Table 2. Modified Specification, Simulation and Test Result
A higher compression ratio results in lower SFC, higher NOx emission. Also, retarded fuel injection timing results in higher SFC, lower NOx emission. These tendencies were also continned by actual engine test.
It is considered that NOx emission reduction while maintaining Pmax at the conventional level is possible by the method mentioned below and Fig.3. Increases in Pmax due to increased charge air pressure could be prevented by the retardation of the intake valve closing timing. The decrease in the effective compression ratio (which impairs smooth engine starting) due to the retardation of the intake valve closing timing is compensated by raising the compression ratio. And it tuned fine by controlling the fuel injection timing.
Table 2 shows the changes in design parameter, prediction of NOx reduction effect through simulation, and the test results for an engine in which the modified engine components was installed. Heavy, oil was used as the fuel in all tests, including the steam addition tests mentioned later.
Based on the simulation results, the intake valve closing timing was delayed by 30 degCA in engine A, for which the NOx emission reduction goal was set relatively high, and 20 degCA in engines B and C, for which the goals were not set as high as for engine A. Since in engine B the effective compression ratio was relatively high even before the optimization, so it is considered that it is possible to start the engine smoothly even if the effective compression ratio is decreased due to the retardation of the intake valve closing timing, and compression ratio was not changed. Since a valve stamp due to an increase in the compression ratio in engine C may become a concern, its overlap period was shortened by 10 degCA.
Although there were some differences in the influence of each parameter upon NOx emission between the engines, it is concluded that the measurement results implied that simulation result was capable of predicting changes in NOx emission on a quantitative basis. As shown in Fig.4, which indicates the NOx level (in E3 mode) measured in tests before and after the optimization, NOx emission were reduced to below the IMO standard in all three engines.
3. NOx REDUCTION THROUGH STEAM ADDITION
As previously mentioned, optimized specifications for NOx reduction that meet the present IMO NOx requirements was first provided through simulation, and the simulation results were later verified through measurement using an actual engine. However, in the future, the obtained NOx level of optimized specifications will not satisfy the future revised requirements of the IMO, or more strict regulations by public organizations or municipalities. Some methods for further NOx emission reducion will therefore be necessary.
Fig.5 shows the relationship between NOx emission from an engine with a bore of 190 mm and a speed of 900 rpm and absolute humidity over a period of approximately 10 months, including both summer and winter season. The engine was operated continuously under 100% load. The measurement results indicate that the levels of NOx emission fluctuate by +20% to -20% when the absolute humidity changes from 2 g/kg to 19 g/kg. In Fig.5, the reciprocals of the IMO and EPA NOx correction factor are shown with terms other than absolute humidity regarded as 0.
