Uh, How soon you guys forget! I've been talking and planning this for years! I've got most of the parts suppliers lined up, and some CAD drawings. When I have about 10k laying around that I need to spend, I will finally build it.
I spent most of a 6 month deployment on a ship scheming.
There is a 200hp 2000rpm 4 cylinder double expansion engine 'kit' you can purchase, it just has to be machined and built by you, the 'kit' is just the castings. Put a brownie box on it, and your good to go power and RPM wise. You would lose some of that hp, but a deuce engine is only 138 or so anyway.
You don't need a transmission. A steam engine produces maximum torque from a standstill, and it curves down from there. You control the speed simply with the throttle. The steam engine stated above, would fit in the frame rails and just forward of where the transmission sits now- you would have to modify the cab floor some, and put the brownie box between the engine and the transfer case. You Could fit a flywheel in there, but it isn't totally necessary.
This leaves almost the entire engine compartment open. Plenty of room for a water tube boiler. There are several styles, but I designed a composite of two very successful and proven designs. The Merry weather quick steaming boilers of the old 'fire' engines. You can have a full head of steam in less than 5 min- you’re supposed to warm a multi up as well before driving, so what's a few extra minutes? Also a Doble boiler, for it's free steaming and higher pressures. A water tube boiler does not have the disaster potential of a fire tube boiler.
Mount a water tank on the driver side where the spare is, and put several condensers stacked where the radiator is now, and you can reuse most of the water. Several old steam cars and British lorries could get close to 15mpg on kero- not shabby for a deuce at all.
Like I've said, I've put serious work and engineering into building one. I plan to actually do it one day.
I think the capabilities of one would surprise you, built with modern techniques and materials.
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