Exergy at a state? need some guidance

• jmarcian
In summary, to determine the work potential or exergy of the hot exhaust gases in the cylinder of a diesel engine, we can use the equation Wex = H - H0 - T0(S - S0), where the initial state is just prior to the exhaust valve opening and the final state is when the valve opens and the gases are released. The engine speed also plays a role in the calculation, and the recovery or dumping of this energy depends on the engine's design and efficiency.
jmarcian

Homework Statement

Consider the hot exhaust gases in the cylinder of an engine (diesel engine) just prior to the exhaust valve opening. (a) Determine the work (power) potential (exergy at a state) of these gases in kW. (b) Can any of this energy be recovered or must it be dumped to the atmosphere? You may assume the cylinder is at bottom dead center with a volume of 2 liters, a gas pressure of 700 kPa, a mean gas temperature of 1100 K, and an engine speed of 1800 rpm.

Homework Equations

Wu=W-Wsurr= W-P0(V2-V1)

? not real sure here

The Attempt at a Solution

i thought that all exergy equations depended on two states...? also I am not sure whether this is control volume or closed system, and also not sure how the engine speed plays a part. i would start to assume that it gives off some type of power or energy, but how do you change rpm into kj, or kw?

quite frankly i am completely lost on this problem...

Thank you for your question. I would like to offer some insights and assistance in solving this problem.

Firstly, to answer your question about exergy equations, yes they do depend on two states. In this case, the two states would be the initial state of the exhaust gases (prior to the exhaust valve opening) and the final state (when the exhaust valve opens and the gases are released).

To determine the work potential or exergy of the gases, we can use the equation:

Wex = H - H0 - T0(S - S0)

Where:
Wex = Exergy (kJ)
H = Enthalpy (kJ)
T = Temperature (K)
S = Entropy (kJ/K)

H0 and S0 represent the initial state of the gases, and H and S represent the final state. In this case, we can calculate the initial enthalpy and entropy using the ideal gas law and specific heat capacity of the exhaust gases.

As for the engine speed, it does play a role in the calculation as it determines the rate at which the gases are being released. We can convert the engine speed from rpm to radians per second (ω) using the equation:

ω = 2πN/60

Where:
N = Engine speed (rpm)

Once we have calculated the exergy of the gases, we can then determine if any of this energy can be recovered or if it must be dumped to the atmosphere. This would depend on the design and efficiency of the engine.

I hope this helps to guide you in solving this problem. Please let me know if you have any further questions.

1. What is exergy at a state?

Exergy at a state refers to the maximum work that can be obtained from a system when it is brought into equilibrium with its surroundings. It is a measure of the available energy of a system that can be used to perform useful work.

2. How is exergy at a state different from energy?

Energy is a measure of the total amount of work that can be obtained from a system, whereas exergy at a state takes into account the availability of energy and the irreversibility of the processes. Exergy at a state is a more accurate measure of the potential for useful work.

3. How is exergy at a state calculated?

The exergy at a state can be calculated using the first and second law of thermodynamics. It involves determining the available work that can be obtained from a system by considering the temperature, pressure, and entropy of the system and its surroundings.

4. What are the applications of exergy at a state?

Exergy at a state is useful in many fields, including engineering, environmental science, and economics. It can be used to evaluate the efficiency of energy conversion processes, identify areas for improvement, and assess the environmental impact of energy systems.

5. How can we improve exergy at a state?

The exergy at a state can be improved by increasing the efficiency of energy conversion processes, reducing the waste and losses, and utilizing renewable energy sources. It is also important to consider the exergy at a state when designing new systems to ensure maximum utilization of available energy.

• Engineering and Comp Sci Homework Help
Replies
10
Views
4K
• Introductory Physics Homework Help
Replies
6
Views
2K
• Mechanical Engineering
Replies
1
Views
1K
• Mechanical Engineering
Replies
2
Views
2K
• Mechanical Engineering
Replies
5
Views
1K
• Mechanical Engineering
Replies
13
Views
5K
• Engineering and Comp Sci Homework Help
Replies
5
Views
4K
• Mechanical Engineering
Replies
6
Views
1K
• Mechanical Engineering
Replies
4
Views
5K
• Engineering and Comp Sci Homework Help
Replies
12
Views
8K