hydrogen combustion


I drive a Mazda RX-8, and ever since I heard about the RX-8 Hydrogen RE, I’ve wished it had made it into production in the United States. Every once in a while, I’ve googled to see if there’s anybody who converts RX-8’s to dual-fuel hydrogen, but I’ve not found any yet.

It was this recent thought which prompted the electrolysis experiment, ostensibly with my kids.

And here’s some raw notes to myself that I made when trying to figure out what mileage I might get (comparing combustion numbers on gasoline [octane] and hydrogen).

octane:     114.23g/mol     0.703g/cm3  255.68J/K/mol
hydrogen:   2.0162g/mol                 14.3kJ/K/kg

.703g/cm^3 * 1cm^3/0.001L = 703g/L = 2661.14448g/gal
1 gal => 23.2963712 mol

From GasolineVsHyFuelCell: 2 mol of octane combusted -> 10224kJ released
-> 5112kJ/mol.
23.296 => 119 MJ

1 gallon of gasoline produces around 119MJ, and gets me 18miles

2 mol of hydrogen releases 486kJ, so 243kJ/mol
To combust 119MJ of Hydrogen, at 119MJ / 243kJ/mol, need 490mol of H2.
At 2.0162g/mol, 

Plug 
	(0.703g/cm^3) * (1gal) / (114.23g/mol) * (5112kJ/mol) / (243kJ/mol) * (2.0162g/mol)
into google search, which handles unit conversions for you

988.1g -- or about 1kg of H2 will produce as much energy as 1gal of gas

So, assuming it’s purely energy that causes the rotation, I would use about 1kg of hydrogen per day

OTOH, assume it’s really pressure (of PV=nRT fame) that causes the pushing. I’d need the specific heat to be able to convert the energy (assuming all heat) into a change in temperature… which would then change the pressure, causing the pushing.

But, I was able to find the DOE Hydrogen and Fuel Cells Program Record PDF, which says that 1kg of H2 is approximately 1 gallon-gasoline-equivalent on an energy content basis. Which means my calculations were reasonably accurate.

Some of my original links: