Exhaust thermal management in a dual-fuel marine engine via fully variable valve actuation and wastegate lambda control

With the growing focus on maritime decarbonization, dual-fuel combustion emerges as a promising solution due to its adaptability and minimal system modifications. However, alternative fuels like natural gas pose challenges such as methane slip (CH4), which has a greater impact on global warming than CO2. To mitigate methane emissions, conventional aftertreatment systems might incorporate a methane oxidation catalyst. However, effective methane oxidation requires high temperatures of approximately 450 °C. Therefore, exhaust thermal management (ETM) is crucial for maintaining high exhaust gas temperatures and ensuring high conversion efficiency. This study investigates the effectiveness of fully variable valve actuation (VVA) including variable exhaust valve opening (VEVO) and early intake valve opening (EIVC) along with lambda control via wastegate control. The primary objective is to evaluate the impact of each strategy on elevating exhaust gas temperature (EGT) while considering potential trade-offs with efficiency. Using a model-based approach, the research focuses on a state-of-the 6-cylinder natural gas (NG)/diesel dual-fuel marine engine (Wartsila 6L20 DF), employing two-stage turbocharger with wastegate. Numerical simulations are conducted using a one-dimensional (1-D) engine model within GT-SUITE across four different load conditions. The model is validated using baseline valve timings and a comprehensive dataset of experimental data, adhering to strict accuracy targets with a deviation threshold of 2% for all performance parameters. Results indicate that all strategies contribute to EGT elevation. VEVO raises temperature by 73 K but reduces brake thermal efficiency by 3.85%. In contrast, EIVC increases temperature by 100 K with a 1.1% drop, while lambda control achieves a 90 K gain with negligible loss. Thus, VVA-based ETM and lambda control facilitate rapid warm-up of exhaust aftertreatment system (EATS) in large bore engines with minor efficiency penalty, enabling compliance with stricter emission regulations. Eventually, this will enhance air quality and maritime ecosystem and contribute to maritime decarbonization.