There are two types of marine diesel engines: crosshead and trunk piston. In crosshead marine diesel engines, the cylinder and crankcase are separated by a partition, and the pressure pushing the piston down is imparted to the crankshaft through the piston rod and the connecting rod.
In trunk piston marine diesel engines, the cylinder and crankcase are not separated and the pressure pushing the piston down is transmitted to the connecting rod directly where it is again imparted to the crankcase (in a manner similar to typical automotive engines). The crosshead variant is found in many large low-RPM 2-cycle diesel engines. It uses a long stroke for increased torque and improves fuel efficiency using low-grade burning oil As for the lubrication of each component in a crosshead 2-cycle marine diesel engine where the cylinder and crankcase are separated (for longer stroke), lubricating oil should be fed separately into cylinder liner that is in contact with piston head inside the cylinder and crankcase. Oil fed into the cylinder liner is referred to as Cylinder Oil and the other oil supplied to lubricate each component within the crankcase is called System Oil