Thursday, November 8, 2012

Diesel Engine Proper 4

(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 400oC. 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 - 330oC and the bottom of thetopof ring groove is designed to below 230 - 250oC.
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.

Saturday, October 20, 2012

Diesel Engine Proper 3

My apologies to the loyal readers because it has been a long time I did not continue writing about diesel engines. I hope I can still continue writing until the end. And now I will continue from Diesel Engine Proper 2.

[4] Pistons
(1) Functions of the Pistons
A piston is one part of combustion chamber. Its function is to transmit the pressure generated by combustion to the crankshaft through a connecting rod. In addition, it has intake, compression and exhaust functions.
The Piston and the cylinder must form a completely enclosed space in order to ensure efficient intake compression, and exhaust.To maintain complete enclosure under every type of operating condition, the material and the structure of the piston must have high heat conductivity and its thermal expansion must be small even when exposed to high temperature (up to about 1,000degree Celcius, even though momentarily). A piston must have enough strength to withstand the combustion pressure (about 80 kg/cm2) so as to transmit the combustion pressure correctly to the crankshaft.

(2) Piston Structure

Pistons have various types of structures. The basic structure is shown below :
Piston head....Since the piston head directly recieves the combustion pressure, its must be very durable so that it is able to withstand the pressure. Some pistons have reinforcing ribs toincrease strength.
Ring Land.....Piston rings are inserted into rings lands. Ordinarily,a diesel engine has 3 compression rings and 1 or 2 oil rings. Some recent models have 3 rings :  2 compression rings and 1 oil ring.
Oil drain hole.....The oil rings groove has an oil drain hole. Oil that is scrapped off by the ring passes through this hole and drops into the oil pan.
Piston pin mounting hole.....This holds the piston pin that connects the piston to the connecting rod.
Skirt.....The part below the piston pin center.

Oval Piston

Conical Piston
(3)Piston Pin
The piston pin connects the piston and the small end of the connecting rod, and has a hollow cylindrical shape. Piston pin ca be classified into the full floating type and the half floating type, depending on the mounting method. The full folating type is ordinarily used on diesel engine.

(4)Offset Piston
In an offset piston, the piston pin hole is slightly offset from the piston center.
Adequate clearence must be maintened  betwen the piston and the cylinder bore. When the piston changes from the compression stroke to the combustion stroke, as the connecting rod inclination changes the piston moves from right to left. The distance of this movement is equal to the clearence. This is type of shock, which produce of hammering sound. This shock can be decreased by decreasing the clearence of the bore. However, the lubricating oil will be unable to pass through if the clearence is too small. If the rings can not be lubricated, air-tightness cannot be maintened. Or the clearence may become too small due to expansion of the piston. Therefore, the specified minimum clearence must be maintened.