The diesel engine is widely used for marine vessel propulsion due to its relatively high efficiency compared to existing alternative propulsion systems. The majority of these engines are slow speed two stroke ones. Despite the improvement of their efficiency there now exists a demand for drastic reduction of daily fuel oil consumption as a result of the global financial situation and continuously increasing fuel prices. Towards this effort, slow steaming is a promising solution for the drastic reduction of daily and specific fuel consumption when expressed in tn/mile. This requires engine operation in the low load (low speed) range where these engines are not designed to operate for long term. The main problem related to slow-steaming, is the lack of air which has a negative impact on the engine and its subsystems. A promising solution to the problem is turbocharger (T/C) cut-out at low load when more than one T/C exists. In the present work a combined computational and experimental investigation is conducted to evaluate the operation potential of a large two stroke marine diesel engine equipped with two T/Cs using T/C cut-out, for which the specific technology presents various challenges. This is achieved using an in-house engine simulation model and measurements with and without T/C cut-out. From the results it is revealed that using this technique the scavenging air and peak firing pressure increase while the specific fuel consumption decreases. In this way, some major problems related with the long term operation of the engine under low load conditions, i.e. accumulation of carbon deposits on the exhaust gas side and continuous operation of the auxiliary air blowers, are surpassed. Moreover, a theoretical investigation is conducted considering fuel injection retard to minimize the peak firing pressure penalty while taking care to limit the corresponding negative impact on specific fuel consumption. For NOx emissions the effect of T/C cut-out is also considered using tail pipe emission data measured during the official shop tests. From the analysis conducted it has been revealed that the technique of turbocharger cut–out (one of two) is technically feasible and could offer certain advantages when slow-steaming is implemented. Moreover, comparing the calculated with the measured results, it has been revealed that the simulation model successfully estimates engine operation with and without T/C cut-out, being a valuable tool for the engineers to investigate combustion and pollutant formation mechanisms under various engine configurations.

In the last years many research studies have been focused on the features of MILD combustion that is a stable form of combustion, obtained with high temperature reactants and high exhaust gas recirculation and characterized by low flame temperature and, consequently, low Nox emissions. This form of combustion is also characterized by low light emissions (for this reason it is also called “flameless” combustion) and a large range of stable operation. MILD combustion has been already applied in industrial furnaces where ceramic regenerators provide to raise the temperature of the entering diluted air, the main advantages being high efficiency and low emissions. The introduction of MILD combustion in power plants would allow for increasing the temperature of the entering reactants beyond the self-ignition temperature without increasing the NOx emission. The main goals of this technique are low combustion exergy losses, large range of stable combustion, and low NOx emissions. Some experiments have shown that the flameless conditions can be obtained using diluted reactants, even using heavy fuel oil. Good results in terms of NOx emissions and soot formation have been obtained for heavy oil combustion in a 10% oxygen concentration of reactants and combustion chamber inlet temperature of about 900K. In order to meet these conditions, a semiclosed CCGT cycle with high recirculation ratio, suitable for the use of heavy fuel oil, is proposed here, assuming state-of-the-art technologies for gas turbine and steam plant and steam cooling of the turbine blades. The thermodynamic analysis shows that the overall plant efficiency of the new scheme is close to 60% that is about the efficiency that can be obtained in modern CCGT power plant fuelling natural gas.