Design of Piston Head

Piston is one of the key components in an Internal Combustion Engine and it closely

relates to the machine performance, carbon emissions and the economy. With the

higher speed of engines and strength development, higher pressure ratio and higher

power improvements work conditions of pistons have become more and more bad, so

its reliability now relies on the key factors. Structure and working environments of

pistons are very complex. In the working environment, the pistons will produce stress

and deformation because of the periodic load effect, which are from high gas pressure,

high speed reciprocating motion from the inertia force, lateral pressure, friction and so

on. Burning of the high pressure gas produces high temperature, which makes piston

move out due to expansion thus its interior produces thermal stress and thermal

deformation. The thermal deformation and mechanical deformation will cause piston

cracks, tortuosity, etc. Therefore, it is essential to analyse the stress field, temperature

field, heat transfer, thermal load and mechanical load coupling of piston, etc. in order

to lower the heat load and improve the thermal stress distribution and improve its

working reliability during operation. Analysis method of the finite element provides a

powerful calculation tool, which is better than test and theory analysis method and has

become an important means for internal combustion engine performance study.

By analysis of the piston working process, we find that stress and deformation of the

piston is most serious under the steady speed conditions when the gas-fired pressure

is the maximum. At the same time, the strength of piston has a limit. Therefore, it is

essential to choose the piston under the rated power and we only analyse distribution

force in the axis of the force, including the maximum explosion pressure and

reciprocating inertia force. Pressure load of piston is that gas pressure affects piston

top surface due to high pressure in the cylinder. For simplified analysis, we can use

the steady state process, but cannot ignore the effect that combustion power stroke

produces, i.e. impact load for piston.

Piston is affected by gas explosion pressure and the reciprocating inertia force and

their common feature is that they have line of effect along the axis direction of the

piston, so the axis direction of piston bears the bigger load. The maximum stress

appears the centre of piston pin top, which is accordance to engine design manual.

Piston pin top is the part most susceptible to fracture.

When Pistons are operating, they directly touch the high temperature gas and their

transient temperature can reach more than 2500K and generates the 18KW power.

Piston is heated seriously and its heat transfer coefficient is 167 W/(m°C) and its heat

dissipation coefficient is poor, so the piston temperature can reach 600 ~ 700 K

approximately and the temperature distributes unevenly. On the basis of these

conditions, we will make thermal analysis for the piston.

Main factors under consideration:

1. Thermodynamic Aspects:

a. Basic state equation - Pressure, Temperature, and the Volume to be swept

inside an engine cylinder.

b. Heat addition or rejection in a cycle.

c. Number of fins required.

d. Speed of the engine.

e. Thermal properties (like heat expansion and contraction of metals).

f. Compression ratio and Adiabatic index.

2. Mechanical Aspects:

a. Radial type.

b. Bore diameter (d).

c. Speed of the engine (N).

d. Fuel volume/displacement (V).

e. Pistons should get enough lubrication - so proper cooling should be

there.

f. The geometry of combustion chamber.

g. Shape of the piston - sometimes an elliptical shaped piston is used.

h. Mechanical properties like materials, strength/hardness coefficients,

and factor of safety.

i. Type of engine:

I. V type.

II. W type.

A tentatively correct method of designing a piston would be to consider the thermal

stresses and normal stresses acting on the face of the piston. The method is

rudimentary but would provide us a fair approximation as to the nature of durability we

need to provide in the material property of the piston.

We need to find the force being exerted on the cross sectional area of the piston by

the hot gases. For that we can multiply the maximum pressure existing in the

chamber (pressure at the end of compression stroke and before the power stroke)

with the cross sectional area of the piston head. That would give us the maximum

force being exerted on the surface of the piston head. We can then determine the

maximum normal stress being developed. There should not be any shear stress

under normal circumstances. For calculating the effects of the thermal stress more

research is required which I’m currently trying to find out and probably include in the

next report.

Design of Cylinder:

1. Thickness of wall:

Longitudinal Stress:

Radial Stress:

Net longitudinal stress:

Net Circumferential stress:

The thickness of the cylinder:

• Thickness varies from 4.5mm to 25mm depending on size.

• The thickness can also be found by using t = 0.0450+ 1.6mm

• Thickness of the dry liner = 0.030 to 0.0350.

• Water Jacket wall thickness = 0.0320 + 1.6mm

• Water space between outer cylinder wall and inner jacket wall is 0.080

+ 6.5mm or 10 mm for 75 mm cylinder and 75mm for 750 mm cylinder.

2. Bore and Length of Cylinder:

Indicated Power (I.P.) = W

Pm = Indicated Mean Effective Pressure

n = Number of working strokes per minute (N – 2, N/2 – 4)

Length of the stroke (l) = 1.25 D to 2D

Length of Cylinder = 15% of length of stroke = 1.15l

Indicated Power = Brake Power / Mechanical Efficiency

Maximum Gas Pressure = 9 to 10 times of Pm

3. Cylinder Flange and Studs:

The thickness of the flange should be 1.2t - 1.4t. The diameter of the studs can be found by equation:

ns is the number of studs = 0.01D+4 to 0.02D+4. Tensile stresses are in

between 35MPa - 70MPa. The nominal diameter of studs lies 0.75tf to tf.

d≤16. The distance of the flange from centre of hole for Stud should not

be less than d+6mm and not be more than 1.5d

4. Cylinder Head Design:

is the radial stress: 30 MPa to 50 MPa.

C is a constant and its value is 0.1

The pitch circle diameter Dp = D+3d.

Piston

Element that moves inside the cylinder by receiving the impulse from the expanding gas and transmits energy to crankshaft through connecting rod. Design Considerations: • It should have enormous strength. (Pressures & Forces)

• Minimum mass. (inertia forces)

• Form effective gas and oil sealing.

• Provide bearing area. (prevent wear)

• Disperse the heat quickly.

• High speed reciprocation. (without noise)

PISTON MATERIALS

• Cast Iron

• Cast Aluminium

• Forged Aluminium

• Cast Steel

• Forged Steel

Design of Piston Head I Crown

The thickness of the piston is (in mm)

P.T.O.

H = Heat flowing through the piston head

k = Heat conductivity factor in W/m/°C

= 46.6 W/m/°C for Grey Cast Iron

= 51.25 W/m/°C for Steel

= 174.75 W/m/°C for Aluminium alloys

Tc = Temperature at Centre, Te = Temperature at edge

Tc ─ Te = 220 °C for Cast Iron & 75 °C for Aluminium

Heat flowing through the head H = C×HCV×m×B.P.(kW) C = Constant = 0.05 HCV = Higher Calorific Value of the Fuel in kJ/kg = 45x103 kJ/kg for Diesel & 47x103 kJ/kg for Petrol m = Mass of the fuel used (kg/B.P./sec)

B.P = Brake Power of the engine per Cylinder