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.
