UNIT 5

Design of I.C Engine Components

An Internal Combustion engine (IC engine) is an engine in which the combustion of fuel, such as petrol or diesel, takes place inside the engine cylinder. In petrol engine, air and petrol is mixed in correct proportion in the carburetor and then passed into the cylinder. This mixture is ignited by means of a spark produced by the spark plug. Since the ignition is done by spark, the petrol engine is called Spark Ignition engine (SI Engine). In the diesel engine, the air entrapped in the cylinder during the suction stroke is highly compressed during compression stroke. This compression increases the air temperature beyond the self-ignition temperature of diesel. The desired quantity of diesel in the form of ﬁne spray is then admitted into the cylinder near the end of the compression stroke. The turbulent hot air ignites the diesel. Since the ignition is done by compression of air, the diesel engine is called Compression Ignition engine (CI engine).

In internal combustion engine thermal, vibrations loads, dynamic loads and other load should be considered:

The important properties should be considered while selecting materials are:

Thickness of Cylinder Wall

There are two types of stresses which are produced by gas pressure inside cylinder they are

Let, Do = Outside diameter of the cylinder in mm

D = Inside diameter of the cylinder in mm

p = Maximum pressure inside the engine cylinder in N/mm2

1/m = Poisson ratio (= 0.25)

Longitudinal Stress,

And circumferential stress is given by,

Net longitudinal stress =

Net circumferential stress =

Thickness of cylinder wall (t) is obtained by using a thin cylinder formula

Where,

p = Maximum pressure inside the cylinder in N/mm2

D = Inside diameter of the cylinder or cylinder bore in mm

σc = Permissible circumferential or hoop stress for the cylinder material in

MPa (35 – 100MPa depending upon the size and material)

C = Allowance for reboring

The thickness of the cylinder wall (t) may also be obtained from the following empirical relation

= 10 mm to 75 mm

= 0.08 D + 6.5 mm

Bore and Length of the Cylinder :

The indicated power produced inside the engine cylinder

where,

pim = Indicated mean effective power

D = Cylinder bore in mm

A = Cross-sectional area of Cylinder in mm2

A =

l = Length of stroke in mm

N = Speed of the engine in rpm

n = Number of working strokes per min

= N for two strokes

= N/2 for four strokes

K = Number of cylinders

Length of Stroke is generally taken as 1.25D to 2D

Length of cylinder, L = 1.15 x Length of stroke

L = 1.15l

Cylinder Flange and Studs :

Diameter of studs or bolts may be obtained by equating the gas load due to the maximum pressure in the cylinder to the resisting force offered by all the studs or bolts.

where,

D = Cylinder bore in mm

p = Maximum pressure in N/mm2

ns = Number of studs (0.01D +4 to 0.02D +4)

dc =Core diameter

σt = Allowable tensile stress in MPa

d = Nominal Diameter of stud (0.75tt to tf)

tf = Thickness of flange

d + 6 = Distance of flange from the centre of the hole for the stud

Cylinder Head thickness (th) :

The cylinder head may be approximately taken as a flat circular plate whose thickness (th)

May be determined from the following relation

where,

D = Cylinder bore in mm

p = Maximum pressure inside the cylinder in N/mm2

σc = Allowable circumferential stress in MPa (30 to 50 MPa)

c = constant (value = 0.1)

There are two types of cylinder liners—dry liner and wet liner as shown in Fig. A dry liner is a cylinder liner which does not have any direct contact with cooling water in the jacket. A wet liner is a cylinder liner which has outer surface in direct contact with cooling water in the jacket.

The desirable properties of materials for cylinders and cylinder liners are as follows:

Cylinders and cylinder liners are usually made of grey cast iron with homogeneous and close grained structure. They are centrifugally cast. For heavy- duty cylinders, nickel cast iron and nickel chromium cast iron are used. In some cases, cast steel and aluminum alloys are used for cylinders.

The piston is a reciprocating part of IC engine that performs a number of functions. The main functions of the piston are as follows:

The most commonly used materials for piston are cast iron, cast aluminum, forged aluminum, cast steel and forged steel.

The design requirements for the piston are as follows:

Considering bending stress

The thickness of the piston head (tH) according to Grashoff’s formula is given by

where,

D = Cylinder bore in mm

p = Maximum pressure inside the cylinder in N/mm2

σb = Permissible bending stress in MPa

Considering thermal or heat transfer

The heat absorbed by the piston due to combustion of the fuel is quickly transferred to the walls of the cylinder.

