Apply a fixed support boundary condition to the main bearing journals of the crankshaft. This represents the rigid connection between the crankshaft and the engine block. If you want to consider frictional effects, you can apply friction coefficients to the contact surfaces between the crankshaft and other components, such as bearings and seals. This helps simulate the resistance and energy losses due to friction. It's important to note that the specific boundary conditions may vary depending on the type of analysis you are performing and the objectives of your study.
Please check the below course for more details on structural boundary conditions.