The Significance of The Turbine Housing A/R
Turbocharger turbine casings perform the function of increasing the engine exhaust gas velocity and then distributing the high velocity exhaust gas around the periphery of the turbine wheel. In order to perform these functions, the design of the turbine casing must include an initial nozzle section to increase the exhaust gas velocity, followed by a volute section to distribute and maintain the high velocity of the exhaust gas around 360° of the turbine wheel outside diameter.
A cross section of a typical turbine casing is diagrammatically illustrated below:
The turbine inlet flange is connected to the engine exhaust manifold and mounts the turbocharger on the engine. As can be seen in the above diagram, the nozzle section of the casing decreases in cross sectional area from the inlet flange to the nozzle throat. The nozzle throat area, in square inches, perpendicular to the gas flow is the "A" of the casing A/R. The radius, in inches, to the center of the throat area is the "R" of the casing A/R.
Divided turbine casings are designed the same as undivided casings: i.e., they both have nozzle sections to increase the exhaust gas velocity and volutes to distribute the high velocity gas around the turbine wheel outside diameter. The divided casings have parallel gas passages created by a central dividing wall that extends from the Turbine inlet flange and continues around 360° of the volute. The divided casings also have an A/R designation where the "A" is the sum of the areas of the two parallel nozzle passages at their throats, at the termination of their nozzle sections.
A velocity diagram at the entrance of the turbine wheel is shown below:
C1 is the exhaust gas velocity generated by the nozzle section of the turbine casing. U1 is the tip velocity of the turbine wheel and W1 is the relative gas velocity entering the turbine wheel blades.
A large area, A, at the nozzle throat produces a lower value of exhaust gas velocity and results in a lower value of C1 entering the wheel. Thus, the turbocharger will be running at a lower RPM, the value of U1 will be lower, and the velocity triangle will be of similar shape.
A small area, A, at the nozzle throat produces a higher value of C1, and this higher gas velocity entering the turbine wheel blades drives the turbine to a higher speed when mounted on the engine. The value of U1 will be higher and the turbine inlet triangle remains a similar shape, indicating little change in turbine efficiency.
Small values of the turbine casing A/R that operate the turbocharger at high speeds (and high boost pressures) cause a high back pressure to exist in the engine exhaust manifold and very small values will result in a negative differential pressure across the engine; from boost pressure to back pressure. This hurts engine fuel consumption.
Large values of the turbine casing A/R will operate the turbocharger at lower speeds, produce lower boost pressures, and normally will produce a positive differential pressure across the engine. This improves engine fuel consumption.
In summary, the value of the turbine casing A/R is an indication of the size of the casing and provides a means of adjusting the speed of the turbocharger when mounted on an engine. Intake manifold boost pressure can be adjusted higher or lower by selecting appropriate A/R values of the turbine casing.