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Steam/Air Hybrid

indirect internal combustion / compressed air ( / electric) powertrain
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(+6, -1)
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(This post contains two major ideas:
1) placing the burner inside the boiler for more efficient energy usage, and
2) using water as a heat-sump in the tank of an air-car to mitigate having to cool the air when filling it up.)

The Steam/Air Hybrid combines an external combustion system with a compressed-air system to produce a powertrain that combines the instant-start and "plug-in" capabilities of an air-powered car with the long-distance capabilities of an all-fuel steam-engined vehicle.


Boiler/Air Tank: this is a heavily reinforced and insulated tank: the holding tank for both the compressed air and the steam. Additionally and importantly it also holds the water used both as heat ballast during the compressed-air charging cycle and boiler water for steam generation. The burner unit is also inside the tank. Burner set to "low" heats the air in the tank; "high" increases burner output and turns on a water spray pump to enable fast absorption of heat from the air to the water. Note that exhaust is retained at this point inside the tank.
[edit: in order to avoid NOx production we can use catalytic "burning" to provide a lower temperature "flame" while retaining all the energy]

Regen Tank: a smaller tank used to temporarily store reversed-air from braking.

Engine: a compound expansion engine <link>. In this case each cylinder also has discrete intakes and outlets directly connected to the Main and Regen tanks as well as the Exhaust/Recycling system.

(Also an Electric Motor/Generator: Clutched to the engine to provide electrics, also runs the fuel and air pumps while the engine is disengaged. Inverter/transformer for in/out household current.)


Operating: whatever air-steam combination happens to be in the tank at the time runs the engine. A valve controls the pressure, thus the speed. No transmission necessary, but if you want reverse you have to get out and push (or use the electric motor)

Regenerative Braking: play musical valves and air runs backwards through the engine into a regen-tank, then forwards when we want acceleration. Note that mixing and matching cylinders can provide various levels of deceleration.

Water Recycling: end-product is filtered, gas is exhausted and water fed back into the tank.

Charging the Air tank: powered by the mains running the Electric Motor, the Engine pumps fresh air into the Main Tank. Due to the water ballast, no cooling during pressurization is necessary.

(Stand-alone electrical generation: the engine runs the motor-generator.)

(assume starting with a load of compressed air which by itself gives a range of 40km)

To the store and back (1-40km): jump in, drive to the store and back on compressed air, without turning the burner on.

To the next city and back (40-say80 km): jump in, drive for awhile, turn the burner on low: this will heat the pressurized air and allow for an extended range without generating any new steam.

Cross-country (80-xxx km): jump in, drive off and turn the burner on high; before the air runs out the steam will be online.

Driving back from somewhere where there's no electrical outlet to air-charge and the tank's run out: Turn the burner on and go for a coffee; come back, jump in, drive off.


( Sep 02 2009 - post was originally "Air Car Range Extender"; this title is more accurate. Though the mechanism is the same, the post was rewritten entirely from a "hybrid point of view" )

( Jun 5 2011 - incorporated use of catalysis instead of burning to provide lower NOx levels )

FlyingToaster, Sep 09 2008

Air Car http://en.wikipedia.org/wiki/Air_car
[FlyingToaster, Sep 01 2009]

compound (steam) engine http://en.wikipedia...iki/Compound_engine
[FlyingToaster, Sep 02 2009]

Stem-Electric hybrid Steam-Electric_20Hybrid
The biggest problem with any steam engine is the need to dump unused heat (typically by a radiator). [Vernon, Jun 06 2011]


       i suspect mostly a steam car. Tanks of adequate size are pretty heavy. Ok on the flat but a real killer on the hills.
WcW, Sep 09 2008

       hmm... I'm gonna work the math a bit tomorrow, but I think if you toss some water into an air-car's airtank, you won't have to worry about cooling the air when you recharge.
FlyingToaster, Sep 09 2008

       i like the hybrid concept of a gas pressure car that can use stored pressure or steam but i suspect there are some logistical issues. Using boiling to recapture waste heat seems really elegant in a pressure based system.
WcW, Sep 10 2008

       I don't know why there is all this fuss about air powered cars; compressed air is a horribly inefficient, expensive way to store energy.
Other than that, this is just a steam engine with a compressed-air-only option, and if steam is that good, why are most trains now diesel-electric instead?
neutrinos_shadow, Sep 10 2008

       Isn't the water vapour in the air tank going to freeze into mechanically unfriendly ice when it bursts forth from its hyperbaric habitat?   

