Two-stage boost system for engines

1. A method for an engine, comprising:

flowing exhaust through each of a first turbine of a first turbocharger and a second turbine of a second turbocharger, the first turbine and the second turbine connected in parallel; and

pressurizing intake air flowing through an intake passage at each of a first compressor of the first turbocharger and a second compressor of the second turbocharger, the first compressor and the second compressor connected in series, wherein the first turbine is equal in size with the second turbine, and wherein the first compressor is larger than the second compressor.

2. A method for an engine, comprising:

flowing exhaust through each of a first turbine of a first turbocharger and a second turbine of a second turbocharger, the first turbine and the second turbine connected in parallel; and

pressurizing intake air flowing through an intake passage at each of a first compressor of the first turbocharger and a second compressor of the second turbocharger, the first compressor and the second compressor connected in series, wherein the first turbine is coupled to a first bank of cylinders while the second turbine is coupled to a second bank of cylinders, the first and second bank including an equal number of cylinders.

3. The method of claim 2 , wherein the first turbine is equal in size with the second turbine, and wherein the first compressor is larger than the second compressor.

4. The method of claim 2 , wherein the engine includes an I6 or a V6 configuration with the first bank including three cylinders firing at intervals of 240° crank angles and the second bank including three cylinders firing at intervals of 240° crank angles.

5. A method for an engine, comprising:

flowing exhaust through each of a first turbine of a first turbocharger and a second turbine of a second turbocharger, the first turbine and the second turbine connected in parallel;

pressurizing intake air flowing through an intake passage at each of a first compressor of the first turbocharger and a second compressor of the second turbocharger, the first compressor and the second compressor connected in series; and

during a temperature of an emission control device being lower than a threshold temperature and a target level of boost being lower than a threshold boost, opening each of a first waste gate coupled across the first turbine and a second waste gate coupled across the second turbine, adjusting geometry of each of the first turbine and the second turbine to closed positions, and flowing exhaust from an exhaust manifold to the emission control device.

6. The method of claim 5 , further comprising, in response to at least one of the temperature of the emission control device being greater than the threshold temperature and the target level of boost being greater than the threshold boost while a boost pressure is lower than the target level of boost, closing each of the first waste gate and the second waste gate, adjusting geometry of the first turbine and the second turbine to decrease opening of each of the first turbine and the second turbine, and pressurizing the intake air at each of the first compressor and the second compressor.

7. The method of claim 6 , further comprising, in response to at least one of the temperature of the emission control device being greater than the threshold temperature and the target level of boost being greater than the threshold boost while a boost pressure being higher than a target boost, closing each of the first waste gate and the second waste gate, adjusting geometry to increase opening of each of the first turbine and the second turbine, and pressurize the intake air at each of the first compressor and the second compressor.

8. The method of claim 7 , further comprising, in response to one of the temperature of the emission control device being greater than the threshold temperature and the target level of boost being greater than the threshold boost, adjusting the geometry of each of the first turbine and the second turbine is identically to maintain an equal turbine inlet exhaust manifold gas pressure at a first inlet of the first turbine and a second inlet of the second turbine.

9. The method of claim 7 , further comprising cooling the pressurized air exiting the first compressor at a first charge air cooler positioned between the first compressor and the second compressor, and then further cooling the pressurized air exiting the second compressor at a second charge air cooler positioned downstream of the second compressor.

10. The method of claim 7 , further comprising, in response to a first compressor ratio increasing to a first surge limit, opening a first compressor recirculation valve coupled across the first compressor, and/or in response to a second compressor ratio increasing to a second surge limit, opening a second compressor recirculation valve coupled across the second compressor.

11. A system for an engine in a vehicle, comprising:

a first turbocharger including a first turbine configured to receive exhaust gas from a first bank of engine cylinders, and a first, larger compressor; and

a second turbocharger including a second turbine configured to receive exhaust gas from a second bank of engine cylinders, and a second, smaller compressor, the first turbine parallel to the second turbine while the first, larger compressor in series with the second, smaller compressor, wherein the first turbine and the second turbine are equally sized variable geometry turbines, the first turbine and the second turbine adjustable to a same opening position.

12. The system of claim 11 , further comprising:

a controller storing instructions in non-transitory memory that, when executed, cause the controller to:

flow an entire volume of intake air entering an intake manifold through the first, larger compressor, a first charge air cooler positioned between the first, larger compressor and the second, smaller compressor, the second, smaller compressor, and a second charge air cooler positioned downstream of the second, smaller compressor prior to entering an intake manifold supplying intake air to each engine cylinders of the first bank and the second bank.