Turbo
Engineering Explained
The Engineering
behind Turbo Technology and Testing:
Turbo Testing explained:
All existing turbocharger units
still undergo testing using guidelines, by the ASME and SAE
societies. We will bypass the explanations of the Test stand
parameters, cause they are irrelevant to the theory we are
here to explain. Below is just one version of a test
stand, which is usually referred to as the (Loop Hot gas
stand).
Above while in testing a
turbocharger unit is typically monitored by no less then 30 to
50 points of data, using thermo wells, flow meters, pressure
sensors, and optical speed sensors. Final product of testing
is documented and will yield results like the one below.
This is a raw compressor map
that has been derived and has not been finished with
efficiency calculations, and choke limit line, which we will
show you further down how they are derived. With a final
commercial version of the compressor map finished you can then
use our
Turbos and Turbo Selection 101 page
which discusses how to use the map to
your advantage for making the most street able and efficient
horsepower for your needs.
The
Calculations and Corrections used:
Raw data:
Above we explained the raw aspect of a
turbocharger and its test process. Taking the raw data from a
test is useless until the data can be interpreted to a
standard which can then be used for comparison analysis
against other units. Once this is performed then performance
and efficiency can then be determined.
The typical data that is corrected is
calculations for compressor and turbine wheel speed,
compressor airflow rates, turbine exhaust flow rates,
compressor pressure ratios and turbine expansion ratio. Below
are some raw numbers from a compressor wheel test. From left
to right the numbers are (Wheel rpm, Mass flow, pressure ratio
and efficiency)
Similarly you will have raw
data compiled for a turbine wheel as well.
SAE standardizing Calculations:
Now that we have shown some of
the raw data, we will show how this data is now converted to
SAE standards and also how we can derive what is missing from
the above maps. First after testing, SAE set guidelines
explain that all compressor data (compressor speed, pressure
ratio, efficiency and flow rate) are corrected to 60 DegF and
14.696psia. The same guidelines explain that the turbine data
must be corrected to 800DegF and 24.696psia. Once these
numbers have been corrected then we may use them to determine
the performance of a turbocharger at all operating conditions.
To normalize the data we will
use the correction formulas below.
The formulas will give us the information
necessary to correct the raw data that was recorded at
uncorrected conditions to a specific inlet temperature ,
pressure and standard inlet condition that was noted above
(60DegF and 14.696 psia for the compressor side, and 800DegF
and 24.696 psia for the turbine side). With these corrections,
now the compressor data and map can be used to gauge the
possible performance that the turbocharger may achieve at
conditions other than the standard inlet conditions.
Efficiency calculations:
Once we have corrected for SAE operating
conditions, we now need to look at plotting the efficiency, or
at least calculating them for plotting purposes. Since a
turbocharger will not operate adiabatically, the heat transfer
gains or loses must be included into our calculations. The
reason for this is the isentropic or adiabatic efficiency of a
compressor is the ratio between the power that would be
necessary to operate the compressor in an ideal adiabatic
situation and the actual power that is necessary to operate
the compressor. The constraint is that the inlet temperature,
pressure, and the exit pressure are the same for the ideal
adiabatic compressor and the real compressor. Deriving from
this we get the equation for a compressor being:
For the turbine side the equation will only
differ slightly:
The heat transfer rate from the turbine and
compressor can be calculated by determining the heat rejection
to the water and oil systems, and then calculating the heat
transferred from the turbocharger CHRA and housings. The
housing heat transfer rate is described in GTI02/0156, a
final report describing the correlations and heat transfer
paths within large turbochargers. Once these calculations are
performed you will now achieve the additional information that
is missing from the compressor map above. A final product
would then be a compressor map that is commercially used.
Stay Tuned, more coming soon
