Quote:
Originally Posted by drmiller100
I'm seeing 1.2 grams per liter for air, and you look like you are calculating 1.2 liters per gram? Which obviously makes the numbers even worse.
An interesting thing to consider is we are not adding 14 times as much WATER - we are adding 14 times as much STEAM.
Steam is an inert gas, already vaporized, just like exhaust gas in EGR systems is an inert gas.
I don't understand why the extra water/vapor would hurt anything?
Also, FWIW, no one said this has to be a total loss system, and as you pointed out earlier, burning gasoline produces water.
The water tank might not be nearly as big as you are thinking!
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Hello drmiller100,
Good thinking, I was looking at air at 100 C not 25 C. 51 g/m of fuel, 14 times more steam.
I wouldn't call steam inert, it is just about the best cleaning solvent. But I know what you mean, it should have little or no effect on ignition. It might increase the breakdown voltage a little. It shouldn't participate in the chemistry at all. It does absorb a lot of heat. I don't know why it seems to lead to misfire at only 20% mixtures. Just solve that and get your boiler ready.
I do most of my driving at cruise. But probably other drivers don't. Cruise conditions would require the most steam. At full power no steam and at idle the rpm should be much lower. So maybe a smaller tank of water for you. After adding a boiler maybe it wouldn't be much more to add a condenser to the exhaust pipe too. You would need to heat the boiler before starting the engine or else use a throttle during warm-up.
Getting back to your earlier post, I don't think there is any benefit in adding steam to the intake. Of course I make lots of mistakes, my math might be off. Thermodynamic efficiency is truly the result of operating at elevated temperatures, the higher the better. If you consider the same model here, an ideal Otto cycle, cruising power at 1500 rpm with about 1/3 of the charge air+fuel and 2/3 steam. Compressing the whole volume by 10:1 compresses the air by 3.3 times adiabatically, and allowing for the addition of compression heat, the pressure is about 0.55 MPa at 330 C. The steam portion compresses by 6.7 times, using a graph of T-S for saturated steam, adiabatic compression from 0.1 MPa at 100 C to 0.67 MPa goes to 330 C, lucky! But also funny, although the cylinder is full the temperature is the same as it would be with no steam added. The new pressure with steam is much higher, 1.2 MPa instead of 0.55 MPa, which is a harder push for the crankshaft.
Heat is added by burning fuel; 51 g = 2244 kJ. The heat added will be divided between the steam and the exhaust (mostly N2, CO2 and H2O) I'm going to add the exhaust water to the steam mass for the calculation, because the specific heat at constant volume of the N2 and CO2 are relatively close, the average is about 0.73. Exhaust mass is about 746 g. But the cV of steam is 1.76 at these conditions. The equilibrium temperature added is 1158. The combustion temperature is 1488 C. If there had been no steam added it would be much higher (but less mass).
Ugh, now shift gears. I'm doing the adiabatic work per mole, sorry. The exhaust contains 585 grams of N2, about 41 moles, and 161 g of CO2, 3.7 moles. There is 801 g of steam, 44 moles. The 89 moles are contained in a volume of 187.5 liter at a temperature of 1488 C (1761 K).
Using 1.33 for the ratio of specific heats (g) on the whole mixture the work of expansion is 2100 kJ. Compressing the mixture took 460 kJ for the air (g=1.4) and about 410 kJ for the steam (g=1.28) (This is a very rough estimate based on expecting the steam fraction to behave as an ideal gas.) The work available is 1230 kJ.
Without steam added the work of compressing the air + fuel (48 moles) is 460 kJ. The combustion temperature is 4400 C, the work of expansion (g=1.38) is about 2700 kJ. The work done to pump the air through a throttle is less than 80 kJ
The work available is 2160 kJ.
The steam added engine would need about 50% more fuel to produce the same work.
Here is a page that calculates some of these things.
-mort