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Steam Jet Thrust Hybrid

 Use engine waste heat for steam jet propulsion
  (+2, -3)
(+2, -3) 		  [vote for,
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Start with a hybrid electric car (edit: Diesel non-hybrid would be a better starting point) and direct the exhaust to boil water or heat a metal chamber for a steam jet. When the water is hot enough, speed up with the electric motor (a fixed amount of thrust is worth more power at high speed), fire the steam jet, and then slow down with regenerative braking (edit: just try to use it mostly at high speeds).

A steam jet should be relatively simple for a waste heat recovery device. A low volume water pump and maybe a shut-off valve and no other moving parts.

—caspian, Dec 14 2006

Four stroke Atkinson cycle engine. http://en.wikipedia...kinson_cycle_engine
[BJS, Dec 14 2006]

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       My Prius has relatively little waste heat. The engine controller has to run the engine to provide EXTRA heat on startup just to get the catalytic converter to run properly.
—Galbinus_Caeli, Dec 14 2006
   

       The Toyota Prius hybrid, Ford Escape hybrid, and Toyota Camry Hybrid all use the Atkinson cycle, which creates less waste heat than conventional Otto cycle engines. And almost all hybrids have relativly small engines anyway so they would't have very much waste heat.   

       So basically this specific idea would be very non-effective.
—BJS, Dec 14 2006
   

       So how big would the tank be to carry all that water? I'm guessing that to carry any signifigant amount of water it would have to be at least a few gallons and the last thing cars need is more weight
—acurafan07, Dec 14 2006
   

       I was thinking of it using about as much water as fuel, so either an equal sized tank or about a quarter as big. Running out of jet water doesn't prevent the car from working, so it can be a bit smaller. The weight could well be a problem which limits how much water is carried.   

       Amount of waste heat: any heat engine is technically going to have a significant amount of waste heat, proportional to its power output, but maybe the exhaust temperature would be too low if the Atkinson cycle engine is efficient enough.   

       If it is high enough temperature, even if only to make the catalytic converter work, there should be plenty of heat.   

       My device features several hazards to make it more interesting, but unfortunately less practical.   

       1: the speeding up and slowing down could be dangerous driving. 2: getting blasted directly by the steam jet. 3: possible low visibility in clouds from the steam.   

       However, number 3 could be useful in car chases.   

       Also, people could attach food to the front of their cars, and if the driver in front was feeling generous, they could steam another car's food while stopped at traffic lights.
—caspian, Dec 16 2006
   

       Well in that case, [+]. If it was mounted either really high up on the car or really low, it probably wouldn't blast steam on anything important. But unless regenerative braking is an important part of the idea, my suggestion would be to lose the electric part of it and go with a turbo diesel engine and steam jet. Diesels are about 15% more efficient than gasoline engines and generally produce more heat, so they would be better suited for a steam jet than either an Atkinson or Otto cycle gasoline engine.
—acurafan07, Dec 17 2006
   

       The electric hybrid was so you could go fast while the jet was activated and then slow down before losing much energy to drag. It's probably not worthwhile, motor/generator/battery losses could wipe out any gains. I'll change that.   

       The getting blasted I was thinking of was if someone was standing behind the car. If the jet was too low it might damage road surface too.
—caspian, Dec 17 2006
   

       What if you used the steam jets primarily to reduce drag instead of provide thrust?   

       This would reduce the water requirements so smaller tanks could be used. The system could be setup to only activate the jet(s) when the cruise control is active.   

       At highway speeds the steam would quickly evaporate under most common atmospheric conditions, so blinding other drivers in a cloud of steam would be a smaller concern. This may actually be a nice feature as it would deter people from "tail-gating".   

       Another positive is that it would help you to visualize the airflow behind your car so aerodynamic improvements could be easily evaluated.
—moots13181, Feb 09 2009
   

       Great in theory, although I'd rather expand the steam through a turbine than into the atmosphere.   

       The problem with this is that driving doesn't tend to be very constant in terms of engine speed or load, even on the motorway. This means that exhaust gas flow rate (and temperature in a diesel) will change - thus modifying the amount and quality of steam available for the turbine (more flexible turbine means lower peak efficiency). This would also allow you to run a closed cycle, meaning no water top-ups required.   

       You'd also have to be careful about the exhaust back-pressure trade-off for the exhaust.
—Skrewloose, Feb 09 2009
   

       Maybe I should open another thread with this idea, but I'm thinking for simplicity's sake the system should be open but should not mix directly with the exhaust. Also, turbines tend to be expensive.   

       I'm envisioning that there would be multiple steam jet outlets on various portions of the car where a low density and medium velocity boundary layer would decrease induced and skin drag. (edit - I was thinking about this some more and it occured to me, a vehicle equipped with something like this would likely be extremely noisy/loud even if the jets were kept to a minimum and strategically placed!)   

       The steam could be generated from a shell-in-tube type heat exchanger somewhere downstream from the catalytic converter(s). In this way the exhaust would only be cooled after its heat is no longer needed. The exhaust tubing could be insulated up to that point to maximize the heat available and thus the steam.   

       Utilizing the steam in this way would not require much of any control systems. The downside is that the benefit would probably also be fairly small and only worth the added water weight if the majority of your miles were accumulated on the highway.   

       As with any steam system, there's also the unavoidable issue of freezing in colder climates. This detail makes most steam power ideas impractical (including my idea).
—moots13181, Feb 10 2009
   


 

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