BACKGROUND OF THE INVENTION A turbocharger is a turbine-driven forced induction device that increases an internal combustion engine's efficiency and power output by forcing extra compressed air into the combustion chamber. The turbocharger comprises a turbine and a compressor. Engine exhaust drives the turbine, which in turn drives the compressor. A turbocharged engine’s improved power output is due to the fact that the compressor can force more air -- and proportionately more fuel -- into the combustion chamber than atmospheric pressure alone. As an alternative to increasing power output for a given engine, turbocharging can increase fuel efficiency by allowing an engine with smaller displacement.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: FIG. 1 illustrates an internal combustion engine having two cylinder banks with cross-boosting in accordance with the invention. FIG. 2 illustrates a method of operating a cross- boosted internal combustion engine, such as the engine of FIG. 1. FIG. 3 illustrates an engine similar to that of FIG. 1, but in which one of the cylinder banks has an exhaust gas recirculation (EGR) loop.

DETAILED DESCRIPTION OF THE INVENTION The following description is directed to the recognition that in conventional turbocharged engines, a turbocharger’s boosting performance is directly related to the engine’s combustion. This relationship does not allow for engine strategies in which the requirements of the engine do not match the available output of the turbocharger. At conditions in which exhaust temperatures are low and hence turbine output is low, an engine might require high boost. Conversely, at conditions in which there is plenty of exhaust energy, an engine may require only a moderate level of boosting. This mismatch between engine demands and boosting power availability limits the possible operation range of advanced combustion strategies. FIG. 1 illustrates an internal combustion engine 100 having six cylinders 101. Engine 100 is represented schematically by its cylinders 101; it is assumed that engine 100 has all other elements for operating as an internal combustion engine. The type of fuel used for engine 100 may be any one of a number of available fuels, and it is particularly assumed that each cylinder 101 has fueling control to control the type and amount of fuel delivered to it, as well as ignition and charge compression equipment if appropriate for any of the engine operating modes described below. In the example of this description, engine 100 is a six-cylinder engine. However, the concepts described herein apply to internal combustion engines having any number of cylinders. Engine 100 is equipped with two turbochargers 102 and 103, each having its own turbine and compressor. It is not necessary that the turbochargers have the same specifications in terms of boosting ability or be the same type of turbocharger. In the example of FIG. 1, each turbocharger 102 and 103 serves one-half the engine’s cylinders. Thus, for engine 100 having six cylinders, each turbocharger serves a “bank” of three cylinders. This results in two cylinder banks, identified as Bank 1 and Bank 2. The exhaust lines of Bank 1 are connected as input to the turbine of turbocharger 102. The exhaust lines of Bank 2 are connected as input to the turbine of turbocharger 103. The connection lines are identified as exhaust connection lines 102a and 103a, respectively. The output of the compressor of turbocharger 102 is connected to the air intake of Bank 1. The output of the compressor of turbocharger 103 is connected to the air intake of Bank 2. The connection lines are identified as air boost connection lines 102a and 103a, respectively. Although not explicitly shown, cylinders 101 each have exhaust ports for delivering exhaust gas to lines 102a and 103a, as well as intake ports for receiving compressed air via lines 102b and 103b. The intake and exhaust manifolds to the cylinder banks are “split” so that each cylinder bank may be cxnnected to a desired number of intake ports and exhaust ports. In FIG. 1, where each bank of cylinders has three intake ports and three exhaust ports, shared manifolds are identified as manifolds 102c and 103c. In other embodiments, the cylinder banks need not have the same number of cylinders. For example, an eight- cylinder engine could have one bank with three cylinders and another bank with five cylinders. It is expected that the typical application of the invention will be implemented with two cylinder banks, but more are possible. In operation, exhaust gas from Bank 2 drives the turbine of turbocharger 103. Because of the cross connected air boost connection lines 102b and 103b, the compressed air from turbocharger 103 is delivered to Bank 1. Similarly, exhaust gas from Bank 1 drives the turbine of turbocharger 102. The compressed air from turbocharger 102 is delivered to Bank 2. The cross-boosting operation of turbochargers 102 and 103 allows combustion to be separated from boosting. In other words, Bank 1 can be operated in a different