1.  Field of the Invention
The present invention relates generally to air induction systems for providing increased airflow to the intake of an engine.  More particularly, the present invention concerns a supercharger system having an inlet valve that varies the supply of air to the supercharger in response to air pressure conditions downstream from the supercharger.  The downstream air pressure conditions can vary depending on the application and could include for example pressure conditions in the engine intake or pressure conditions in the inlet to a downstream turbocharger. 
2.  Discussion of Prior Art
Centrifugal superchargers that increase air flow to an engine (thereby increasing the power generated thereby) are known in the art and typically utilize a compressor powered by the engine""s crankshaft to supply pressurized air, or xe2x80x9cboostxe2x80x9d to the engine.  Known prior art centrifugal superchargers, however, are problematic and have several limitations.  For example, conventional centrifugal superchargers have a somewhat linear boost response curve, therefore, if optimal boost is provided at higher engine rpms, the boost provided at lower engine rpms is significantly less than optimal.  Conversely, if the typical centrifugal supercharger is set up to provide optimal boost at lower engine rpms, it will provide more boost than the engine can handle at higher engine rpms. 
Known centrifugal superchargers are also problematic in that they suffer from undesirable surge conditions that are counterproductive to the operation of both the supercharger and the engine.  For example, when a downstream restriction occurs, such as the closing of the engine throttle, the large pressure loads created can destroy the internal parts of the supercharger.  Some prior art superchargers utilize a bypass valve that dumps the load into a recirculating line during these surge conditions; however, this bypass valve solution is problematic in that an undesirable pressure spike still occurs and the resulting pressure fluctuations can cause the mass flow meter to feed too much fuel into the engine. 
Turbochargers, similar to superchargers, increase air flow to an engine; however, turbochargers utilize a compressor powered by the exhaust output by the engine.  Known turbochargers also suffer from low-end boost problems, similar to those described above.  Some prior art turbochargers utilize a supercharger upstream from the turbocharger, and powered by the engine""s crankshaft, to supplement its low-end boost.  These supercharged turbochargers, however, are problematic and have several limitations.  For example, known supercharged turbochargers have an undesirable variance in the pressure supplied to the inlet of the turbocharger.  That is, they are only supercharged at lower engine rpms and not operable to be supercharged at higher engine rpms.  This is particularly problematic in high altitude applications wherein turbochargers typically do not supply sufficient boost for the desired engine power output.  For example, diesel powered trucks frequently use turbochargers to boost engine power and these trucks are commonly used to transport heavy loads through high altitude conditions wherein the boost provided by the turbocharger at higher engine rpms is insufficient to desirably power the engine.  Additionally, high altitude conditions lower volumetric efficiency and increase exhaust emissions. 
The present invention provides an improved air induction system that does not suffer from the problems and limitations of prior art systems set forth above.  The inventive system provides an inlet valve that varies the supply of air to the supercharger in response to air pressure conditions downstream from the supercharger.  The valve can be used to both throttle the supercharger as well as substantially eliminate undesirable surge conditions therein by selecting pressure conditions in the intake to the engine as the controlling downstream pressure conditions.  The inventive system can also be used to provide supercharged air at a substantially constant pressure to a turbocharger by selecting pressure conditions in the inlet to the turbocharger as the controlling downstream pressure conditions. 
A first aspect of the air induction system of the present invention concerns a system for inducing airflow into the intake of an internal combustion engine including and broadly includes a centrifugal supercharger presenting an air supply opening and a spaced pressurized air exhaust opening and being operable to pressurize air between the supply and exhaust openings, and a valve in communication with the supply opening to control air supply thereto. 
A second aspect of the air induction system of the present invention concerns a valve assembly to be used in an air induction system that induces airflow into the intake of an internal combustion engine, wherein the induction system includes a compressor presenting an air supply opening and a spaced pressurized air exhaust opening communicating with the engine intake.  The valve assembly broadly includes a valve fluidly connectable to the supply opening to control air supply thereto, and a valve control mechanism including an air pressure sensor adapted to sense air pressure downstream from the air exhaust opening.  The valve control mechanism is operable to cause the valve to vary the air supply depending on the air pressure sensed. 
Another aspect of the air induction system of the present invention concerns an air induction system in a powered vehicle including an engine.  The induction system broadly includes a turbocharger operable to pressurize air and deliver it to the engine and including an inlet operable to receive air.  The induction system further includes a supercharger in communication with the inlet and being operable to pressurize air and deliver it to the inlet, wherein the supercharger includes an air supply opening operable to receive air.  In addition, the induction system includes a valve in communication with the supply opening and being operable to vary air supply thereto so that air pressure of the pressurized air delivered to the inlet remains substantially constant. 
Other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments and the accompanying drawing figures.