Unit 1.pdf

Basics of Internal Combustion Engines
ENERGY CONVERSION:
At one time, the primary source of power for the work of peace or war was chiefly man's
muscles. Later, animals were trained to help and afterwards the wind and the running stream
were harnessed.
But, the great step was taken in this direction when man learned the art of energy conversion
from one form to another. The machine which does this job of energy conversion is called an
engine.
DEFINITION OF 'ENGINE‘:
An engine is a device which transforms one form of energy into an-other form.
However, while transforming energy from one form to another, the efficiency of conversion
plays an important role.
Normally, most of the engines convert thermal energy into mechanical work and therefore
they are called 'heat engines'.

DEFINITION OF 'HEAT ENGINE‘:
Heat engine is a device which transforms the chemical energy of a fuel into thermal energy and
utilizes this thermal energy to perform useful work. Thus, thermal energy is converted to
mechanical energy in a heat engine.
Heat engines can be broadly classified into two categories: (i) Internal Combustion Engines (IC
Engines) (ii) External Combustion Engines (EC Engines)
CLASSIFICATION:
Internal Combustion or External Combustion are of two types, viz.
(i) Rotary engines (ii) Reciprocating engines
The most widely used engines are the reciprocating internal combustion engine, the gas
turbine and the steam turbine. The steam engine is rarely used nowadays.

EXTERNAL COMBUSTION AND INTERNAL COMBUSTION ENGINES:
External combustion engines are those in which combustion takes place outside the engine.
For example, in a steam engine or a steam turbine, the heat generated due to the combustion
of fuel is employed to generate high pressure steam which is used as the working fluid in a
reciprocating engine or a turbine.
In internal combustion engines combustion takes place within the engine. For example, in case
of gasoline or diesel engines, the products of combustion generated by the combustion of fuel
and air within the cylinder form the working fluid.

BASIC ENGINE COMPONENTS AND NOMENCLATURE:
Even though reciprocating internal combustion engines look quite simple, they are highly
complex machines. There are hundreds of components which have to perform their functions
satisfactorily to produce output power. There are two types of engines, viz., spark-ignition (SI)
and compression-ignition (CI) engine.
Let us now go through the important engine components and the nomenclature associated
with an engine.
Engine Components:
A cross section of a single cylinder spark-ignition engine with over-head valves is shown in Fig.
The major components of the engine and their functions are briefly described below.

Cylinder Block:
The cylinder of a multi-cylinder engine are cast as
single unit, called cylinder block. The cylinder
head is mounted on the cylinder block. The
cylinder head and cylinder block are provided
with water jackets in the case of water cooling or
with cooling fins in the case of air cooling.
Cylinder head gasket is incorporated between the
cylinder block and cylinder head. The bottom
portion of the cylinder block is called crankcase. A
cover called crankcase which becomes a sump for
lubricating oil is fastened to the bottom of the
crankcase.

Cylinder: It is a cylindrical vessel or space in which
the piston makes a reciprocating motion. The
varying volume created in the cylinder during the
operation of the engine is filled with the working
fluid and subjected to different thermodynamic
processes.
Piston: It is a cylindrical component fitted into the
cylinder forming the moving boundary of the
combustion system. It fits perfectly (snugly) 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.

Combustion Chamber: The space enclosed in the upper
part of the cylinder, by the cylinder head and the piston
top is called the combustion chamber. The combustion
of fuel and the consequent release of thermal energy
results in the building up of pressure in this part of the
cylinder.
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 is called the inlet
manifold.
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 is called the exhaust manifold.

Inlet and Exhaust Valves: Valves are commonly
mushroom shaped poppet type. They are
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) from the cylinder.
Spark Plug: It is a component to initiate the
combustion process in Spark-Ignition (SI) engines
and is usually located on the cylinder head.

Connecting Rod: It interconnects the piston and
the crankshaft and transmits the gas forces from
the piston to the crankshaft. The two ends of the
connecting rod are called as small end and the big
end. Small end is connected to the piston by
gudgeon pin and the big end is connected to the
crankshaft by crankpin.
Crankshaft: It converts the reciprocating motion of
the piston into useful rotary motion of the output
shaft. The balance weights are provided for static
and dynamic balancing of the rotating system. The
crankshaft is enclosed in a crankcase.

Piston Rings: Piston rings, fitted into the slots
around the piston, provide a tight seal between
the piston and the cylinder wall thus preventing
leakage of combustion gases.
Gudgeon Pin: It forms the link between the small
end of the connecting rod and the piston.
Cams: These are made as integral parts of the
camshaft and are designed in such a way to open
the valves at the correct timing and to keep them
open for the necessary duration.

