Heat Engine
Heat engine is a device that converts heat energy into mechanical energy or more exactly a system
which operates continuously and only heat and work may cross its boundaries. Heat engine forms the
very crux of internal combustion engine. A firm understanding of Internal Combustion Engines cannot be
possible without understanding working principle of Heat engines and concepts of Energy.
Heat engines are classified into two broad types:
• External combustion engine
• Internal combustion engine
An external combustion engine is a heat engine where an (internal) working fluid is heated by
combustion of an external source, through the engine wall or a heat exchanger. The fluid then, by
expanding and acting on the mechanism of the engine produces motion and usable work. The fluid is
then cooled, compressed and reused (closed cycle). For example ( like water is for a steam engine,
where the heat is used to generate steam from water, which in turn is used to power the piston or a
turbine).
In an Internal Combustion Engine, the products of combustion would directly move the piston of the
engine. Gasoline, Diesel, Wankel engines and open gas turbines are all examples of an Internal
Combustion Engine.
Internal combustion engines are most commonly used for mobile propulsion in vehicles and portable
machinery. In mobile equipment, internal combustion is advantageous since it can provide high power‐
to‐weight ratios together with excellent fuel energy density.
If the combustion of the air‐fuel mixture is initiated by spark plug then engine is called spark ignition (SI)
engine. If air‐fuel mixture is initiated by itself, the engine is called the compression ignition (CI) engine.
Heat Engine
Internal Combustion Engine External Combustion Engine
SI Engine Steam Engine
CI Engine Stirling Engine
Gas Turbine Gas Turbine
Lecturer: Ram Chandra Sapkota [Note prepared by: Ram Krishna Singh]
Otto ccycle:
Otto cyclee is ideal cycle
e for SI engin
nes, named affter Nikolaus A. Otto. In m most SI enginees, piston exeecutes
four complete strokess and crankshhaft completee two revoluttions for each thermodynnamic cycle. These
T
engines are called fourr‐stroke(IC) engine.
The schem
matic diagramms of the stro
okes and P‐v diagrams forr both actual and ideal 4 sstroke enginees are
shown in fig below.
Leecturer: Ram Chandra Sap
pkota [Note prepared by: R
Ram Krishna SSingh]
Initially, both the intake and the exhaust valves are closed and the piston is at the lowest position (BDC).
Compression stroke: The piston moves upward, compressing the air‐fuel mixture. Shortly before the
piston reaches its highest point (TDC), the spark plug fires and the mixture ignites, increasing the
pressure and temperature of the system.
Expansion or power stroke: The high‐pressure gases force the piston down. The piston in turn forces the
crankshaft to rotate, producing a useful work output.
Exhaust stroke: At the end of power stroke, the piston is at its lowest position and the cylinder is filled
with combustion products. During the exhaust stroke the piston moves up, purging the exhaust products
through the exhaust valve.
Intake stroke: The piston moves down a second time, drawing in fresh air‐fuel mixture through the
intake valve.
In two stroke engines, all the four functions described are executed in just two strokes, namely power
stroke and compression stroke.
It may seem that two stroke engine will put out twice as much power as comparable four stroke cycle
engine because there are twice as many power strokes. However this is not the case. Because to get rid
of the burnt gases in the cylinder from last power stroke, it must be relied upon the force of air and fuel
mixture entering the cylinder, there is some dilution of the mixture. The mixing of the intake mixture
with the exhaust gases reduces the potential power output.
Also with inlet and exhaust port opened together, a certain amount of fuel and air mixture is lost. There
is also much shorter period in which inlet port is open. These factors reduce the amount of power from
each power stroke.
However they have the advantages of being simple and inexpensive, and have high power to weight and
power to volume ratio. These features make them suitable for applications requiring small size and
weight such as motor cycle.
