The term “turbocharging” refers to methods of increasing the air or air/fuel mixture density of a motor vehicle engine by increasing the pressure of the intake air stream prior to its entering the engine cylinder using an air intake compressor powered by the engine exhaust stream. Increasing the density of air in the cylinder is desirable because it represents a relatively simple method of increasing the power generated by the engine.
Turbocharging is a favored method of increasing intake air pressure because current turbocharger designs are very efficient at harvesting the energy in the exhaust stream. This increased efficiency translates into an increase in the engine power output without a significant decrease in fuel economy.
In its basic form, a turbocharger consists of a turbine wheel and a compressor wheel mounted on the same shaft. The turbine wheel and the compressor wheel are each isolated in a housing. A gas inlet and a gas outlet in the turbine housing permit the exhaust stream from the engine to be used to spin the turbine wheel. As the turbine wheel spins, so does the shaft and the compressor wheel which pulls air into the compressor housing where it is pressurized and then directed to the engine intake manifold.
Because the speed of the compressor is dependent on the pressure of the exhaust gas stream, there is generally not enough pressure at the beginning moments of vehicle acceleration, causing turbo “lag” and too much pressure at the final moments. Because most turbochargers are capable of delivering enough pressure at peak engine rpm's to damage the engine and the turbocharger a wastegate is commonly used to vent this extra pressure.
A well known solution to more closely matching the pressure generated by the turbo with engine rpm is a variable geometry turbocharger using a plurality of adjustable vanes or nozzles (referred to herein as a variable nozzle turbocharger). The theory of such turbos is relatively simple: vary the size of the turbine housing, the compressor housing, or both by varying the position of the vanes to permit increased pressure at low engine rpm's and decreased pressure at high engine rpm's.
U.S. Pat. Nos. 4,490,622; 4,973,223; and 6,543,994 describe such variable nozzle turbos having a plurality of nozzle vanes that can be simultaneously moved to change the geometry of turbine housing or the compressor housing. Such variable nozzle turbochargers are commonly used with large diesel engines particularly in commercial truck applications and, more recently, in passenger car applications in combination with common rail, direct injection diesel engines. However, because of the relative complexity of these turbos, they have yet to be successfully adapted for use in the significantly hotter operating environment of gasoline engines.
One engineering solution that offers some of the advantages of variable nozzle turbochargers for a gasoline engine is to use multiple chambers in the turbine housing. A control valve is initially positioned to limit the flow area by closing off chambers to permit the turbocharger to deliver greater initial boost. As engine speed and/or load increases, the control valve is positioned to increases the flow area by opening additional chambers and thus limit the amount of top-end boost.
Examples of turbochargers having multi-chambered turbine housings are found in U.S. Pat. Nos. 4,177,006; 4,512,714; 4,781,528; and 4,544,326. All of these designs focus on the configuration of the turbine housing and/or the kind and operations of the control valve and fall short of describing designs that address the operation of the turbocharger/engine as an entire system. In addition, the described designs do not appear durable enough to deliver sustained, problem free operation when coupled to a gasoline fueled engine. Thus, it would be advantageous to provide an improved design for a variable geometry turbocharger that would consider the turbocharger/engine as a whole system, as well as have a simple and durable design suitable for operation in conjunction with a gasoline engine.