Camshaft: The camshaft and its associated parts
control the opening and closing of the two valves.
The associated parts are push rods, rocker arms,
valve springs and tappets. This shaft also provides
the drive to the ignition system. The camshaft is
driven by the crankshaft through timing gears.

Cylinder Bore (d): The nominal inner diameter
of the working cylinder is called the cylinder
bore and is designated by the letter d and is
usually expressed in millimeter (mm).
Piston Area (A): The area of a circle of
diameter equal to the cylinder bore is called
the piston area and is designated by the letter
A and is usually expressed in square
centimeter (cm2).
Stroke (L): The nominal distance through
which a working piston moves between two
successive reversals of its direction of motion
is called the stroke. It is shown as ‘L’.

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 is called the dead
center. There are two dead centers in the
engine as indicated in Fig.
(i) Top Dead Centre (TDC): It is the dead center
when the piston is farthest from the crankshaft.
It is designated as TDC for vertical engines and
as Inner Dead Centre (IDC) for horizontal
engines.

(ii) Bottom Dead Centre (BDC): It is the dead
center when the piston is nearest to the
crankshaft. It is designated as BDC for vertical
engines and as Outer Dead Centre (ODC) for
horizontal engines.
Displacement or Swept Volume (Vs): The
nominal volume swept by the working piston
when travelling from one dead center to the
other is called the displacement volume. It is
expressed in terms of cubic centimeter (cc) and
given by

Cubic Capacity or Engine Capacity: The
displacement volume of a cylinder multiplied
by number of cylinders in an engine will give
the cubic capacity or the engine capacity. For
example, if there are K cylinders in an engine,
then
Clearance Volume (Vc): The nominal volume
the combustion chamber above the piston
when it is at the top dead center is the
clearance volume. It is designated as Vc and
expressed in cubic centimeter (cc).

Compression Ratio (r): It is the ratio of the
total cylinder volume when the piston is at the
bottom dead center, VT, to the clearance
volume, Vc. It is designated by letter r.

THE WORKING PRINCIPLE OF ENGINES:
If an engine is to work successfully then it has to follow a cycle of operations in a sequential
manner. The sequence is-quite rigid and cannot be changed. Even though both engines have
much in common, there are certain fundamental differences.
The credit of inventing the spark-ignition engine goes to Nicolaus A. Otto (1876) whereas
compression-ignition engine was invented by Rudolf Diesel (1892). Therefore, they are often
referred to as Otto engine and Diesel engine.

In a four-stroke engine, the cycle of operations is completed in four strokes of the piston or two
revolutions of the crankshaft.
During the four strokes, there are five events to be completed, viz., suction, compression,
combustion, expansion and exhaust.
Each stroke consists of 180° of crankshaft rotation and hence a four-stroke cycle is completed
through 720° of crank rotation.
The cycle of operation for an ideal four-stroke SI engine consists of the following four strokes :
(i) suction or intake stroke; (ii) compression stroke; (iii) expansion or power stroke and (iv)
exhaust stroke.
The details of various processes of a four-stroke spark-ignition engine with overhead valves are
shown in Fig. When the engine completes all the five events under ideal cycle mode, the p-V
diagram will be as shown in Fig.

Suction or Intake Stroke: Suction stroke (0-4) starts when the
piston is at the top dead center and about to move downwards. The
inlet valve is open at this time and the exhaust valve is closed. Due to
the suction created by the motion of the piston towards the bottom
dead center, the charge consisting of fuel-air mixture is drawn into the
cylinder. When the piston reaches the bottom dead center the suction
stroke ends and the inlet valve closes.

Compression Stroke: The charge taken into the cylinder during the
suction stroke is compressed by the return stroke of the piston. During
this stroke both inlet and exhaust valves are in closed position. The
mixture which fills the entire cylinder volume is now compressed into
the clearance volume. At the end of the compression stroke the mixture
is ignited with the help of a spark plug located on the cylinder head.
Ideally burning should takes place instantaneously when the piston is at
the top dead center and hence the burning process can be
approximated as heat addition at constant volume. During the burning
process the chemical energy of the fuel is converted into heat energy
and the pressure at the end of the combustion process is considerably
increased due to the heat release from the fuel. (process 2-►3).

Expansion or Power Stroke: The high pressure of the burnt gases
forces the piston towards the BDC, (stroke 3-4) Fig.1.5. Both the valves
are in closed position. Of the four-strokes only during this stroke power
is produced. Both pressure and temperature decrease during expansion.

