INTRODUCTION 1.1 Energy Conversion

• Definitions:

i. Engine – a device which transforms one form of energy into another form.

ii. Heat Engine – a device which transforms the chemical energy of

fuel into thermal energy and utilizes this thermal energy to perform useful work (mechanical energy).

• Classification of heat engines - Engines whether internal Combustion or

External Combustion are of 2 types:

i. Rotary engines

ii. Reciprocating engines

• External engine – combustion takes place outside the engine. • Internal engine – combustion takes place within the engine e.g. steam

engine or turbine, and gasoline or diesel engines. • The most widely used engines are:

i. The reciprocating internal combustion engines (have been found

suitable for the use in automobiles, motor-cycles and scooters, power boats, ships, slow speed aircraft, locomotives and power units of relatively small output.)

ii. The gas turbine

iii. The steam turbine

• Advantages of reciprocating internal combustion engines compare to the

steam engine are:

i. Mechanical simplicity and improved efficiency due to the absence of heat exchangers in the passage of the working fluid (boilers and condensers in steam turbine plant).

ii. Higher thermal efficiency due to:

a) All its components are worked at an average temperature which is much below the maximum temperature of the working fluid in the cycle.

b) Moderate maximum working pressure of the fluid in the cycle

produces less weight to power ratio.

c) The possibility of developing a small power output reciprocating internal combustion engines.

• The main disadvantages of reciprocating internal combustion engines are:

i. The problem of vibration caused by the reciprocating components.

ii. Only liquid or gaseous fuels of given specification, which are relatively more expensive, can be efficiently used.

1.2 Basic Engine Components and Nomenclature

Fig. 1.2 Cross-section of a Spark Ignition Engine

The major components of the engine and their functions are:

No Components Definition/Function 1. Cylinder Cylindrical vessel or space in which the piston makes a

reciprocating motion. It is filled with the working fluid and subjected to different thermodynamic processes during the engine’s operation.

2. Piston A cylindrical component fitted perfectly into the cylinder providing a gas-tight space with the piston rings and the lubricant. It forms the first link in transmitting the gas forces to the output shaft.

3. Combustion Chamber

The space enclosed in the upper part of the cylinder, by the cylinder head and the piston top during the combustion process. The combustion of fuel and the and the consequence release of thermal energy results in the building up of pressure in this part of the cylinder.

4. Inlet Manifold The pipe which connects the intake system to the inlet valve of the engine and through which air or air-fuel mixture is drawn into the cylinder.

5. Exhaust manifold

The pipe which connects the exhaust system to the exhaust valve of the engine and through which the products of combustion escape into the atmosphere.

*6. Inlet and Exhaust Valve

Provided either on the cylinder head or on the side of the cylinder for regulating the charge coming into the cylinder (inlet valve) and for discharging the products of combustion (exhaust valve) fro the cylinder.

7. Spark Plug A component to initiate the combustion process in spark ignition (SI) engine and is usually located on the cylinder head.

8. Connecting Rod

Interconnects the piston and the crankshaft and transmit the gas forces and is usually from the piston to the crankshaft.

9. Crankshaft Converts the reciprocating motion of the piston into useful rotary motion of the output shaft. There are a pair of crank arms and balance weights which provide static and dynamic balancing for the rotating system. The crankshaft is enclosed in a crankcase [].

10. Pistons Rings Fitted into the slots around the piston to provide a tight seal between the piston and the cylinder wall thus preventing leakage of combustion gases.

11. Gudgeon Pin Forms the link between the small end of the connecting rod and the piston.

12. Camshaft Control the opening and closing of the two valves with its associate parts which are push rods, rocker arm, valve springs and tappets. Also provides the drive to the ignition system and is driven by the crankshaft through timing gears.

*13. Cams Integral parts of the camshaft and are design to open the valves at the correct timing and to keep them open for necessary duration.

Fly Wheel A mass inertia attach to the output shaft in the form of wheel to achieve a uniform torque.

• The general nomenclatures are: Cylinder bore (d): The nominal inner diameter of the working cylinder and is

usually expressed in mm.

Piston area (A): The area of a circle of diameter equal to the cylinder bore and is usually expressed in cm2.

