- 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
Thursday, November 8, 2012
Saturday, October 20, 2012
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.
(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
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.
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.
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.