combustion mode than Bank 2 and use a different boosting level. Examples of various internal combustion modes are rich, lean, or stoichiometric combustion, which having varying equivalence ratios. Another group of combustion modes includes CCI (charge compression ignition) modes, in which fuel and oxidizer (typically air) are compressed to the point of auto-ignition. These CCI modes include HCCI (homogenous charge compression ignition) and PCCI (premixed charge compression ignition) combustion modes. Thus, for example, Bank 1 could be operated at one equivalence ratio (rich, lean, or stoichiometric) and Bank 2 could be operated at another. Bank 1 operating at stoichiometric equivalence ratio only requires a moderate level of boosting from turbocharger 103 but provides a generous level of exhaust gas for turbocharger 102 for boosting Bank 2. As another example, Bank 1 could be operated in a CCI mode. A highly diluted CCI mode requires a high level of boost, but the exhaust output is low temperature. However, the required boost for Bank 1 could be made available from its associated turbocharger 103 operating at a mode that produces a high level of exhaust energy. Thus, at the same time, Bank 2 could be operated at a mode, such as stoichiometric, that does not require a high level of boosting from its associated turbocharger 102 but does produce high energy exhaust for turbocharger 103. FIG. 2 illustrates a method of operating an engine having cross-boosting in accordance with the invention. The engine is configured as described above, with cross- connected turbochargers. The result is two banks of cylinders, each having an associated turbocharger. A first bank of cylinders is operated in a first mode, that requires only low boost. It delivers high energy exhaust to an associated turbocharger. The other bank of cylinders is operated in a second mode that requires high boost. It delivers low energy exhaust to an associated turbocharger. The turbochargers do not deliver compressed air to the same cylinder bank that drives their turbines. Instead, their compressed air outputs are cross-connected so that compressed air is delivered to the other cylinder bank. The terms “low” and “high” are used in a relative sense. A “low” boost that is optimal for one mode is lower than the boost optimal for the other mode. Similarly, the “high” energy exhaust output from one mode is exhaust that is hotter and will drive the turbine to more power than the exhaust output of the other mode. In sum, features of the invention are that “banks” of cylinders are defined by being cross-connected to different turbochargers, and may operate in different modes with different air boost requirements. Each bank of cylinders is boosted with a different turbocharger. It could be the case that the banks of cylinders are operated in modes that have different air boost requirements, but not necessarily different exhaust energy outputs. The different engine operating modes need not be operated continuously. In other words, a cylinder bank might run in a mode that is a “part-time mode”, occurring only during certain engine conditions. For example, upon engine start-up, one cylinder bank might be run in a rich mode and the other cylinder bank be run in stoichiometric mode. In other operating conditions, both cylinder banks might operate in stoichiometric mode. FIG. 3 illustrates an embodiment of the invention in which at least one bank of cylinders has an exhaust gas recirculation (EGR) loop. In the example of FIG. 3, one cylinder bank, Bank 2, has an EGR loop 310. Exhaust gas is recirculated from the cylinders of Bank 2 back to the intake of those cylinders. An EGR valve 311 is used to control when Bank 2 operates in EGR mode and how much EGR is provided. When valve 311 is open and Bank 2 is operating in EGR mode, Bank 2 has reduced exhaust temperature (energy) for boosting. However, the operation of Bank 2 in EGR mode is optimal with a higher level of boosting so that fresh air and EGR is adequately pushed into the intake. Thus, during the EGR mode of Bank 2, Bank 1 is operated at a mode that provides a sufficient level of boosting for Bank 2. During the EGR mode of Bank 2, one appropriate mode for Bank 1 is a stoichiometric mode. Referring again to FIG. 1 and 3, both engines 100 and 300 are equipped with a controller 120 and 320, respectively, which delivers “mode control” signals to Bank 1 and Bank 2. The method of FIG. 2 -- operating cylinders in different modes with cross-boosted turbochargers -- is implemented with this controller. Controller 120 or 320 has appropriate hardware and programming to carry out these tasks. It may be part of a larger and more sophisticated engine control unit. The mode control signals cause each cylinder bank to operate in a selected mode, based on input representing engine operating conditions. These mode control signals may vary depending on the particular mode selected. They may include for example, fueling signals. In the case of EGR mode, the mode control signals would control EGR valve 311.