Stroke (L): The nominal distance through which a working piston moves between two successive reversals of its direction of motion and is usually expressed in mm.

Dead centre: The position of the working piston and the moving parts which are mechanically connected to it, at the moment when the direction of the piston motion is reversed at either end of the stroke. There are two dead centres in the engine:

i) Top dead centre (TDC):

The dead centre when the piston is farthest from the crankshaft.

ii) Bottom dead centre (BDC):

The dead centre when the piston is nearest to the crankshaft.

Displacement or swept volume (Vs):

The nominal volume swept by the working piston when travelling from one dead centre to the other. It is expressed in terms of cubic centimetre (cc):

LdLAVs2

4π

=×= (1.1)

Clearance volume (VC):

The nominal volume of the combustion chamber above the piston when it is at the top dead centre is the clearance volume and it is usually expressed in cubic centimetre (cc).

Compression Ratio (r):

The ratio of the total cylinder volume when the piston is at the bottom dead centre, VT, to the clearance volume, VC:

C

s

C

sC

C

T

VV

VVV

VV

r +=+

== 1 (1.2)

Fig. 1.3 Top and Bottom Dead Centre

1.3 THE WORKING PRINCIPLE OF ENGINES 1.3.1 Four-Stroke Spark Ignition Engine

• The cycle of operations is completed in 4 strokes of the piston (2 revolutions of the crankshaft).

• 4-stroke cycle is completed through 720 °C of crank rotation as each

stroke consists of 180 °C of crankshaft rotation.

• Five events to be completed, viz., suction, compression, combustion, expansion and exhaust.

• An ideal 4 stroke SI engine consists:

i. Suction/intake stroke - ii. Compression stroke iii. Expansion/power stroke iv. Exhaust stroke

• Suction/Intake Stroke

Ø 0 => 1 Ø Piston at top dead center (TDC) moving downwards to bottom dead

center (BDC). Ø Inlet valve opens and exhaust valve is closed Ø The charge consisting of fuel-air mixture is drawn into the cylinder.

Fig. 1.4 Working principle of a Four-Stroke SI Engine

• Compression Stroke

Ø 1 => 2 Ø The return stroke of the piston compresses the mixture into the

clearance volume. Ø Both inlet and exhaust valves are closed. Ø The mixture is ignited with the help of an electric spark between the

electrodes of a spark plug at the end of the process. Ø Burning takes place almost instantaneously when piston at TDC

and the chemical energy of fuel is converted into heat energy (2 => 3).

Ø The pressure at the end of the combustion process is increased due to the heat release at constant volume.

• Expansion/power Stroke

Ø 3 => 4 Ø High pressure of the burnt gasses forces the piston downward the

BDC thus produces power. Ø Both inlet and exhaust valves remain closed. Ø Both pressure and temperature decrease.

Fig. 1.5 Ideal Indicator Diagram of a Four-stroke SI Engine

• Exhaust Stroke

Ø 5 => 0 Ø The piston moves from BDC to TDC and sweeps the burnt gases

out from the cylinder almost at atmospheric pressure. Ø The exhaust valve opens and the inlet valve remains closed at the

end of the expansion process. Ø The exhaust valve closes at the end of this stroke and some

residual gasses trapped in the clearance volume remain in the cylinder.

1.3.2 Four-Stroke Compression Ignition Engine

• Suction/Intake Stroke

Ø Air alone is inducted Ø Intake valve is open and the exhaust valve is closed.

• Compression Stroke

Ø Air inducted is compressed into the clearance volume. Ø Both valves are closed.

Fig. 1.6 Cycle operation of a Four Stroke CI Engine.

• Expansion/power Stroke

Ø Fuel injection starts at the end of the compression stroke. Ø The piston movement on its expansion stroke increases the

volume and heat is assumed to have been added at constant pressure.

Ø After the injection of fuel is completed, the products of combustion explode.

Ø Both valves remain close.

• Exhaust Stroke

Ø The piston travelling from BDC to TDC pushes out the products of combustion.

Ø The exhaust valve is open and the intake valve is closed.

1.3.3 Comparison of SI and CI Engines Description SI Engine CI Engine

Basic cycle Otto cycle or constant volume heat addition cycle

Diesel cycle or constant pressure heat addition cycle.