Considering piston as a circular plate, Its thickness is given as

where,

tH = thickness of piston head (mm)

H = amount of heat conducted through piston head (W)

k = thermal conductivity factor (W/m/°C)

Tc= temperature at the center of piston head (°C)

Te= temperature at the edge of piston head (°C)

Heat flowing through the piston head (H) determined by the following expression

H =C x HCV x m x BP in kW

C = Constant

HCV = Higher calorific value in kj/kg

m = mass of fuel used in kg per brake power per sec

BP = Brake power of the engine

D = Cylinder bore in mm

σb = Allowable bending stress in MPa

pw = Pressure of gas on the cylinder wall in N/mm2

The maximum axial thickness (t+2+) may also be obtained from the following empirical relation

nR = Number of rings

b1 = tH to 1.2 tH

b2 = 0.75 t2 to t2

The gap between the free ends of the ring is given by 3.5 t1 to 4 t1

The cylindrical portion of the piston between the last scrapper ring and the open end is called the piston skirt. The piston skirt acts as a bearing surface for the side thrust. The length of the skirt should be such that the bearing pressure due to side thrust is restricted to 0.25 MPa on the projected area. In high speed engines, the bearing pressure up to 0.5 MPa is allowed to reduce the weight of the reciprocating piston. The maximum side thrust will occur during expansion stroke.

We know that, maximum gas load on the piston,

Maximum side thrust on the cylinder,

The side thrust (R) is also given by,

Generally the length of the piston skirt is taken as 0.65 to 0.8 times the cylinder bore.

Therefore the total length of the piston (L) is given by

L = Length of skirt + Length of ring section + Top land

Design of piston pin on basis of bearing pressure

Let,

do = Outside diameter of the piston pin in mm

lp = length of the piston pin in the bush of the small end of connecting

rod in mm. the value is taken as 0.45D

pb1 =Bearing pressure at the small end of the connecting rod bushing

in N/mm2. Value may be taken as 25 N/mm2

We know that, load on the piston due to gas pressure or gas load,

Maximum gas load =

and load on the piston pin due to bearing pressure or bearing load

= Bearing pressure x Bearing area

Maximum gas load = pb1 x do x lp

Design of piston on the basis of bending

Bending moment is calculated as,

also,

Outside diameter of pin in mm

Design of piston pin on the basis of shear strength

Gas load =

Gas load =

Total Length of piston pin is taken as

Lpt = 0.9 D

The basic function of the connecting rod is to transmit the push and pull forces from the piston pin to the crank pin. The connecting rod transmits the reciprocating motion of the piston to the rotary motion of the crankshaft. It also transfers lubricating oil from the crank pin to the piston pin and provides a splash or jet of oil to the piston assembly. The connecting rod is subjected to the force of gas pressure and the inertia force of the reciprocating part. It is one of the most heavily stressed parts of the IC engine. The materials used for the connecting rod are either medium carbon steels or alloy steels.

Design procedure of connecting rod

Dimensions of Cross-section of the connecting rod

According to Rankine’s formula

A = Cross-sectional area of the connecting rod

L = Effective length of the connecting rod

σc = Crippling or Buckling stress

WB = Buckling load

a = Rankine’s constant

K2xx = 3.18 t2

Width of the section, B = 4t

Depth/Height H = 5t

Depth near the small end is taken as H1 = 0.75 H to 0.9 H

Depth near the bigger end(Crank end) is taken as H2 =1.1 H to 1.25 H

Dimensions of the big end and small end of connecting rod

Maximum gas force,

D = Cylinder bore

p = Maximum gas pressure

dc = Diameter of the crank pin

lc = Length of crank pin

Load on the crank pin = Projected area x Bearing pressure

= dc. lc . pbc

Similarly, load on the piston pin = dp . lp . pbp

Equating the above equations

Fmax = dc . lc . pbc

Taking lc = 1.25 dc to 1.5dc, the values of dc and lc are determined from the above expression

Fmax = dp . lp . pbp

Taking lp = 1.5 dp to 2dp, the values of dp and lp are determined from the above expression

