Turbocharger matching
• While the operating engineer will not
normally be involved in turbo-charger
matching, a familiarity with the procedure
will lead to a better understanding of the
interdependent relationship between
engines and their turbochargers, and of
the effects, in service, of operation off the
design point.
1.
2.
Inlet conditions of the
compressor P1 (after
pressure drop across air
suction filter) and T1 are
selected.
An estimate is made of the
power of the engine at a
particular engine RPM.
Also, an estimate is made
of amount of air, ma, the
engine would require at
above conditions.
ma = Vsw x a x vol x N (for
two stroke diesel engine).
The pressure in the air
manifold and temperature
are estimated to get the a.
3.
4.
A drop across air cooler is
assumed and added to air
manifold pressure to get
the compressor discharge
pressure P2. The
compressor pressure ratio
P2/P1 can now be
calculated.
The compressor frame size
and its diffuser can now be
selected by entering the
family of compressor maps
with the values of P2/P1 and
Va or (maT0/P0). The
operating point must have
adequate margin from
surge limit i.e. it must be
10% to 20% to the right of
the surge limit at the value
of P2/P1.
5. From the performance
map of the selected
compressor and
diffuser, the
compressor efficiency
(c) and turbocharger
RPM are read at the
operating point.
6. The required power to
drive the compressor
is given by
Wc=maCpaT1[(P2/P1)(-1)/  -1]/ c
The required turbine
power Wt = Wc / mech
where mech is the known
characteristic of the
bearings.
7. Again an estimate is made of the gas
conditions at the turbine inlet i.e. P3 and T3
from basic principles and empirical data
including previous performance.
8. The turbine outlet pressure P4 is also
estimated by adding an amount to
atmospheric pressure to allow for typical
losses through exhaust gas economiser.
Then expansion ratio P3/P4 is calculated.
9. The mass flow rate of gas, mg, is calculated
by adding mass flow rate of fuel, mf, to
mass flow rate of air, ma, which was
estimated previously.
10. In general, the selection of a
compressor wheel diameter
predetermines turbine
characteristics, which
includes wheel diameter and
blade length. With values of
P3/P4 and Vg, a turbine blade
and nozzle angle selection
curve, can be entered for the
frame size under
consideration, to select nozzle
opening and blade angle.
11. The following are calculated:
Mean tangential velocity of blade,
Um =  x Dm x RPM
where Dm = mean diameter of
turbine wheel
Ideal gas speed at nozzle exit
Cg = [2Cpg T3 {1- (P4/P3)-1/]
12. The turbine efficiency  t
can then be obtained from
figure.
13. The available power of the
turbine
Wt = mgCpgT3 [1–(P4/P3)(-1)/] t
14. A comparison is made
between the turbine power
available and turbine
power required. If available
power is greater than
required power, then the
estimate of air manifold
pressure, made in step 2,
can be raised and the
procedure is repeated with
this new assumption. If the
available power is lower, a
lower value of air manifold
pressure is assumed.
• This iterative process is continued till the
required turbine output matches the
achieved value within a percent or two,
with the final matching to be done by
actually running the turbocharger and
engine on a test-bed and making final
adjustments by changing compressor
diffuser vanes and turbine nozzle rings.