Design of IC Engine Cylinder

DESIGN OF CYLINDER

In designing a cylinder for an I. C. engine, it is required to determine the following values : 

1. Thickness of the cylinder wall-The cylinder wall is subjected to gas pressure and the piston side thrust. The gas pressure produces the following two types of stresses : 

(a) Longitudinal stress, and (b) Circumferential stress.

Since these two stresses act at right angles to each other, therefore, the net stress in each direction is reduced. 

The piston side thrust tends to bend the cylinder wall, but the stress in the wall due to side thrust is very small and hence it may be neglected. 

Let D0 = Outside diameter of the cylinder in mm,

D = Inside diameter of the cylinder in mm,

p = Maximum pressure inside the engine cylinder in MPa, 

t = Thickness of the cylinder wall in mm, and 

1/m = Poisson’s ratio. It is usually taken as 0.25.

The thickness of a cylinder wall (t) is usually obtained by using a thin cylindrical formula,i.e

Where 

σc = Permissible circumferential or hoop stress for the cylinder material in MPa. Its value may be taken from 35 MPa to 100 MPa depending upon the size and material of the cylinder. 

C = Allowance for reboring.

The allowance for reboring (C ) depending upon the cylinder bore (D) for I. C. engines is given in the following table : 

The thickness of the cylinder wall usually varies from 4.5 mm to 25 mm or more depending upon the size of the cylinder. The thickness of the cylinder wall (t) may also be obtained from the following empirical relation, i.e. 

t = 0.045 D + 1.6 mm 

The other empirical relations are as follows : 

Thickness of the dry liner = 0.03 D to 0.035 D

Thickness of the water jacket wall = 0.032 D + 1.6 mm or t / 3 m for bigger cylinders and 3t /4 for smaller cylinders 

Water space between the outer cylinder wall and inner jacket wall = 10mm for a 75 mm cylinder to 75 mm for a 750 mm cylinder or  0.08 D +  6.5 mm 

2. Bore and length of the cylinder-The bore (i.e. inner diameter) and length of the cylinder may be determined as discussed below : 

Let pm = Indicated mean effective pressure in MPa

D = Cylinder bore in mm

A = Cross-sectional area of the cylinder

Length of stroke in metres, 

N = Speed of the engine in r.p.m., and 

n = Number of working strokes per min 

N, for two stroke engine

N/2, for four stroke engine. 

We know that the power produced inside the engine cylinder, i.e. indicated power,

From this expression, the bore (D) and length of stroke (l) is determined. The length of stroke is generally taken as 1.25 D to 2D. 

Since there is a clearance on both sides of the cylinder, therefore length of the cylinder is taken as 15 % greater than the length of stroke. In other words, Length of the cylinder, L = 1.15 × Length of stroke = 1.15 l 

3. Cylinder flange and studs-The cylinders are cast integral with the upper half of the crankcase or they are attached to the crankcase by means of a flange with studs or bolts and nuts. The cylinder flange is integral with the cylinder and should be made thicker than the cylinder wall. The flange thickness should be taken as 1.2 t to 1.4 t, where t is the thickness of cylinder wall. The diameter of the 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. Mathematically,

where D = Cylinder bore in mm

p = Maximum pressure in MPa 

ns =Number of studs. It may be taken as 0.01 D + 4  to  0.02 D + 4 

dc = Core or minor diameter, i.e. diameter at the root of the thread in mm,

σ = Allowable tensile stress for the material of studs or bolts in MPa. It may be taken as 35 to 70 MPa. 

The nominal or major diameter of the stud or bolt (d ) usually lies between 0.75 tf  to  tf, where tf is the thickness of flange. In no case, a stud or bolt less than 16 mm diameter should be used. The distance of the flange from the centre of the hole for the stud or bolt should not be less than d + 6 mm and not more than 1.5 d, where d is the nominal diameter of the stud or bolt. In order to make a leak  proof  joint, the pitch of the studs or bolts should lie between 19 d to 28.5 , d where d is in mm. 

4. Cylinder head- Usually, a separate cylinder head or cover is provided with most of the engines. It is, usually, made of box type section of considerable depth to accommodate ports for air and gas passages, inlet valve, exhaust valve and spark plug (in case of petrol engines) or atomiser at the centre of the cover (in case of diesel engines). 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 MPa

cσ = Allowable circumferential stress in MPa. It may be taken as 30 to 50 MPa, and 

C = Constant whose value is taken as 0.1. 

The studs or bolts are screwed up tightly along with a metal gasket or asbestos packing to provide a leak proof joint between the cylinder and cylinder head. The tightness of the joint also depends upon the pitch of the bolts or studs, which should lie between 19 Root d to 28.5 Root d The pitch circle diameter (Dp) is usually taken as D + 3d.