Piston Assembly Gas and Inertia forces are applied. Gas and Inertia forces are applied. Both of these forces are variable in Both of these forces are variable in – Magnitude – Direction There are also lateral forces which press the piston against the cylinder walls There are also lateral forces which press the piston against the cylinder walls Uneven heating (radially and axially) causes thermal stresses Uneven heating (radially and axially) causes thermal stresses There are also local forces There are also local forces – The lands are under different pressures and inertia forces from piston rings – Upper and lower edges are subjected to shock as the piston tilts at TDC

Piston Assembly

Heat rejection is by Heat rejection is by – Piston rings – Skirt to the liners – oil on the inner side of the crankcase – Air circulation and radiation % of heat rejection is through the piston rings % of heat rejection is through the piston rings H/D is greater in Diesel engines (more compression and oil rings are used) H/D is greater in Diesel engines (more compression and oil rings are used)

Piston Assembly Height of top land (h) affects the temperature in the zone of the first compression ring Height of top land (h) affects the temperature in the zone of the first compression ring Permissible temperatures in this zone are C Permissible temperatures in this zone are C Thickness of Piston crown depends on the maximum combustion pressure Thickness of Piston crown depends on the maximum combustion pressure The land between the first and second rings are higher The land between the first and second rings are higher holes are drilled in the oil ring groove holes are drilled in the oil ring groove

Piston Assembly Skirt take lateral loads and should be checked for deformation Skirt take lateral loads and should be checked for deformation Position of the pin axis Position of the pin axis – Uniform distribution of lateral force – Maximum pressure applied to compression rings for safe lubrication. – Tilting at TDC Tilting can be eliminated by offsetting the axis of piston pin towards its most loaded side surface. Tilting can be eliminated by offsetting the axis of piston pin towards its most loaded side surface. e/D = e/D =

Piston Material Cast Iron pistons for Diesel Engines Cast Iron pistons for Diesel Engines Aluminium Alloy Pistons for Spark Ignition Engines and automotive Diesel engines Aluminium Alloy Pistons for Spark Ignition Engines and automotive Diesel engines Coefficient of Linear Expansion Coefficient of Linear Expansion – Aluminium Alloy Pistons : (16-21)x10 -6 – Steel and Cast Iron Pistons : (11-12)x10 -6 The crown diameter is less than the skirt diameter due to difference in temperature The crown diameter is less than the skirt diameter due to difference in temperature The piston is OVAL at the crown and this ovality continues down to he piston pin position. The piston is OVAL at the crown and this ovality continues down to he piston pin position. Ovality is to compensate for the uneven expansion of the piston at the pin bosses. Ovality is to compensate for the uneven expansion of the piston at the pin bosses.

Piston Crown Thickness for cast iron pistons for cast iron pistons

Piston Diameter at the Top and Skirt

Piston Pin Location Check for oil pressure P max < ( ) kPa P max < ( ) kPa H 1 = x 2 and H - H 1 = x 1 H 1 = x 2 and H - H 1 = x 1

Bending Stresses in Piston Skirt The skirt takes the lateral loads The skirt takes the lateral loads Ensures coaxial position of piston in cylinder Ensures coaxial position of piston in cylinder t s = uniform piston t s = uniform piston thickness thickness (mean piston skirt (mean piston skirt thickness ) thickness )

Bending Stresses in Piston Skirt Difference between piston and cylinder radii Difference between piston and cylinder radii Outer radii of Piston Outer radii of Piston Modulus of Elasticity Modulus of ElasticityTake