Ic Engine Component Design 2u641o

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 of process Type of cycle Number of cylinders Arrangements of cylinders Single and double acting

1. Engine speed  Rotational speed  Piston speed  Speed factor

2. Stroke bore ratio 3. Principle of similitude According to the principle of similitude, similar engine are constructed of the same material and their dimensions are proportional to some dimensions, usually the cylinder bore. 4. General design consideration The size 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 maximum 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.

    

Due to gas pressure in the cylinder Due to inertia and centrifugal forces Due to torsional moment reaction and weight Due to vibration especially of the crank shaft Thermal loads

The stresses due to vibration are very important, especially, in the case of crankshaft. 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 Damping in the shaft should be increased by the use of suitable materials

There will be thermal loads also on the engine parts since about one half of unused heat leaves through the water jacket. The stresses due to the thermal loads will be less if the section is thinner. While deg the various parts of the internal combustion engine, the following temperature effect should be considered:    

Expansion of metals Stresses induced in metals due to temperature difference Reduction in the strength of metals at high temperature Determination of the metal surfaces which are subjected to high and varying

temperature  Growth of cast iron

Lecture 2 Cylinder design Design of cylinder

The cylinder of an internal combustion engine performs a number of duties. Its primary function is to contain the working fluid and the secondary function to guide the trunk piston.

A) Construction  Small engine  Larger engine

Liners are made in two types

Wet liner and Dry liner

Fig.2.1 Wet liner

B) Advantage and disadvantage of dry and wet liner

Fig.2.2 Dry liner

C) Qualities of good liner D) Liner material E) Surface treatment of cylinder liner F) Total liner stress G) Liner distortion H) Cylinder lubrication (I) Liner ts J) Wall thickness The wall thickness is usually calculated by applying the formula for a thin cylinder t=

P max D +k 2σ c

Where, t = wall thickness in (mm) Pmax =maximum gas pressure , N /mm2 D= cylinder bore in (mm) σ c =maximum hoop stress(N /mm 2) K= Reboring factor

K) Cylinder flanges and studs

L) Cylinder head The general design of the cover is governed by the following factors along with the strength considerations:

 Air and gas ages.  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 approximately a flat circular plate, the thickness can be determined by the relation: t= D

√

C pmax ft

C= constant, in this case equal to 0.1 2

f t=allowable stress , taken¿ be 35 ¿ 56 N /mm

Lecture 3 Piston Design Function 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 stroke. Seals the inside portion of the cylinder from the crankcase by means of piston rings. It takes the side thrust resulting from obliquity of connecting rod.

Materials

Commonly used material for IC engine are cast iron, cast steel, forged steel, cast aluminium alloy and forged aluminium alloy.

Piston head

Fig.3.1 Piston

Trunk type piston, as shown in fig.3.1 is used in IC engines. It consists following parts:     

Piston head or crown Piston rings Oil scraper ring Piston skirt Piston pin

Thickness of piston head

There are two types of piston heads 1. Flat plate type

Fig.3.2 Flat plate type head 2. Cup type

Fig.3.3 Cup type head

There are two criteria for calculating the thickness of piston head – strength and heat dissipation. According to the Grashoff’s formula, the thickness of piston head is given by,

th = D

√

3 Pmax 16 σ b

Where, t h = thickness of piston head (mm) D = cylinder bore (mm) Pmax =maximum gas pressure (N /mm2) σ b =permissible bending stress (N/ mm2 )

The piston head absorbs the heat during combustion of fuel and transmits it to the cylinder wall. It should have sufficient thickness to quickly transfer the heat to the cylinder wall.

th =

H 12.56 k (T c −T e )

Where, t h = thickness of piston head (mm) H= amount of heat conducted through piston head (W) K= thermal conductivity factor (W/m/ oC )

T c =temperatureat thecentre of the piston head ( oC ¿ T c =temperature at theedge of the piston head ( oC ¿ Piston Ribs

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.

Piston rings

There are two types of piston rings 1. Compression rings 2. Oil scraper rings Piston 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.

Piston barrel The thickness of the piston barrel at the top end is given by, t 3 =(0.03 D+b+ 4.9) Where, t 3 =thickness of piston barrel at thetop end ( mm ) b= radial width of the ring (mm)

The thickness 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 piston barrel at open end (mm)

Fig. 3.4 Piston barrel Piston skirt As shown in the fig.3.1, the cylindrical portion of the piston between the last scrapper ring and the open end is called the piston skirt. The piston skirt act as a bearing surface for the side thrust.

Maximum gas force on the piston head = (

Side thrust =

μ (

π D2 ¿ Pmax 4

π D2 ¿ Pmax 4

Where, μ=coefficient of friction (0.1) The side thrust taken by the skirt is also given by, Side thrust =

Pb D l s

Pb=¿ Allowable bearing pressure l s=¿ Length of skirt

Piston pin or gudgeon pin The connection between the piston and the connecting rod small end can be following types:  Full floating  Semi floating  Fixed Piston pin material

The usual material is nitrogen hardened or case hardened steel alloy containing nickel , chromium , molybdenum or vanadium . Piston clearance The clearance between the cylinder liner and the piston is provided to take care of thermal expansion and distortion under load.

Piston troubles The following are some of the troubles encountered in pistons:     

Seizure of piston Cracking of piston crowns and side walls Burning of piston crowns Excessive lubricating oil consumption Sticking of piston ring

 Breaking of the piston rings  Scuffing of piston rings and cylinder liners Piston cooling The usual medium of cooling is oil.

