Abstract

Lubrication is a crucial aspect when talking about efficiency and durability of any moving machine parts and, in particular, for large internal combustion engines. As per definition a lubrication system is designed to deliver a reasonably stable and clean oil film at the correct temperature and with the proper flow. It must accomplish several purposes like prevent direct contact between moving parts, reduce friction, reduce wear, provide cooling, sealing and cleaning effect, adsorb shocks and reduce noise. All these functions together, contribute to components and systems lifetime and ultimately to the overall engine operation. Predicting and simulating engine lubrication performances is therefore particularly challenging both in terms of oil splash in the sump and in terms of forced flow in the oil circulation system. Referring to the latter, a CFD model of the oil channels needs to take into account the inertial effect due to the complex motion of the engine parts, and it has to be able to simulate the transient nature of the flow with far different spatial scales at the same time. In large combustion engine having some meter-long crankshaft, the flow inside bearings and the leakage through small gaps strongly affect the oil flow and pressure behaviour. In this study the attention was focused on the big-end-bearings oil feeding system of an 18-cylinders engine configuration, which is one of the biggest existing gas 4-stroke engine ideal for base load application. Wärtsilä and EnginSoft built a Moving Particle Simulation model, a mesh-less method to solve the Navier-Stokes equations, which allows simulating very complex geometries with moving parts. The Wärtsilä engine model included all the relevant moving items, like crankshaft, bearings, connecting rod and pistons with their motion. The modelling and simulation of the engine using Finite Volume CFD techniques would be unfeasible due to the geometrical complexity of the oil system and above all due to the motion of the engine parts, which would make the management of the mesh motion not practicable. By simulating the flow through the channels, the Moving Particle Simulation model allowed calculating the transient pressure behaviour in crucial areas. The comparison of two bearing configurations highlighted differences in terms of pressure stability, peaks and low values that could potentially lead to cavitation issues. The results of the comparative analysis are presented and explained, together with accompanying illustrations.