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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 GTI-02/0156, 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




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