nicklockard
Torque Dorque
What effect does water injection have in the diesel cycle?
In trying to understand this, I have read up on use of water injection from WWII warplane days. I'm trying to understand exactly what happens within either the intake tract or within the cylinder when you have saturated the air with water vapor--as it pertains to either the diesel cycle or the otto cycle.
In trying to find helpful information, I went hunting for a good steam table. I have found a most useful Java applet from the National Institutes of Standards and Technology (NIST); it is linked/listed in my signature.
What I'm specifically after is a model of cylinder pressure-versus crank angle with and without water vapor. I'd like to know if water injection can allow for running aggressive boost and timing and reduce emissions at the same time.
Using the applet in isobaric properties mode, I put in:
pressure 185 bar (guess at peak pressure)
T-low (immediately prior to fuel injection)= 410 C
T-high (peak temperature) = 2177 C
for a stock 1.9L Tdi. It gives:
http://webbook.nist.gov/cgi/fluid.cgi?P=185&TLow=410&THigh=2177&TInc=10&Applet=on&ID=C7732185&Action=Load&Type=IsoBar&TUnit=C&PUnit=bar&DUnit=mol%2Fl&HUnit=kJ%2Fmol&WUnit=m%2Fs&VisUnit=cP&STUnit=dyn%2Fcm&RefState=ASH
And for a very modded Tdi reaching > 221 bar PCP
If it is possible for a modded Tdi to run > 221 bar PCP, then injected water will be in the form of supercritical steam:
http://webbook.nist.gov/cgi/fluid.cgi?P=225&TLow=350&THigh=3400&TInc=20&Applet=on&ID=C7732185&Action=Load&Type=IsoBar&TUnit=C&PUnit=bar&DUnit=mol%2Fl&HUnit=kJ%2Fmol&WUnit=m%2Fs&VisUnit=cP&STUnit=dyn%2Fcm&RefState=ASH
Moreover, the supercritical steam has lower Thermal Conductivity than normal steam beyond 1137 C which should lower heat losses to cylinder walls during that first fraction of the expansion stroke when pressures > 221 bar until temps drop < 1137 again/ and when pressures < 221 bar again:
I'm hypothesizing that if heat losses are lower during that time, it will preserve peak pressures a smidge longer. If coupled with aggressive Start-of-Injection timing, this could yeild some greater BMEP under some loads and give more useful P-V work upon the crank without the same NOx production the equivilant BMEP-sans-water-injection would produce. (see Michael Willmann's comments in reference 2.) Michael's reference 1 below goes into more detail on emissions effects from flame front temperature and rate of heat rise (RHOR)
Some good reading:
reference 1: http://www.ilot.edu.pl/Journal%20of%20KONES%202001/JOK2001%20NO%201-2/R5.pdf
reference 2: http://www.ilot.edu.pl/Journal%20of%20KONES%202002%20No%201_2/01/str275.pdf (relevant point: "there is a high amount of ...water vapor..which cools the reaction zone very well. Apart from this, the combustion process takes more time. As a result the flame temperature is lower."
Q: is PCP lower as well or is it largely insensitive to this?
Finally, the last question of mine is what effect does injecting the water pre or post-turbo have, thermodynamically?
Adding injected water should increase the heat transfer through the intercooler, making it more effective. In a modded Tdi running 2.25 bar absolute (post turbo), water will boil to steam at 125 C. Whether the temperatures post turbo ever get this high, I don't know. I have seen my stock IC temps as high as 65C on a hard run, but I have no idea what pre-IC temps were.
In trying to understand this, I have read up on use of water injection from WWII warplane days. I'm trying to understand exactly what happens within either the intake tract or within the cylinder when you have saturated the air with water vapor--as it pertains to either the diesel cycle or the otto cycle.
In trying to find helpful information, I went hunting for a good steam table. I have found a most useful Java applet from the National Institutes of Standards and Technology (NIST); it is linked/listed in my signature.
What I'm specifically after is a model of cylinder pressure-versus crank angle with and without water vapor. I'd like to know if water injection can allow for running aggressive boost and timing and reduce emissions at the same time.
Using the applet in isobaric properties mode, I put in:
pressure 185 bar (guess at peak pressure)
T-low (immediately prior to fuel injection)= 410 C
T-high (peak temperature) = 2177 C
for a stock 1.9L Tdi. It gives:
http://webbook.nist.gov/cgi/fluid.cgi?P=185&TLow=410&THigh=2177&TInc=10&Applet=on&ID=C7732185&Action=Load&Type=IsoBar&TUnit=C&PUnit=bar&DUnit=mol%2Fl&HUnit=kJ%2Fmol&WUnit=m%2Fs&VisUnit=cP&STUnit=dyn%2Fcm&RefState=ASH
And for a very modded Tdi reaching > 221 bar PCP
If it is possible for a modded Tdi to run > 221 bar PCP, then injected water will be in the form of supercritical steam:
http://webbook.nist.gov/cgi/fluid.cgi?P=225&TLow=350&THigh=3400&TInc=20&Applet=on&ID=C7732185&Action=Load&Type=IsoBar&TUnit=C&PUnit=bar&DUnit=mol%2Fl&HUnit=kJ%2Fmol&WUnit=m%2Fs&VisUnit=cP&STUnit=dyn%2Fcm&RefState=ASH
Moreover, the supercritical steam has lower Thermal Conductivity than normal steam beyond 1137 C which should lower heat losses to cylinder walls during that first fraction of the expansion stroke when pressures > 221 bar until temps drop < 1137 again/ and when pressures < 221 bar again:
I'm hypothesizing that if heat losses are lower during that time, it will preserve peak pressures a smidge longer. If coupled with aggressive Start-of-Injection timing, this could yeild some greater BMEP under some loads and give more useful P-V work upon the crank without the same NOx production the equivilant BMEP-sans-water-injection would produce. (see Michael Willmann's comments in reference 2.) Michael's reference 1 below goes into more detail on emissions effects from flame front temperature and rate of heat rise (RHOR)
Some good reading:
reference 1: http://www.ilot.edu.pl/Journal%20of%20KONES%202001/JOK2001%20NO%201-2/R5.pdf
reference 2: http://www.ilot.edu.pl/Journal%20of%20KONES%202002%20No%201_2/01/str275.pdf (relevant point: "there is a high amount of ...water vapor..which cools the reaction zone very well. Apart from this, the combustion process takes more time. As a result the flame temperature is lower."
Q: is PCP lower as well or is it largely insensitive to this?
Finally, the last question of mine is what effect does injecting the water pre or post-turbo have, thermodynamically?
Adding injected water should increase the heat transfer through the intercooler, making it more effective. In a modded Tdi running 2.25 bar absolute (post turbo), water will boil to steam at 125 C. Whether the temperatures post turbo ever get this high, I don't know. I have seen my stock IC temps as high as 65C on a hard run, but I have no idea what pre-IC temps were.
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