Parametric investigation of diesel–methanol dual fuel marine engines with port and direct injection

• Port injection combustion of methanol is limited to 50% methanol energy fraction. • Direct injection combustion of methanol allows for 95% methanol energy fraction. • NOx emissions reduce between 1% and 84% for all methanol cases. • Indicated thermal efficiency increases up to 4% comparing to diesel operation. Methanol is identified as a transition fuel to improve the environmental footprint of shipping operations. However, high methanol shares cannot be achieved in premixed combustion marine dual-fuel engines. This study aims at parametrically investigating the impact of two types of methanol injection, namely port and direct injection, on marine engines by employing CFD modelling. A large medium speed marine engine with nominal power of 10.5 MW at 500 rev/m is considered. CFD models are developed for the engine diesel and dual fuel modes with methanol port or direct injections. Several cases with methanol energy fractions ranging up to 50 % for the port injection and 95 % for the direct injection cases are investigated. The developed CFD models were validated for the investigated engine operation in the diesel gas modes, as well as for a small engine with methanol port injection. A parametric study considering the engine settings and methanol energy fraction is performed to identify the engine settings and limits for the combustion knock-free operation for high methanol shares. Subsequently, the comparative assessment of the investigated marine engine performance and emissions parameters is performed for the considered cases. This study results reveal that the methanol direct injection can use up to 95 % methanol energy fraction retaining knock-free combustion conditions, whilst reducing NOx emissions by 85 %. The engine indicated thermal efficiency increases at higher methanol energy fractions for direct injection, whereas opposite trade-offs are exhibited for premixed combustion. Methanol use shortens the combustion durations compared to the diesel mode, reducing the maximum temperature by 1–3 %. This study provides valuable insights delineating the impact of the settings to the marine engines performance and emissions trade-offs, hence contributing to developing methanol fuelled marine engines.