Analysis of Otto cycle
The actual cycle is pretty hard to analyze. Utilizing the air‐standard assumptions, the analysis can be
simplified. Otto cycle is the ideal cycle for spark ignition engines. It consists of the following processes:
1. Isentropic compression
2. Constant volume heat addition
3. Isentropic expansion
4. Constant volume heat rejection
Lecturer: Ram Chandra Sapkota [Note prepared by: Ram Krishna Singh]
Heat transfer to the wo
orking fluid:
Heat transferred from the working fluid:
Net work done
Thermal eefficiency:
Relationsh
hips for the issentropic pro
ocesses 1‐2 an
nd 3‐4:
γ being compression ratio, γγ = =
If analysiss is done on a cold air‐stan
ndard basis, fo
ollowing exprressions may be used:
Leecturer: Ram Chandra Sap
pkota [Note prepared by: R
Ram Krishna SSingh]
1
1
1
Effect of compression
n ratio on peerformance
Otto cycle thermal efficiency
e increases as th
he compressiion ratio inccreases. But the possibility of
premature ignition of the fuel (kno own as auto ignition, which produces an audible n noise called eengine
knock), puts
p an uppe
er limit on the compresssion ratios th
hat can be used
u in sparrk‐ignition intternal
combustio on engines. EEngine knock affects perfoormance and damages thee engine. Fuels formulated d with
tetraethyl lead have goood antiknock characteristtics, i.e. they allow higher compression n ratios. But leeaded
gasoline fforms compounds during tthe combustiion process that are hazarrdous to heallth and pollutte the
environment.
king prin
Work nciple of diesel engine: modell
This cyclee is also known as constaant pressure cycle. Dieseel engine is mostly
m emplo
oyed in Stationary
Power plaants, Ships, Heavy Motor V Vehicles.
In Petrol EEngine, the air‐fuel mixturre is compresssed in the en
ngine cylinderr to a high preessure, and then it
is ignited by an electricc spark from aa spark plug.
In diesel engine, dieseel oil is used
d as fuel. Thiss fuel is ignitted by injectting it into th
he engine cylinder
containingg air compre essed to a veery high pressure. The tem mperature off this air is sufficiently
s hiigh to
ignite thee fuel. That is why therre is no sparrk plug used d in diesel engine.
e This high temperrature
compresssed air is injeccted at a con ntrolled rate iin the form o of very fine sp pray so that tthe combustiion of
fuel proceeeds at constaant pressure..
Leecturer: Ram Chandra Sap
pkota [Note prepared by: R
Ram Krishna SSingh]
Diesel Engine is mainly worked on below strokes.
01) Intake Stroke:‐
In this stroke, the piston moves down from the top dead centre. As a result, inlet valve opens and air is
drawn into the cylinder. After sufficient quantity of air with pressure is drawn, suction valve closes at the
end of the stroke. The exhaust valve remains closed during this stroke.
02) Compression Stroke:‐
In this stroke, piston moves up from the bottom dead centre. During this stroke both inlet and exhaust
valve are closed. The air drawn into the cylinder during intake stroke is entrapped inside the cylinder
and compressed due to upward movement of the piston. In diesel engine, the compression ratio used is
very high. The air is compressed to a very high pressure up‐to 40 kilogram per centimeter square. At this
pressure, the temperature of the air is reached to 1000 degree centigrade which is enough to ignite the
fuel.
03) Power Stroke:‐
In this stroke, the fuel is injected into the hot compressed air where it starts burning, maintaining the
pressure constant. When the piston moves to its top dead center, the supply of fuel is cut‐off. It is to be
said that the fuel is injected at the end of compression stroke and injection continues till the point of
cut‐off, but in actual practice, the ignition starts before the end of compression stroke to take care of
ignition tag.
04) Working or Power Stroke:‐
In this stroke, both inlet and exhaust valve remain closed. The hot gases (which are produced due to
ignition of fuel during compression stroke) and compressed air now expand adiabatically, in the cylinder
pushing the piston down and hence work is done. At the end of stroke, the piston finally reaches the
bottom dead center.
Lecturer: Ram Chandra Sapkota [Note prepared by: Ram Krishna Singh]
ust Stroke:‐
05) Exhau
In this strroke, the pistton again mo oves upward. The exhaustt valve opens, while inlet and fuel valvve are
closed. A greater part o of the burnt ffuel gases esccape due to their own expansion. The uupward moveement
of the pisston pushes the remainingg gases out th hrough the oopen exhaust valve. Only aa small quanttity of
exhaust ggases stay in the combustion chamber. At the end of exhaust sttroke, the exxhaust valve ccloses
and the cyycle is thus coompleted.
