Gas Power Cycles (Applications of Thermodynamics)

Air Standard Assumption:

The assumptions are as follows:

1. The working fluid is air which continuously circulates in a closed loop and always behaves as an ideal gas.
2. All the process are internally reversible.
3. The combustion process is replaced by a heat addition process from an external source.
4. The exhaust process is replaced by a heat rejection process that restores the working fluid to its initial state.

Cold Air Assumption:

This assumption us to simplify the analysis even more by assuming that air has constant specific heats whose values are determined at room temperature.

An Overview of the Reciprocating Engines:

The important topics you should understand to make this topic clear are:

• Top Dead Centre (TDC) ➤ top's maximum point of piston having minimum volume.
• Bottom Dead Centre (BDC) ➤ bottom's maximum point of piston having maximum volume.
• Bore ➤ Internal Diameter of piston.
• Stroke ➤ Distance travelled between TDC and BDC by piston.
• Intake Valve ➤ It is actually to bring air fluid mixture in the cylinder.
• Exhaust Valve ➤ It us use to throw exhaust Gases out of  the cylinder.
• Clearence Volume ➤ The minimum volume form in the cylinder when the piston is at TDC.
• Displacement Volume ➤ The volume displaced by the piston as it moves between TDC and BDC.

Displaced Volume = Piston Area × Stroke

Compression Ratio:

The ratio of maximum volume to the minimum volume is called Compression Ratio.

Mean Effective Pressure:

It us a fictitious pressure that if it acted upon the piston during the entire power cycle, would produce the same amount of net work as that produce during the actual cycle.

Types of Reciprocating Engines:

There are two types of reciprocating engines which are:

1. Spark ignition or Otto Engine 
2. Compression ignition or Diesel Engine 

Spark Ignition Or Otto Engine

Ignition is started by spark plug and air and fuel mixture is compressed and is small engine.

》 Actual Four Stroke Spark Ignition Engine:

1. First Stroke or Compression Stroke:

It is a compression stroke in which both valve is closed and piston will go from BDC to TDC. At the end of compression  stroke, ignition takes place.

2. 2nd Stroke or Power Stoke:

In 2nd stroke, piston will go from TDC to BDC, both valve is closed.

3. Exhaust Stroke:

In 3rd stroke, piston will go from BDC to TDC. Exhaust valve opens, exhaust gases go out of the cylinder.

4. Intake Stroke:

In 4th stroke, piston will go from TDC to BDC. Intake Valve opens, air-fuel mixture come into the cylinder.

Note:

• One-stroke means 180° shaft rotation.
• Four-stroke means 729° or 2 shaft rotation.
• In 4-stroke engine, we get power after 2nd revolution.

》Ideal Four Stroke Spark Ignition  Engine (Using Air standard assumptions):

The diagram shows different processes as follows:

• 1 ➤ 2: Isentropic Compression 
• 2 ➤ 3: Constant Volume Heat Addition 
• 3 ➤ 4: Isentropic Expansion 
• 4 ➤ 1: Constant Volume Heat Rejection 

》Two-Stroke Spark Ignition Engine:

In two stroke engine, there are no intake and exhaust valve but there are intake and exhaust manifolds (opening).

Comparison of 2-Stroke and 4-Stroke Engine:

• In 2-stroke engine compression, ignit ion,  expansion, intake and exhaust  take place. We get power after each revolution of the shaft.
• 2-stroke engine are relatively simplementation,  inexpensive and they have high power to weight ratio (P:W).
• They are commonly used in applicationS requiring small size and weight.

Efficiency of Otto Engine:

Efficiency of an Engine is given by:

The otto engine is executed in a closed system and disregarding the kinetic and potential energy changes, the specific energy balance is:

Process 1 ↝ 2 and 3 ↝ 4 are Isentropic and from the figure

The derivation of Efficiency of Otto Engine is given in the video below:

Relation Between Compression Ratio and Efficiency of Otto Engine:

Compression Ratio is directly propertional to the efficiency of Otto Engine.

Compression Ignition Or Diesel Engine 

Only air is compressed to the self ignition temperature of the fuel then the fuel is sprayed and the ignition is started automatically and is large engine.

The figure shows that:

• 1 ↝ 2 : Isentropic Compression 
• 2 ↝ 3 : Constant Pressure Heat Addition 
• 3 ↝ 4 : Isentropic Expansion 
• 4 ↝ 1 : Constant Volume Heat Rejection 

Cut-off Ratio:

It is defined as;

The ratio of volume after ignition to the volume before ignition.

Derivation of Efficiency of Diesel Engine:

 

Efficiency of Diesel Engine is given by:

Note that the Diesel cycle is executed in a piston cylinder device, which forms a closed system and disregarding the kinetic and potential energy changes, the specific energy balance is given by:

Now, 

For 1 ↝ 2 (Isentropic compression) and  3 ↝ 4 (Isentropic expansion) 

Now, changing equation (3), we get 

The derivation of Efficiency of Diesel Engine is given in the video below:

Comparison of Diesel Engine and Otto Engine:

The efficiencies of Otto and Diesel Engine is given by:

They are different by factor 

Dual Cycle

It is a thermodynamic cycle that combines the Otto and Diesel cycle. In the Dual cycle, combustion occurs partly at Constant volume and partly at Constant pressure.

• 1 ➤ 2 : Isentropic Compression 
• 2 ➤ 3 : Constant Volume Heat Addition 
• 3 ➤ 4 : Constant Pressure Heat Addition 
• 4 ➤ 5 : Isentropic Expansion 
• 5 ➤ 1 : Constant Volume Heat Rejection 

The derivation involves the following steps in the important steps are as follows:

And the derivation for dual engine cycle is given in the video below:

Types of Fuel used in Heat Engine:

There are three types of fuel commonly used which are as follows:

1. Solid Fuel:

• It is used in past because of storage problem, handling, transportation, etc.
• They need to be pulverized  (crushed into particles/products).
• They produce ash and toxic emission. For example: coal, etc.

2. Liquid Fuel:

• It also have storage problem.
• There is a problem of making proper air-fuel Gaseous mixture.
• Liquid fuel should be highly volatile.

3. Gaseous Fuel: 

• They are ideal fuel and are commonly used nowadays.
• They can easily male proper Gaseous air-fuel mixture.
• Gases have higher volume so they also have storage problem. This exact reason why natural gas is compressed and saved as CNG (Compressed Natural Gas) in cylinders.

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