# How Do You Calculate Temperature and Pressure in Isentropic Compression?

• bruce999
In summary, the conversation is about a student seeking help with an exam question on thermodynamics. The question involves estimating the temperature and pressure at the end of compression for a charge entering a spark ignition engine at 330K and 1 bar. The charge can be either pure air with constant specific heat or a stoichiometric mixture of air and octane with variable specific heats. The student is unsure how to approach the second part of the question and is seeking assistance.
bruce999
Exam revision question. Help!

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).

For part a: you will use the constant specific heat relationships. They are something like P1/P2 = (V2/V1)^k-1 note: That equation is probably not true, but the three formulas are in that same format. Knowing that compression ratio is V1/V2, you can easily plug that number into the formula given initial T and P to find final T and P.

for b: hmmmm...partial pressures and stuff...wow, I have no idea. I guess one would find the stoichiometric ratio, I believe it's ~14:1, then go from there. That's about all the help I got for that part.

Hi there,

I can understand the stress and pressure of preparing for an exam, especially when facing a challenging question. Don't worry, I am here to help!

For part a), you can use the isentropic compression equation to solve for the final temperature and pressure. This equation is T2/T1 = (P2/P1)^((γ-1)/γ), where T is temperature, P is pressure, and γ is the ratio of specific heats (1.4 for air). You can plug in the given values and solve for T2 and P2.

For part b), you will need to use the specific heat ratio (γ) for a mixture of air and octane, which can be calculated using the mole fractions of each component. You can then use the same isentropic compression equation to solve for the final temperature and pressure.

Remember to pay attention to units and double-check your calculations. Also, make sure to review any relevant formulas and concepts before the exam. Good luck!

## 1. What is the best way to prepare for exams?

The best way to prepare for exams is to start early and create a study schedule. This will help you cover all the material and avoid cramming. Also, practice past exam questions and make sure to review your notes regularly.

## 2. How can I stay motivated during exam revision?

Staying motivated during exam revision can be challenging, but it's important to remind yourself of your goals and why you are studying. Take breaks, reward yourself, and surround yourself with positive and supportive people.

## 3. How do I know which topics to focus on for the exam?

It's important to review all the topics covered in class, but also pay attention to any hints or emphasis your teacher gives on certain topics. Practice past exam questions to get a better understanding of which topics are frequently tested.

## 4. Is it helpful to study in groups or alone?

This depends on your personal learning style. Some people find it helpful to study in groups and discuss concepts with others, while others prefer to study alone and focus on their own pace. Experiment with both methods to see what works best for you.

## 5. How can I manage my time effectively during exam revision?

Create a study schedule and stick to it. Break down your study sessions into smaller chunks and take breaks in between. Prioritize your studying based on the difficulty of the material and allocate more time to challenging topics. Avoid distractions and stay organized to make the most out of your study time.

• Engineering and Comp Sci Homework Help
Replies
8
Views
1K
• Thermodynamics
Replies
5
Views
1K
• General Engineering
Replies
3
Views
6K
• Classical Physics
Replies
7
Views
1K
• Engineering and Comp Sci Homework Help
Replies
2
Views
3K
• Engineering and Comp Sci Homework Help
Replies
33
Views
6K
• Mechanical Engineering
Replies
9
Views
3K
• Mechanical Engineering
Replies
1
Views
1K
• Thermodynamics
Replies
11
Views
7K
• General Engineering
Replies
11
Views
8K