Potential of a two-stage variable compression ratio downsized spark ignition engine for passenger cars under different driving conditions

• Published in: Energy conversion and management 2020-01, Vol.203, p.112251, Article 112251
• Main Authors: López, J. Javier, García, Antonio, Monsalve-Serrano, Javier, Cogo, Vitor, Wittek, Karsten
• Format: Article
• Language: English
• Subjects: Actuation; Air pollution; Automobile industry; Automobiles; Carbon dioxide; Carbon dioxide emissions; Catalysts; Compression; Computer simulation; Connecting rods; Downsized internal combustion engines; Driving ability; Driving conditions; Driving cycles; Eddy current testing; Eddy currents; Emissions; Engines; Fuel consumption; Internal combustion engines; Knock; Load; Nitrogen oxides; Pollutants; Pollution control; Spark ignition; Superchargers; Switching; Thermodynamic efficiency; Variable compression ratio
• ISSN: 0196-8904; 1879-2227
• DOI: 10.1016/j.enconman.2019.112251

•Transient time of the variable compression ratio system characterized experimentally.•Compression ratio switching time described through a polynomial function.•Non-instantaneous behavior of the system evaluated in seven driving cycles.•The non-instantaneous behavior leads to fuel consumption penalties between 1–2%.•3–4% lower fuel consumption compared to the fixed compression ratio. With the aim of reducing pollutant emissions from internal combustion engines (ICE), the application of stoichiometrically operated spark ignition (SI) engines, for light-duty vehicles, has been overcoming the compression ignition (CI) engines market share throughout the past years. The ability of a substantial reduction of the primary harmful emissions (HC, CO, and NOx) through the use of the simple three-way catalyst (TWC) is the main reason for that. Nonetheless, with increasing attention to CO2 emissions, the development of highly efficient downsized SI engines turn to be of enormous interest. The synergies of multiple systems such as direct injection, turbocharger, and variable valve actuation are able to lead the SI efficiencies closer to those of CI engines. However, to enable high load operation on such downsized engines, the compression ratio (CR) must be reduced due to knock limitations, reducing the partial-load operations efficiency. The implementation of two-stage variable compression ratio (VCR) systems enables the extraction of high thermal efficiency with high CR at lower loads and extended knock-free high load operation with low CR. In this study, the evaluation of a two-stage VCR system applied to a state-of-the-art downsized SI engine was made through standard driving cycle simulations. The VCR mechanism is composed of an eccentric element in the small end of the connecting rod, which is rotated to increase/decrease the effective connecting rod length, achieving the CRs of 12.11:1 and 9.56:1. The engine was run in an eddy-current dynamometer test bench throughout the essential operating range to obtain the brake specific fuel consumption (BSFC) map. The VCR mechanism CR switching delay was also experimentally characterized to derive a function of the operating conditions. The measured map was entered into the map-based driving cycle simulation with a sub-model to account for the isolated effects of the transient period encompassing the compression ratio switching. The results show that slow CR transitions lead to fuel consumption penalties, which suggests th