Fuel Gasoline, a highly volatile fuel. Self-ignition temperature is high.

Diesel oil, a non-volatile fuel. Self-ignition temperature is comparatively low.

Introduction of fuel

A gaseous mixture of fuel and air is introduced during the suction stroke. A carburator is necessary to provide the mixture.

Fuel is injected directly into the combustion chamber at high pressure at the end of the compression stroke. A fuel pump and injector are necessary.

Load control Throttle controls the quantity of mixture introduced.

The quantity of fuel is regulated in the pump. Air quantity is not controlled.

Ignition Requires an ignition system with spark plug in the combustion chamber. Primary voltage is provided by a battery or a magneto.

Self-ignition occurs due to high temperature of air because of the high compression. Ignition system and spark plug are not necessary.

Compression ratio

6 to 10. Upper limit is fixed by antiknock quality of the fuel.

16 to 20. Upper limit is limited by weight increase of the engine.

Speed Due to light weight and also due to homogeneous combustion, they are high speed engines

Due to heavy weight and also due to heterogeneous combustion, they are low speed engines

Thermal efficiency

Because of the lower CR, the maximum value of thermal efficiency that can be obtained is lower.

Because of higher CR, the maximum value of thermal efficiency that can be obtained is higher.

Weight Lighter due to lower peak pressures.

Heavier due to higher peak pressures.

1.3.4 Two-Stroke Engine

• Based on the concept where the two unproductive strokes, viz., the suction and the exhaust could be served by an alternative arrangement, especially without the movement of the piston, the two-stroke engine was invented by Dugald Clark (1878).

• The cycle is completed in 1 revolution of the crankshaft.

Fig. 1.7 Cycle operation of a two-stroke engine.

• No piston strokes are required for the two unproductive strokes, viz., the

suction and the exhaust.

i. The filling of fresh charge is accomplished by the charge compressed in crankcase or by a blower.

ii. The induction of the compressed moves out through exhaust ports.

• Therefore, two strokes are sufficient to complete the cycle, one for

compressing the fresh charge and other for expansion/power stroke.

• The air or charge is inducted into the crankcase through the spring-loaded inlet valve when the pressure in the crankcase is reduced due to the upward motion of the compression stroke.

• The ignition and expansion takes place after the compression.

• The charge in the crankcase is compressed during the expansion stroke.

• The piston uncovers the exhaust ports and the cylinder pressure drops at atmospheric pressure as the combustion products leave the cylinder near the end of the of the expansion stroke.

Fig.1.8 Indicator diagram of a two-stroke SI engine.

• Further movement of the piston uncovers the transfer port, permitting the

slightly compressed charge in the crankcase to enter the engine cylinder.

• The top of the piston has usually a projection to deflect the fresh charge towards the top of the cylinder before flowing to the exhaust ports.

1.3.5 Comparison of 4-stroke and 2-stroke engines

Table 1.2 Comparison of Four and Two-Stroke Cycle Engines

Four-Stroke Engine Two-Stroke Engine

The thermodynamic cycle is completed in four strokes of the piston or in two revolutions of the crankshaft. Thus, one power stroke is obtained in every two revolutions of the crankshaft.

The thermodynamic cycle is completed in two strokes of the piston or in one revolution of the crankshaft. Thus one power stroke is obtained in each revolution of the crankshaft.

Because of the above, turning moment is not so uniform and hence a heavier flywheel is needed.

Because of the above, turning moment is more uniform and hence a lighter flywheel can be used.

Again, because of one power stroke for two -revolutions, power produced for same size of engine is less, or for the same power the engine is heavier and bulkier.

Because of one power stroke for every revolution, power produced for same size of engine is more (theoretically twice; actually about 1.3 times), or for the same power the engine is lighter and

more compact.

Because of one power stroke in two revolutions lesser cooling and lubrication requirements. Lower rate of wear and tear.

Because of one power stroke in one revolution greater cooling and lubrication requirements. Higher rate of wear and tear.

The four-stroke engine contains valves and valve actuating mechanisms to open and close the valves.

Two-stroke engines have no valves but only ports (some two-stroke engines are fitted with conventional exhaust valve or reed valve).

