Thermo Exam Question: Estimate Temp & Pressure After Compression

• bruce999
In summary, the charge is compressed from 330K to 1 bar, and then is isentropically compressed to 7 bar. The temperature is then estimated using the appropriate equation.
bruce999
Hello.
I have a thermodynamics exam tomorrow. This is a past exam question and I'm not doing very well with it. If anyone has any ideas please help!

A charge enters a spark ignition engine at 330K and 1 bar, and is isentropically compressed through a ratio of 7:1. Estimate the temp and pressure at the end of the compression, taking the charge to be:
a) pure air with constant specific heat.
b) a stoichiometric mixture of air and octane(C8H18) with variable specific heats (neglect residual gases).

Well, I guess the test was yesterday. How'd you do?

Anyway:

1) First of all, you must understand the significance of isentropic.

Then the compression is 7:1 so the charge, air, is compressed to 7 bar.

Then solve the temperature using the appropriate equation.

2) similar to 1) but now a mixture of stoichiometric mixture of air and octane(C8H18) - so determine the partial pressures and composition, which influences specific heat. Same compression ratio.

Astronuc said:
Well, I guess the test was yesterday. How'd you do?

Anyway:

1) First of all, you must understand the significance of isentropic.

Then the compression is 7:1 so the charge, air, is compressed to 7 bar.

Then solve the temperature using the appropriate equation.

2) similar to 1) but now a mixture of stoichiometric mixture of air and octane(C8H18) - so determine the partial pressures and composition, which influences specific heat. Same compression ratio.

That's not true. You need to use the isentropic relations. Compression ratio is a ratio of volumes, not pressures. The three isentropic relations are:
(T2/T1) = (v1/v2)^(k-1)
(T2/T1) = (P2/P1)^(k-1)/k
(P2/P1) = (v1/v2)^k
These are estimations based on constant specific heats. To be more accurate and use variable specific heats, you will need to use vr and Pr, relative specific volume and relative pressure. However, since the question says estimate, using the isentropic relations should be good. So, if (v1/v2) = 7, then P2 = P1*7^1.4 (k = 1.4 for air). Likewise, T2 = T1*7^0.4

As said in the other thread, I'm not 100% sure how to do part b off the top of my head.

1. What is the purpose of estimating temperature and pressure after compression?

The purpose of estimating temperature and pressure after compression is to understand how the properties of a gas change when it is compressed. This information is important in many scientific and industrial applications, such as in the design of engines and compressors.

2. How is temperature estimated after compression?

Temperature can be estimated after compression by using the ideal gas law, which states that the pressure, volume, and temperature of a gas are related. By measuring the initial temperature and pressure, and knowing the volume before and after compression, the final temperature can be calculated.

3. What factors affect the estimation of temperature and pressure after compression?

The factors that affect the estimation of temperature and pressure after compression include the initial temperature and pressure, the type of gas being compressed, the volume before and after compression, and any heat transfer or work done during the compression process.

4. How accurate are the estimates of temperature and pressure after compression?

The accuracy of the estimates of temperature and pressure after compression depends on the accuracy of the initial measurements and the assumptions made in the calculations. In real-world scenarios, there may be other factors that can affect the accuracy, such as variations in gas properties and external conditions.

5. What are some potential applications of estimating temperature and pressure after compression?

Some potential applications of estimating temperature and pressure after compression include designing and optimizing engines, predicting the behavior of gases in industrial processes, and understanding the effects of compression on gases in various scientific experiments. This information can also be useful in the development and improvement of gas laws and equations of state.

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