Size of bolts for Securing the Big End Cap

Ft = Inertia load acting on bolts

dcb = Core diameter of the bolt

σt = Allowable tensile stress for the material of the bolts

nb = Number of bolts

The nominal or major diameter of bolt is given by

Thickness of the big end cap

Maximum bending moment acting on the cap will be taken as

x = Distance between the bolt centres

bc = Width of cap

σb = Allowable bending stress for the material of cap

Section modulus for the cap,

Bending stress

The crankshaft is an important part of IC engine that converts the reciprocating motion of the piston into rotary motion through the connecting rod. The crankshaft consists of three portions—crank pin, crank web and shaft. The big end of the connecting rod is attached to the crank pin. The crank web connects the crank pin to the shaft portion. The shaft portion rotates in the main bearings and transmits power to the outside source through the belt drive, gear drive or chain drive. There are two types of crankshafts—side crankshaft and center crankshaft the side crankshaft is also called the ‘overhung’ crankshaft. It has only one crank web and requires only two bearings for support. It is used in medium-size engines and large-size horizontal engines. The center crankshaft has two webs and three bearings for support. It is used in radial aircraft engines, stationary engines and marine engines.

The popular materials used for crankshafts are plain- carbon steels and alloy steels. The plain carbon steels include 40C8, 45C8 and 50C4. The alloy steels used for making crankshafts are nickel–chromium steels such as 16Ni3Cr2, 35Ni5Cr2 and 40Ni10Cr3Mo6.

The forces acting on the center crankshaft at the top dead center position are shown in Fig. 25.20. The crankshaft is supported on three bearings 1, 2 and 3.

Assumptions

Bearing Reactions

(a) The reactions at the bearings 1 and 2 due to force on the crank pin (Pp) are denoted by R1 and R2 followed by sufﬁx letters v and h. The vertical component of reaction is denoted by the sufﬁx letter v such as (R1) v. The horizontal component of reaction is denoted by the sufﬁx letter h such as (R1)h.

(b) The reactions at the bearings 2 and 3 due to weight of the ﬂywheel (W) and sum of the belt tensions (P1 + P2) are denoted by R¢2 and R¢3 followed by sufﬁx letters v and h such as (R2¢)v or (R3¢)h.

Suppose,

Pp = force acting on crank pin (N)

D = diameter of piston (mm)

pmax.= maximum gas pressure inside the cylinder (MPa or N/mm2)

W = weight of ﬂywheel (N)

P1 = tension in tight side of belt (N)

P2 = tension in slack side of belt (N)

b = distance between main bearings 1 and 2

c = distance between bearings 2 and 3

At the top dead center position, the thrust in the connecting rod will be equal to the force acting on piston.

It is assumed that the portion of the crankshaft between bearings 1 and 2 is simply supported on bearings and subjected to force Pp. Taking moment of forces,

Similarly,

It is also assumed that the portion of the crankshaft between bearings 2 and 3 is simply supported on bearings and subjected to a vertical force W and horizontal force (P1 + P2). Taking moment of forces,

The resultant reactions at the bearings are as follows:

R1 = (R1)v

Design of Crank Pin

The central plane of the crank pin is subjected to maximum bending moment. Suppose,

dc = diameter of crank pin (mm)

lc = length of crank pin (mm)

sb = allowable bending stress for crank pin(N/mm2)

The bending moment at the central plane is given by,

Substituting,

Design of Left-hand Crank Web

Suppose,

w = width of crank web (mm)

t = thickness of crank web (mm)

The dimensions of crank web are calculated by empirical relationships and checked for the stresses. The empirical relationships are as follows:

t = 0.7 dc

w = 1.14 dc

where,

dc = diameter of crank pin (mm)

The left-hand crank web is subjected to eccentric load (R1)v. There are two types of stresses in the central plane of the crank web, viz., direct compressive stress and bending stress due to eccentricity of reaction (R1)v.

The direct compressive stress is given by,

The bending moment at the central plane is given by,

Substituting,

The total compressive stress is given by,

(σc)t = σc + σb

The total compressive stress should be less than the allowable bending stress.

Design of Right-hand Crank Web The right- hand and left-hand webs should be identical from balancing considerations. Therefore, the thickness and width of the right-hand crank web are made equal to that of the left-hand crank web.

Design of Shaft Under Flywheel The forces acting on the shaft under the ﬂywheel. The central plane of the shaft is subjected to maximum bending moment. Suppose,

ds = diameter of shaft under ﬂywheel (mm)

The bending moment in the vertical plane due to weight of ﬂywheel is given by,

The bending moment in the horizontal plane due to resultant belt tension is given by,

Similarity

The diameter of shaft under ﬂywheel (ds) can be calculated from above equations.