       I would have thought the cryogenic cooling effect of using compressed air is difficult enough to deal with when it's dry.
Texticle, Sep 10 2008

       You're assuming that the tank would provide a sufficient amount of air to start. I doubt enough air could be stored initially to have a significant amount of energy to use up, and efficient is going to be garbage considering the low pressures you're likely to encounter. (Think about why modern IC engines have such high compression ratios.)
kevinthenerd, Aug 31 2009

       [ktn] this is a range extender, not a replacement for the compressed air fillup. (You could run it steam-only but then like you said, if it had been sitting idle for awhile you couldn't drive it right away until it warmed up)   

       Shameless elf promotional <links> added for prerequisite-reading posts.   

       [edit: all in all I like the idea of calling it a Steam-Air Hybrid", like an ICE/electric and even has a "plug-in" option.
FlyingToaster, Sep 01 2009

       Bump for pastry. Remember the major advantages of this are a) much less energy gets wasted because the burner and exhaust products are inside the boiler: in this respect it's more efficient than a regular steam engine, and b) using it in air-only mode means you don't have to wait for boiler pressure to rise.   

       The problem that I had envisioned is high NOx production due to the high internal pressure adding to the high temperature of burning, but if a catalytic "burner" is used then I think we can keep it to a minimum, as low or even lower than an ICE engine.   

       (Of course, as posted, this is a step "backwards" in boiler design, but a tube boiler system inside an air tank might work also, with a merge valve between air(/exhaust) and steam)
FlyingToaster, Jun 06 2011

       //Boiler/Air Tank// = bomb. One that slices and dices and cooks all in one go. Watch the mythbusters episode where they superheated a hot water system. Safer steam designs produce steam in narrow tubes, rather than a big boiler, to reduce the size of the explosion.   

       It seems crazy to burn fuel to heat the air directly. You're using high grade fuel to produce low grade heat. It seems better to use the waste heat in the exhaust from the steam engine to heat the air; that's a useful synergy, which increases the efficiency of both processes (assuming the steam engine is closed-cycle).
spidermother, Jun 06 2011

       I don't see why it would seem crazy to burn fuel to heat air since that's what you're going to do, and much more indirectly, ie: less efficiently, no matter what other process you use.   

       I do see what you mean though, I think, but running the whole thing strictly from exhaust pressure isn't one of the operating cycles (that would work of course but be pretty lossy compared to an ICE engine): assumed is that you'd start off with a full load of compressed air, having plugged in and charged it up in a similar fashion to a normal air-car (post reworded).   

       I'm not too sure about the "bomb" bit in comparison to other boiler designs: for one thing short and medium range trips would be air-only. I can (sort of) envision a tube design inside an air-tank... that might work better.   

       The steam bit would be closed cycle'ish, recovered after the engine; "ish" because burning a Hydrocarbon produces steam, so you wouldn't have to recover *all* the original water.   

       The post incorporates two major ideas which were originally posted separately: 1) putting a burner inside of a boiler, and 2) using water as a heat-sump/temp-maintainer for an air-tank.
FlyingToaster, Jun 06 2011

       By closed-cycle I meant that the spent steam† is condensed and reused. This has the advantage that you don't have to carry as much water, and a potential efficiency gain if you can maintain a pressure below atmospheric on the exhaust side, but the disadvantage that you have to dump the waste heat somewhere (open cycle steam engines simply dump it into the atmosphere).   

       The obvious place to dump it is into the compressed air - that way, you effectively heat the air for free and cool the steam for free! You've got two systems with classic problems that complement each other.   

       There's still nothing stopping you from keeping a reserve of hot water to heat the air, but I think it should be the condensate (at 60ºC or whatever). That way you increase the safety, and the thermodynamic efficiency.   

       † The working fluid steam, that is. Collecting the water produced by combustion is probably not necessary or worthwhile.
spidermother, Jun 06 2011

The burner/catalyser is *inside* the tank and exhausts there, so fuel-produced steam is part of the steam that runs through the engine. At the end of the day you (could, if you wanted,) have more water than what you started with.

       The steam is recycled from the engine exhaust (not the burner-exhaust, the engine exhaust): the gas (air/CO2) is tossed, and enough water is recycled back into the boiler to keep it level.   

