Quote:
Originally Posted by RedDevil
I like the suggestion of combining (increased?) EGR with water injection.
I saw heat was a limiting factor on EGR flow.
I am confused though as why it has to be a true vapor.
I feel the coarseness of the spray can be an issue, of benefit or otherwise. Deliberately using tiny droplets instead of fine spray could help as they would evaporate more slowly, like after combustion instead of during, not hindring the combustion process too much but still add to the volume of the exhaust gas. But it is just an idea and I realize how much that depends on the other parameters (like revs, pressure, fuel mixture, temperature) to work or be detrimental.
I overstep my knowledge on this, but pray the corral will be mild to me.  |
Here in the Corral, good questions are welcome.
It would seem only logical that if increased EGR is good, and water injection can do the same, wouldn't the cumulative sum of both be even better?
Unfortunately, it isn't so simple. EGR is mainly composed of water, CO2 and the N2 that makes up the bulk of air. It is relatively un-reactive so it's sole purpose is to act as a heat buffer via it's inherent heat capacity and it's dilution of the oxygen percentage so that the fuel doesn't burn fast and hot since rate of fuel oxidation is reduced. Water vapor does the same thing. The steam displaces the oxygen bearing fresh air and reduces the rate of oxidation in the same way EGR does. If, as T Vago is doing, you increase the EGR to the point of wide throttle opening with low torque output for cruising, you may be up against the wall of combustion flame out. At this point, the addition of more EGR may be detrimental as the combustion flame becomes erratic and torque falls off. The addition of water vapor would do the same - variable combustion ensues and power is reduced while emissions increase.
I differentiate between water vapor and water droplets because as you correctly implied, the absorption of heat by water droplets and the subsequent evaporation can be dramatic. This can be useful in high load situations where extreme heat results in uncontrolled detonation and the evaporating water droplets provide considerable buffering, much the same as the cooled EGR performed in the SwRI paper link. An interesting aside is the phase change advantage of water droplets adding to the pressure above the piston. Ricardo observed this back in the 30s when testing water injection in boosted aircraft engines. Coarsely injected droplets added a measurable (2-3%) amount of power above that of water vapor even with the phase change absorption of heat. He surmised it was the added pressure of the liquid water turning into steam. Over fueling an engine results in much the same gain. By adding extra fuel which vaporizes and quenches the combustion, you can control detonation but also you can add to the effective pressure above the piston. Of course, economy goes down and emissions go up.
One COULD use EGR and water injection together, but EGR is available and free so it's use only makes sense from a complexity and economic point of view.