1. An engine or a motor is a machine designed to convert
energy into useful mechanical motion. Heat engines,
including I.C. engines and E.C. engines burn a fuel to create
heat, which then creates motion. Motors convert electrical
energy into mechanical motion.
2. Engine was originally a term for any mechanical device
which converts energy into motion. Engine comes from old
French Ingenium meaning ability. In modern terms, engine
is a device which burns or consumes fuel to perform
mechanical work by exerting a torque or a linear force to
drive machinery. A heat engine also works as a Prime
Mover- a component that transforms the flow or changes in
pressure of a fluid in to mechanical energy
TYPES OF ENGINES
Engines are classified on the following basis:
Number of cylinders: single cylinder engines , multi cylinder
Arrangement of cylinders: Row giving-in-line engine, V type
Arrangement of valves: Overhead valve engine, T-head engine.
Number of strokes: Two stroke engine, Four stroke engine.
Type of cycle: Otto engine, Diesel engine.
Type of cooling: Air cooled engine, Water cooled engine.
Type of fuel used: Gas engine, Petrol engine etc.
Field of application: Marine engines, Stationary engines etc.
1. The internal combustion engine is an engine in which the combustion of a fuel (normally
a fossil fuel) occurs with an oxidizer (usually air) in a combustion chamber that is an integral
part of the working fluid flow circuit. In an internal combustion engine (ICE) the expansion of
the high-temperature and high-pressure gases produced by combustion apply direct force to
some component of the engine. The force is applied typically to pistons, turbine blades, or
a nozzle. This force moves the component over a distance, transforming chemical energy into
useful mechanical energy. The first commercially successful internal combustion engine was
created by Étienne Lenoir.
2. The term internal combustion engine usually refers to an engine in which combustion is
intermittent, such as the more familiar four-stroke and two-stroke piston engines, along with
variants, such as the six-stroke piston engine and the Wankel rotary engine. A second class of
internal combustion engines use continuous combustion: gas turbines, jet engines and
most rocket engines, each of which are internal combustion engines on the same principle as
The ICE is quite different from external combustion engines(E.C.E.), such as steam or Stirling
engines, in which the energy is delivered to a working fluid not consisting of, mixed with, or
contaminated by combustion products. Working fluids can be air, hot water, pressurized
water or even liquid sodium, heated in some kind of boiler. ICEs are usually powered by
energy-dense fuels such as gasoline or diesel, liquids derived from fossil fuels. While there are
many stationary applications, most ICEs are used in mobile applications and are the dominant
power supply for cars, aircraft, and boats.
Two Stroke Engine
Four Stroke Engine
FOUR STROKE ENGINE
A four-stroke engine (also known as four-cycle) is
an internal combustion engine in which
the piston completes four separate strokes which
comprise a single thermodynamic cycle. A stroke refers to
the full travel of the piston along the cylinder, in either
The name four stroke refers to the
intake, compression, combustion and exhaust stroke that
occurs during two crankshaft rotations per power cycle.
The cycle begins at Top Dead Centre, when the piston is
farthest away from the crankshaft. A stroke refers to the
full travel of the piston from Top Dead Centre to Bottom
TYPES OF FOUR STROKE ENGINE
There are two types of four stroke engines. They are closely
related to each other, but they have major differences in
1. First type of four stroke engine is known as petrol or
gasoline engine named after the fuel. First created by
Nikolaus A. Otto, they are called Otto engines. They
require a spark plug to ignite the combustible material
inside the chamber. So they are also called spark ignited
2. The other type of four stroke engine is the Diesel engine,
named after the fuel and its inventor Rudolf Diesel. It
employs the technique of self ignition by compressed air
and so are also called compressed ignition engines (C.I.).
Nikolaus O. Otto And
The Otto Engine
Rudolf Diesel And
The Diesel Engine
FOUR STROKE CYCLE
1. As their name implies, four-stroke internal
combustion engines have four basic steps that
repeat with every two revolutions of the
2. (1) Intake/suction stroke (2) Compression
stroke (3) Power/expansion stroke and (4)
• The first stroke of the internal combustion
engine is also known as the suction stroke
because the piston moves to the maximum
volume position (downward direction in the
cylinder) creating a vacuum (negative pressure).
The inlet valve opens as a result of the cam lobe
pressing down on the valve stem, and the
vaporized fuel mixture is sucked into the
combustion chamber. The inlet valve closes at
the end of this stroke.
