Bear in mind I haven't built an engine so I'm talking theory.
I use the term "steam" loosely because water steam may not be the ideal working media.
I'm also reluctant to use the term "external combustion" because not all steam engines are combustion engines.
You can probably use sterling cycle engines in place of steam engines but I haven't been impressed with what I've seen so far.
It seems to me that converting biomass into liquid fuel will be less efficient than simply burning the stuff.
It might not be as convenient and it may not go mainstream but as conventional fuel become very expensive steam will become more attractive.
Of course steam power can be conveniently oil fired but that isn't where we want to end up.
One interesting engine can be found at http://www.greensteamengine.com/
There are plenty of steam sites on the web - another to check out is http://www.pritchardpower.com/
I don't think any of these engines have been perfected and there would be scope to improve efficiency further with computer controlled engine management - in particular the use of computerized valve timing.
Other steam concepts which interest me - do away with linear to rotary conversion. Linear engines could produce electricity, compressed gas/air or pressurized liquid - potential very high pressure water for water jet cutting.
People sometimes argue that steam is obsolete old technology. This argument is crap. Our power grid here is driven by steam - coal fired in this case. Even nuclear power plants are steam engine driven - fission just supplies the heat. These big power plants use turbines but piston engines may still have their place in smaller applications.
People also point out we have a water shortage.
Water is not as scarce as oil will be and steam engines can be closed cycle to recover the water.
I've researched alternative fuels and was exploring making methanol via pyrolysis when I detoured and started making turbo-stoves and turbo furnaces.
Now I'm thinking we've be better off burning biomass directly rather than try to convert it to something we can run in a petrol or diesel engine.
Even if we are burning fossil fuel oil we'd make better use of the resource by burning the crude.
Every refining process consumes some of the energy contained in the oil.
You would possibly need to remove the sulfur first but the less processing the better.
Even burning dry corn would yield more energy that you get by processing it into ethanol.
If people saw corn being burned it might change their perspective on corn as an alternative fuel.
Energy densityThere will be times when the energy density is important (eg aviation) but most times we could happily use fuel with a lower energy density than refined petroleum.
Our current petroleum based fuels are very energy density at all around 45 MJ/kg.
Most woody biomass fuel are well under 20 MJ/kg.
Even methanol is down to 22.7 MJ/kg but it is still used are racing fuel and model aircraft fuel.
It looks to me like we could use something like wood (15ish MJ/kg) if we had to.
The energy content of wood varies quite a bit with type and water content - it could be much higher than 15.
Fats are approaching the energy density of crude oil (38.9 and 41.9) and charcoal is not bad at 29.
If we were to grow a fuel crop, (and I'm not saying we should) perhaps it could be a oil rich plant of some sort but we burn the whole plant (or the harvested parts) so we don't waste the cellulose and lignin.
Jatropha is one plant people are excited about using for fuel while others want it eradicated.
Maybe we can pelletize grass clipping or something for starters.
I want a steam powered pelletizer.
I had a quick look at grain prices and it looks like burning grain would already be a cheaper fuel than oil. We don't want to do this but it shows we don't have to wait for bio-fuel to be economical.
Later I discovered I'm not the first person to figure this out. http://www.iburncorn.com/
Apparently some people don't mind burning food.
Further research also showed there is already an interest in grass pellets for heating fuel but I haven't seen anyone suggest we use them as a transportation fuel.
With wood pellets selling for $187 a ton in the US a pellet burning steam engine (eg a car) would be cheap to run. If everyone did this there would be a supply problem.
A liquid fueled steam car could be just as convenient as a one using an internal combustion engine.
Warm up times are short but if you needed instant start then you might want a hybrid of some sort.
Perhaps less effort should be going into finding a petrol substitute and more should go into developing an automated delivery system for solid fuel.
There are some fundamental difference between steam and IC engines.With steam you can have a steady flame which is cleaner than having a series of small explosions.
Steam engines are the ultimate multi-fuel engine, they can run on almost any fuel or heat source.
Another important difference is IC engines need cold intake air - preheating the intake air will reduce the engine power.
With a steam engine you can preheat the air and also the water/steam to improve power and efficiency.
In other words what is waste heat in an IC engine can be recovered in a steam engine.
You also don't need to cool the cylinders - you would probably actually insulated them.
You can't beat the carnot cycle efficiency but you can get closer.