Diesel Engine Proper 4

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(5) Piston Material

Piston must be made of a material that meets the following requirements :

• Low Thermal expansion. The coefficient of thermal expansion must be low. It is best to use the same material for both pistons and cylinders.
• High heat conductivity.
• Low spesific gravity (to decrease inertia during high speed operation).
• Sufficient strength and large abrasion resistance even at high temperatures.
• Easy to cast

Alumunium alloys is currently used because they satisfy all of the above requirements. Special cast iron is used as well. A piston made of special cast iron has the same coefficient of thermal expansion as the cylinder, but tends to be heavy.

Alumunium alloys has a larger coefficient of thermal expansion than iron,but has high heat conductivity, therefore the temperature of the piston head can be lowered.

However, alumunium alloy has a weak point (poor lubricating oil retention). For this reason,pistons are usually plated with lead to eliminate this shortcoming. Seizure can be prevented by lead plating.

Some pistons have a special cast iron ring carrier that is cast into the top ring groove to prevent abrasion. A piston usually tin plated to improve initial breaking in performance and to prevent rusting.

(6) Thermal Problem of Pistons

The strength and hardness of the alumunium alloy used for manufacturing pistons will suddenly decrease when temperature exceeds 400^oC. As a result, abrasion and cracking will begin to occur. When Lo-Ex alloy is used, the piston head cavity temperature is designed to be 300 - 330^oC and the bottom of thetopof ring groove is designed to below 230 - 250^oC.

The overheating of piston can be prevented by various methods.  For example the cooling efficiency can be raised to lower the temperature of the cylinder liner. The thermal flow type shape (dome shape that promotes the flow of heat from the top of the piston to the ring) can be adopted for the back of pistons so that the piston temperature will be even. Pistons can also be oil cooled.

(7) Handling of Piston

Clearence between piston and cylinder

When the piston is installed in the cylinder, there must be a specified clearence between them. Insufficient clearence will cause seizure due to thermal expansion, while excessive clearence will lead to compression leakage, inefficient heat radiation by the piston, over-consumption of lubricating oil, and piston slap.

Measurement of piston dimensions

A piston is designed to maintain an even clearence with the cylinder during operation when thermal expansion is taken into consideration. Therefore the dimensions of the piston in the cold stage are supposed to be smaller than in the operating state by the amount of  thermal expansion that takes place. The upper part of the piston is heated more than the lower part. Therefore its diameter isthe smallest and the top and increases toward the bottom. In other words, a piston has conical shape. Since heat is transmitted through the ribs that connect the bosses of the piston head and the piston pin, the ribs and bosses are heated more than the other parts. This mean that the expansion in the axial direction of the piston is larger. Therefore the diameter in the pin direction is smaller than the diameter in the perpendicular direction. (this called Ovality) (A cast iron piston is exactly round). Since a piston has the complicated shape explained above, its maximum diameter is measured when finding its dimension measurements.