Supercharger - Isentropic charts - Pressure Ratio and temperature

In summary, the conversation discusses the use of a supercharger's isentropic chart to calculate volume and pressure ratios for a given engine input. It also mentions the adiabatic efficiency and the impact of power usage on the engine's performance. The conversation concludes with a question about whether the 17 kW power usage should be included in the pressure ratio calculation. The answer is that it comes from the engine and should not be iterated again in the calculation.
  • #1
Billy22Bob
1
0
I am trying to understand my superchargers isentropic chart.
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 won't converge - it simply runs away out of control...
 

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  • #2
Billy22Bob said:
note - there is no indication on the chart what inlet temp it uses.
Black text. Top centre of the page. Inlet temperature = 20 C

Also the flows are in m3 per minute, NOT m3 per hour.

Question 1.
17 kW is small compared to the additional power generated. If it is not, then you should specify a lower capacity supercharger.

Question 2.
Do not iterate again. The 17 kW comes from the engine.
 

1. What is a supercharger?

A supercharger is a device that forces more air into an engine, allowing it to burn more fuel and produce more power. It is commonly used in high-performance engines to increase their horsepower and torque.

2. How does a supercharger work?

A supercharger works by using a belt or chain connected to the engine to rotate a compressor, which compresses the air entering the engine. This compressed air is then forced into the engine's cylinders, allowing it to burn more fuel and produce more power.

3. What are isentropic charts?

Isentropic charts are graphical representations of the thermodynamic properties of a gas, such as air, at different pressures and temperatures. They are commonly used in the design and analysis of superchargers and other gas compression systems.

4. How is pressure ratio related to temperature in a supercharger?

The pressure ratio in a supercharger is directly related to the temperature of the compressed air. As the pressure ratio increases, the temperature of the compressed air also increases. This is due to the fact that as the air is compressed, its molecules are forced closer together, resulting in an increase in temperature.

5. What is the ideal pressure ratio for a supercharger?

The ideal pressure ratio for a supercharger depends on the specific engine and its operating conditions. In general, a higher pressure ratio will result in more power, but it may also put more strain on the engine and reduce its longevity. It is important to carefully consider the engine's needs and design a supercharger system that balances power and reliability.

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