Lecture 1 Introduction Selection of type of I.C engine The selection of the type of I.C engine depends upon the power output needed and the type of service required. The following factors may be considered while arriving at the type of the engine.
Types Types of process Type of cycle Number of cylinders Arrangements of cylinders Single and double acting
1. ngin nginee spee speed d
!otational speed "iston speed Speed factor
#. Stro Stro$e $e bore bore rati ratio o %. "rin "rincip ciple le of simi similit litud udee According to the principle of similitude& similar engine are constructed of the same material and their dimensions dimensions are proportion proportional al to some dimensions& dimensions& usually the cylinder bore. '. (ene (eneral ral desig design n consi conside derat ratio ion n The si)e of the engine as a whole is determined on the basis of mean cylinder pressure and the speed& but the design of the individual parts depends on the ma*imum cylinder pressure and the acceleration of the moving parts. The stresses in the various parts of the engine come from the following types of loads.
+ue to gas pressure in the cylinder +ue to inertia and centrifugal forces +ue to torsional moment reaction and weight +ue to vibration especially of the cran$ shaft Thermal loads
The stresses due to vibration are very ver y important& especially& in the case of cran$shaft. To reduce their effects& the designer should observe the following points,
The dimensions should be suitably selected to avoid resonance The parts should be made quite stiff to reduce the amplitude of vibration The material and shape of the parts should be such as to avoid failure by fatigue +amping in the shaft should be increased by the use of suitable materials
There will be thermal loads also on the engine parts parts since about one one half of unused heat leaves through the water -ac$et. The stresses due to the thermal loads will be less if the section is thinner. hile designing the various parts of the internal combustion engine& the following temperature effect should be considered,
*pansion of metals Stresses induced in metals due to temperature difference !eduction in the strength of metals at high temperature +etermination of the metal surfaces which are sub-ected to high and varying
temperature (rowth of cast iron
/ecture # Cylinder design +esign of cylinder
The cylinder of an internal combustion engine performs a number of duties. Its primary function is to contain the wor$ing fluid and the secondary function to guide the trun$ piston.
A0 Construction
Small engine /arger engine
/iners are made in two types
et liner and +ry liner
ig.#.1 et liner
20 Advantage and disadvantage of dry and wet l iner
ig.#.# +ry liner
C0 3ualities of good liner +0 /iner material 0 Surface treatment of cylinder liner 0 Total liner stress (0 /iner distortion 40 Cylinder lubrication 5I0 /iner -oints 60 all thic$ness The wall thic$ness is usually calculated by applying the formula for a thin cylinder
P max D t7
2 σ c
+ k
here& t 7 wall thic$ness in 5mm0 2
Pmax = maximumgas pressure, N / mm +7 cylinder bore in 5mm0 2
σ c = maximumhoopstress ( N / mm ) 87 !eboring factor
80 Cylinder flanges and studs
/0 Cylinder head The general design of the cover is governed by the following factors along with the strength considerations,
Air and gas passages. Accommodation of valves and their gear. Accommodation of the atomiser at the centre cover in the case of diesel engines.
If the cylinder head is appro*imately a flat circular plate& the thic$ness can be determined by the relation,
t7 D
√
C pmax f t
C7 constant& in this case equal to 9.1 2
f t = allowablestress , taken ¿ be 35 ¿ 56 N / mm
/ecture % "iston +esign unction It is the reciprocating part of I.C engine that performs number of functions. The main functions are as follows,
Transmits the forces due to the gas pressure. Compresses the gas during the compression stro$e. Seals the inside portion of the cylinder from the cran$case by means of piston rings. It ta$es the side thrust resulting from obliquity of connecting rod.
:aterials
Commonly used material for IC engine are cast iron& cast steel& forged steel& cast aluminium alloy and forged aluminium alloy.
"iston head
Fig.3.1 Piston
Trun$ type piston& as shown in fig.%.1 is used in IC engines. It consists following parts,
"iston head or crown "iston rings ;il scraper ring "iston s$irt "iston pin
Thic$ness of piston head
There are two types of piston heads 1. lat plate type
Fig.3.2 Flat plate type head #. Cup type
Fig.3.3 Cup type head
There are two criteria for calculating the thic$ness of piston head < strength and heat dissipation. According to the Grashoff’s formula& the thic$ness of piston head is given by&
t h 7 +
√
3 Pmax 16 σ b
Where,
t h = thickness of piston head (mm ! = cylinder "ore (mm 2
Pmax = maximumgas pressure ( N / mm ) σ b = permissible bending stress (#$ mm2
The piston head absorbs the heat during combustion of fuel and transmits it to the cylinder wall. It should have sufficient thic$ness to quic$ly transfer the heat to the cylinder wall.
