Negative impact of constant RPM control strategy on ship NOx emission in waves

Abstract

In severe wave conditions, the ship propulsion system is loaded with high fluctuations due to external disturbances. The highly fluctuating loads enforce radical changes in the main engine torque, which in turn demands variation of the fuel rate injected into the cylinders if a constant rotational speed strategy is applied. Therefore, the temperature of gases varies to a large extent during the combustion process in the cylinders. The emitted NOx is a function of this highly fluctuating temperature. The main goal of this study is to investigate NOx emission under the aforementioned conditions when a usual constant RPM control strategy is applied in waves similar to the calm water condition. The paper presents a mathematical model of the whole system, which is applied to a selected ship both in regular waves and in calm water conditions. The results show that the sea waves, in comparison with the calm water condition, can radically increase the emitted NOx under the constant rotational speed strategy. This change can reach even 1014 times more, averagely. The results also show that the higher the wave height the higher the emitted NOx. It is concluded that the control strategy of keeping the engine rotational speed in waves at a constant level is the most important reason for the significantly increased NOx emission in waves in comparison with the calm water condition.