Engine Performance Impact for Water Injection

reduction in turbine inlet temperature. A 1:1 water-to-fuel ratio was selected as the optimal ratio to reduce NOx 80 percent and still maintain combustor stability. While holding a constant takeoff thrust, figure 48

illustrates the modeled results of water injection. As the water flow rate is increased, the engine spool speeds (N1 and N2), the compressor exit temperature (T3), and turbine inlet temperature (T4) all decrease.

Figure 48.—Water injection directly into the combustor was modeled to provide a 120 °F reduction in T4.

Figure 49.—Water injection resulted in LPC and HPC surge margin losses of 0.4 and 1.6 percent, respectively.

Figure 50.—A turbine rematch would be required with water injection to correct for compressor surge and would result in a 0.4 percent SFC penalty for a retrofit engine. The LPC and HPC showed less deterioration in surge margin than for LPC water misting. Figure 49 shows that at the optimal 1:1 water-to-fuel injection ratio, the LPC loses 0.4 percent in surge margin while the HPC loses 1.6 percent.

For some types of engines, this level of deterioration can be tolerated without further intervention. However, for other engines that are more sensitive to compressor surge, some action will need to be taken to resolve the issue.

To compensate for the loss in surge margin for the PW4062 engine, the high-pressure turbine would need to be opened up to rematch the new compressor flow rate with water injection. However, figure 50

shows that so doing will result in a 0.4 percent SFC penalty. For long-range airplanes that are very sensitive to engine fuel efficiency, this level of efficiency would be quite onerous.

However, for a new engine that is specifically designed around water injection, this penalty may be eliminated. This again suggests that water injection would be best suited for newly designed airplanes and retrofit options are less suitable.

The EGT coming out of the engine’s turbine is typically also used to control the operation of the engine. As the parts of a gas turbine engine wear and the performance begins to deteriorate, the EGT increases. Therefore, an upper limit is set for each engine at which time the engine must be overhauled to avoid over-temperature situations, which would damage the turbine blades. The difference between operating temperature and the limit is referred to as the “EGT margin.”

Figure 51 shows that when water injection is used during takeoff (for the PW4062 engine with rematched turbine), a 25 °F decrease in EGT is achieved, which will increase the EGT margin. This means that the engine can be operated for a longer period of time before overhaul, which could be of substantial savings for an airline operator. However, once the normal EGT margin is reached, the water- injection system would lose its “optional use” status and need to be operated at each takeoff to avoid turbine over temperature conditions. As this study considered the use of water injection as an option to the operator (to avoid any potential safety issues with a flight critical system), the cost benefits of the

increased EGT margin were not considered in this study.

As the water tends to slightly quench the flame temperature in the combustor, there is an accompanying loss in thermal efficiency of the engine when water injection is used during takeoff. Figure 52 shows that during takeoff, there is a 1.2 percent increase in fuel consumption when water injection is used.

This thermal efficiency loss is reflected in the decrease in combustor exit temperature. This

temperature is shown in figure 53 and is slightly higher than T41 turbine inlet temperature for the takeoff profile of the 747–400ER aircraft. During takeoff and climbout, the combustor exit temperature (CET) is about 135 °F cooler when water injection is used. At the point where water injection is discontinued, about 4 n mi from takeoff roll, the CET returns to temperatures that are almost the same as the baseline PW4062 engine. It is a few degrees cooler at this point due to the turbine rematch that was required to correct the compressor surge margin issue.

Figure 52.—SFC increases 1.2 percent during takeoff when water injection is used.

Figure 53.—CET decreases 135 °F when water injection is used.

One of the benefits of decreasing combustor exit and turbine inlet temperatures is that it can improve the life of the engine hot section. When looking at the normal T41 temperature profile from takeoff to climb in figure 54, one can see from the top solid line that the turbine is exposed to its peak temperature during the 90 sec takeoff and climbout phase of flight, which takes the airplane to about 2 n mi from takeoff roll.

The bottom dashed line shows the T41 profile of an engine with 1:1 water-to-fuel injection rate. Once the aircraft reaches 3000 ft altitude (about 4 n mi) the water-injection system is turned off and the T41 temperature returns to normal. Thus, using water injection during the takeoff roll and climbout will reduce the most life-demanding peak temperature level to about that of normal climb condition.

Figure 54.—Using water injection cuts peak blade temperature right were it is needed most—during takeoff.

3.4.5 Engine Performance Impact for Water Misting.—Due to the more-limited industrial engine