Piston Troubles: The following are some of the troubles encountered in pistons:      

Seizure of piston Cracking of piston crowns Burning of piston crowns Excessive lubricating oil consumption Sticking of piston rings Breaking 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 carbon steel to alloy steel and manufactured by drop forging.

Shape of connecting rod The usual shapes of the connecting rod are: rectangular, circular, tubular, I section and H-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. 2. Friction of the piston rings and of the piston. 3. Inertia of connecting rod. 4. The friction of the two end bearings i.e of the piston pin bearing and the crank pin bearing.

a) b) c) d)

Load due to gas pressure and piston inertia Force due to the function of piston rings and of piston Inertia of connecting rod Friction of the two end bearing

Design procedure f cu∗A

Buckling load =

1+ a

( kl ) 2

Lubrication There are two methods for the lubrication of connection rod and bearings: 1. Splash lubrication 2. Pressure feed lubrication

Crush

Crush

Lecture 5 Crank shaft Function The function of the crank shaft is to transform reciprocating motion into rotary one or vice- versa. Crank shaft consist of the shaft parts which revolve in the main bearing

Types: the crank shaft can be divided into two types: a) Crank shaft with a side crank or overhung crank b) Crank shaft with a centre crank

Types of crank shaft

Materials

 Enough strength to withstand the forces to which it is subjected i.e the bending and twisting moments.  Enough rigidity to keep the distortion a minimum.  Stiffness to minimise, and strength to resist, the stress due to torsional vibrations of shaft.  Minimum weight, especially in aero engines.

Manufacturing Drop forging Casting

Stresses Stress induced in the crank shafts are bending and also shear stresses due to torsional moment of shaft.

Design procedure 1. Determine the magnitude of the various load acting in the crank shaft. 2. Determine the distance between s. The will depends upon the length of the bearing. The length and the diameter of the bearing are determine on the basis of maximum permissible bearing pressure, L/D ratio and acting loads. 3. For the sack of simplicity and safety the shaft is considered to be ed at the centres of the bearings. 4. Now calculate the distance between the s. 5. The thickness of the crank webs is assumed about 0.5D to 0.6D, where D= diameter of the shaft or from 0.25D to 0.32D, where D is the cylinder bore. 6. Assume allowable bending and shearing stresses. 7. Compute necessary dimensions of the crank shaft. Note: all the forces and reactions are assumed to be acting at the centres of the bearings.

Design calculation 1. Analysis of centre crank shaft

a) Crank on dead centre b) Crank pin c) Left hand crank web 2. Crank at angle of maximum twisting moment 

Analysis of side crank shaft

Lecture 6 Valve Gear Mechanism Definition The term Valve gear mechanism designates the combination of those parts which operate or actuate the various intake, exaust and air starter valves, open and close them at the proper time with respect to the position of the piston and crank pin and keep them open during the required time.

Valve gear arrangement for vertical engine

Valve gear arrangement for horizontal engine

Valves Valves used in internal combustion engine are of two types: 1. Poppet valve or mushroom valve 2. Sleeve valve

Poppet valve

Size of valve ports Valve seat angles, lifts, valve seat and inserts

Design of valves The thickness of the valve disk or head can be determined by considering it a flat plate subjected to maximum gas load taken as uniformly distributed. Thus

t= k* d p∗ where, k= constant

d p= port diameter(mm)

√

p max ft

pmax =maximum gas pressure (

N ) 2 mm

f t=allowable stress∈bending ,(

N ) mm 2

The valve therefore, should be designed so as to absorb least heat and dissipate most.  Sodium cooled air craft engine exhaust valve  Proportions of valve

Cam A cam is an eccentric projection on a revolving disk used for the opening and closing of a valve through the intermediate parts. The cams are of the following four types:    

Tangent cams Concave cams Convex flank cams Proportional cams

Lecture 8 Flywheel A flywheel is a heavy rotating body that act as a reservoir of energy. The energy stored in the flywheel in the form of kinetic energy. The flywheel act as energy bank 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 work cycle  To reduce the power capacity of the electric motor  To reduce the amplitude of speed fluctuations

Flywheel and Governors

Flywheel material

Traditionally the flywheel are made of cast iron. From design consideration, cast iron flywheel offers following advantages:

 Cast iron flywheels are the cheapest  Cast iron flywheel can be given any complex shape without machining operations  Cast iron flywheel has excellent ability to damp vibrations. More recently, flywheel are made of high strength steels and composites in vehicle applications. Graphite- Fibre Reinforced Polymer (GFRP) is considered as an excellent choice for flywheels fitted on modern car engines.

Torque Analysis

 (dw/dt) = T i −T o

Where,  = mass moment of inertia of flywheel (Kg- m2 ) T i = driving or input torque (N-m) T o=¿ Load or output torque (N-m) w= angular velocity of shaft (rad/s)

Coefficient of fluctuation of energy

The coefficient of fluctuation of energy is defined as the ratio of the maximum fluctuation of energy to the work done per cycle. It is denoted by Ce .

Solid disk flywheel

The simple type of flywheel is a solid circular disk as shown in the fig. the mass moment of inertia of this disk is given by,

=

mR 2

2

Where, = mass moment of inertia of disk (kg-m2 ) m=mass of disk (kg) R= outer radius of disk (m)

Solid disk flywheel