Because of the heavy weight and complicated valve mechanism, the initial cost of the engine is more.

Because of lightweight and simplicity due to the absence of valve mechanism, initial cost of the engine is less.

Volumetric efficiency is more due to more time for induction.

Volumetric efficiency is low due to lesser time for induction.

Thermal efficiency is higher; part load efficiency is better than two-stroke cycle engine.

Thermal efficiency is lower; part load efficiency is poor compared to a four-stroke cycle engine.

Used where efficiency is important, viz., in cars, buses, trucks, tractors, industrial engines, airplanes, power generation etc.

Used where low cost, compactness and light weight are important, viz., in mopeds, scooters, motorcycles, hand sprayers etc.

1.4 ACTUAL ENGINES

• Actual engines differ from the ideal engines because of various constraints in their operation.

Ideal Indicator Diagram Actual Indicator

Fig. 1.9 (a) Four-stroke Engine

• The indicator diagram also differs considerably from the ideal indicator diagrams as given in Fig. 1.9 (a) and 1.9 (b) respectively.

Ideal Indicator Diagram Actual Indicator Fig. 1.9 (b) Two-stroke Engine

1.5.1 Number of strokes

• Classification on the number of piston strokes for completing a cycle of operation: eg. 2-stroke or 4 stroke

1.5.2 Cycle of operation

i) Constant volume heat addition cycle or Otto cycle engine (Spark Ignition engine or Gasoline engine)

ii) Constant-pressure heat addition cycle engine or Diesel cycle engine

(also call a Compression Ignition CI engine or diesel engine). 1.5.3 Type of Fuel Used

i) Volatile liquid fuels like gasoline, alcohol, kerosene, benzene etc. ii) Gaseous fuels like natural gas, petroleum gas, blast furnace gas, town

gas and biogas etc.

iii) Solid fuels like charcoal, powdered coal etc.

iv) Viscous fluid like heavy and light diesel oils.

v) Two fuels (Dual –fuel engines)

1.5.4 Method of Changing

i) Naturally aspirated engines: admission of air or air-fuel mixture at near atmospheric pressure.

ii) Supercharged engines: admission of air or air-fuel mixture under

pressure i.e. above atmospheric pressure. 1.5.5 Type of Ignition • Two types of ignition system are normally used to produce the required high

voltage for the initiation of spark plug:

i) Battery ignition system ii) Magneto ignition system

1.5.6 Type of cooling

i) Air-cooled engine ii) Water-cooled engine

1.5.7 Cylinder arrangement

i) Cylinder row ii) Cylinder bank

1.6.2 Two-Stroke Diesel Engine • Very high power used for ship propulsion (brake power can be up to 37 000

kW). 1.6.3 Four-Stroke Gasoline Engines • Most important application of small four-stroke engine is in automobiles

(typical output is in the range of 30-60 kW at a speed about 4500 rpm).

• Also used for busses and trucks (4000 cc) and big motorcycles with sidecars. • Small pumping sets and mobile electric generating sets. • Small aircraft (200 kW range) 1.6.4 Four-Stroke Diesel Engines • One of the most efficient and versatile prime movers (manufactured in sizes

form 50-1 000 mm of cylinder diameter and engine speeds ranging from 100-4 500 rpm while delivering outputs from 1-35 000 kW).

• Development is going in the use of diesel engines in personal automobiles, as they are more efficient compare to gasoline engines. However the vibrations from the engine and the unpleasant odor in the exhaust are the main drawbacks.

• Diesel engine are used both for mobile and stationary electric generating

plants of varying capabilities:

i) Tractors for agricultural application (30 kW), jeeps, buses and trucks (40 – 100 kW).

ii) Engine with higher output like supercharged engine in the output range

of 200 to 400 kW.

iii) Locomotive applications (output 600 – 4 000 kW).

iv) Marine applications, from fishing vessels to ocean going ships (100 – 35 000 kW).

v) Small diesel engines are used in pump sets, construction machinery,

air compressors, drilling rigs and many miscellaneous applications. 1.7 First Law of Engine Cycle • According to first law energy can neither be created nor destroyed. It can only

transform from one form to another. Therefore there must be an energy balance of input and output to a system.

