The adiabatic combustion temperature for natural gas is approximately 2000 C. I believe that gasoline engines have roughly the same combustion chamber temps, although this will vary depending on engine design.
shushuwa,
What is your point? The most thermal energy you could recover by dissociating water to hydrogen and oxygen would be when it recombines as water. And, even if your combustion temperature were astronomically high enough to make water dissociate, you certainly would not want recombination to blow apart the exhaust system.
Consider the temperatures achievable with gas-oxygen torches:
2200°C = 3992°F, for Propane/Oxygen
2927°C = 5300°F, for MAPP Gas/Oxygen
2700°C = 4892°F, for Acetylene/Oxygen
3200°C = 5792°F, for Hydrogen/Oxygen
Of course, hydrogen/oxygen couldn't reach 3200°C if the reaction began to go backwards at 2500°C!!! It might also be a problem for NASA's liquid fueled rockets, too.
But as a practical matter, water injection was used on some supercharged fighter aircraft piston engines during WW II. It allowed more power w/o burning up the engines by creating some steam power.
Here is another combustion plot that indicates how the flame temperature changes with an increase of the oxidant preheat temperature.
I should have indicated that the plots are valid when the oxidant is standard air -- on mole/volume basis standard air composition is 79% N2 and 21 % O2.
Feric posted "flame temperature increase with an increase of
the oxidant preheat temperature". Several other factors
influence flame temperature including including cylinder
pressure (this raises oxidant temp.), residual gas, air/fuel
ratio and fuel type.
So yes, as the flame front progresses the pressure and
temperature ahead of the flame rises resulting in increased
flame temperature.
Try this link to see the influence various factors have on
flame temperatures.
My plots are just for an ideal and complete combustion case.
The more time you spend dealing with the subject matter, the more you realize the complexity of the subject matter and that other factors that need to be considered and taken into consideration.
It is good to participate in such a forum and contribute as much as I can.
This is a highly complex subject with so many variables.
I have a personal interest in the effects of moisture
content on flame temperature and cylinder pressure.
Racing at night in Louisiana we see extreme levels of
humidity. I have seen several instances when it appeared
that cylinder pressure was increased with a rise in humidity
using auto-ignition as an indicator. I’m not saying what
I saw was dissociating water to hydrogen and oxygen. I
suppose we see enough humidity here to generate cylinder
pressure with steam.
Using the Wiley Adiabatic Flame Temperature Applet I
developed this chart of theoretical flame temperatures.
Fuel – Gasoline
Cylinder Pressure; variable 1200 kPa to 4800 kPa
(1200 being theoretical cranking compression and 4800 peak cylinder pressure)
Unburned gas temperature; variable 298K to 519K
(298 inlet A/F temperature and 519 the auto-ignition point of gasoline)
Typically values of residual gas fraction are in the range 3% to 12% for the SI
engine. This can be higher depending on camshaft overlap, operating RPM and
other factors.
Recalculating the table with residual mass fraction constant at 0.01 generates
different results. What becomes apparent is we can control flame temperature
with engine design and operating conditions.
