Thermodynamic analysis of the turbocharged marine two-stroke engine cycle with different scavenging air control technologies

Abstract

Scavenging air control has been recognized as a practical approach to optimizing the marine two-stroke engine performance under off-design conditions. This paper aims at providing a comprehensive comparison of different tuning technologies from thermodynamic perspectives. A new theoretical model for the marine two-stroke diesel engine is first built with all assumptions of an air-standard cycle relaxed. Thermodynamic characteristics of the two-stroke engine cycle integrating the turbocharged scavenging process are then studied, followed by parametric studies of different scavenging air control methods. The results show that there exists an optimal exhaust valve closing timing to minimize the so-called “Miller loss” with the high-pressure tuning. Despite similar fuel saving potential of 2.5 g/kWh is observed at low to medium loads, the high-pressure tuning is superior to the exhaust gas bypass tuning because of a lower engine thermal load. The power turbine bypass appears to be the best solution when the engine frequently operates at loads higher than 60%, while the sequential turbocharging system shows better performance at engine loads lower than 50%. Thus, factors including the engine thermal and mechanical limitations, ship’s operational profile, cost and package should be considered for selecting the optimal scavenging tuning method in a practical case.