In addition, fuel consumption remains unchanged and NOx emissions is further reduced. The reduction in NOx emissions may be contributed by the decrease of premixed combustion fraction.
5.2.4 Maximum pressure, Pmax
The test results generally show that the maximum pressure increases with water addition. The relative increase in Pmax is more significant at high loads and seems to grow with an increase in the water content. Because the ignition delay is increased with water addition, more fuel is injected prior to combustion. As a result, the premixed combustion increases leading into an increase in peak pressure. However, the peak firing pressure may be reduced at low loads and/or high water quantities as a result of the dilution of the charge molar and energy densities by water vapour. An increase in pressure is often a deskable feature for high power output. Moreover, the time when Pmax occurs is retarded, (generally 1-2｡? with water addition) but despite this change, the process is still efficient since the peak pressure reaches very close to the TDC. No clear relationship is found between water rate and position of Pmax, since the ignition change (1-2｡?) remains the same even with a 70% W/F ratio.
5.2.5 The maximum rate of pressure rise, (dp/dθ)max
The maximum rate of pressure rise at the beginning of combustion (dp/dθ)max is a function of fuel quantity, which reaches combustible conditions at the end of ignition delay period [23]. It follows that a long ignition delay as a result of water addition can lead into an increase in (dp/dθ)max
As shown in Fig.4, there is a small influence of the longer ignition delay on (dp/dθ)max , which seems to increase proportionally to the water content. The rate of pressure rise may be as high as 11 bar per crankshaft angle in this engine. In addition the test results indicate that the relative changes become more significant at high loads and water rate around 50% of the fuel volume.
The higher rate of pressme rise may cause a rough combustion of the engine, leading into engine "Knocking". However this is not observed during the tests.
5.2.6 Heat release process
Fig.5 shows the heat release pattern and the heat release rate (dQ/dθ) at different water rates as a function of speed-load.
The findings from this analysis indicate that the maximum heat release rate increase with water addition. Notice that the cumulative heat release at any stage of combustion process is directly proportional to the pressure rise. However, this stage of combustion can be responsible for a disproportionately large production of NOx emissions. It was already discovered [17] that the major sources of NOx were always associated with regions of intense combustion activity. Analysis of the test results shows the effect being less pronounced at low loads.
Combustion duration shows no significant change with water rate changes. The results at high power levels show that the timing of combustion end is not affected. On the basis of these considerations it is indicated that the rate of heat release at premixed combustion period after ignition, becomes higher while a small increase is also recognised at later combustion regions, leading into shorter combustion duration. On the other hand at low power, the whole heat release pattern may be shifted by almost 2 degrees CA (depending on the ignition timing) regardless of the W/F ratio increase, while an increase in the combustion duration may be obtained.
6. THE EFFECT OF WATER ON INDICATED POWER AND MECHANICAL EFFICIENCY
The indicated power is found to increase when W/F ratio increases. Judging from the results shown in Fig.5 and Fig.6, it seems that the increase in the indicated power is contributing to the shorter combustion duration in conjunction with an improvement in fuel-combustion efficiency.
Furthermore, with water rejection in the retake air manifold, it is inevitable that some water, either drops or vapour, will contact the cylinder surfaces, hence, disintegrating the lubricating oil film. As a result the friction losses may increase leading to a lower mechanical efficiency. This is consistent with the results shown in Fig.4, where at 70% water rate, SFOC is increased due to the increase in friction losses. The effect of water strike on the cylinder wall, investigated by Torpey et al [26], indicates that 10% dilution of the lubrication off occurred in 50 hrs of engine running with water addition.
7. CONCLUSIONS
This research is primarily carried out to assess the potential of water to reduce NOx concentration in the exhaust plume, based on the idea that lower in-cylinder temperature leads to lower NOx formation. The secondary reason is to investigate the effect of water on other harmful emissions, combustion efficiency and engine reliability. The main outcomes of the investigation are smnmarised as follows.
・As much as 35% of NOx reduction has been achieved without any increase in SFOC.
・CO concentration decreases at high power levels.
・Ignition delay increases slightly with water injection in both low and high loads; this tendency is more pronounced at low loads.
・The percentage of CO2 emission tends to increase when the percentage of excess oxygen tends to reduce.
・The experimental findings also indicate that as the mount of water injected increases the exhaust temperature decreases.
・The increase in ignition delay with water addition leads into an increased fraction of premixed burning favouring higher rates of heat release, pressure rise, hence higher Pmax, is resulted. Moreover, at high power, the combustion duration is shorter resulting improved combustion.
・The indicated power increases with W/F ratio increase leading to a lower mechanical efficiency as the brake power remained unchanged.
The experiment is also conducted to determine the amount of water the engine can absorb. From our results we suggest that the optimum water content varies from 35 to 50% W/F ratio.
