5. EXPERIMENTAL RESULTS
Water injection rate is controlled in proportion to the fuel consumption as a basis for evaluating NOx reduction. The water rate varies from 0% to 70% of the value of the fuel consumed. Tests are carried out at four different speeds (1030, 1500, 1800, 2000 rpm) combined with different levels of output shaft torque (30, 60, 90, 120 Nm).
5.1 Effect of Water Content on Emission Fig.3 shows the relationship between the water content and NOx, CO, CO2, and O2.
5.1.1 NOx
It is predictable that water injection is effective in reducing NOx at all W/F ratios. In fact the highest reductions are always achieved with the highest W/F ratio used. The maximum reduction in NOx is as much as 45% at 70% W/F ratio and 2000rpm-120Nm. The average reduction in NOx from dry to 20, 35, 50 and 70% water was 17.5, 25, 30 and 37% respectively. It must also be pointed out that NOx reduction varied with engine operating conditions. It is recognised that the NOx reduction effect improves when the engine load and water content increases. This is because that with an increase in load the fuel consumed increases thus the absolute water injection amount is higher for a given W/F ratio. It is also noted that water injection has a greater effect in NOx reduction at high to medium loads.
If NOx is analysed separately as NO and NO2, the results show that the water reduction is more effective in reducing NO2 than NO. Notice that NO2 is a product of further oxidation of NO at high temperatures. It is concluded that as the water content increases the NOx suppressing effect increases too.
5.1.2 CO
In general, CO emissions show an increase trend with water addition. A significant increase is observed at low loads. However, at high power a tendency of CO reduction is detected. The average increase in CO at 30Nm, from dry to 20, 35, 50, 70% W/F ratio is 13, 19, 23 and 24% respectively. For a given shaft torque, CO emission levels vary with engine speed. For example, at 120 Nm torque, with the same water rates as above, the average change of CO emissions is -1, +7, -3 and +8%, respectively.
Most efforts in reducing emissions of CO are concentrated on the completion of oxidation of CO to CO2. The oxidation reactions would be incomplete, if there is a lack of oxidants or insufficient reaction temperature. At light loads the addition of water will effectively reduce the compression temperature (chilling and dilute the reaction zones). As a result, the rate of reaction is not high enough to carry out the oxidation process, thus enhances the amounts of partial oxidation products [18]. However at high loads the temperature levels are significantly higher due to the combined effect of faster burning rates and increased cylinder pressure, despite the impact of water addition. As a result, a reduction in CO emissions is observed.
5.1.3 CO2
It is found that carbon dioxide exhaust emissions increased with W/F ratio increases under almost all the test conditions. The average increasein CO2 from dry conditions to 20, 35, 50 and 70% water is 2.4, 3.5, 3.4 and 5%, respectively. The amount of fuel injected into the engine cylinder per cycle is nearly the same thus the increasein CO2, can be attributed to the variation in total mole number of exhaust gases. For example, when the charge air is induced at a lower rate (less N2 & O2), the total amount of exhaust gases decreases, which leads to higher CO2 concentration. With high-pressure atomisafion of water, the rates of vaporisation may be sufficiently rapid to displace some of the air charge before the suction valve closes. Concentration of CO2inexhaust gases may also increase because of the water gas reaction.
5.1.4 O2
The results also present that the percentage of O2 concentration in the exhaust decreases with an increase in W/F ratio. As mentioned earlier the reason for this reduction is the dilution of the inlet air by the water vapour. The average reduction in O2 from dry to 20, 35, 50 and 70% W/F is 0.9, 1.3, 1.9 and 3.5% respectively.
The effect of water on the formation of CO2 is nearly opposite to those on the excess oxygen concentration. It is found that the sum of the exhaust concentration of oxygen and carbon dioxide is approximately constant for each change in W/F ratio.
5.1.5 Particulate
Particulate emissions are not measured in this study. Previous tests [12] showed that particulate emissions were increased considerably with water addition. The increase was from 32% at 1500 rpm and 60 Nm to 178% at 2000 rpm, 120 Nm.
5.1.6 HC
No data were obtained in respect of the effect of water addition on HC emissions. However examinations carried out by other researchers indicate a strong increase in HC emissions with water injection to the intake air manifold [11][19]. This is related to the fact that the ignition delay increases (expanded lean flame zone, which governs HC emission concentration) and the cylinder cycle temperatures decreases (suppressed oxidation rates)[20][21]. Moreover, water impingement on the cylinder wall exacerbates this problem as a result of the flame quenching process[22].