As there is some resisstance while operating in inlet and exhaust valve and
a the somee portion of burnt
gases remmains inside tthe cylinder d during the cycle, resultingg the pumpingg losses. These pumping llosses
are treateed as negative e work and th herefore subttracted from actual work done during the cycle. This will
give us neet work done from the cycle.
Air staandard d
diesel cyccle
The dieseel cycle is the
e ideal cycle ffor compresssion ignition eengines, nammed after it’s proposer Rudolph
Diesel. Th
he spark plug is replaced byy fuel injector in diesel engines. In diesel engine it iss assumed thaat the
heat addittion occurs during a consttant pressure process that starts with th he piston at ttop dead centter.
The air staandard diesel cycle consistts of the follo
owing internally reversible processes in series:
1. Issentropic com
mpression
2. Constant presssure heat adddition
3. Issentropic expansion
4. Constant volum me heat rejecction
In diesel eengine,
Leecturer: Ram Chandra Sap
pkota [Note prepared by: R
Ram Krishna SSingh]
1 1
1
1
1
Comparing η of Otto cycle and η of diesel cycle when both cycle operate at the same compression ratio,
ranges from 35 – 40 percentages.
Lecturer: Ram Chandra Sapkota [Note prepared by: Ram Krishna Singh]
Two stroke engine
Two‐stroke engine is an internal combustion engine which completes the thermodynamic cycle in two
movements of the piston. This increased efficiency is accomplished by using the beginning of the
compression stroke and the end of the combustion stroke to perform simultaneously the intake and
exhaust functions. In this way two‐stroke engines often provide strikingly high specific power. Petrol
(spark ignition) versions are particularly useful in lightweight (portable) applications such as chainsaws
and the concept is also used in diesel compression ignition engines in large and non‐weight sensitive
applications such as ships and locomotives.
Instead of placing the intake and exhaust ports in the combustion chamber, they are placed in the
cylinder wall. In this engine piston goes through a power stroke every time it moves from top dead
center to bottom dead center. The downward stroke is also an intake and exhaust stroke. As the piston
moves from bottom dead center to the top dead center, it is going through compression stroke.
Working of two stroke engine
Lecturer: Ram Chandra Sapkota [Note prepared by: Ram Krishna Singh]
It is easier to understand if you look at what is happening in the crankcase, then consider what is
occurring above the piston in the cylinder.
Two things are always happening at the same time in the two stroke cycle.
Upward stroke:
During upward stroke, there is compression on the top of the piston and induction of the mixture below
the piston.
In crankcase
• As the piston rises the volume increases and pressure drops below atmospheric pressure.
• The inlet port is uncovered by the skirt of the piston.
• Atmospheric pressure forces air through carburetor where it is mixed with fuel and enters the
crankshaft.
In the combustion chamber
• as the piston rises all ports are closed off.
• Mixture previously transferred from the crankcase into the cylinder is compressed.
Downward stroke:
During downward stroke there is pressure due to combustion above the piston and fuel/air mixture is
being compressed below the piston in the crankcase.
In crankcase
• As the piston rises the volume increases and pressure drops below atmospheric pressure.
• The inlet port is uncovered by the skirt of the piston.
• Atmospheric pressure forces air through carburetor where it is mixed with fuel and enters the
crankshaft.
In the combustion chamber
• as the piston rises all ports are closed off.
• Mixture previously transferred from the crankcase into the cylinder is compressed.
Advantages and usage:
Two stroke engine is most extensively used in very small equipment. It is lightweight and able to run at
very high speed due to the absence of mechanical valve train.
Lecturer: Ram Chandra Sapkota [Note prepared by: Ram Krishna Singh]
Comparison of 4‐stroke and 2‐stroke cycle engines
S.N Aspects Four stroke cycle engine Two stroke cycle engine
1 Completion of The cycle is completed in 4 –strokes of The cycle is completed in 2 strokes of
cycle the piston or in 2 revolution of the the piston or in one revolution of the
crankshaft. Thus one power stroke is crankshaft. Thus one power stroke is
obtained in every 2 revolutions of the obtained in each revolution of the
crankshaft. crankshaft.
