B.E -First YearSUBJECT CODE:-2110006

ELEMENTS OF MECHANICAL ENGINEERING

Puneet MathurAssistant Professor

M.Tech (Machine Design)MEFGI, Rajkot

Contact info:puneet_mathur87@rediff.compuneet.mathur@marwadieducation.edu.in

Reference Books1. Basic Mechanical Engineering by Pravin Kumar,

Pearson 2. Thermal Science and Engineering by Dr. D.S.

Kumar, S.K. Kataria & sons, Publication New Delhi 3. Fundamental of Mechanical Engineering by G.S.

Sawhney, PHI Publication New Delhi 4. Elements of Mechanical Engineering by Sadhu

Singh, S. Chand Publication 5. Introduction to Engineering Materials by B.K.

Agrawal, Tata McgraHill Publication, New Delhi 6. Applied Thermodynamics by Omkar Singh, New

Age Publication7. Thermal Engineering by R.S. Khurmi, S. Chand

Publication

UNIT- I (Internal Combustion Engine)

Internal Combustion Engines

Introduction and Classification,

Engine details,

four-stroke/ two-stroke cycle

Petrol/Diesel engines,

Indicated power, Brake Power, Efficiencies Course Outcome: After learning the course the students should be able to To understand the fundamentals of mechanical

systems To understand and appreciate significance of

mechanical engineering in different fields of engineering

Introduction“Engine refers to a device which transforms one form of

energy into the other form”. “Heat engine is a modified form of engine used for

transforming chemical energy of fuel into thermal energy and subsequently for producing work”. Based on the mechanism used for adding thermal energy they can be classified into the following:

(a) External combustion engine(b) Internal combustion engine.External combustion engine have combustion occurring

outside engine and adding heat to the working fluid used in the engine.

Thus, in external combustion engines heat released during combustion is indirectly utilized by the working fluid in external combustion engine.

Internal combustion engines have combustion occurring in engine itself and heat released during combustion is directly utilized for getting shaft work

Internal Combustion Engines (IC-engines) produce mechanical power from the chemical energy contained in the fuel, as a result of the combustion process occurring inside the engine.

IC engine converts chemical energy of the fuel into mechanical energy, usually made available on a rotating output shaft.

Chemical energy of the fuel is first converted to thermal energy by means of combustion or oxidation with air inside the engine, raising the T and p of the gases within the combustion chamber.

The high-pressure gas then expands and by mechanical mechanisms rotates the crankshaft, which is the output of the engine.

Crankshaft is connected to a transmission/power-train to transmit the rotating mechanical energy to drive a vehicle.

Advantages of I.C. Engine over E.C. Engine

Internal combustion engines have numerous advantages over external combustion engines

such as lower weight to power output ratio, simplicity, smaller initial cost,higher efficiency etc.

ApplicationsInternal combustion engines are exhaustively

used in automobiles, gas turbine etc. AndExternal combustion engines are used in steam turbine, steam engine, nuclear power plant etc.

CLASSIFICATION OF IC ENGINESBased on number of strokes

Based on thermodynamic cycle

Based on mechanism of ignition

Based on type of fuel used

Based on fuel admission

Based on type of cooling

Based on type of motion

CLASSIFICATION OF IC ENGINES

Internal combustion engines can be classified on the following basis.(a) BASED ON NUMBER OF STROKES : Number of strokes involved in

a cycle of IC engine can be two strokes or four strokes. Such engine can be;

(i) Two stroke engines(ii) Four stroke engines

(b) BASED ON THERMODYNAMIC CYCLE : Depending upon thermodynamic cycle used in the internal combustion engines these can be classified as:

(i) Engines based on Otto cycle (‘Spark-Ignition engine’)(ii) Engines based on Diesel or Dual cycle (‘Compression-Ignition engine’)

(c) BASED ON MECHANISM OF IGNITION: Internal combustion engines have combustion as the basic process.

(i) Spark ignition engines (S.I. Engines)(ii) Compression ignition engines. (C.I. Engines)The spark ignition engines may have “magneto ignition system” or

“battery ignition system” for creating necessary electric potential for producing spark.

(d) BASED ON TYPE OF FUEL USED: IC engines may be classified depending upon the type of fuel being used. These can be:

(i) Petrol engines (petrol being used as fuel)(ii) Gas engines (gaseous fuel being used)(iii) Diesel engines (diesel being used as fuel)(iv) Multi-fuel engines (more than one fuel being used)

(e) BASED ON FUEL ADMISSION: IC engines can be of different types depending upon arrangement used for fuel admission:

(i) Carburettor type engines (use carburettor fuel metering)(ii) Injection type engines (use fuel injector and injection system)

(f) BASED ON TYPE OF COOLING: IC engines have inherent requirement of continuous cooling of engine. Based on type of cooling these can be classified as: (i) Air cooled engines (Generally used in small sized engines)

(ii) Water cooled engines (Generally used in large sized engines)

(g) BASED ON TYPE OF MOTION: IC engines may have reciprocating motion of piston or it may also have rotary motion. Such engines can be:

(i) Reciprocating engines(ii) Rotary engines

Reciprocating engines may have different cylinder arrangements such as:

(i) Opposed cylinder engines(ii) Inclined cylinder engines(iii) V-shaped cylinder arrangement.

