Engine braking method and control system varying engine braking power within cylinder-number braking mode

1. A method of braking an engine comprising:

operating an engine in a cylinder-number braking mode where exhaust valves for some combustion cylinders in the engine are operated in an engine braking timing pattern and exhaust valves for some combustion cylinders in the engine are operated in a combustion timing pattern;

determining a control term indicative of at least one of an intake air pressure different from a present intake air pressure or a change to an intake air pressure from a present intake air pressure, that varies a braking power of the engine;

commanding varying a position of an exhaust-impinged surface in an exhaust turbine for the engine based on the determined control term;

varying a speed of an intake air compressor for the engine driven by the exhaust turbine, based on the commanded varying of a position of the exhaust-impinged surface; and

decreasing the braking power of the engine, within the cylinder-number braking mode, based on a decrease to intake air pressure in the engine occurring in response to the varied speed of the intake air compressor; and

the decreasing the braking power including decreasing the braking power via closed loop control using intake air pressure as a targeted variable.

2. The method of claim 1 further comprising:

receiving an engine braking request requesting the cylinder-number braking mode from among a plurality of available cylinder-number braking modes each braking the engine using a different number of combustion cylinders; and

commanding the operation of exhaust valves in the engine braking timing pattern, for a number of the combustion cylinders that is dependent upon the cylinder-number braking mode requested.

3. The method of claim 2 wherein the receiving of the engine braking request includes receiving an engine braking request from an engine braking control switch in a cab of a machine having a driveline coupled to the engine.

4. The method of claim 2 wherein the determining of a control term includes determining the control term using a map associated with the requested cylinder-number braking mode.

5. The method of claim 4 wherein the map is one of a plurality of maps each corresponding to one of the plurality of available cylinder-number braking modes.

6. The method of claim 1 further comprising receiving an engine speed signal and receiving an altitude signal, and the determining of the control term includes determining the control term based on the engine speed signal and the altitude signal.

7. The method of claim 1 wherein the determining of the control term includes determining a desired intake manifold pressure (IMAP).

8. The method of claim 7 further comprising monitoring an IMAP, determining an IMAP error based on a difference between the monitored IMAP and the desired IMAP, and commanding further varying of the position of the exhaust-impinged surface in the exhaust turbine based on the IMAP error.

9. The method of claim 1 wherein the commanding varying of a position of an exhaust-impinged surface includes commanding varying a turbine vane position in the exhaust turbine.

10. The method of claim 9 further comprising receiving a braking power modulation request, and the determining of the control term is based on the braking power modulation request.

11. An engine braking control system comprising:

an engine braking controller structured to couple to an engine braking control switch, and further structured to:

receive an engine braking request from the engine braking control switch indicative of a requested cylinder-number braking mode in an engine having a plurality of combustion cylinders;

command operation of exhaust valves for a number of the combustion cylinders that is dependent upon the requested cylinder-number braking mode in an engine braking timing pattern;

determine a control term that is indicative of at least one of an intake air pressure different from a present intake air pressure or a change to an intake air pressure from a present intake air pressure that varies a braking power of the engine within the requested cylinder-number braking mode;

command varying a position of an exhaust-impinged surface in an exhaust turbine coupled to the engine, based on the determined control term, to vary a speed of a compressor driven by the exhaust turbine;

decrease the braking power of the engine based on a decrease to a pressure of intake air supplied to the engine in response to the varied speed of the compressor; and

control the decreasing of the braking power of the engine via closed loop control using intake air pressure as a targeted variable.

12. The engine braking control system of claim 11 further comprising an engine speed sensor, and the engine braking controller is further structured to determine the control term based on an engine speed signal produced by the engine speed sensor.

13. The engine braking control system of claim 12 wherein the engine braking controller is further structured to determine the control term using a map having as coordinates engine speed and altitude.

14. The engine braking control system of claim 13 wherein the map is one of a plurality of stored maps each associated with a different cylinder-number braking mode of the engine.

15. The engine braking control system of claim 11 wherein the control term includes a desired intake manifold air pressure (IMAP).

16. The engine braking control system of claim 15 further comprising an IMAP sensor structured to produce an IMAP signal, and the engine braking controller is further structured to determine an IMAP error based on the IMAP signal and the desired IMAP, and to command further varying of the position of the exhaust-impinged surface in the exhaust turbine based on the IMAP error.

17. The engine braking control system of claim 11 further comprising:

the engine braking control switch, and the engine braking control switch having a finite number of switch configurations each corresponding to one of a plurality of available cylinder-number braking modes; and

a second controller structured to produce a braking power modulation request, and the engine braking controller is further structured to determine the control term based on the braking power modulation request.

18. An engine braking system comprising:

a plurality of engine braking actuators structured to adjust timings of exhaust valves for a plurality of combustion cylinders in an engine;

an exhaust turbine actuator structured to couple with an exhaust-impinged surface in an exhaust turbine;

an engine braking control system including an engine braking control switch, an engine speed sensor, and an engine braking controller coupled to the engine braking control switch and to the engine speed sensor;

the engine braking controller is structured to:

command, using the respective engine braking actuators, transitioning at least some of the exhaust valves from a first timing pattern to an engine braking timing pattern, based on an engine braking request from the engine braking control switch indicative of a requested cylinder-number braking mode of the engine;

determine, based on an engine speed signal produced by the engine speed sensor, a control term that is indicative of at least one of an intake air pressure different a present intake air pressure or a change to an intake air pressure from a present intake air pressure that varies a braking power of the engine within the requested cylinder-number braking mode;

command, using the exhaust turbine actuator, varying a position of an exhaust-impinged surface in the exhaust turbine based on the determined control term, such that a speed of a compressor rotated by the exhaust turbine is varied;

decrease the braking power of the engine based on a decrease to intake air pressure occurring in response to the commanded varying of a position of the exhaust-impinged surface; and

control the decrease of the braking power of the engine via closed loop control using intake air pressure as a targeted variable.

19. The engine braking system of claim 18 further comprising an intake manifold air pressure (IMAP) sensor structured to monitor an IMAP, and the engine braking controller is further structured to command a further change to a position of the exhaust impinged surface based on the monitored IMAP.

20. The engine braking system of claim 18 further comprising a transmission controller structured to produce a braking power modulation request, and the engine braking controller is further structured to determine the control term based on the braking power modulation request.