• In this stroke, both valves are closed and the
piston starts its movement to the minimum
volume position (upward direction in the
cylinder) and compresses the fuel mixture.
During the compression process, pressure,
temperature and the density of the fuel mixture
• When the piston reaches a point just before top
dead center, the spark plug ignites the fuel
mixture. The point at which the fuel ignites
varies by engine; typically it is about 10 degrees
before top dead center. This expansion of gases
caused by ignition of the fuel produces the
power that is transmitted to the crank shaft
• In the end of the power stroke, the exhaust
valve opens. During this stroke, the piston starts
its movement in the maximum volume position.
The open exhaust valve allows the exhaust gases
to escape the cylinder. At the end of this
stroke, the exhaust valve closes, the inlet valve
opens, and the sequence repeats in the next
cycle. Four-stroke engines require two
An Otto cycle is an idealized thermodynamic cycle which
describes the functioning of a typical spark
ignition reciprocating piston engine, the thermodynamic
cycle most commonly found in automobile engines.
The Otto cycle is constructed out of:
1. Top and bottom of the loop: a pair of quasiparallel adiabatic processes
2. Left and right sides of the loop: a pair of parallel isochoric
3. The adiabatic processes are impermeable to heat: heat
flows into the loop through the left pressurizing process and
some of it flows back out through the right depressurizing
process, and the heat which remains does the work.
PROCESSES IN OTTO CYCLE
1. Process 1-2 is an isentropic compression of the air as the piston moves from
bottom dead centre (BDC) to top dead centre (TDC).
2. Process 2-3 is a constant-volume heat transfer to the air from an external source while
the piston is at top dead centre. This process is intended to represent the ignition of
the fuel-air mixture and the subsequent rapid burning.
3. Process 3-4 is an isentropic expansion (power stroke).
4. Process 4-1 completes the cycle by a constant-volume process in which heat is
rejected from the air while the piston is a bottom dead centre.
1. The Otto cycle consists of adiabatic compression, heat addition at constant
volume, adiabatic expansion, and rejection of heat at constant volume.
2. In the case of a four-stroke Otto cycle, technically there are two additional
processes: one for the exhaust of waste heat and combustion products
(by isobaric compression), and one for the intake of cool oxygen-rich air (by
isobaric expansion); however, these are often omitted in a simplified analysis.
first law is rewritten as:
Applying this to the Otto cycle the four process equations can be derived:
The net work can also be found by evaluating the heat added minus the heat leaving or
From analyzing efficiency equation it is evident that the Otto cycle efficiency depends
directly upon the compression ratio
Since the for air is 1.4, an increase in will produce an increase in . However,
the for combustion products of the fuel/air mixture is often taken at approximately
1.3. The foregoing discussion implies that it is more efficient to have a high
The standard ratio is approximately 10:1 for typical automobiles. Usually this does
not increase much because of the possibility of auto ignition, or "knock", which places
an upper limit on the compression ratio.
During the compression process 1-2 the temperature rises, therefore an increase in
the compression ratio causes an increase in temperature. Auto ignition occurs when
the temperature of the fuel/air mixture becomes too high before it is ignited by the
flame front. The compression stroke is intended to compress the products before the
flame ignites the mixture. If the compression ratio is increased, the mixture may autoignite before the compression stroke is complete, leading to "engine knocking". This
can damage engine components and will decrease the brake horsepower of the
Otto engines are about 30% efficient; in other words, 30% of the energy generated by
combustion is converted into useful rotational energy at the output shaft of the
engine, while the remainder being losses due to waste heat, friction and engine
The maximum amount of power generated by an engine is determined by the
maximum amount of air ingested. The amount of power generated by a piston
engine is related to its size (cylinder volume), whether it is a two-stroke or four-stroke
design, volumetric efficiency, losses, air-to-fuel ratio, the calorific value of the
fuel, oxygen content of the air and speed (RPM).
The speed is ultimately limited by material strength and lubrication. Valves, pistons
and connecting rods suffer severe acceleration forces.
At high engine speed, physical breakage and piston ring flutter can occur, resulting in
power loss or even engine destruction.
The Thermal efficiency of a Otto engine is directly related to the compression ratio .
Since the for air is 1.4, an increase in will produce an increase in .
However, the for combustion products of the fuel/air mixture is often taken at
The foregoing discussion implies that it is more efficient to have a high compression
The standard ratio is approximately 10:1 for typical automobiles.
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