       I can't think of anything to do with the excess engine-exhaust water though: the stuff inside the tank is pretty well by definition going to be hotter, and if you preheat combustion-intake air then you just have to work that much harder to pump it into the tank (but maybe not, thought required here). It could definitely be used to pre-heat liquid fuel if that's what's being used.   

       One tank. The tank contains a burner (fed both air and fuel from the outside) and water and air.   

       The (original) air in the tank is pumped in by (electrically) running the engine backwards while the car is parked for the night (or day). This is the "air car plug-in" bit and it's almost the same as a normal air car cycle.   

       For this type of operation the water in the tank is being used as a heat sump so you don't have to cool the intake air. If the water wasn't there then the compressed air would be really really hot and the heat would escape quickly even through the insulation. With the water there it's just medium-hot, and there's not as much for the insulation to do and it stays hot longer. (This might not be useful if you're going to plug it in a couple days in advance, so maybe there's an auxiliary shunt so you can heat the water in your house or something, but that's not the main idea)   

       The water in the tank is there as a heat-sump as previously mentioned, but if it's heated up enough it will vaporize and then you get steam, and now the motive power for the engine-cylinders is steam + burner exhaust (plus air until you run out).   

       The burner in the tank which is fed air and fuel from the outside but exhausts into the tank, you wouldn't use at all if you had enough compressed air in the tank for your (short) journey. For a medium-length run you'd set it to "low" to heat the air already in the tank to make it last longer. This is also normal'ish operation for some air-cars (except for the part where the burner exhausts into the tank).   

       The burner exhausts into the tank because that's a neat way of making sure as much energy available is used, as opposed to letting it escape through an exhaust tube.   

       I'm currently ignoring two things though: the possibility of CO2 dissolving in water, and that, for a certain small temperature range under the boiling point of water inside the tank, the energy required to pump in air and fuel for the burner might be mitigated somewhat by the end product of liquid water instead of vapour (only "somewhat" though, I think overall it's a good idea).
FlyingToaster, Jun 06 2011

       Tank you.   

       But compressed air needs a source of low-grade heat, and steam needs a place to dump waste heat. Not letting them play nicely makes baby Carnot cry.   

       You can probably still do it with a single tank, plus a couple of heat exchangers.
spidermother, Jun 06 2011

       I think you read the draft-anno.   

       C jr. would be giggling hisself silly: combustion-exhaust gas from a normal steam boiler is at best at the temperature inside the boiler. For this one the final effluent exhaust has been run through the engine: it's at a much lower temperature.   

       (okay, for the "hot air" cycle it might be advantageous to run the burner-exhaust straight out instead of exhausting inside the tank, but for steam operation it's a definite plus AFAICT)   

       ... wait... dammit.   

       ... still...
FlyingToaster, Jun 06 2011

       A note of clarification: running the burner inside the tank, presumably by burning a hydrocarbon, would be advantageous or not depending on the temperature.   

       If pressure/temperature conditions are high enough then the exhaust-product H2O will be a vapour, if not then it's a liquid and you've wasted energy pumping in the fuel and air... not that much considering that air is 80% N2 of course, but still.   

       But it works like that on an external burner too: there if the boiler temperature is > 100C (which makes the optimum exhaust temperature > 100C) then the water in the exhaust will be wasted as a gas (it carries a pretty full load of energy).   

       So I was right the first time :) more or less.   

       [edit: also known as the high and low heating value]
FlyingToaster, Jun 06 2011

       Combustion exhaust from a well designed steam boiler can be made not much hotter than the feed water - and much cooler than the steam - by using countercurrent exchange.   

       There have been domestic hot water systems with an internal flame. A friend of mine had one, but unfortunately I never got to see it. It had a rose like a shower head at the top, and a burner at the bottom, protected from directly getting wet by a metal umbrella. I thought about the possibility of using something similar to generate steam.
spidermother, Jun 06 2011

       //countercurrent exchange// That would transfer some of the energy, more if the fuel source was a compressed gas, but still a drop in the bucket compared to latent heat of evaporation of water which forms a pretty, white, cloud exiting the burner stack, that cost you 10% of your energy (for methane).
FlyingToaster, Jun 07 2011

       //countercurrent exchange//(more)   

       Perhaps a series of pumps to inject the water into the boiler, instead of just one. The exhaust gas could heat the variously pressurized water stages, ie: the stage just before injection could be heated to a pretty high temperature if it was pressurized, without vapourizing the water.
FlyingToaster, Jun 08 2011


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