I think that I may have figured out why steam went out of use in transportation. It's too simple, too easy. I used to think of steam engines as a quaint, old-fashioned thing, but recently they came to my attention again in a way that made me go into the matter a little more, and I learned that there were steam engines in the 1920's that had total efficiencies that were as good as or better than most internal combustion automobiles have now. Doble put out models that weighed over 5000 pounds that got fifteen miles to the gallon. That can't be converted directly to what a 1500 to 2000 pound automobile would get, but it is an indicator.
Steam engines for automobiles are lighter, more efficient, more durable, and have fewer parts and less friction than internal combustion engines.
In the past arguments were made against steam that gave steam a bad name. People cited such things as explosions, great weight, slow starting, etc., but those are complaints that go back to the early days of steamboats and early steam railroads. The explosions were caused by large boilers that were filled with super heated steam. For later era automobiles and traction engines there are pipes that are filled with small amounts of steam that goes to the engine then to the condenser. You can't blow one up, even if you try. If you heat the to the maximum and jam the relief valve, then some tubing joints will give out and there'll be a cloud of steam until the boiler runs dry. You wouldn't want to walk into the steam, but it would quickly dissipate. A modern boiler for a steam car is about the size of microwave oven, and there is no large reservoir of steam. Steam stays in tubes that cycle through the engine and back to the water tank.
The engine itself is compact, light, and efficient. To power a passenger car or a pickup truck one would use a bi-directional two piston engine that is less than two feet long and a foot square in cross section, and it weighs less than a hundred pounds. There would be two slide valves for the intake and the piston would act as the exhaust valve. There is a connecting rod from the piston to the crankshaft, and in many models the crankshaft was integral with the rear axle.
There would be a few other moving parts like a water pump, fuel pump, check valves and shut-offs, but there are a few dozen parts instead of the hundreds of moving parts in an internal combustion engine. The difference in the number of moving parts is major, because all those parts create friction, which cuts into efficiency.
Another way that steam engines are more efficient is because they don't need a transmission. Internal combustion engines have a minimum speed, generally about eight hundred or a thousand rpm, below that speed they stall. Steam engines can operate down to one rpm, or even less. In a typical gasoline powered automobile the transmission gears down the drive so that it can be driven at slower speeds. I once discovered that my car went forty MPH at idle speed in high gear. With steam engine you just throttle back the steam, so less goes to the engine, and the engine slows.
There are some sources that give the thermal efficiency of steam engine as 5 to 10 percent, but those figures were for the open pot boilers that were used in steamboats and early railroads Those made no attempt at using water or fuel efficiently. the steam was simply exhausted into the air. Later designs put that stem into use preheating the water and condensed it so that it wouldn't be necessary to reload the water every thirty miles. An even more efficient model could use the exhausted steam to run a lower pressure engine, either piston or turbine, that could provide auxiliary power for lights and other electrical appliances. Either use would act to condense the steam, so that it could return to the reservoir.
Early steam engines also neglected simple fuel efficiency measures that could save lot of fuel. Simply insulating the boilers would greatly reduce fuel usage.
I wasn't around when there were steam cars being made, so I didn't figure out why they all went out of business, and there were a lot of companies in that business over the years. It might be that they tried to do too much. The Doble Steam Car Company in the 1920's was charging $5000 for chassis before the body and engine were added. That was a whole lot of money then, and it would translate into about $100,000 now. There wasn't all that much demand for their product then, because it was in a completely different class from what Dodge, Ford, Chevrolet, and others were building and selling for $1000 or less. If someone had bothered to put the same Uniflow engine into a Model T body, then it could have sold for about $1000 and made money. Today that same engine could be put into a typical car, and the car could sell for a lot less than an internal combustion model, because there are so many add-ons to the engine that are designed to make it burn cleaner and more efficiently. Those add-ons aren't necessary with steam, because it burns at atmospheric pressure, which creates none of the pollutants that are formed in the combustion chamber of an internal combustion engine. And the engine is efficient by its nature.
Many people will disagree with me, but I believe that an inexpensive, safe, clean steam powered automobile could be produced in America and sold for less than most automobiles that are on the market. The technology has been on the shelf for decades, and it is out of patent protection. The steam generators and steam engines that were invented decades ago wouldn't require much research and development. Initially, the engine could be put into cars that already are being made, which would avoid having to go through as many government hoops.
If I were to take an existing car and change it to steam, then it would have to be rear wheel drive, which would also allow for there to be a disconnect in the drive-train and for a limited slip differential, so that the drive wheels could turn at different rates on turns, etc. Steam cars had a reversing switch, which reversed the crankshaft, and that would have to be included, and it might be possible, or desirable, to install a two speed transmission (normal and high speed) that might make high speed driving even more efficient. Otherwise, modern brakes, steering, etc. would be perfectly good for a steam car. Electrical generators could run off the crankshaft or from a spent steam turbine, but there would be less electricity needed for engine operation.
Fuel would be more optional than in internal combustion engine, because it would just have to burn, rather than explode in the combustion chamber. In the past kerosene (number 1 fuel oil) was the common fuel; although gasoline (number 0 fuel oil) was often used as the pilot and sometimes used as the general fuel. There is not reason why the boiler couldn't use either of those or a gaseous fuel (propane or natural gas), or anything that could burn. Being flexible on the fuel would be cost efficient, especially if it just meant that the nozzle had to be changed.
There is some information about steam engines on the internet, but not as much as I would like. Try using search terms "uniflow steam engine" and "Doble uniflow engine". There are a few relevant articles in Wikipedia. Unfortunately, there aren't any detiled technical drawings.
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