Trends and Limits of Two-Stage Boosting Systems for Automotive ...

Section 2.2 Chapter 2
important effort was devoted to the components definition and particularly
the valves of the system, including new functional characteristics and specific
physical definition of the different components [275]. System layout is shown
in figure 2.6 where left-hand plot shows valves positions in 1T operation mode
and right-hand plot shows valves positions in 2T operation mode.
Figure 2.6: System functional architecture [116].
Turbocharger matching is a difficult task for the parallel sequential turbocharging
system. In fact turbocharger surge and over-speed limits impose
range limits for the different engine operating modes. In 2T operation, these
limits are dependant of the flow repartition between 1T and 2T and therefore
the relative area ratio between the two turbines. In addition, turbines selection
are normally based on engine torque response, but a trade-off between
dynamic response and transition range at full load must be considered [116].
Figure 2.7: Definition of operating zones - A:2T mode is not possible (surge risk) -
B:1T mode is recommended - C: switching zone - D:2T mode recommended (fuel
efficiency) - E:1T mode is not possible (overspeed risk) [116].
Transition zones shown in figure 2.7 are defined respecting two critical
criteria: over-speed and surge that enforce the mode transition due to risk in
system damage, and two mild criteria: efficiency and system dynamics [115].
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