MECHANICAL THEORY

SI Engine:Working of 4 stroke SI Engine, Otto Cycle-Derivation,P-V & T-S Diagram

SI Engine:Working of 4 stroke SI Engine,Otto Cycle-Derivation,P-V and T-S Diagram:

SI engines are used in passenger cars, motorcycles, aircrafts, agricultural equipments etc.

In a four stroke engine, the cycle of operation is completed in four strokes of the piston or Two revolutions of the crankshaft.

During the 4 strokes of the piston, there are five events to be completed and they are:Suction, compression, combustion, expansion and exhaust.

Each stroke consists of 180 degrees of Crankshaft rotation and hence a four stroke cycle is completed through 720 degrees of crankshaft rotation.

The cycle of operation for the four stroke engine consists of 

  1. Suction stroke
  2. Compression stroke
  3. Expansion or Power stroke
  4. Exhaust stroke

Diagram of 4 Stroke SI engine:

The diagram of 4 Stroke SI Engine was drawn below. Here in this diagram,one need to concentrate on the Movement of Piston under four strokes and the opening & closing of Inlet and Exhaust Valves.

SI Engine-Working of 4 stroke SI Engine, Otto Cycle-Derivation,P-V and T-S Diagram
SI Engine-Working of 4 stroke SI Engine, Otto Cycle-Derivation,P-V and T-S Diagram

Working of 4 Stroke SI Engine:

The working of 4 Stroke petrol(SI) engine was explained below.

1.Suction stroke:

It starts when the piston is at the Top Dead Centre (TDC) and about to move downwards. During suction stroke, the inlet valve is open and exhaust valve is closed.

suction stroke in SI Engine
suction stroke in SI Engine

Piston moving downwards creates a partial vacuum inside the cylinder i.e. the pressure inside the cylinder is less than the atmospheric pressure. Due to pressure difference, the Air-Fuel mixture is sucked into the engine cylinder.

2.Compression Stroke:

 During this stroke, the inlet and exhaust valves are in closed position.The charge taken into the cylinder during the suction stroke is compressed by the return stroke of the piston.

compression stroke in SI Engine
compression stroke in SI Engine

At the end of the compression stroke, the mixture is ignited with the help of an electric spark between the electrodes of a spark plug located on the cylinder head.

Burning takes place, when the piston is at the Top Dead Centre (TDC). During burning, the chemical energy of the fuel is converted into heat energy and the pressure at the end of combustion process is considerably increased due to heat release.

3.Expansion Stroke:

This is the Power stroke or Working stroke in which both valves are closed and high pressure gas expands. At the end of the expansion stroke, exhaust valve opens.

expansion stroke in SI Engine
expansion stroke in SI Engine

Some of the burnt gases come out through the exhaust valve. Due to this,the pressure decreases nearly at constant volume.

4.Exhaust Stroke:

During this stroke, exhaust valve is open and inlet valve remains closed. Piston moving upwards (BDC to TDC) forces the burnt gases through exhaust valve.

At the end of this stroke, exhaust valve closes and inlet valve opens.

exhaust stroke in SI Engine
exhaust stroke in SI Engine

The fresh charge is taken into the engine cylinder while the Piston moving downwards. All the above events are repeated for the next cycle.

This is the detailed explanation of 4 Stroke SI Engine. Now, we need to learn about the OTTO cycle.


OTTO Cycle-Derivation:

This is a constant volume cycle on which petrol and gas engines work.The Otto cycle consists of 4 processes and are as follows.

Process 1-2: Reversible Adiabatic Compression or Isentropic Compression

Process 2-3: Constant Volume heat supply

Process 3-4: Reversible Adiabatic Expansion or Isentropic Expansion

Process 4-1: Constant Volume heat Rejection

PV and TS Diagram of otto cycle
PV and P

The explanation of above processes are as follows.

Process 1-2: Reversible Adiabatic Compression or Isentropic Compression:

The air is compressed into the clearance volume of the cylinder and thus raising the pressure and temperature of the air.But, entropy remains constant as there is no heat transfer across the cylinder walls

Process 2-3: Constant Volume heat supply (or) Heat Addition:

heat is supplied to air from hot body while the volume of the air remains constant. Thus the work done is zero but the pressure and temperature of the air increases further to the maximum value attained in the cycle.Due to heat supply(addition) there is an increase in entropy.

Qs = Cv(T3-T2)

Process 3-4: Reversible Adiabatic Expansion or Isentropic Expansion:

Air expands adiabatically causing the work energy to be transferred to the surroundings at the expense of internal energy of the air.

Entropy remains constant as there is no heat transfer but the Pressure, Temperature and Volume decreases from stage 3 to 4.

Process 4-1: Constant Volume heat Rejection

Heat is rejected from air to cold body. During the heat rejection, air is maintained at constant volume and due to heat rejection, entropy decreases, pressure and temperature also decreases to initial state.

QR = Cv(T4-T1)

Thus, the cycle is completed. 

The efficiency of the OTTO Cycle can be calculated as follows.

Just follow the below derivation in order to calculate the efficiency of Petrol engine by Otto Cycle.

otto cycle-PV Diagram and TS Diagram
otto cycle-PV Diagram and TS Diagram
Derivation of OTTO CYCLE
Derivation of OTTO CYCLE

This is the detailed explanation of 4 stroke SI Engine along with the derivation of Otto Cycle.

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