2 Flywheel Because of the above turnin – More uniform turning movement and
required movement is not so uniform and hence lighter flywheel is needed.
heavier or hence heavier flywheel is needed
lighter
3 Power Because of one power stroke for 2 Because of one power stroke for one
produced for revolutions, power produced for same revolution, power produced for same
same size of size of engine is small or for the same size of engine is more (theoretically
engine power the engine is heavy and bulky. twice, actually about 1.3 times) or for
the same power the engine is light and
compact.
4 Cooling and Because of one power stroke in two Because of one power stroke in one
lubrication revolutions lesser cooling and revolution greater cooling and
requirements lubrication requirements. Lesser rate lubrication requirement. Great rate of
of wear and tear. wear and tear.
5 Valve and The four stroke engine contains valve Two stroke engines have no valves but
valve and valve mechanism. only ports.
mechanism
6 Initial cost Because of heavy weight and Because of light weight and simplicity
complication of valve mechanism, due to absence of valve mechanism,
higher is the initial cost. cheaper in initial cost.
7 Volumetric Volumetric efficiency more due to Volumetric efficiency less due to lesser
Efficiency more time of induction. time for induction.
8 Thermal and Thermal efficiency higher, part load Thermal efficiency lower, part load
part‐load efficiency better than two stroke efficiency lesser than four stroke cycle
efficiencies engine. engine.
9 Applications Used where efficiency is important, in In two stroke petrol engine some fuel
cars, buses, trucks, tractors, industrial is exhausted during scavenging. Used
engines, airplane, power generators where (a)low cost, and (b)
etc. compactness and light weight
important.
Lecturer: Ram Chandra Sapkota [Note prepared by: Ram Krishna Singh]
Comparison of spark ignition (SI) and combustion ignition
(CI) engines
S.N Aspect S.I. Engine C.I. Engine
1 Thermodynamic Otto Cycle Diesel cycle: for slow speed engines.
Cycle Dual cycle: for high speed engines.
2 Fuel used Petrol Diesel
3 Air‐Fuel ratio 10:1 to 20:1 18:1 to 100:1
4 Compression ratio Up to 11; Average value 7 to 9 12 to 24; Average value 15 to 18
Upper limit of compression ratio Upper limit of compression ratio is
fixed by anti‐knock quality of fuel limited by thermal and mechanical
stresses.
5 Combustion Spark ignition Compression ignition
6 Fuel supply By carburetor: cheap method By injection: expensive method
7 Operation pressure
i.compression 7 bar to 15 bar 30 bar to 50 bar
ii.Max pressure 45 bar to 60 bar 60 bar to 120 bar
8 Operating speed High speed: 2000 to 6000 rpm Low speed: 400 rpm; Medium
speed: 400‐1200 rpm; High speed:
1200‐ 3500 rpm
9 Control of power Quantity of fuel: governed by Quantity of fuel: governed by rack
throttle
10 Heating value of 44 MJ/kg 42 MJ/Kg
fuel
11 Running cost High Low
12 Maintenance cost Minor maintenance required Major overall required by less
frequently.
13 Supercharging Limited by detonation. Used only in Limited by blower power and
aircraft engines mechanical and thermal stress.
Widely used.
14 Two stroke Less suitable, fuel loss in No fuel loss in scavenging. More
operation scavenging. But small two stroke suitable.
engines are used in mopeds,
scooters and motorcycles due to
their simplicity and low cost.
15 High powers No Yes
16 Uses Mopeds, scooter, motorcycles, Buses, trucks, locomotives, tractors,
simple engine passenger cars, earth moving machinery and
aircrafts etc. stationary generating plants.
