Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 3: Internal Combustion engines introductions. working of Two stroke petrol engine.

Two stroke petrol engines

•In a 2-stroke engine, ports are present in the cylinder in place of valves. The ports are the openings in the cylinder opened and closed by the movement of piston with in the cylinder. There are three ports, namely

•Inlet port: Through which admitting of charge in to the crankcase takes place.

•Transfer port: Through which the charge is transferred from the crankcase to the cylinder.

•Exhaust port: Through which the burnt gases are discharged out of the cylinder.

•In a 2-stroke engine, piston performs two different strokes or crankshaft completes one revolution to complete all the operations of the working cycle. In these engines there are no suction and exhaust strokes, instead they are performed while the compression and power strokes are in progress.

First stroke(Downward stroke)

vAt the beginning of this stroke, the piston is in the TDC as shown in the figure(a). At this position, inlet port is opened and hence fresh air petrol mixture enters into the crankcase. At this position, compressed air-petrol mixture present in the cylinder in the previous cycle is ignited by the spark generated by the spark plug. The combustion of fuel releases hot gases which increases the pressure in the cylinder. The high pressure gases exerts a pressure on the piston and hence the piston moves from TDC to BDC. Thus piston performs power stroke. The power impulse is transmitted from the piston to the crankshaft through the connecting rod. This causes the crankshaft to rotate at high speeds. Thus work is obtained in this stroke.

vAs the piston moves downwards, it uncovers the exhaust port and hence burnt gases escape out of the cylinder as shown in the figure(b). As piston moves downwards further, opens the transfer port and the charge in the crank case is compressed by the underside of the piston as shown in figure(b). the compressed charge from the crankcase rushes into the cylinder through the transfer port as shown in fig(b). the charge entering the cylinder drives away the remaining exhaust gases through the exhaust port.

vThe process of removing the exhaust gases with the help of fresh charge is known as scavenging. The piston is provided with a projection at its top known as ‘deflector’. The purpose of providing a deflector is to deflect the fresh charge coming through the transfer port to move towards the top end of the cylinder. By doing this, the fresh charge will be able to derive the entire burnt gases out of the cylinder.

Second stroke(Upward stroke):

vAt the beginning of the stroke, piston is in BDC and it covers the inlet port as shown in figure(c) and stops the flow of fresh charge into the crankcase. During the stroke, piston ascends and move towards TDC. As the piston moves upwards, it closes the transfer port, there by stopping the flow of fresh charge into the cylinder kas shown in figure(d).

v  Further upward movement of the piston closes the exhaust port and actual compression of the charge begins. In the meantime, the inlet port is opened, and the upward movement of piston creates a suction in the crankcase. Fresh charge enters into the crankcase through the inlet port as shown in the figure(a). the compression of the charge in the cylinder continues till the piston reaches the TDC. This completes the cycle.

Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 3: Internal Combustion engines introductions. working of four stroke diesel engine.

4-stroke Diesel Engine

The working principle of a 4-stroke diesel engine is based on theoretical diesel cycle. Hence it is also called diesel cycle engine.

Suction stroke: 

At the beginning of the stroke piston is in TDC and during the stroke, piston moves from TDC to BDC. The inlet valve opens and the exhaust valve will be closed. The downward movement of the piston creates a suction in the cylinder and as a result, fresh air is drawn into the cylinder through the inlet valve. when the piston reaches the BDC, the suction stroke completes and this is represented by the line AB on P-V diagram.

Compression stroke: 

At the beginning of the strike piston is in the BDC and during the stroke piston moves from BDC to TDC. Both inlet and the exhaust valves are closed. As the piston moves upwards, air in the cylinder is compressed to a high pressure and temperature. The compression process is adiabatic in nature and is shown by the curve BC in P-V diagram. At the end of the stroke , the fuel(diesel) is sprayed into the cylinder by fuel injector. As the fuel comes in contact with the hot compressed air, it get ignited and undergoes a combustion at constant pressure. This process is shown by the line CD on PV diagram. At the point D fuel supply is cutoff. The compression ratio ranges from 16:1 to 20:1.

