Variable valve actuation (VVA) for next-generation marine and off-road engines: a comprehensive review for meeting future emissions legislation

The marine and off-road sectors face challenges in complying with strict emission regulations. While zero‑carbon fuels and advanced combustion concepts are promising, variable valve actuation (VVA) technology offers immediate benefits for reducing emissions and increasing powertrain efficiency. Contemporary VVA reviews predominantly focus on stoichiometric spark ignition (SI) engines. The possibilities that VVA offers for modern compression ignition (CI) and dual-fuel engines with advanced combustion concepts have not yet been systematically explicated and evaluated. The present study decouples VVA's impact on three critical areas in CI and dual-fuel engines: exhaust thermal management (ETM), gas exchange, and combustion. Each has been thoroughly explored and the trade-offs between emission reduction potential and efficiency have been quantified. Using a systematic literature review (SLR) methodology, findings from 78 core research papers are corroborated, focused on marine and off-road applications. There is special emphasis on VVA in heavy-duty, medium- and high-speed engines. Key conclusions highlight that cylinder deactivation and intake modulation (Miller cycle) elevate exhaust temperature at low loads without compromising fuel efficiency. This is due to adjusting in-cylinder heat capacity and heat release rate. Modulating intake valve opening (IVO) and closing (IVC) are most promising in reducing pumping losses and enhancing volumetric efficiency and in-cylinder flow dynamics in high-speed engines. In medium-speed engines, a positive pressure gradient hinders the direct pumping loop effect, but the strategies can support the functioning of contemporary lambda control subsystems. Intake modulation and VVA-invoked exhaust gas recirculation allow fast combustion-phasing control by adjusting in-cylinder thermodynamic conditions. This extends the operational limits of ultra-clean, low-temperature combustion used in marine dual-fuel engines. The review underscores that VVA could be a key enabling technology for the next-generation of marine and off-road powertrains.