Of all the pre-war cars I have been in or around... only steam had the power to throw you back in the seat. It would be interesting to see a modern multifuel interpretation of one. Might be well suited to long runtimes and alternate fuels.
The problem with steam is that external combustion tends to be very fuel inefficient compared to internal combustion. This is true in general, but becomes even more true when you constrain the size and weight of the engine.
> external combustion tends to be very fuel inefficient compared to internal combustion. This is true in general
No, that's not true at all. Big power plants stayed with steam for efficiency reasons, and they are pretty close to theoretical limits.
Cars and aeroplanes, and later ships and trains, went to internal combustion because size and weight are very real concerns for them, and this trade-off against pure efficiency (heat-to-torque) was worth making. In the name of overall efficiency, if you like -- smaller engines meant more cargo room, so in oil-to-cargo terms you could come out ahead.
More electrical now is gas-turbine, but again this is about trading efficiency for other things -- peak-hour electricity is worth much more than 3am electricity, etc.
Doble got pretty close to solving that problem actually. His water-tube boilers were extremely efficient at the time (probably surpassing the contemporary ICEs). He also did some work on a “wet heater” which is a boiler that
mixed the fuel with the water, burned the mix, and used the exhaust gas+steam mix for the engine. Doble really pushed the boundaries for steam cars.
I am not sure it's less efficient by default or simply because of lack of R&D into making steam engines better.
I read about a government program in maybe the 70s (oil crisis?) where steam engines were reviewed to replace gas engines. The reason being is that burning fuel at atmospheric pressure means you burn it more cleanly and fully. This sounds like an efficiency balance at some level, certainly a pollution advantage.
You run into Carnot cycle [1] physical limits. The efficiency of any heat engine is bounded by 1 - Tc/Th, i.e. the ratio of the absolute temperatures of the cold reservoir (usually the exhaust gases) to the hot reservoir (combustion temperature). To make an engine more efficient, it needs to run as hot as possible. Non-superheated steam engines are bounded by the boiling point of water (373 C); assuming 21 C outside air, you get a max theoretical efficiency of about 22%. ICEs are typically bounded by the thermal limit of the cylinder material; for iron engines, this gives about 37% efficiency.
Superheated steam or closed-cycle (eg. Stirling) engines can do a lot better than this - Stirling engines can reach 50% efficiency, and this is nearly matched by superheated steam engines as used in power plants. But then you run into weight & safety problems. Superheated steam is great in a nuclear reactor or battleship, but in a car where a crash could easily break the engine piping? You're turning survivable crashes into death traps.
It seems something is not possible until someone figures a way to do it. I could see steam/sterling engines combined with electric could do amazing things. Primarily because of low pollution (from what I read, almost none in some cases) of burning almost any combustible fuel.
Even if there are some efficiency limitations, a small steam generator (range extender) combined with an electric vehicle could be ideal.
I really don't think efficiency is the primary problem with steam/heat tech, I think it's politics and society and possibly greed.
Edit: a quick search, it seems the Carnot limit applies to ICE engines as well. So your argument seems to be contradictory. Can you clarify how ICE engine efficiency is different from steam, related to the Carnot limit?
The Carnot limit is a simple statement about temperatures: the thermodynamic efficiency of a heat engine is 1 - the ratio of cold/hot temperatures in Kelvin. In practice that means that efficiency goes up the hotter you can make the engine's working fluid, and that (because you start at 294K) you have to go pretty high to get really good efficiencies.
The relevance to ICE vs. steam is largely about material science. You're limited first of all by mechanism by which the working fluid is heated and second by the materials used to contain it. Regular non-superheated steam never gets past 100C (373K), because once it does it boils off and the steam transmits the heat away from the heat source into the engine.
Superheated steam (as in a nuclear reactor or military-grade steam turbine) can get significantly higher than that, and reach corresponding efficiencies, but you have to figure out how to continue applying the heat source to the steam after it has boiled, and that steam will be under correspondingly high pressure (because of the ideal gas law: PV = nRT), so you need materials that can both contain the high pressure and don't degrade under heat. The working fluid within an ICE is entirely contained within the engine; thus, the primary constraint is that the material used to construct the cylinders can't melt or deform under the heat of combustion. The big advantage of ICEs is that you don't need any piping, though, so you can machine the engine out of a solid block of iron or similar material and get all the strength that results.
Electric or solar-thermal Stirling engines actually do have very good efficiency ratings. But the key here is for stationary uses. They are big, bulky things, because they have to be to provide sufficient heating to the working fluid and then move it to and through the engine without any pipes bursting.
Moving vehicles have a large constraint: any engine adds to the weight of the vehicle, and has to be accelerated along with the payload. So power-to-weight is crucial: an engine that has equal efficiency but weighs as much as a car has effectively half the efficiency, because you need to move twice as much weight around. That's why most of the interest is in either smaller (and hence lighter) ICE cars or in electric drive: electric motors have very good power-to-weight ratios if you can make the battery storage light enough.
Ha, I remember reading about that in university. One of the options my group considered for a culminating project in our engineering degree was a 6-stroke rotary (Wankel) engine that would have a water injection phase after the exhaust was pressed out.
I don't think manufacturing a working model of such a thing would have been within reach for a couple of undergrads, although the project we went with was almost as ambitious— we ended up building a mostly functional laundry washer from scratch.
