Cylinder Liner, Piston
The cylinder liner is tightened against the top of the cylinder frame by the cylinder cover. It expands freely downwards when the temperature increases during the running of the engine.
The part of the cylinder liner, located in the scavenge air space of the cylinder frame has a number of scavenging airports. The piston uncovers these ports when it is in its bottom position.
The cylinder liner is provided with bores circumferentially for injecting cylinder lubrication oil at regular timed intervals. The cylinder liners are centrifugally cast to reduce porosity and other casting defects. The typical material used is pearlitic grey cast iron with 3% to 3.5% carbon. Graphite flakes have inherent lubricant properties.
Diesel engine pistons possess sufficient strength to withstand the fluctuating gas forces due to combustion, the inertia of moving parts, and high operating temperatures. Piston material also requires good thermal conductivity to conduct heat, preventing thermal stresses.
The diesel engine “Piston” is made up of two parts. The upper part is called the crown and the lower part is the skirt. The top of the crown is concave.
If the piston crown is flat, it will flex under the fluctuating loads leading to fatigue failure. Since the piston is subjected to combustion heat, it has to be sufficiently cooled. Cooling of the piston is necessary, To prevent excessive metal temperatures.
To limit thermal expansion to maintain correct clearances between the piston and the cylinder liner. In a Two-stroke crosshead engine, the piston rod connects the piston to the crosshead. The stuffing box isolates the scavenge space from the crankcase.
The piston crown and skirt are made of cast steel and the piston rod is made of forged steel. Crown houses piston rings, whose function is to seal the combustion space.
The piston rings are made of grey cast iron. The liner fitted in the entablature is provided with O’rings, which avoid leakage of jacket water into the under-piston space. Cylinder water in the entablature jacket enters from the bottom and leaves from the top to avoid the formation of air pockets or airlocks.
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