Rotary engines may be further classified as single rotor engines or multi rotor engines i.e.

(i) Single rotor engine(ii) Multi rotor engine

Single cylinder, 4-stroke engine 4- cylinder, 4-stroke engine

IN-Line Engiine

Opposed Piston Engine

V-Engine

Rotary Engine

Engine Details1. Cylinder: It is a cylindrical block having cylindrical space inside

for piston to make reciprocating motion. Upper portion of cylinder which covers it from the top is called cylinder head. This is manufactured by casting process and materials used are cast iron or alloy steel.

2. Piston and Piston rings: Piston is a cylindrical part which reciprocates inside the cylinder and is used for doing work and getting work. Piston has piston rings tightly fitted in groove around piston and provide a tight seal so as to prevent leakage across piston and cylinder wall during piston’s reciprocating motion. Pistons are manufactured by casting or forging process. Pistons are made of cast iron, aluminum alloy. Piston rings are made of silicon, cast iron, steel alloy by casting process.

3. Combustion space: It is the space available between the cylinder head and top of piston when piston is at farthest position from crankshaft (TDC).

4. Intake manifold: It is the passage/duct connecting intake system to the inlet valve upon cylinder. Through intake manifold the air/air-fuel mixture goes into cylinder.

5. Exhaust manifold: It is the passage/duct connecting exhaust system to the exhaust valve upon cylinder. Through exhaust manifold burnt gases go out of cylinder.

6. Valves: Engine has both intake and exhaust type of valves which are operated by valve operating mechanism comprising of cam, camshaft, follower, valve rod, rocker arm, valve spring etc. Valves are generally of spring loaded type and made out of special alloy steels by forging process.

7. Spark plug: It is the external ignitor used for initiating combustion process. Spark plug is activated by electrical energy fed by electrical system with engine. It delivers spark with suitable energy to initiate combustion at appropriate time for suitable duration.

8. Bearing: Bearings are required to support crank shaft. Bearings are made of white metal leaded bronze.

9. Connecting rod: It is the member connecting piston and crankshaft. It has generally I section and is made of steel by forging process.

10. Crank: It is the rigid member connecting the crankshaft and connecting rod. Crank is mounted on crankshaft. Crank transfers motion from connecting rod to crankshaft as it is linked to connecting rod through crank pin.

11. Crankshaft: It is the shaft at which useful positive work is available from the piston-cylinder arrangement. Reciprocating motion of piston gets converted into rotary motion of crankshaft. Crankshaft are manufactured by forging process from alloy steel.

12. Crankcase: Crankcase actually acts like a sump housing crank, crankshaft, connecting rod and is attached to cylinder. These are made of aluminium alloy, steel, cast iron etc. by casting process.

13. Gudgeon pin: It is the pin joining small end of the connecting rod and piston. This is made of steel by forging process.

14. Cams and Camshafts: Cams are mounted upon camshaft for opening and closing the valves at right timings and for correct duration. Camshaft gets motion from crankshaft through timing gears.

15. Carburettor: Carburettor is device to prepare the air fuel mixture in right proportion and supply at right time.

I.C. Engine Terminology

1. Bore: It is nominal inner diameter of the cylinder.

2. Piston area: It is the area of a circle of diameter equal to bore.

3. Stroke: It is the nominal distance travelled by the piston between two extreme positions in the cylinder. It is denoted by L.

4. Dead centre: It refers to the extreme end positions inside the cylinder at which piston reverses it’s motion. Thus, there are two dead centres in cylinder, called as ‘top dead centre’ or ‘inner dead centre’ and ‘bottom dead centre’ or ‘outer dead centre’. Top dead centre (TDC) is the farthest position of piston from crankshaft. It is also called inner dead centre (IDC). Bottom dead centre (BDC) refers to the closed position of piston from crankshaft. It is also called outer dead center (ODC).

5. Swept volume : It is the volume swept by piston while travelling from one dead centre to the other. It may also be called stroke volume or displacement volume.Mathematically, Swept volume = Piston area × Stroke

6. Clearance volume: It is the volume space above the piston inside cylinder, when piston is at top dead centre. It is provided for cushioning considerations and depends, largely upon compression ratio.

7. Compression ratio: It is the ratio of the total cylinder volume when piston is at BDC to the clearance volume.

Compression ratio (r)= Total cylinder Volume/Clearance volume

For Petrol engine r varies from 6 to 10 For diesel engine r varies from 14 to 20

8. Piston Speed: It is the average speed of the pistonVp=2LN/60 m/s

4- Stroke Petrol Engine- Working

4- Stroke Petrol Cycle or Otto Cycle

4- Stroke Diesel Engine- Working

4- Stroke Diesel Cycle or Diesel Cycle

Two Stroke Petrol EngineIt is a modified form of 4-stroke petrol

engine.All the four processes required for

completion of one cycle of SI engine get completed in two strokes.

