design of piston(basics)

PISTON MATERIALS

              Cast iron is the most popular material used for the construction of piston for low speed engines. Mostly closed grain pearlitic CI is used.

              Limitation of CI is high weight density, which in turn increases the inertia force.

              Modern high speed engine uses aluminium.

DESIGN OF PISTON:

1.          Piston Crown or Piston Head

              Simplest piston crown is a flat type.

              The selection of piston crown depends up on the requirement of volume for the combustion chamber and valve arrangement.

The thickness of the piston crown can be calculated by using eqn. assuming the crown to be a flat plate of uniform thickness and fixed at the edges;

            

Besides this, the piston crown has to carry the heat that is generated during combustion of fuel.

            

2.          PISTON RINGS:

              Generally two types of piston rings are used

·                 Compression rings

·                 Oil rings

The radical thickness of cast iron piston ring may be computed by

            

Minimum depth of piston ring or axial thickness (h) can be found out by equation

            

The distance from top to the first groove tg from [equ. 10.30 pg 363]

PISTON BARREL

              This the cylindrical portion of the piston.

              Maximum thickness can be found out using equation [18.20 page 362]

             t₃ = 0.03 D + b + 4.5

PISTON SKIRT

              The portion below the ring section is known as piston skirt.

              The length of piston skirt should be such that the bearing pressure on the piston barrel due to side thrust does not exceed 0.25 N/mm².

Max. gas load on the piston

            

Max. side thrust on cylinder; [R]

              [Usually side thrust [R] is taken as 1/10 of gas load].

R = Bearing Pressure × Projected area of piston skirt.

             R = P_b × b × l

Total Length (L) of piston may be taken as

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

PISTON PIN:

             The design of the piston pin is based on;

Bearing Failure:

              The piston pin is subjected to maximum force bring the sum of the gas force and Inertia force.

             Bearing Pressure,

             P       Maximum gas force

             l_s       Length of small end of the connecting rod

The permissible value of the bearing pressure is (8-16) N/mm² for petrol and diesel engine.

If the pin is employed as floating on bosses.

            

             l_p       Length of piston pin

             b       the distance b/w the boss ends.

The ratio  is usually taken as 1.5 –2.5

 Bending Failure:

             Maximum bending moment

            

            

             di  hollow dia of the piston pin.

Shear Failure of pin:

            

Ovalization of pin:

            

                Ratio of inner dia to outer dia

             Maximum value of ovalization lies b/w 0.02 and 0.05mm.

References

1.      Mechanical Engineering Design – Joseph Shigley

2.      Machine Design – Mubeen

3.      Machine Design – Black

4.      Principles of Lubrication – Cameron A.