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

Section 2.2 Chapter 2
in torque between 1T and 2T, and exhaust valve pre-lift actuation strategies.
An example of resulting transitions with and without corrections can be
observed in figure 2.8.
2.2.2 Serial Boosting Systems
Serial two-stage turbocharging systems were primary designed to increase
the absolute boost pressure in power production and marine applications [41].
In fact, there is a limit to the boost that can be achieved by a single compressor
due to efficiency decrease and map width requirements when the pressure ratio
is increased. Moreover the compressor outlet temperature increases with pressure
ratio and it gives way to more expensive forgings or the use of titanium
for the impeller material [33]. So at higher pressure ratios, two stages of compression
become more attractive and cheaper because existing turbochargers
can be used and new turbocharger development is not required.
Ex xpansionn
Ratio
2.6
22.4 4
2.2
2
11.8 8
1.6
1.4
1.2
HP Turbine
LP Turbine
1
0.75 0.85 0.95 1.05 1.15
HP Turbine Flow Parameter
Figure 2.9: Left: cross section of two-stage turbocharger - Right: LP turbine
swallowing capacity curve [322].
An example of a compact series turbocharger developed by Holset [322]
comprising two single stage turbocharging is shown in figure 2.9 (left side). It
comprises on the same axis a conventional turbocharger for the high pressure
stage and a larger centrifugal compressor for the low pressure stage. The
low pressure turbine was a custom-designed axial stage fed directly from the
exhaust of the high pressure turbine. Utilizing the close coupled turbines, duct
losses were kept to a minimum resulting in a high overall turbine performance.
The turbocharger was designed for a high output diesel engine requiring a
pressure ratio of 6:1 and operating at around 30 bar BMEP in a narrow range.
26