Thus, obviously in each stroke two processes get completed.

Here all four processes occur during two strokes and one revolution of crank shaft.

Thermodynamic cycle followed by 2-stroke SI engine is Otto cycle.

Scooter engines are generally two stroke engines.

2-stroke SI engines are used for smaller applications.

Performance EvaluationInternal combustion engines have combustion taking place

inside and power is available at crankshaft.The shaft work available is less than the total energy

released inside the cylinder due to frictional and other losses.

For performance evaluation of internal combustion engine one is interested in following different powers.

(a) INDICATED POWER (I.P): It refers to the power available inside the cylinder

i.e. the power provided to piston. It is the power actually developed by the engine

cylinder. It is measured from the indicator diagram which is

obtained using indicator mechanism.

Mathematically,

A = cross-sectional area of cylinder. A = π /4D2, where D is bore.

L = length of stroke.

N = Speed of the engine in rpm

N=N (For 2-stroke cycle)

N=N/2 (For 4-stroke Cycle)

k = no. of cylinders.

Pm = Mean effective pressure= as/l

b) Friction power: It refers to the power lost due to friction and

other reasons. It is quantified by the difference between

indicated power and brake power.

Friction power = Indicated power – Brake power

(C) BRAKE POWER: It refers to the power available at crankshaft i.e. it is the useful shaft work.

It is usually measured by means of brake mechanism (Prony brake or rope brake)

Brake power is usually measured by absorption or transmission type dynamometers. It can be given as:

Let, W= Net load acting on the brake drum, N R=Radius of the brake drum, m

N= R.P.M. of crank shaft T= resisting torque N-m

Efficiencies

(i) Indicated thermal efficiency:=

∴ ƞit=

Where, mf = mass of fuel supplied, kg/sec,

C.V. = Calorific value of fuel, J/kg

(ii) Brake thermal efficiency =

∴ ƞbt =

Also ƞbt= ƞmechx ƞit

(3) Relative efficiency:It is the ratio of indicated thermal efficiency of an engine to air standard cycle efficiency ∴ ƞrel=

(4) Air Standard efficiency: It is the efficiency of thermodynamic cycle of engine.

for petrol engine, ƞair= 1 -

for diesel engine, ƞair= 1 -

where, r = compression ratio, = Cut-off ratio, V3/V2

and = Adiabatic index, cp/cv

(5) Volumetric efficiency: It is the ratio of the volume of charge/air actually sucked at atmospheric condition to swept volume of engine. It indicates breathing capacity of the engine.

Ƞvol=

(6) Specific output : The specific output of the engine is defined as the power output per unit piston area. specific output = (7) Specific fuel consumption: Specific fuel consumption (SFC) is defined as the amount of fuel consumed by and engine for one unit of power production.

SFC = kg/kWhWhere mf= mass of fuel consumed in kg/hr,

B.P. = Power produced in kW

Difference between SI and CI engine

Comparison of 2-stroke with 4-stroke engine

Q1. A four cylinder diesel engine of 4-stroke type has stroke to bore ratio as 1.2 and the cylinder diameter is 12 cm. Estimate indicated power of the engine using the indicator diagram arrangement. Indicator card shows the diagram having area of 30 cm2 and length as half of stroke. Indicator spring constant is 20 × 103 kN/m3 and engine is running at 2000 rpm. Also find out mechanical efficiency of engine if 10% of power is lost in friction and other losses.

Q2. A four cylinder two stroke cycle petrol engine develops 23.5 KW brake power at 2500r.p.m. The mean effective pressure on each piston is 8.5 bar and the mechanical efficiency is 85%. Calculate the diameter and stroke of each cylinder, assuming the length of stroke equal to 1.5 times the diameter of cylinder.

Q3. A four stroke single cylinder petrol engine has a bore of 150 mm and stroke of 250mm. At 500 r.p.m and full load, the net load on friction brake is 435N and torque arm is 0.45m. The indicator diagram gives a net area of 580 mm2 and a length of 70mm with a spring rating of 0.85 bar/mm. Determine indicted power, brake power, Friction power and mechanical efficiency.

Q4. During trial of four stroke single cylinder engine the load on dynamometer is found 20 kg at radius of 50 cm. The speed of rotation is 3000 rpm. The bore and stroke are 20 cm and 30 respectively. Fuel is supplied at the rate of 0.15 kg/min. The calorific value of fuel may be taken as 43 MJ/kg. After some time the fuel supply is cut and the engine is rotated with motor which required 5 kW to maintain the same speed of rotation of engine. Determine the brake power, indicated power, mechanical efficiency, brake thermal efficiency, indicated thermal efficiency, brake mean effective pressure, indicated mean effective pressure.

Q5. Determine the power required to drive a double acting reciprocating pump having indicator diagram with area 40 cm2 and length 8 cm. Bore and stroke of the pump are 15 cm and 20 cm. The pump motor runs at 100 rpm. Indicator spring constant is 1.5 × 108 Pa per m.