Lecturer: Ram Chandra Sapkota [Note prepared by: Ram Krishna Singh]
Comparison between a petrol engine and a diesel engine
S.N Aspects Petrol Engine Diesel Engine
1 Suction stroke Air petrol mixture is sucked in the Only air is sucked during suction stroke
engine cylinder
2 Fuel ignition Spark plug is used Employs an injector
device
3 Power stroke Power is produced b spark ignition Power is produced by compression
ignition
4 Thermal Up to 25% Up to 40%
efficiency
5 Size Occupies less space Occupies more space
6 Running cost High running cost Low running cost
7 Weight Light Heavy
8 Fuel cost Fuel(Petrol) costlier Fuel(diesel) cheaper
9 Volatility of fuel Petrol being volatile is dangerous Diesel is no‐dangerous as it is non‐
volatile
10 Pre‐ignition Pre‐ignition possible Pre‐ignition not possible
11 Working cycle Works on Otto cycle Works on Diesel cycle
12 Dependency Less dependable More dependable
13 Applications Used in cars and motor cycles Used in heavy duty vehicles like trucks,
buses and heavy machinery
Lecturer: Ram Chandra Sapkota [Note prepared by: Ram Krishna Singh]
Comparison between a gas turbine and IC engine
S.N Aspects Gas Turbine IC Engine
1 Balancing of Perfect balancing of rotating parts Difficult to balance perfectly
components
2 Mech. efficiency Mech. Efficiency is high(95%) Mech. Efficiency is lower(85%)
3 Flywheel Torque on the turbine shaft is Needs flywheel
continuous(no need of flywheel)
4 Weight to power The weight of gas turbine is 0.15 The weight of IC engine is 2.5 kg/kW
ratio kg/kW
5 Engine rpm High rpm(40,000 rpm) It works on low rpm compared to gas
turbine
6 Max pressure Is low (5 bar) Is 60 bar or more
7 Weight of Components are lighter Components are heavier
components
8 Fuel Can use cheaper fuel(paraffin Higher grade fuels are used to avoid
type, residue oils) knocking
9 Thermal Low (15‐20%) Around 25‐ 30%
efficiency
10 Part load Poor thermal efficiency at par Comparatively better thermal efficiency
efficiency loads(air quantity remains same) at part load
11 Max gas temp Temp. of gases supplied to the Gas temperature can be higher
turbine is limited to 1100K
12 Cost Blades(Ni‐Cr alloy) are difficult to Piston assembly is cheaper
manufacture and costly
13 Cooling system Special cooling system is required No need of special cooling
for the turbine blades
14 Engine starting The starting of gas turbine is Easy to start IC engine
difficult
15 Exhaust gas The exhaust gas less polluting but The exhaust gas is polluting and needs
produced in large quantity treatment
Lecturer: Ram Chandra Sapkota [Note prepared by: Ram Krishna Singh]
Merits of two stroke cycle
1. A two stroke engine has twice as many power strokes as a four stroke cycle engine, at the same
engine speed. Theoretically, therefore a two‐stroke engine should develop twice the power of
four stroke engine of the same dimensions. However, the extra power developed is only 70 to
90%( greater value applicable in slow speed engines) due to the power absorbed in compressing
the charge, reduction in the effective stroke and the compression ratio due to the valve ports
and due to short time available for the exhaust of gases, in high speed engines.
2. For the same power a two stroke cycle engine is lighter and occupies less floor area. This makes
it more suitable for use in marine engines.
3. As the number of working strokes are double than in four stroke engine, the turning moment is
more uniform and hence a lighter flywheel is required.
4. The more uniform turning moment results in lighter foundation of the engine.
5. The mechanism is very simple, as there are no valves. In some cases mechanically operated
valves may be provided.
6. In the absence of valves, a simple arrangement can be used for reversing the engine.
Demerits of two stroke cycle
1. In two stroke engines, particularly high speed ones scavenging (driving out of exhaust gases
from engine cylinder) is not complete due to short time available for exhaust and hence the
fresh charge is polluted. This pollution of the charge has been reduced in opposed piston tow
stroke diesel engines by uni‐directional scavenging.
2. As inlet and exhaust ports open simultaneously some fresh, charge containing fuel in the case of
petrol and gas engines and compressed air in the case of diesel engines is lost.
The thermal efficiency of the two strokes engines is likely to be lower than four stroke engine
due to above reasons and due to lower effective compression ratio.
3. As the number of power stroke is double than four stroke cycle, cooling system presents
difficulty.
4. Consumption of lubricating oil is greater.
5. As the number of power stroke is twice, there is more wear and tear.
6. The exhaust is noisy due to short time available.
Lecturer: Ram Chandra Sapkota [Note prepared by: Ram Krishna Singh]
