Turbocharger systems are used with internal combustion engines to supply pressurized intake air to the cylinders for improving combustion which decreases undesirable emissions and increases performance and efficiency. Internal combustion engines have a wide range of operating speeds and loads which produce widely differing volumes of exhaust gases. A single turbocharger is not capable of efficiently operating with widely differing volumes of exhaust gases. Therefore, known turbocharging systems using a single turbocharger limit their operating range, for example, by not functioning at low engine speeds.
In order to resolve the problems resulting from the difference in operating range between the engine and turbocharger, multiple turbocharger systems have been developed. Known multiple turbocharger applications have difficulties in controlling which of the system's turbochargers are to be used under various ranges of operating conditions and how the flow of intake air and exhaust gases are to be controlled in transient modes of operation. For example, if two similar sized turbochargers are used in series, then each turbocharger has an operating range which does not correspond to the entire operating range of the engine. Thus, the turbocharging system will either fail to provide optimal pressurized intake air during low exhaust gas mass flow conditions or lack capacity during high exhaust gas mass flow conditions. If two similar sized turbochargers are used in parallel, transitional performance is detrimentally effected by a loss in compression of the intake air when exhaust gases are diverted from the first turbocharger to the second turbocharger.
Other turbocharging systems have used two turbochargers of different size. These systems require complex controls which involve the use of electronically or mechanically operated air flow management valves in the intake air and exhaust gas systems, and additional means for avoiding the problems of surging and exceeding the operational limits of the individual turbochargers. Moreover, these complex controls increase the overall system costs, maintenance, weight, and structural volume, as well as reduce the durability of the system.