3.5 Analysis Results
Analysis was conducted for the following two cases: [A], absolute humidity (Ha), intercooled air temperature (Tsc), and engine load (Load) were used as correction factors; [B]-F1, fuel oil density (F1) is used as well; [B]-F2, cetane index (F2) was used in addition; and [B]-F3, CCAI (F3) was used in addition. Table 4 shows the analysis result of Kfci and multiple correlation coefficient on CGE1. In case [A], the multiple correlation coefficient is 0.985. In cases [B]-F1, -F2, -F3, where fuel oil properties were added to the correction factors, the multiple correlation coefficient was between 0.986 and 0.988. This shows that the multiple correlation coefficient is almost the same irrespective of whether fuel oil properties are considered. The authors also conducted analysis about the case [C]-F2 where ambient temperature (Ta) factor was added to the case [B]-F2. As a result, regarding Kfci of the individual correction factors, there was no significant difference attributed to the combination of correction factors as far as the results of this analysis were concerned. In comparison between the case of NOx Technical Code and the results of this analysis, the Kfci of ambient temperature(Ta) was almost the same, while those of absolute humidity (Ha) and intercooled air temperature (Tsc) were greater in this analysis, nearly double and triple, respectively. Fig.4 shows the results of corrected NOx-concentration data using the correction formula in eases [A] and [B]-F1. The results obtained by the correction formula of NOx Technical Code indicate that the values include a seasonal fluctuation of 200 ppm, while the results of correction obtained by correction formula [A] and [B]-F1 showed a fluctuation of only 100 ppm. There was no significant difference between the results calculated by formula [A] and [B]-F1, in the latter of which fuel oil density was considered.
4. COMBUSTION TEST BY SINGLE CYLINDER ENGINE
4.1 Test Engine Specifications
The engine used in the combustion test was Daihatsu Diesel's direct-injection medium-speed single-cylinder diesel engine(SCE), with cylinder bore diameter of 200 min. Table 5 provides the specifications of the test engine. The engine power was set to 160 kW at 900min-1. As fuel oil, MDO (density, 0.873; calculated cetane index, 45) was used. In this system, the fuel injection rate was adjustable by controlling the rack stroke of fuel injection pump. Intake air was supplyed to the engine after compressed by screw compressor and reduced the pressure by control valve. And the intake air temperature was controlled by air heater. In this combustion test, fuel injection rate and supply pressure were set for determined engine load levels specified in the test cycle type D2(E2), in reference to the performance data of engines with commercial six cylinders 200 mm in bore diameter, which is equivalent to the bore diameter of SCE.
4.2 Humidity Adjustment of Intake Air
The moisture in the air supplied to the engine is below 3 g/kg as in absolute humidity because of pressurized by a compressor, cooled, condensed and drained. To vary the absolute humidity of supply air, as shown in Fig.5, water was injected through a spray nozzle installed at the air intake port. The injection rate was controlled by obtaining the rate of air supply to the engine, to vary up to 20 g/kg maximum in absolute humidity.
4.3 Test Results
4.3.1 Change of Absolute Humidity of Supply Air
The moisture content in supply air was changed by controlling the quantity of water injected to air intake port. At the individual engine load levels specified in the test cycle type D2(E2), NOx concentration in emissions was measured to determine its fluctuation with varying moisture content.
Fig.6 shows variation in NOx concentration at changing absolute humidity of supply air. The abscissa axis represents water content g/kg of dry air, which is the sum of the humidity before water injected and the humidity added by water injecting. According to auxiliary-engine test cycle D2, data is shown for the individual load levels of 10, 25, 50, 75 and 100%. The data of main-engine constant-speed test cycle E2 are shown for all the load levels except 10% load.
