Entropy and Cycles

Lab Day 7: Entropy and Cycles

Purpose:

In today's class, the purpose was to analyze entropy and different types of engine cycles including reversible and irreversible processes. The different type of cycles in comparison to the carnot cycle include the otto cycle, diesel cycle, stirling cycle, and the brayton cycle. These cycles have different types of conditions which often makes them more or less efficient.

Experiment:

Entropy was measured to be the ratio of Heat energy absorbed over the thermal energy and the process is determined using the states of equilibrium. This is also considered as the amount of disorder where we can see that it is useful in explaining how heat input does not always cause a change in temperature. 

Diesel Engine: there is a 4-step cycle

1.Adiabatic compression
2. Constant pressure
3. Adiabatic expansion
4. Constant Volume

In the first step there is the compression stroke, where there is the adiabatic compression of gas and diesel fuel in the cylinder. The second is the ignition of gas fuel mixture where large compression causes burning to happen when pressure is constant. The third is the expansion stroke where it is adiabatic (no heat transfer). There is an isentropic expansion of gases and there is positive work. The last part is the exhaust in which it returns to the same volume and is the same at the initial and ending of the exhaust and intake stroke.

Otto Cycle: 

 Next there is the 4 stroke cycle engine which is called the Otto Cycle where it was able to carry out four piston strokes in one combustion cycle.The Otto engine is the closest to a real engine of an automobile.

1. The intake stroke consist of the piston moving down where the intake valve opens due to low pressure of air/fuel mixture. 

2. The compression stroke is when Bottom Dead Center is at maximum volume and the piston moves up, the pressure increases and volume decreases. At the end, ignition starts rapidly causing the piston to go downward. 

3. The power stroke is the force that cause the piston to go downward when the valves are closed. There is no energy where internal energy decreases along with temperature. 

4. Exhaust Stroke- at BDC exhuast valve is open which cause the piston to move up and pressure outside cylinder decreases. Gas leaves the cylinder and volume decreases.

Stirling Cycle: 

The Stirling cycle is an example of a Carnot Engine where we have work done due to differences in temperature. 

1. Isothermal compression of working gas

2. Constant Volume where heat is absorbed from energy storing device

3. Isothermal expansion of gases and there is positive work done.

4. Heat transfer from working gas to energy storage at constant pressure. heat stored is same as second step of cycle. 

This is an example of a Stirling engine where ice is put on top and the bottom is a beaker of water that is heated. The blades in fact moved counterclockwise due to the Stirling engine. We did the same experiment but had the temperatures reversed and it was seen to move the other direction. 

As a result, we calculated the efficiency of the cycle using Carnot's efficiency. The efficiency was calculated to be 23%  for our example of a Stirling engine. 

In this reversible engine, we identified that the change in entropy is zero S=0 in which we can calculate the final temperature by setting the system to zero. The final temperature is 46 degrees which is low and reasonable because it shows that some of the heat has become work. The work done in this reversible process is therefore the heat from the hot minus the heat from the cold. If we compare its efficiency to a carnot engine, it is less because thermal efficiency is much less due to the fact that a carnot cycle produces the highest possible efficiency in which temperatures do not change because their reservoirs are infinite. 

The density of bubble is higher and inside the bubble is hot air. As professor mason blows into the tube, the bubble comes up and pops. The second experiment showed how the flame went up when the bubble ignited. Since the bubble is less dense, it floated up and once ignited, it led to a rapid combustion as seen in the demonstration where flame went up. 

Conclusion:

Based on calculations and experiments on the new engines discussed, the different cycles all represent different types of conditions in which the efficiency is different. The entropy of a system is measured as the amount of disorder in a system. In fact as we briefly discussed the cycles in class, the Wankel and Brayton cycles both fell short because in brayton cycle it was suppose to have constant pressure but due to heat combustion it did not. On the other hand, The Wankel engine had flaws because it was suppose to be even more efficient than the otto cycle. We can conclude that the best heat engines can be determined on the basis of comparson to an ideal Carnot engine.