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

Section 1.2 Chapter 1
Conventional thermomechanical limits such as maximum cylinder pressure,
exhaust manifold temperature and pressure, compressor outlet temperature,
etc. . . become rapidly limiting factors. Modifying their levels represents major
investments not only in new engine parts but as well in complete validation
processes to guarantee engine reliability. So it is important to analyze how
these limits restrain the downsized-downspeeded engine performance and when
substantial benefits can be achieved. Another limiting factor is the small turbine
size requirement to fulfill the high specific torque density at low engine
speeds. Nowadays, very small turbines are not present in the automotive market
as turbine efficiency significantly decreases with turbine diameter [348].
When combustion efficiency improvements through higher gas density in the
cylinders are lower than pumping losses deterioration caused by the small turbine
design, limitations appear in the utilization of turbocharging technologies.
Reaching this point, if other solutions like mechanical or electrical assistances
cannot efficiently provide the required boost pressure, downsized-downspeeded
techniques cease to be efficient. These limits have therefore to be defined to
quantify the maximum CO2 benefits that can be raised by these techniques.
The right design and coupling of the boosting system to the internal combustion
engine have capital importance to obtain the best overall performance.
Chargers characteristics, components and control valves specifications have to
be carefully optimized to match a given engine size and to fulfill as optimum
as possible project objectives. Doing such optimization process experimentally
would be very expensive and of little efficiency. That is why a correct
combination of experimental techniques and simulation tools is required. Generally,
wave action models are used for engine performance prediction [119,
134]. Through numerical methods, they solve the unsteady, non-linear and
non-homentropic gas flow equations assuming one-dimensional flow in intake
and exhaust piping system. Nevertheless as they predict engine performance
from a specific boosting system configuration, they need a lot of iterations to
perform matching calculations and are too time-consuming. With the complexity
arising from multistage architectures, large parametric studies have to
be carried out to optimize the large number of parameters, to understand the
important interactions between systems and to check influences of particular
designs. Their computational costs are therefore inappropriate to undertake
these studies especially under current fast engine development process.
Furthermore, when turbochargers and engine architecture are not completely
defined, few intake and exhaust lines geometrical data are usually available.
Without information such as intake manifold runners diameters, pipe lengths,
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