Power stroke/Expansion stroke/working stroke:

At the beginning of this stoke, piston is in TDC and during the stroke, piston moves from TDC to BDC. Both inlet and exhaust valve remain closed. As combustion of fuel takes place, the burnt gases expand and exert a large force on the piston. Due to this, piston is pushed from TDC to BDC. The power impulse is transmitted down through the piston to the crank shaft through the connecting rod. This causes the crankshaft to rotate at high speeds. Thus work is obtained in this stroke.

The expansion of gases is adiabatic in nature and this is shown by the curve DE on P-V diagram. When the piston reaches the BDC, the exhaust valve opens. A part of burnt gases escapes through the exhaust valve out of the cylinder due to self expansion. The drop in pressure at constant volume is shown by the line EB on P-V diagram.

Exhaust Stroke:

At the beginning of the stroke piston is in BDC and during this stroke, piston moves from BDC to TDC. The inlet valve is closed and the exhaust valve is opened. As the piston moves upward, it forces the remaining burnt gases out of the cylinder through the exhaust valve. this is shown by the line BA on P-V diagram. When the piston reaches the TDC the exhaust valve closes. This completes the cycle.

In the next cycle the piston which is at the TDC moves to BDC there by allowing fresh air to enter into the cylinder and the process continues.

Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 3: Internal Combustion engines introductions. working of four stroke petrol engine.

Four Stroke Engines

•In four stroke engines, piston perform four different strokes to complete all the operation of the working cycle.

–Suction stroke

–Compression stroke

–Power stroke (Expansion stroke or Working stroke)

–Exhaust stroke

Stroke	Position of the piston		Inlet valve	Exhaust Valve	Crank rotation
	Initial	Final
Suction stroke	TDC	BDC	Open	Close	00 – 1800
Compression stroke	BDC	TDC	Close	Close	1800 – 3600
Power stroke	TDC	BDC	Close	Close	3600 –  5400
Exhaust stroke	BDC	TDC	Close	Open	5400 –  7200

Four Stroke petrol engine

•The Working principle of four stroke petrol engine is based on theoretical ottocycle. Hence it is also known as a otto-cycle engine. A four stroke petrol engine performs 4 different strokes to complete one cycle. The working of each stroke are discussed below.

Suction stroke:

At the beginning of the stroke, piston is in TDC and during the stroke , piston moves from TDC to BDC. The inlet valve opens and exhaust valve will be closed. As the piston moves downwards, suction is created in the cylinder as result, fresh air-petrol mixture(charge) is drawn in to the cylinder through the inlet valve. As the piston reaches BDC, the suction stroke completes and inlet valve closes. The suction stroke is represented by the line AB on PV diagram.

Compression stroke:

At the beginning of the stroke, piston is in BDC and during the stroke the piston moves BDC to TDC. Both inlet and exhaust valve are close. As the piston moves upwords the air-petrol mixture in the cylinder is compressed adiabatically. The pressure and temperature of charge increases and this is shown by the curve BC on the P-V diagram. When the piston reaches the TDC, the spark plug ignites the charge. The combustion of fuel takes place at the constant volume and is shown by a line CD on the P-V diagram. The compression ratio in petrol engine raging from 7:1 to 11:1.

Power stroke/Expansion stroke/Working stoke: 

At the beginning of the stroke, piston is in TDC and during the stroke the piston moves form TDC to BDC. Both inlet and exhaust valve remain close. The combustion of fuel liberates gases and these gases starts expanding. Due to  expansion, the hot gases exerts a large force on the piston and as result the piston is pushed from TDC to BDC. The power impulse is transmitted down through the piston to the crank shaft through the connecting rod. This causes crankshaft to rotate at high speeds. Thus work is obtained in this stroke. Hence, this stroke is also called an expansion stroke.

The expansion of gases is adiabatic in nature and this is shown by the curve DE on the P-V diagram. As the piston reaches the BDC, the exhaust valve opens. A part of the burnt gases escape through the valve out of the cylinder due to their own expansion.

