A Two-Stage Variable Compression Ratio System for Large-Bore Engines with Advanced Hydraulic Control Circuit and Mechanical Locking Device

In order to meet upcoming emission targets, an increasing number of ships using Liquefied Natural Gas (LNG) as fuel have been put into service. In this context, many shipowners are particularly interested in the dual-fuel (DF) large-engine technology, which enables ships to operate with both gaseous and conventional liquid fuels. The use of different combustion principles in DF engines requires a layout of the base engine with a relatively low compression ratio (CR) for the gas mode to prevent unstable combustion (knocking). However, this layout leads to disadvantages in the Diesel operation mode, which requires a higher CR for optimal fuel efficiency. Therefore, a two-stage variable compression ratio (VCR) system is a technology particularly suitable for DF engines. It allows to reduce fuel costs by approximately 5.5%.

This article presents an innovative VCR connecting rod (conrod) design for modern DF engines that adapts the piston position by changing the effective conrod length. The VCR system is developed by the Institute for Combustion Engines of the Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen University together with Forschungsgesellschaft für Energietechnik und Verbrennungsmotoren (FEV) Europe GmbH. It is equipped with a novel functional principle inside the conrod’s small eye specifically tailored to large engine boundary conditions. The system includes an advanced hydraulic circuit combining the function of a hydraulic freewheel, the oil supply for piston cooling, and a mechanical locking device (LD) for both CRs. In a comprehensive simulation study, the layout and the system behavior of the new hydraulic circuit are presented using a one-dimensional (1D) hydraulic-mechanical simulation model, which was validated in advance with measurement data from a passenger car (PC) engine. The study intends to examine the functional behavior of the VCR system during engine operation. The focus is on the switching process between the two CRs, as well as on the fixed CR operation. The aim is to provide a deeper understanding of the hydraulic-mechanical behavior and to identify special requirements on the system.