t h 7
H 12.56 k ( T c − T e )
Where,
t h = thickness of piston head (mm %= amount of heat conducted through piston head (W &= thermal conducti'ity factor (W$m$ oC
T c =temperatureat the centre of the pistonhead 5 oC ¿ T c =temperatureat theedge of the pistonhead 5 oC ¿ "iston !ibs
The piston head is provided with number of ribs for the following reasons, It strengthen the piston head against the gas pressure. They increase the rigidity and prevent the distortion of piston head.
"iston rings
There are two types of piston rings 1. Compression rings #. ;il scraper rings "iston ring material
The material for the piston ring usually cast iron and alloy cast iron due ti their good wearing qualities and also retain the spring characteristics even at high temperature.
"iston barrel The thic$ness of the piston barrel at the top end is given by&
t 3 =(0.03 D + b + 4.9 ) here&
t 3 =thickness of pistonbarrel at thetop end ( mm ) b7 radial width of the ring 5mm0
The thic$ness of piston barrel at the lower or open end is given by&
t 4=( 0.25 t 3 ) to
(0.35 t ) 3
t 4=thickness pistonbarrel at openend ( mm )
Fig. 3. Piston "arrel "iston s$irt As shown in the fig.%.1& the cylindrical portion of the piston between the last scrapper ring and the open end is called the piston s$irt. The piston s$irt act as a bearing surface for the side thrust.
:a*imum gas force on the piston head 7 5
Side thrust 7 μ 5
πD 4
2
¿ Pmax
πD 4
2
¿ Pmax
here&
μ= coefficient of friction 59.10 The side thrust ta$en by the s$irt is also given by& Side thrust 7 Pb D l s
Pb=¿ )llo*a"le "earing pressure l s=¿ +ength of skirt
"iston pin or gudgeon pin The connection between the piston and the connecting rod small end can be following types,
ull floating Semi floating i*ed "iston pin material
The usual material is nitrogen hardened or case hardened steel alloy containing nic$el & chromium & molybdenum or vanadium . "iston clearance The clearance between the cylinder liner and the piston is provided to ta$e care of thermal e*pansion and distortion under load.
"iston troubles The following are some of the troubles encountered in pistons,
Sei)ure of piston Crac$ing of piston crowns and side walls 2urning of piston crowns *cessive lubricating oil consumption Stic$ing of piston ring
2rea$ing of the piston rings Scuffing of piston rings and cylinder liners "iston cooling The usual medium of cooling is oil.
"iston Troubles, The following are some of the troubles encountered in pistons,
Sei)ure of piston Crac$ing of piston crowns 2urning of piston crowns *cessive lubricating oil consumption Stic$ing of piston rings 2rea$ing of piston rings
Lecture 4 Connecting Rod Function The main function of the connecting rod is to transmit the push and pull from the piston pin to the crank pin.
Connecting rod
Materials Connecting rod of I.C engine are mostly mild or medium caron steel to alloy steel and manufactured y drop forging.
Shape of connecting rod The usual shapes of the connecting rod are! rectangular" circular" tuular" I section and #$section.
Stresses in connecting rod The stresses in the connecting rod are set up by a combination of forces. The various forces acting on the connecting rod are, 1. The combined effect of gas pressure on the piston and the inertia of the reciprocating parts. #. riction of the piston rings and of the piston. %. Inertia of connecting rod. '. The friction of the two end bearings i.e of the piston pin bearing and the cran$ pin bearing.
a0 b0 c0 d0
/oad due to gas pressure and piston inertia orce due to the function of piston rings and of piston Inertia of connecting rod riction of the two end bearing
%esign procedure f cu∗
2uc$ling load 7
1
+a
() l k
2
Lurication There are t&o methods for the lurication of connection rod and earings! 1. Splash lurication '. (ressure feed lurication
Crush
Crush
Lecture ) Crank shaft Function The function of the crank shaft is to transform reciprocating motion into rotary one or *ice$ *ersa. Crank shaft consist of the shaft parts &hich re*ol*e in the main earing
Types, the cran$ shaft can be divided into two types, a0 Cran$ shaft with a side cran$ or overhung cran$ b0 Cran$ shaft with a centre cran$
Types of cran$ shaft
Materials
+nough strength to &ithstand the forces to &hich it is su,ected i.e the ending and t&isting moments. +nough rigidity to keep the distortion a minimum. Sti-ness to minimise" and strength to resist" the stress due to torsional *irations of shaft. Minimum &eight" especially in aero engines.
