Unformatted text preview: pistons of aluminium alloys are coated with aluminium oxide by an
electrical method. 32.8 Piston Head or Crown
The piston head or crown is designed keeping in view the following two main considerations, i.e.
1. It should have adequate strength to withstand the straining action due to pressure of explosion
inside the engine cylinder, and
2. It should dissipate the heat of combustion to the cylinder walls as quickly as possible.
On the basis of first consideration of straining action, the thickness of the piston head is determined
by treating it as a flat circular plate of uniform thickness, fixed at the outer edges and subjected to a
uniformly distributed load due to the gas pressure over the entire cross-section.
The thickness of the piston head (tH ), according to Grashoff’s formula is given by
tH = 3 p.D 2
16 σt ...(i) p = Maximum gas pressure or explosion pressure in N/mm2,
D = Cylinder bore or outside diameter of the piston in mm, and
σt = Permissible bending (tensile) stress for the material of the piston in
MPa or N/mm2. It may be taken as 35 to 40 MPa for grey cast ir...
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- Spring '11