Advance thermodynamics (week 1) Part 1.

Advanced Thermodynamics

So, this is my first post. I want to begin it with thermodynamics material, in advance of course. Although, it was labelled as an advance, but for the early meeting still manage about basic thermodynamics review. Hopefully you can enjoy it. 

 Week 1&2

Gas Power Cycle

Ref : Thermodynamics (Cengel)

Thermodynamics is a subject that study about energy and it form. Energy define as an ability to do work. First, I want to review about the law of thermodynamics. We study about energy in thermodynamics, but here we must know what kind of energy and its scope. There was 2 kind of energy in a thermodynamics approach, stored energy and energy in transition. Stored energy is an energy that carried by mass, It could be an entalphy, kinetic energy, potential energy. Energy in transition is an energy that pass and moving in and out into system. What is system? System is a part of universe that need to be studied and it is bordered by a boundary. So, energy in transition is an energy that income and outcome into/from a system in form of heat and work. According to the cengel, he stated that there are two important area in thermodynamics, namely, power generation and refrigeration. Both area was involving a cycle and divided into 2 cycles: power cycle and refrigeration cycle. Power cycle can be categorized as gas power cycle and vapor power cycle depending of its working fluid. Here, we will study about gas power cycle.

GAS POWER CYCLE

When we talk about gas power cycle, it means that the working fluid for entire process remains a gas form. Example of gas power cycle are Spark-Ignition Engines, Auto Ignition Engine, and conventional gas turbine. Before we go advance into an engine approach, we need to know that in every engineering problems involve a complex analysis. To make it easier, an engineer must use a great trick, an assumption and an approximation. We don’t need to analyze it using actual phenomena, but we can approach it using idealization. So, many engineering problems was studied under idealization process.

Back to gas power cycle, gas will always be used in analysis. So, we need to make an ‘air-standard assumption’ on the analysis. Air-standard assumption consist of:

1.The working fluid is air, which continuously circulates in a closed loop and always behaves as an ideal gas.

2.All the processes that make up the cycle are internally reversible. 

3.The combustion process is replaced by a heat-addition process from an external source (Fig. 9–9).

4.The exhaust process is replaced by a heat-rejection process that restores the working fluid to its initial state. [ cengel ]

So, in this analysis we need to use properly assumption using air-standard assumption.

OTTO CYCLE : IDEAL CYCLE FOR SPARK-IGNITION ENGINES

The Otto cycle is the ideal cycle for spark-ignition reciprocating engine and it’s named after Nikolaus A. Otto. Otto cycle

From the figure above we can see how the otto cycle operate. There are 2 cycle, actual and ideal cycle. To make it easier to imagine, I want you to follow me. Imagine that you carry your motorcycle go out from garage. After that you sit above your motorcycle and plug your key. Before your motorcycle is on, you know that inside your motor there is an engine with a piston like the figure above. Your piston is on the lower level, after you plug your key the piston will compress an air and quickly after you started your motor, the spark will ignite the combustion chamber. (figure a). and that’s how the motorcycle active. 

So, to make it easier again, let’s look at the figure b. On the point 1, the piston is on the lower level, and it will be compressed to make a volume of air smaller and its pressure higher. After it reach TDC (Top dead center) the fuel is injected into piston chamber and spark is ignite the combustion between an air and a fuel. So in this process the pressure will significantly increase and suddenly after combustion occurring the piston will be pulled down and the pressure is decreasing. So, after piston at its lower level again, the gas as a result of combustion is thrown away and fresh air is injected again and the process will be same again.  Piston that pull up and down will rotate the shaft and the wheel.

Let’s look at the process, when the fuel is injected. Process 2-3, we can call it heat addition process, because there was a heat energy is go inside the system in form of combustion. After do an energy balance analysis, it can be obtained that heat addition is determine by the difference of internal energy of process 2&3. We know that, process 2-3 occurs on constant volume, so the formulation can be arranged into heat capacity form times temperature. Then, after combustion process, the engine will release residual gas and heat, this is gas will release from muffler and the heat will heat the muffler. Because process 4-1 is also in constant volume, so the amount of heat can be defined as internal energy difference between 4-1.

So, what about work? The main important of engine is to produce a work. Work in otto cycle is in form of rotating wheel. So if your motorcycle move, it because of engine work. How to measure work? It can be analyzed by pressure and volume, or can be simplified by reduce heat in (qin) with heat out (qout). You can check this analysis more advance using thermodynamics literature. So, how to calculate how efficient our engine is? There are an efficiency analysis and for otto cycle it define as the relationship between temperature operation of each process, see equation below. Please refer to cycle process’s figure to know the terms of number.

Then, introducing new variable, namely r and k. r is compression ratio and it’s defined as ratio of maximum volume of piston and minimum volume during operation. And k is defined as specific heat ratio. With some modification, the equation of efficiency can be written as :

Again, please refer to cengel for complete analysis.

So, here we know that compression ratio of engine influence the efficiency of otto engine. Higher of compression ratio will produce a higher efficiency also. But, there are some limitation, if we increase the compression ratio it will produce a knocking in engine and leads to engine problem (broken), another consequence if the r increase is resulting an auto ignition. Every fuel has auto-ignition point. For example gasoline, gasoline has auto-ignition point at some pressure and temperature, so if one of those condition is obtained, there will be ignition without a spark. It will be dangerous. Thus, maybe, in my opinion, great power isn’t always produce a higher efficiency. In designing process, so many factor should be considered until it reach optimum condition and optimum efficiency, that’s more important that only produce power as many as possible.

Continue to part 2