Manufacturing %rop forging Casting
Stresses Stress induced in the crank shafts are ending and also shear stresses due to torsional moment of shaft.
%esign procedure 1. %etermine the magnitude of the *arious load acting in the crank shaft. '. %etermine the distance et&een supports. The &ill depends upon the length of the earing. The length and the diameter of the earing are determine on the asis of maimum permissile earing pressure" L/% ratio and acting loads. 0. For the sack of simplicity and safety the shaft is considered to e supported at the centres of the earings. 4. o& calculate the distance et&een the supports. ). The thickness of the crank &es is assumed aout 2.)% to 2.3%" &here % diameter of the shaft or from 2.')% to 2.0'%" &here % is the cylinder ore. 3. 5ssume allo&ale ending and shearing stresses. 6. Compute necessary dimensions of the crank shaft. ote! all the forces and reactions are assumed to e acting at the centres of the earings.
%esign calculation 1. Analysis of centre cran$ shaft
a0 Cran$ on dead centre b0 Cran$ pin c0 /eft hand cran$ web #. Cran$ at angle of ma*imum twisting moment •
Analysis of side cran$ shaft
Lecture 6 Valve Gear Mechanism %e7nition The term =alve gear mechanism designates the combination of those parts which operate or actuate the various inta$e& e*aust and air starter valves& open and close them at the proper time with respect to the position of the piston and cran$ pin and $eep them open during the required time.
=alve gear arrangement for vertical engine
=alve gear arrangement for hori)ontal engine
8al*es 8al*es used in internal comustion engine are of t&o types! 1. (oppet *al*e or mushroom *al*e '. Slee*e *al*e
(oppet *al*e
Si9e of *al*e ports =alve seat angles& lifts& valve seat and inserts
%esign of *al*es The thickness of the *al*e disk or head can e determined y considering it a :at plate su,ected to maimum gas load taken as uniformly distriuted. Thus
t k; d p∗ &here" k constant
d p= port diameter ( mm)
√
p max f t
pmax =maximumgas pressure (
N mm
f t = allowable stress ∈ bending , (
2
)
N mm
2
)
The *al*e therefore" should e designed so as to asor least heat and dissipate most.
Sodium cooled air craft engine ehaust *al*e (roportions of *al*e
Cam 5 cam is an eccentric pro,ection on a re*ol*ing disk used for the opening and closing of a *al*e through the intermediate parts. The cams are of the follo&ing four types!
Tangent cams Conca*e cams Con*e :ank cams (roportional cams
Lecture < Fly&heel
A flywheel is a heavy rotating body that act as a reservoir of energy. The energy stored in the flywheel in the form of $inetic energy. The flywheel act as energy ban$ between the source of power and the driven machinery.
The functions of flywheel are as follows,
To store and release energy when needed during the wor$ cycle To reduce the power capacity of the electric motor To reduce the amplitude of speed fluctuations
lywheel and (overnors
lywheel material
Traditionally the flywheel are made of cast iron. rom design consideration& cast iron flywheel offers following advantages,
Cast iron flywheels are the cheapest Cast iron flywheel can be given any comple* shape without machining operations Cast iron flywheel has e*cellent ability to damp vibrations. :ore recently& flywheel are made of high strength steels and composites in vehicle applications. (raphite> ibre !einforced "olymer 5(!"0 is considered as an e*cellent choice for flywheels fitted on modern car engines.
Torque Analysis
Ι 5dw?dt0
7 T i −T o
here& Ι 7
mass moment of inertia of flywheel 58g> m # 0
T i 7 driving or input torque 5N>m0 T o=¿ /oad or output torque 5N>m0 w7 angular velocity of shaft 5rad?s0
Coefficient of fluctuation of energy
The coefficient of fluctuation of energy is defined as the ratio of the ma*imum fluctuation of energy to the wor$ done per cycle. It is denoted by C e .
Solid dis$ flywheel
The simple type of flywheel is a solid circular dis$ as shown in the fig. the mass moment of inertia of this dis$ is given by&
Ι7
m!
2
2
here& Ι7
mass moment of inertia of dis$ 5$g>m # 0
m7mass of dis$ 5$g0 !7 outer radius of dis$ 5m0
Solid dis$ flywheel
