Fixed Geometry Turbochargers

Hannu Jääskeläinen

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Abstract: In the simplest turbocharger design, the turbine and compressor geometry are fixed and the boost pressure is entirely determined by the exhaust flow. An exhaust side bypass, or wastegate, is a common means of achieving better boost pressure control with fixed geometry turbines. The wastegate can be built into the turbine side of the turbocharger or it can be a separate valve connected to the external plumbing. Pressurized pneumatic wastegate actuation has been common, but vacuum actuation and electric actuation is utilized in many newer designs.


The simplest turbocharger design from a control perspective is one whose turbine and compressor geometry are fixed and that uses no means to control boost pressure. The boost pressure provided by this type of turbocharger is entirely determined by the engine exhaust flow and the characteristics of the turbocharger. The turbocharger is optimized for a particular operating condition. Turbocharger turbine size and/or A/R ratio tend to be relatively large for a given application because of the need to size the turbocharger so that at the highest flow conditions, the turbocharger does not overspeed or provide excessive boost pressure. While boost pressure near rated conditions can be selected via turbocharger sizing, transient response and boost pressure at lower engine speeds can suffer. Also, at high altitudes, turbocharger speeds would tend to increase which could lead to problems with surge and/or turbocharger overspeeding unless accounted for by oversizing the turbocharger. However, for some engine applications operating primarily at a limited number of steady-state conditions, an uncontrolled turbocharger with a fixed geometry turbine can prove entirely satisfactory.

For applications that experience a wide range of operating conditions and that must provide good dynamic response, for example in passenger car applications, a fixed geometry turbocharger with no control of boost pressure is unsuitable. Two methods can be used to control boost pressure for turbochargers with fixed geometry turbines in these applications:

Exhaust Side Bypass (Wastegate)

Adding a bypass valve that allows some of the exhaust gas to bypass the turbine is the more common means of achieving better boost pressure control with fixed geometry turbines. In most applications, this allows a smaller size or smaller A/R ratio fixed geometry turbine that is able to provide more power to the compressor at lower exhaust flows to be used for a given application, Figure 1 [2629]. Transient response is also improved significantly because of the improved low flow efficiency as well as the lower rotational inertia of the turbocharger.

In Figure 1, the blue line represents a turbocharger with a fixed geometry turbine while the red line represents a turbocharger with a smaller fixed geometry turbine. Neither fixed geometry turbine has a wastegate. Note that the turbocharger with the smaller turbine would overspeed and overboost the engine at relatively low engine speeds. Adding a wastegate to the turbocharger with the smaller turbine can significantly improve boost at lower engine speeds while avoiding overboost and overspeeding of the turbocharger at higher speeds. The amount of improvement depends on how well the wastegate is controlled.

[chart] [chart]
Figure 1. Effect of Turbine Size and Wastegate on Boost Pressure and Turbocharger Speed

(Source: Cummins Turbo Technologies)

Figure 2 shows another example but from the perspective of the compressor map. The full load boost characteristic with a fixed geometry and wastegate controlled turbocharger are shown. Each turbine is sized to provide the engine with the same boost pressure, intake air mass flow and rotational speed at rated power. The fixed geometry turbine without a bypass must be able handle the entire exhaust flow at rated power and tends to provide less boost pressure at lower engine air flow conditions. The advantage of being able to use a smaller turbine/lower A/R ratio with a wastegate is readily apparent. It should be noted that since the turbocharger speed at maximum flow for all the cases is the same, the pressure ratio at high engine speeds across the wastegated turbine, and thus engine pumping losses, must be higher than for the fixed geometry turbine without a wastegate [2538].

Figure 2. Full Load Boost Characteristics with Two Different Turbine Options