1.8 Engine Performance Parameters

i) Indicated thermal efficiency, ηith The ratio energy in the indicated power, ip, to the fuel energy in appropriate units:

[ ][ ]kJ/s second per fuel inenergy

kJ/s ipith =η (1.3)

= fuel of valuecaloric fuel/s of mass ×

ip (1.4)

ii) Brake thermal efficiency, ηbth

The ratio of energy in the brake power, bp, to the input fuel energy in the appropriate units:

fuel of valuecaloric fuel/s of mass ×=

bpbthη (1.5)

iii) Mechanical efficiency, ηm

The ratio of brake power to the indicated power (power provided by the piston):

fpbp

bpipbp

m +==η (1.6)

bpipfp −= (1.7)

It can also be defined as the ratio of the brake thermal efficiency to the indicated thermal efficiency.

iv) Volumetric efficiency, ηv The ratio of the volume of air actually inducted at ambient conditions to the swept volume of the engine:

pressure and etemperatur ambient at volume sweptby drepresente charge of mass

inductedactually charge of mass=vη (1.8)

volume sweptconditions ambient at

stroke per aspirated charge of volume

= (1.9)

v) Relative efficiency or Efficiency ratio, ηrel

The ratio of thermal efficiency of an actual cycle to that of the ideal cycle and it is very useful for indicating the degree of development of the engine:

efficiency standard-Airefficiency thermal Actual

=relη (1.10)

vi) Mean effective pressure, pm

The average pressure inside the cylinders of an internal combustion engine based on the calculated or measured power output. For any given engine, operating at a given speed and power output, there will be a specific indicated mean effective pressure, imep, and a corresponding brake mean effective pressure, bmep. Therefore;

Indicated power:

100060×=

LAnKpip im (1.11)

Indicated mean effective pressure:

LAnK

ippim×

=60000 (1.12)

The brake mean effective pressure:

LAnK

bppbm×

=60000 (1.13)

where

ip = Indicated power (kW) pim = Indicated mean effective pressure (N/m2) L = Length of the stroke (m) A = Area of the piston (m2) N = Speed in revolution per minute (rpm) n = Number of power strokes

N/2 for 4-stroke and N for 2-stroke engines K = Number of cylinders

Alternately, the indicated mean effective pressure can be specified from the knowledge of engine indicator diagram (p-V diagram):

diagram indicator the of Length

diagram indicator the of Area=imp

where the length of the indicator diagram is given by the difference between the total volume and the clearance volume.

vii) Mean piston speed, ps_

An engine parameter for correlating engine behavior as a function of speed.

LNsp 2=_

where L is the stroke and N is the rotational speed of the crankshaft in rpm.

viii) Specific power output. Ps

The power output per unit piston area and is a measure of the engine designer’s success in using the available piston area regardless the cylinder size:

Specific power output, Ps..

AbpPs /= (1.14)

_

constant pbm sp ××= (1.15)

ix) Specific fuel consumption, sfc An important parameter that reflects the fuel consumption characteristic of an engine:

[ ]hour fuel/kW of kg Power

time unit per nconsumptio Fuel=sfc (1.16)

x) Fuel-air (F/A) or air-fuel ratio (A/F)

In the SI engine the fuel-air ratio practically remain a constant over a wide range of operation. The CI engines at a given speed the airflow does not vary with load; it is the fuel that varies directly with load. Therefore the term fuel-air ratio is generally used instead of air-fuel ratio. Stoichiometric fuel-air ratio: a mixture that contains just enough air for complete combustion of all fuel in the mixture. Rich mixture: a mixture that having more fuel than that in a chemically correct mixture. Lean mixture: a mixture that contains less fuel (or excess air). The ratio of actual fuel-air ratio to chemically correct fuel-air ratio is called equivalence ratio:

ratio air-fueltric Stoichiomeratio air-fuel Actual

=φ

accordingly, φ = 1 (chemically correct mixture)

φ < 1 (lean mixture) φ > 1 (rich mixture)

xi) Caloric value of the fuel

The thermal energy released per unit quantity of the fuel when the fuel is burned completely and the products of the combustion is cooled back to the initial temperature of the combustible mixture. Other terms used are the heating value and the heat of combustion.

1.9 Design and performance Data