MECHANICAL THEORY

Diesel Cycle-Working of 4 Stroke CI Engine,P-V & T-S Diagram,Derivation

Diesel Cycle-Working of 4 Stroke CI Engine, P-V & T-S Diagram, Derivation: In this article, the working of 4 Stroke CI engine will be discussed first and later the 4 processes employed in Diesel cycle will be provided along with its derivation to calculate the Efficiency of Diesel cycle.

Working of 4 Stroke CI Engine:

The 4 stroke CI engine is similar to the 4 stroke Si engine but, it operates at much high compression ratio.

The compression ratio of CI engine varies from 16 to 20 whereas SI engine varies from 6 to 10.

Diesel cycle-Working of 4 stroke Diesel Engine
Diesel cycle-Working of 4 stroke Diesel Engine

Due to the high compression ratio employed, the temperature at the end of the compression stroke is sufficiently high to self Ignite the fuel which is injected into the combustion chamber.

The major differences are that the ignition occurs spontaneously due to the high temperature of the compressed air that is ignited by means of a fuel injector.

The sequence of operations in a four-stroke CI engine (diesel engine) is as follows.

1.Suction Stroke

2.Compression Stroke

3.Expansion Stroke

4.Exhaust Stroke

The explanation of the above operations is as follows.

1.Suction Stroke:

Suction stroke
Suction stroke

During suction stroke only, Air is admitted into the cylinder. In this stroke, the inlet valve is opened and the exhaust valve is closed and the piston travels from TDC to BDC.

2.Compression Stroke:

here the inlet valve and the exhaust valve will remain closed. The Piston moving upward compresses the air to high pressure and temperature.

Diesel cycle-Compression stroke
Diesel cycle-Compression stroke

Fuel is injected into the cylinder at the end of the compression stroke and ignition occurs spontaneously due to the high temperature of the compressed air.

3.Expansion Stroke:

Burning of fuel results in the release of chemical energy which increases the pressure and thus results in moving the piston from TDC to BDC.

Diesel cycle-Expansion stroke
Diesel cycle-Expansion stroke

This process is the expansion process where the piston comes down to BDC(Bottom Dead Center) with both inlet and exhaust valves in a closed position.

4.Exhaust Stroke:

Exhaust stroke
Exhaust stroke

The Piston traveling from bottom dead Centre to Top Dead Centre pushes out the products of combustion through the exhaust valve, which is opened.

Diesel Cycle-4 Processes with Derivation:

This cycle was introduced by Dr. D.R.Diesel in 1897.

In the Diesel cycle, heat is supplied at Constant Pressure whereas, in Otto Cycle, heat is supplied at Constant Volume.

Diesel cycle comprises of 4 processes.

Process 1-2:Reversible Adiabatic Compression

Process 2-3:Constant Pressure heat supply

Process 3-4:Reversible Adiabatic Expansion

Process 4-1:Constant Volume heat Rejection.

The explanation of the above processes is as follows.

Process 1-2: Reversible Adiabatic Compression

It is also known as a reversible adiabatic compression process. At point 1, the cylinder is full of air. Let P1, V1, T1 be the corresponding Pressure, volume, and temperature. Air is compressed by the piston to high pressure and temperature till the values become P2, V2, and T2.

Process 2-3: Constant Pressure heat supply

It is also known as a constant pressure heat supply process.

In this process, heat is supplied to air at constant pressure from the hot body and thereby there is an increase in the temperature from T2 to T3 and Volume from V2 to V3.

                                           Qs = mCp(T3-T2)                                                             

Process 3-4: Reversible Adiabatic Expansion

It is also called a reversible adiabatic expansion process. In this process, at point 3, the supply of heat is stopped and this point is known as Point of Cut off. Later, the air expands to the conditions of P4, V4 and T4 respectively corresponding to point 4.

This indicates the work is done by the gas due to the expense of internal energy and there will be no heat transfer in this process.

Process 4-1: Constant Volume Heat Rejection

It is also called a constant volume heat rejection process. In this process, air rejects the heat to the cold body at constant volume till point 1 where it returns to its original state.

In this process, Temperature decreases from T4 to T1 and Pressure decreases from P4 to P1.

QR =mCv(T4-T1)

Derivation of Diesel Cycle:

In this derivation, the efficiency of the Diesel cycle is calculated.

The P-V and T-S Diagram of the Diesel cycle are presented below.

Considering 1 Kg of Air,

Work done = Heat Supplied – Heat Rejected

                 = mCp(T3-T2) – mCv(T4-T1)

Efficiency = Work done/Heat Supplied

The rest of the derivation is written below.

Diesel cycle derivation 1
Diesel cycle derivation 1
Diesel cycle derivation-2
Diesel cycle derivation-2

This is the efficiency of the Diesel Cycle.

For detailed info about Internal Combustion Engines, please read the below articles.

 

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