NOx Reduction Study on Four Stroke, Slow Speed Engine for Marine Propulsion Use
Hiroaki KIMURA*, Satoru GOTO*, Takahisa MIMURA*, Katsuhisa IKAWA**
Genarally, NOx emission from medium-size four-stroke slow-speed engine is high level, comparing to the IMO NOx emission regulation, so some NOx reduction method is needed.
In this study, design parameters such as compression ratio, intake and exhaust valve timing, charge air pressure and injection timing are optimized, at first step. The engine performance tendency including NOx reduction effect is investigated from a simulation method, and the engine test using the modified engine components. The result is that NOx emission level is improved below IMO Regulation without drastic modification to the engine, and keeping same specific fuel consumption (SFC) and maximum cylinder pressure level as it is.
Also, further NOx emission reduction will be necessary, because NOx regulation will become more and more stringent. So, steam addition system, that adds steam to combustion air and to maintain the absolute humidity at a desired level, is developed at second step. NOx emissions were decreased through steam addition, and virtually no decline was observed in SFC.
Key Words: NOx, Diesel Engine, Simulation, Steam, Humidifieation
The level of NOx emission from medium-size four-stroke slow-speed engines is somewhat higher than that from medium-speed engines. Based on the IMO regulation on NOx emission, those engines must be modified to enable their NOx emission levels to be lowered significantly. The authors therefore investigated a simulation method that would reduce NOx emission without sacrificing specific fuel consumption (SFC). Design parameters such as compression ratio, intake and exhaust valve timing, charge air pressure and injection timing are optimized. N0x emission is succeeded in reducing through the installation of modified engine components designed based on the simulation results. NOx emission is successfully reduced to below the IMO regulation with no drastic structural modification or loss of thermal efficiency.
The current IMO regulation can be satisfied through this optimization. However, since NOx regulation will become more stringent and thermal efficiency will need to be increased in the future, method for further NOx emission reduction will be necessary. A system has therefore been developed that adds steam to intake air and to maintain the absolute humidity of the intake air at a desired level, even if the ambient conditions (temperature and relative humidity) are varied.
2. NOx REDUCTION THROUGH OPTIMIZATION OF THE DESIGN PARAMETERS
A general simulation software is applied to four-stroke slow- and medium-speed engines in order to investigate NOx emission reduction by optimizing the engine design parameters. A modified engine components developed based on the simulation results was prepared and installed. Then, engine test was operated to verify the NOx emission reduction. Table 1 shows the major engine specification, and Fig.1 shows the E3-mode NOx emission level for each engines prior to design optimization. Slow-speed engines in particular do not satisfy the IMO requirements, as shown in Fig.1. The goals for NOx reduction of 40% for engine A, 21% for B, and 7% for C are planned.
The compression ratio, intake and exhaust valve timing, charge air pressure and injection timing were studied.
As an example, Fig.2 shows simulation results demonstrating the effects of each parameter upon NOx emission and SFC.
Table 1. Engine Specifications