5.2 Engine performance
During the tests, engine performance are examined in terms of SFOC, inlet air temperature, exhaust gas temperature, Pmax pressure rise rate (dp/dθ)max and ignition delay period. The results are shown in Fig.4.
5.2.1 SFOC
The findings indicate that SFOC is slightly decreased until the water content reaches 50% of the fuel supply rate. The average percentage change of SFOC from dry to 20, 35, 50% W/F ratio is negligible, while at 70% W/F the average increase is 1.2%. The change in SFOC is greater at high loads comparing to low loads. The experimental results show that the maximum increase in SFOC is as much as 2.8% at 70% W/F and 1500 rpm-120 Nm.
For a long time it was believed that low fuel consumption and low NOx contradict each other. However the data obtained in this experiment has revealed that it is possible to lower NOx concentration without adversely affecting fuel consumption.
5.2.2 Inlet air temperature
Test results show that inlet air temperature is reduced with water addition. Upto 30% water rate, the temperature of inlet air drops almost linearly with water rate increases. A further increase in water rate does not have significant effect on inlet air temperature reduction. The results also indicate that inlet temperature change due to water addition varies little with engine speed, i.e. air flow velocity.
The drop of inlet air temperature is mainly caused by heat transfer between water and air. Without water addition, the temperature of air at the inlet manifold is about 31℃. The temperature drops to 22℃ at 30% of water rate and remains at about 20-21℃ for water rates over 30%. The temperature of water before injection is about 15℃. Energy conservation analysis shows that the heat gained by water is about 70% of the heat released by air. This indicates that there is a small portion of water vaporised in the inlet manifold. The humidity of atmospheric air affects the vaporisation of water at the inlet manifold, in terms of vaporisation rate and quantity. However, this effect was not studied in this research.
5.2.2 Exhaust gas temperature
With water addition, the exhaust gas temperature decreases. This tendency is expected due to the reduced cycle temperature and the higher specific heat of the exhaust gas with water addition. The relative reduction in exhaust gas temperature becomes less significant at low power. However at high power and 70% W/F ratio a surpassingly increase in the exhaust gas temperature is observed, which is associated with the later burning.
5.2.3 Ignition characteristics
The results indicate that the ignition delay period increases with water addition. The effect seems to be more pronounced at medium-low power irrespective of W/F ratio. The difference between the ignition start for dry and wet conditions at high power is approximately 1｡?. However, at medium to low power ranges the change is greater, as much as 2｡?.
The reason for this undesirable combustion retardation is attributed mainly due to the reduced compression temperature as well as possible chemical effects. The reduced compression temperature due to the water injection can adversely affect both the physical and chemical processes that take place during the ignition delay. As a result, lower oxygen concentration because of the charge dilution may be responsible for the reduction of reaction rates and retarded ignition [23].
Other tests conducted in the Dept. of Marine technology at the University of Newcastle has indicated that the addition of ignition improvers hastens ignitions and recovers the ignition delay caused by the addition of water [24].
