Piston Rings

Pressure behind the FIRST compression ring is close to that in the cylinder Pressure behind the FIRST compression ring is close to that in the cylinder The pressure behind the SECOND ring is approximately 65 % of this value The pressure behind the SECOND ring is approximately 65 % of this value

Pressure Distribution Around Rings

Piston Tilting About TDC

Piston Tiltin About TDC

Piston Moving Down During Induction

Piston Rings During INDUCTION and EXHAUST pressure in the grooves behind the rings has no time to drop. This helps to reduce leakage. During INDUCTION and EXHAUST pressure in the grooves behind the rings has no time to drop. This helps to reduce leakage. At part load the pressure in the grooves is lower and leakage may increase. At part load the pressure in the grooves is lower and leakage may increase. At higher engine speed the relative leakage decreases. Therefore high speed engines (SI) can use only 2 compression rings, Medium power Diesel engines normally use 3 compression rings. At higher engine speed the relative leakage decreases. Therefore high speed engines (SI) can use only 2 compression rings, Medium power Diesel engines normally use 3 compression rings.

Piston Ring Vibration Vibration of rings increases leakage. Vibration of rings increases leakage. Vibration begins at a certain engine speed. Vibration begins at a certain engine speed. Vibration is caused by; Vibration is caused by; – mainly the irregularities on sleeve surface – loss of elasticity – drop of gas pressure in ring grooves Vibration can be reduced by ; Vibration can be reduced by ; – increasing their elasticity – decreasing the volume of the groove space at the back of the ring – reducing the width of the ring

Heat Transfer by Piston Rings Heat transfer from a piston : Heat transfer from a piston : – % through the compression rings – % through the external surface of the skirt – 5 % through the internal surface into the oil and gases. – Heat transfer is highest at the first and second compression rings

Uneven (corrected) Pressure Distribution When the rings get worn, the pressure in the joint zone reduces and is redistributed onto the adjacent areas When the rings get worn, the pressure in the joint zone reduces and is redistributed onto the adjacent areas This increases the effective life of the ring This increases the effective life of the ring

Design Parameters 1. The ratio of the cylinder diameter to the radial thickness D/t, which determines the radial pressure due to the elasticity of the ring and the bending stress on it 2. The ring width b 3. The relative quantity S o / t where S o is the springing of the ring, i.e. the difference between the free ring and the temperature clearances

Design Parameters RINGS D/t b (mm) S o / t Compression Oil

Analysis of The Piston Ring The mean pressure of the ring on the wall is ; Pm = kN/m 2 (upto 350 kN/m 2 ) (comp.) Pm = kN/m 2 (oil) E = 1.2 x 10 8 kN/m 2

Analysis of The Piston Ring The maximum working stress :

Analysis of The Piston Ring Maximum Sress in a ring whose joint is widened when being fitted onto the piston

Friction Load Due To Piston Rings The average force of friction on the compression and exhaust strokes is nearly the same The average force of friction on the compression and exhaust strokes is nearly the same The average force of friction on the power stroke is about twice that on the suction stroke. The average force of friction on the power stroke is about twice that on the suction stroke. Forces tend to be high just after TDC and BDC due metallic contact of rings and liners Forces tend to be high just after TDC and BDC due metallic contact of rings and liners Friction of piston rings is 80 % of the friction of the piston assembly Friction of piston rings is 80 % of the friction of the piston assembly

Piston Ring Materials Most piston rings are made of GRAY CAST IRON because of its excellent wearing properties in all kinds of cylinder bores. Most piston rings are made of GRAY CAST IRON because of its excellent wearing properties in all kinds of cylinder bores. Where ring leakage is a problem, nodular iron or even steel is used (usually with coated bearing surfaces) Where ring leakage is a problem, nodular iron or even steel is used (usually with coated bearing surfaces) For greatest resistance to wear of both ring and bore rings are faced either by chromium plate or “metallized molybdenum” which is a porous structure of molybdenum oxides. For greatest resistance to wear of both ring and bore rings are faced either by chromium plate or “metallized molybdenum” which is a porous structure of molybdenum oxides. Oil rings may be iron or steel. Oil rings may be iron or steel.