Efficiency Improvement of Large Scale 2-Stroke Diesel Engines Through the Recovery of Exhaust Gas Using a Rankine Cycle

Abstract

Highly increased fuel prices and the need for greenhouse emissions reduction from diesel engines used in marine and stationary applications make diesel engine exhaust gas heat recovery technologies attractive. Such technologies are of special importance for the large two stroke diesel engines due to their large absolute fuel consumption and long operating periods. In this type of engines despite their relatively low exhaust gas temperature there still exists a good potential to recover energy from the exhaust gas. While the use of Rankine cycle is already a well-established technology for energy production with increasing application base, it is still relatively new especially in the marine sector market. This technology offers advantages since it presents relatively high thermodynamic cycle efficiency at both part and full load operation, relatively simple start up procedures, automatic and continuous operation without operator intervention, simple maintenance procedures and long lifecycles. In the present paper, a theoretical study is conducted on a two stroke marine diesel engine equipped with a Rankine cycle to evaluate the potential benefits for fuel consumption. The results with some constraints are also valid for stationary engines used for power generation. A simulation model of the Rankine cycle was developed and employed to estimate the operating parameters of the system. The bottoming cycle considered exploits heat from the exhaust gas stream and the charge air cooler. A comparative evaluation is made for the increase of overall efficiency when using either steam or organic as working medium of Rankine cycle. The analysis reveals that specific fuel consumption is improved for the entire operating range of the installation. Considering the absolute fuel consumption of such engine, it appears to be a promising solution especially if fuel prices remain at the present levels or rise and towards the direction of CO2 emissions reduction.