I am trying to understand my superchargers isentropic chart.(adsbygoogle = window.adsbygoogle || []).push({});

For an example case where a supercharger outputs 10m3/hr but the engine only takes 6.57m3/hr in capacity, I get a volume ratio (VR) of 1.52 - easy....

I can then work out a pressure ratio by VR^k = 2.0^1.4 = 1.8

The theoretical temperature for such a PR is = [inlet Temperature K] x PR^0.2857 =

where 0.2875 is the result of (k-1)/k

note this is all independent of the supercharger....

Now we head to our supercharger isentropic chart.

It tells me for PR=1.8 and 10m3/hr input I should have an adiabatic efficiency of 65%

The formula to work out the actual temperature from the theoretical is;

OutletT = [TheorT-InletT]/Eff. + InletT using Kelvin

so

lets say for an inlet of 27oC = 300K

TheorT = [inlet Temperature K] x PR^0.2857 =355K (82C)

OutletT = [TheorT-InletT]/Eff. + InletT = [355-300]/65% + 300 = 385K = 111oC

And the chart tells me 100C - close.

note - there is no indication on the chart what inlet temp it uses.

Question 1 - Is the above approach sound?

Now the chart also tells me the Supercharger will be using 17kW of poer to execute this task - that it it will steal 17kW of power from the crank to hopefully produce enough increased power in the motor to compensate for this 17kW plus more....otherwise its a waste of time.

Question 2 - do I need to include some of this 17kW in the pressure ratio and iterate again?

After all - it is going to go in as temperature increase in the air stream (approx. 35% of it) and if this was a constant volume - it would increase the pressure and hence the PR....unfortunately if I do this - it wont converge - it simply runs away out of control...

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# Supercharger - Isentropic charts - Pressure Ratio and temperature

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