Exhaust Stroke: At the end of the expansion stroke the exhaust valve
opens and the inlet valve remains closed. The pressure falls to
atmospheric level a part of the burnt gases escape. The piston starts
moving from the bottom dead center to top dead center (stroke 50)
and sweeps the burnt gases out from the cylinder almost at
atmospheric pressure. The exhaust valve closes when the piston
reaches TDC. at the end of the exhaust stroke and some residual gases
trapped in the clearance volume remain in the cylinder. These residual
gases mix with the fresh charge coming in during the following cycle,
forming its working fluid.

Each cylinder of a four-stroke engine completes the above four operations in two engine
revolutions, one revolution of the crankshaft occurs during the suction and compression
strokes and the second revolution during the power and exhaust strokes.
Thus for one complete cycle there is only one power stroke while the crankshaft turns by two
revolutions. For getting higher output from the engine, the heat release (process 2 —> 3)
should be as high as possible and the heat rejection (process 3-►4) should he as small as
possible. So one should be careful in drawing the ideal p-V diagram.

Four-Stroke Compression-Ignition Engine:
The four-stroke CI engine is similar to the four-stroke SI engine but it operates at a much higher
compression ratio.
The compression ratio of an SI engine is between 6 and 10 while for a CI engine it is from 16 to
20. In the CI engine during suction stroke, air, instead of a fuel-air mixture, is inducted. 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.
In CI engines, a high pressure fuel pump and an injector are provided to inject the fuel into the
combustion chamber.
The carburetor and ignition system necessary in the SI engine are not required in the CI engine.
The ideal sequence of operations for the four-stroke CI engine is as follows:

Suction stroke:
Air alone is induced during the suction
stroke. During this stroke, inlet valve is open
and exhaust valve is closed.
Compression stroke:
Air induced during the suction stroke is
compressed into clearance volume. Both
valves remain closed during this stroke.

Expansion Stroke:
Fuel injection starts nearly at the end of the compression
stroke. The rate of injection is such that combustion
maintains the pressure constant in spite of the piston
movement on its expansion stroke increasing the volume.
Heat is assumed to have been added at constant pressure.
After the injection of fuel is completed (i.e. after cut-off) the
products of combustion expand. Both the valves remain
closed during the expansion stroke.

Exhaust Stroke:
The piston travelling from BDC to TDC pushes out the products of
combustion. The exhaust valve is open and the intake valve is closed
during this stroke. The ideal p-V diagram is as shown. Due to higher
pressures in the cycle of operations the CI engine has to be more sturdy
than a SI engine for the same output. This results in a CI engine being
heavier than the SI engine. However, it has a higher thermal efficiency
on account of the high compression ratio (of about 18 as against about
8 in SI engines) used.

Comparison of SI and CI engines:

Two stroke engine:
As already mentioned, if the two unproductive strokes, viz., the suction and exhaust could be
served by an alternative arrangement, especially without the movement of the piston then
there will be a power stroke for each revolution of the crankshaft.
In such an arrangement, theoretically the power output of the engine can be doubled for the
same speed compared to a four-stroke engine.
Based on this concept, DugaId Clark (1878) invented the two-stroke engine. In two-stroke
engines the cycle is completed in one revolution of the crankshaft. The main difference
between two-stroke and four-stroke engines is in the method of filling the fresh charge and
removing the burnt gases from the cylinder. In the four-stroke engine these operations are
performed by the engine piston during the suction and exhaust strokes respectively. In a two-
stroke engine, the filling process is accomplished by the charge compressed in crankcase or by
a blower. The induction of the compressed charge moves out the product of combustion
through exhaust ports. Therefore, no piston strokes are required for these two operations.

Two strokes are sufficient to complete the cycle, one for compressing the fresh charge and the
other for expansion or power stroke. Figure shows the ideal indicator diagram of such an
engine. 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 upward motion of the piston during
compression stroke. After the compression and ignition, expansion takes place in the usual
way.

During the expansion stroke the charge in the crankcase is compressed. Near the end of the
expansion stroke, the piston uncovers the exhaust ports and the cylinder pressure drops to
atmospheric pressure as the combustion products leave the cylinder. Further movement of the
piston uncovers the transfer ports, 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. This serves the
double purpose of scavenging the upper part of the cylinder of the combustion products and
preventing the fresh charge from flowing directly to the exhaust ports.
The same objective can be achieved without piston deflector by proper shaping of the transfer
port. During the upward motion of the piston from BDC, the transfer ports close first and then
the exhaust ports close when compression of the charge begins and the cycle is repeated.

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