal combustion engine comprising at least one cylinder defining a respective bore, within which a piston is arranged to reciprocate, the cylinder having a gas inlet, a gas outlet and a fuel inlet, the internal combustion engine having an inlet and an outlet including an EGR manifold and a Lambda manifold;

an exhaust gas recirculation system comprising an exhaust gas recirculation path arranged to pass at least a portion of the gas exhaust from the engine outlet back to the engine inlet;

a turbocharger comprising a compressor and a turbine;

the turbine comprising a housing, the housing defining at least one turbine inlet in gas communication with the engine outlet, a turbine outlet, a turbine chamber between the at least one turbine inlet and the turbine outlet, an annular inlet passageway arranged around the turbine chamber, an EGR volute having a first turbine inlet for receiving exhaust gas from the engine exhaust manifold, a Lambda volute having a second turbine inlet for receiving exhaust gas from the Lambda manifold,

the EGR volute and Lambda volute communicating with the inlet passageway around the radially outer portion of the inlet passageway and defining respective f lowpaths from the respective turbine inlets to the inlet passageway, and

the turbine further comprising a turbine wheel rotatably mounted within the turbine chamber for rotation about an axis such that it is rotated by gas exhaust from the engine outlet passing from the at least one turbine inlets to the turbine outlet;

the compressor comprising a housing, the housing defining a compressor inlet, in gas communication with an air source, a compressor outlet in gas communication with the engine inlet, a chamber between the compressor inlet and the compressor outlet and an impeller wheel rotatably mounted within the chamber for rotation about an axis such that rotation of the impeller wheel compresses air from the compressor inlet and passes the compressed air to the compressor outlet;

the turbine wheel being coupled to the impeller wheel such that the rotation of the turbine wheel drivably rotates the impeller wheel;

wherein, for at least one engine speed, the relationship between the EGR loop pressure drop (x) along the recirculation path, and the asymmetry ratio representing the ratio of mass flow parameters along the flowpaths, is according to the expression y = -0.131 ln(x) + 0.8523 + δ. (C) where the parameter y is the asymmetry ratio multiplied by a pressure ratio Z of the ambient pressure and the turbine outlet pressure, and δ is a tolerance parameter having a magnitude no greater than 0.1 .

13. An engine system according to claim 12 in which δ has a magnitude less than 0.05.

14. An engine system according to claim 12 in which δ has a magnitude less than 0.04.

15. An engine system according to claim 12 in which δ has a magnitude less than 0.02.

16. An engine system according to any of claims 12 to 15 in which the at least one engine speed includes the peak torque engine speed for the engine system.

17. An engine system according to any of claims 12 to 16 in which the at least one engine speed includes the rated power speed for the engine system.

18. An engine system according to any of claims 12 to 17 in which the at least one engine speed includes one or more engine speeds which are a first engine speed nto plus a respective proportion X of the difference between the first engine speed nto and a second engine speed nhi, where X is selected from the group comprising 15%, 25%, 50% and 75%, and nto and nhi are respectively the lowest engine speed for which a first predetermined power value is achievable by the engine system, and a highest engine speed for which a second predetermined power value is achievable by the engine system.

19. An engine system according to claim 18 in which the first predetermined power value is 50% of the maximum power achievable by the engine system, and the second predetermined power value is 70% of the maximum power achievable by the engine system.
20. An engine system according to any of claims 12 to 19, further comprising:
(a) at least one control valve, the at least one control valve comprising:
(i) a balance value for controlling a flow of exhaust gas between the EGR volute and the Lambda volute, and a control mechanism for the balance valve, and
(ii) a wastegate valve for controlling a diverted flow of gas from the Lambda volute to the output of the turbine avoiding the turbine wheel; and
(b) a valve control system for controlling the balance valve and/or the wastegate valve, the control system being operative to control the at least one control valve to control the symmetry ratio to be according to Eqn. (C) for a range of engine speeds.
21 . An engine system according to claim 20 in which the range of engine speeds includes the peak torque engine speed.
22. An engine system according to claim 20 or 21 in which the range of engine speeds includes the rated engine speed.
23. An engine system according to any of claims 20 to 22 when dependent on claim 18, in which the range of engine speeds includes the one or more engine speeds which are the first engine speed nto plus a respective proportion X of the difference between the first engine speed Π|0 and the second engine speed nhi.