Exhaust stoke: 

At the beginning of the stroke piston is in BDC and during the stroke the piston moves from BDC to  TDC. The inlet is closed and exhaust valve is opened. As the piston moves upward, it forces the remaining burnt gases out of the cylinder to the atmosphere through the exhaust valve. this is shown by the line EB and BA on P-V diagram. When the piston reaches the TDC, the exhaust valve closes and this completes the cycle.

In the next cycle the piston which is at TDC moves to BDC there by allowing fresh charge to enter the cylinder and the process continues.

Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 3: Internal Combustion engines introductions.

Internal Combustion Engines

•A heat engine is a device which transforms the chemical energy of a fuel into thermal energy and uses this energy to produce mechanical work

•It is classified into two types

  (a) External combustion engine

  (b) Internal combustion engine

(a)External combustion engine: An Engine in which combustion of fuel take place outside the engine cylinder is called external combustion engine. These engine generally called EC engines

Ex: Steam engines, steam turbines, closed cycle gas turbines

(b) Internal combustion engine: In this engine, the combustion of air and fuels take place inside the cylinder and are used as the direct motive force. These engine generally called as IC engines

Ex: petrol engines, diesel engines, gas engines.

Classification of IC engines

1)According to the type of fuel used.

a)Petrol engines: In this type of engines, the fuel used is petrol.

b)Diesel engines: In this type of engines, the fuel used is diesel.

c)Gas engines: In this type of engines, the gaseous fuels natural gas, biogas are used.

d)Bi-fuel engines: These engines use a mixture of two fuels.

2) According to the number of strokes per cycle.

a)4- stroke engines : in this type of engines, the working cycle is completed in four different strokes.

b)2- stroke engines : in this type of engines, the working cycle is completed in two different strokes.

3) According to the method of ignition

a)Spark ignition engines (S.I. engines) : In this type of engines, fuel is ignited by an electric spark generated by a spark plug.

b)Compression ignition engines (C.I. engines) : In this type of engines, the fuel gets ignited as it comes in contact with the hot compressed air. 

4) According to the cycle of combustion

a)Otto cycle engine : In this type of engines, combustion of fuel take place at constant volume.

b)Diesel cycle engines : In this type of engines, combustion of fuel takes place at constant pressure.

c)Duel combustion engines : In this type of engines, combustion of fuel first takes place at constant volume and then at constant  pressure.

5) According to the number of cylinders.

a)Single cylinder engines : This type of engines consist of only one cylinder.

b)Multi cylinder engines : This type of engines consist of 2,3,4,6 and 8 cylinders.

6) According to the arrangement of cylinders

a)Vertical Engine : In this type of engines, the cylinder is arranged in a vertical position.

b)Horizontal engine : In this type of engines, the cylinder is arranged in a horizontal position.

c)Inline engine : In this type of engines, cylinder are arranged in line.

d)Radial engine : in this type of engines, cylinders are arranged along the circumference of a circle.

e)V-Engine : In this type of engine, combination of two inline engines equally set an angle.

7) According to the method of cooling 

a)Air cooled engines : In this type of engine, the heated cylinder walls are cooled by continuous flow of air.

b)Water cooled engine : In this type of engine, water is used for cooling the heated cylinder walls.

c)Oil cooled engines : In this type of engine, oil is used for cooling the heated cylinder walls.

Principal parts of an I.C. engines

1) Cylinder

•It Is the cylindrical vessel in which the fuel is burnt and the power is developed. It is considered as heart of the engine. The primary functions of cylinder is

–To contain the working fluid under pressure and

–To guide the piston while reciprocating inside the cylinder.

2)Cylinder head

•The top end of the cylinder is closed by a removable cylinder head. The cylinder head consists of two valves ‘inlet valve’ and ‘exhaust valve’.

3)Piston

•It is a hallow cylindrical plunger reciprocating inside the engine cylinder. It transmits power developed by the combustion of the fuel to the crankshaft through connecting rod. It receives an impulse due to the expansion of gases during power stoke.

4)Piston rings

•The rings which are placed in the grooves cut towards top of the piston are called piston rings. There are two set of rings inserted into the groves. They are compressing rings and oil rings.

