Thank you. I quit my ME degree 10 years ago because I was making more money as a manager selling autoparts, now I'm finishing it off as fast as I can to escape the "Rust Belt" or start my own company here. Maybe I'm finally learnin' something useful!
The "Hobbes Switch" is just an adjustable pressure switch you can get at any NAPA. You could just as easily set it up with a switch on the EGT. The EGT sensor is about $100 and the "Hobbes" is $30. That's why I picked it. The EGT would probably be a better way to control it. But if you set the Hobbes to come on after you exceed a certain boost level, you are doing also telling it to ignore the "normal" use and only turn on when your foot is into it... when the turbo is being spooled up from all that extra fuel you're dumping in at fairly high RPMs. But, the EGT could cool the engine a bit if you were foot to the floor "lugging" the engine as well. At low RPMs and high fuel flow, you make heat, but not a lot of boost... hence the EGT would see the issue and kick in the water. Lugging the engine is bad, and should be avoided like the ex-wife, but it can happen, and the EGT would be better at catching it.
The water is there just to cool when the extra fuel is added... and yes, you can just keep an eye on the EGT and back out of the throttle if you run out of water. But... it's just water. Any gas station that has filtered or distilled water will be able to fix you up. Tap water may have some impurities, but seriously, you would have to run very dirty water for a very long time to have any issues. With a 10 gallon keg, and using 3 gph, at a high average fuel range of 10mpg on a 50gal tank at 60 mph (when the truck would be using it constantly for arguments sake) we have 8 hours of fuel, and 3 hours of water... that's constant use... pedal to the bolt, and needing the water the whole time. Honestly, I think it will last longer because with boost or EGT, it won't be switched on the whole time. But if you want, just remember that you need to check the water every 200 miles. As with all things dependent upon driving style, YMMV.
I haven't even done the other part of the math... the water is converted to steam... it expands. That means it makes some power in it's own right by making working use out of what would otherwise be waste heat energy. I have no idea how large or small that effect might be, I do know the water injection study claimed an increase in overall fuel efficiency, for what that is worth.
It's not free energy, and it won't make the truck a hotrod or a gas-sipping 20mpg wonder... but the water should let us get a little bit more power safely, and keep the engine clean even if we run nasty waste oil or idle too much.
I'm building it for my truck this spring... If someone beats me to it, we can compare notes. jwaller is the guy who knows the most about water injection on the multifuel, and I really would like someone to double check my data. No experiment is without risk, but as long as you have an EGT and a boost gauge, it should be easy to track the system before anything bad could happen.
