Although details were lost as a result of the fact that other factors such as the temperature of the air were disregarded, our experiments suggest that those correction factor may underestimate the influence of the absolute humidity on NOx emission.
As some land- and marine-engine plants use steam stationarily, it is considered that if steam is utilized to maintain the absolute humidity at a specific value, seasonal fluctuations in NOx emission can be eliminated conveniently. It will also be possible to control the NOx emission level by adjusting the absolute humidity to a specific value. Fig.6 shows the diagram of steam addition system.
The amount of the additional steam is controled as following sequence:
1: Determine the absolute humidity, of the environment air based on the temperature, humidity, and atmospheric pressure.
2: Calculate the water content in the intake air based on the above absolute humidity and the intake air volume calculated based on the charge air pressure and the engine speed.
3: Determine the extent to which the discharge-valve should be opened based on the difference between the water content in the intake air and the target value.
The steam content calculated based on the flow rate, pressure, and temperature in the steam line is added to the data on water content in the intake air as feedback to control the valve opening. Since the amount of intake air is determined based on the charge air pressure and the engine speed as explained above, relation between the amount of intake air, the charge air pressure and the engine speed must be clarified in advance based on the specifications of each engine. The EPA correction factor was used for steam control, as it reflects the influence of absolute humidity upon NOx emission in a similar manner to measurement results compared with the IMO correction factor.
If steam is injected into the front of the air cooler, it condenses and is drained, eventually making humidity control impossible. It therefore be determined that steam should be injected into the crankease intake air trunk after the air cooler.
The test results for steam addition using this system are given below. In the test, it first is checked whether this steam addition system would operate correctly corresponding to changes in humidity by randomly changing the ambient humidity. Changes in NOx emission and SFC were analyzed based on the estimated absolute humidity (including injected steam) inside the intake air trunk.
3.1. Test of medium-speed test engine
Fig.7 shows the test results for a medium-speed test engine. Steam was injected under a test condition of 3 g/kg to 4.5 g/kg absolute humidity, which is similar to that typical for February in Kanto region of Japan. The measurement was conducted under loads of 75% and 50% in constant speed and propeller law conditions. The four-cylinder test engine used had a bore of 255 mm, rated output/engine speed of 840 kW/1000 rpm, and BMEP of 1.82 MPa. The horizontal axis represents the estimated absolute humidity inside the intake air trunk calculated as (amount of water in intake air plus injected steam)/(amount of dry air). The vertical axis represents change of NOx based on the convened concentration to reference oxygen concentration in the exhaust gas at 13%, and change of SFC.