–Compression rings: The compression rings press hard with the cylinder walls forming a tight seal between the piston and the cylinder. This prevents escaping of the high pressure gases into the crankcase.

–Oil ring: The function of oil rings is to extract the lubricating oil form the cylinder walls and send it back to oil sump through the holes provided on the piston.

5)Connecting rod

•The connecting rod is a link that connects the piston and crankshaft. Its function is to convert the reciprocating motion of the piston into rotary motion of the crankshaft.

6)Crank

•The crank is a lever with one of its end connected to the connecting rod by  pin joint with other end connected rigidly to the crankshaft. The other end of the crank is connected to the crankshaft. The power required for any useful purpose is taken from the crankshaft

7)Crank case

It encloses the crankshaft and serves as a sump for the lubricating oil.

8)Valves

•The valves are control devices that allow the air/fuel to enter into the cylinder and also to discharge the burnt gases to atmosphere. There are two valves

–Inlet valve: It is the one through which fresh charge(air and fuel) enters into the cylinder.

–Exhaust valve: Through which the burnt gases are discharged out of the cylinder. These valves are actuated by means of cams.

9)Cams

•It is an element designed to control the movement of both the inlet and exhaust valves.

10)Flywheel

•It is a heavy mass of rotating wheel mounted on the crankshaft and is used as an energy storing device. The flywheel stores energy received during the power stroke and supplies the same during other strokes.

I.C. Engine Terminology

•Bore: The inside diameter of the cylinder is called bore.

•Top dead center(TDC): The extreme position of the piston near to the cylinder head is called ‘top dead center’ or ‘TDC’.

•Bottom dead center(BDC): The extreme position of the piston of the piston nearer to the crankshaft is called ‘bottom dead center’ or ‘BDC’.

•Stoke: It is the linear distance travelled by the piston from the TDC to BDC or BDC to TDC.

•Clearance volume(Vc): It is the volume above the top of the piston, when the piston is at the TDC.

Swept volume or stoke volume(Vs): It is the volume swept by the piston as the it moves from BDC to TDC or TDC to BDC.

•Compression ratio(Rc): The ratio of the total cylinder volume to the clearance volume is called compression ratio.

  Total cylinder volume = stroke volume(Vs) + Clearance volume(Vc)

Rc="Vs+Vc" /"Vc"

•Piston speed: The average speed of the piston is called ‘piston speed’.

–Piston speed = 2. L.N                      where L = Stroke length in m

                                                                 N =Speed of the engine in RPM

Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 2: Hydraulic Turbines

Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 2: Hydraulic pumps

Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 2: Boilers part 2

Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 2: Boilers

Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 1 introduction of steam part 2

Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 1, Introduction to steam.

DIFFERENT STATES OF STEAM

•Wet steam

•Dry saturated steam

•Superheated steam

WET STEAM

•A wet steam is defined as a two-phase mixture of water molecules and steam in thermal equilibrium at the saturation temperature corresponding to a given pressure.•Both water molecules and steam will have same saturation temperature.•The dryness fraction of wet steam is less than one.

DRY STEAM

•A dry steam is the steam at the saturation temperature corresponding to a given pressure and having no water particles.

•The dryness fraction of dry steam will be unity.

SUPERHEATED STEAM

•A superheated steam is defined as the steam which is heated beyond its dry saturated state to temperature higher then its saturation temperature at the given pressure.

•The dryness fraction of superheated steam will be more then unity.

Temperature Enthalpy Diagram

DRYNESS FRACTION OF STEAM

•The ration of mass dry particles presents in a known quantity of wet steam is defined as dryness fraction. It is denoted by x, X=mg/(mg+mf )

•It is expressed as fraction or percentage. It indicates the quality of steam i.e., If dryness fraction (x)=1, the steam is stated as dry steam and if x lies between 0 and 1, the steam is stated as wet steam.

Saturation Temperature 

•The boiling temperature is known as the temperature of  formation of steam (or) saturation temperature(ts 0C)

Sensible Heat

•The amount of heat required to change temperature of one kg of water from 00C to saturation temperature is defined as sensible heat and is denoted by hf.

