Heat Made By The Supercharger
To See just how much heat is generated by 1 PSI of boost we can use the following formula:
(PR0.28 x Tabs) - Tabs
PR = Pressure Ratio
Tabs = Ambient Temperature on the absolute scale. ( For example on a sunny day in the UK, we could assume 70oF = 70o + 460o = 530o Absolute.)
Another way of expressing this formula would be:
(PR0.28-1) x Tabs
So using this formula we can work out how much 1 psi of boost
will raise the temperature at an ambient temperature of 70oF
Note: 14.7psi absolute = Atmospheric pressure at sea level
So we have a figure of 9.857o absolute as an ideal temperature gain for 1 psi at an ambient temperature of 70oF. But this does not take into account the efficiency of the supercharger. So to get the REAL temperature gain, we take the ideal temperature gain divided by the thermal efficiency of the supercharger.
So for a
Centrifugal Supercharger such as the Rotrex with a typical thermal efficiency of
75% the Real temperature gain for 1 psi of boost would be:
This is a typical figure to give an idea of the temperatures that can be reached. The Actual temperature gain per psi decreases with boost pressure because of the 0.28 exponent in the formula and the fact that the superchargers efficiency is not linear. For example the temperature gain per psi at 15 psi is about two thirds of the gain at 1 psi boost.
But even taking this into account you can see that with a typical setup producing 10 psi of boost you could easily have a temperature rise in excess of 120oF and that's above what you would see on a normally aspirated engine with all other components being equal.