Latent Heat

•The amount of heat required to convert one kg of water at saturation of temperature into dry saturated steam at the same temperature, is known as latent heat of evaporation and is denoted by hfg.

Total heat of evaporation

•The amount of heat required to convert one kg of water form 00 C to dry saturated steam, is known as total heat of evaporation.

•It is the sum of the sensible heat(hf) and latent heat(hfg).

•It is denoted by hg and hg = hf+hfg

Amount of super heat

•It is the amount of heat required to convert dry steam at (ts) into superheated steam at Tsup.

Superheated Temperature

•If dry steam of temperature ts is heated further its temperature will rise and which is known as superheated temperature and is denoted Tsup.

Degree of super heat

•The amount of rise in temperature when dry steam is converted into super heated steam is termed as degree of super heat i.e.

•degree of super heat= Tsup- ts 0(C)

Enthalpy of Steam

•It is defined as sum of the internal energy and the product of the pressure and volume. It is heat content of the steam and is denoted by h.

        i.e. h = u +PV                                                               where,                

•h is enthalpy of steam

•u is internal energy

•P is pressure

•V is volume

Enthalpy of dry steam

•The amount of heat required to convert one kg of water from 00C to dry steam (at saturation temperature).

• hg = hf + hfg kJ/kg

Enthalpy of wet steam

•Wet steam contains water particles in it. If x is the quality/dryness fraction of wet steam, the enthalpy of wet steam is calculated by,

  hwet = hf +x hfg kJ/kg

Enthalpy of superheated steam

•The amount of heat required to convert 1kf of water (at00C) into super heated steam (at Tsup0C) is defined as enthalpy of super heated steam is denoted by hsup.

hsup = hf  + hfg + Csup(Tsup -ts)        (as hf + hgf= hg)

        = hg + Cps (Tsup -ts)                       

where,

Cps = 2.25 kJ/kg K. is specific heat of super Heated steam

Specific volume

•The volume(m3) occupied by unit mass (1 kg) of a substance is know as specific  volume (or)

•The reciprocal of density is known as specific volume.

•i.e. volume/unit mass is specific  volume.

•It is usually expressed in m3/kg.

Specific volume of saturated water

•The volume occupied by one kg of water saturation temperature at a given pressure is defined as specific volume of saturated water. It is denoted by Vf.

•Note: This value is available from steam tables, corresponding to the given pressure/temperature.

Specific volume of wet steam

•The volume occupied by one kg of wet steam at a given pressure is known as specific volume of wet steam.

If ‘x ’ is the dryness fraction of wet steam

Then x Vg is the specific volume of wet steam

 

Where, Vg is specific volume of dry steam

  (Obtained from steam tables)

Specific volume of superheated steam

•The volume occupied by one kg of superheated steam at a given pressure and super heated temperature is known as specific volume of superheated steam.

  It is denoted by Vsup.

  Vsup=Vg  Tsup/ts       Where, Tsup – Superheated temperature in K

 ts – Saturation temperature in K

Note: Tsup, ts must be in kelvin only.

   K = 0C + 273

External work of evaporation

•Whenever the volume increases from Vf to Vg, the piston inside the cylinder moves upwards. The work utilized to move this piston due to increase in volume is defined as external work of evaporation (work done).

• For dry steam It is  = 100 PVg kJ/kg
•For Wet Steam it is = 100 PxVg kJ/kg and
•For superheated steam it is  = 100 PVsup kJ/kg        
                                       
where, P is constant pressure in bar , x is dryness fraction

Internal latent heat

•The energy spent/ required is internal latent heat. Actually this much amount of heat is required to change the phase.

  It is given by,= (hfg – 100 PVg) kJ/kg               

Where, hfg= latent heat(total)

  100PVg= external work of dry steam

Internal energy of steam

•Actual energy stored in the steam is called internal energy. It is the difference between the enthalpy of steam and the external work of evaporation and is denoted by U.

For dry steam,            Ug = hg  - 100 PVg kJ/kg

For wet steam,            Uf = hf + x hfg -100 P x Vg kJ/kg

For superheated steam  Usup = hsup – 100 PVsup kJ/kg