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Most energy in cars is wasted during acceleration (fuel) and
braking (heat). Various ideas have been around for using
the brake energy to accelerate - like the Ford Tonka's
Hydraulic Launch Assist. Using compressed gas seems to be
the best way to store energy - Ford however needs a turbo
compressor
and a lot of additional stuff to compres the air.
I was thinking it must be possible to use the available piston
engine as air compressor (the engine is already connected
to the wheels). If you would have hydrolic computer
controlled valves one could use the pistons to compress the
air to brake and pump the compressed air back into the
cylinders to accelerate. The only extra weight is a
compressed gas tank.
Here in Europe we have manual transmissions and I can tell
you can brake pretty hard on the compression of the
engine. The Jake brake works the same way - but the power
is not stored.
I can't believe no-one ever thought of this but I can't see
why it couldn't work. Can you?
PS: Of course I just want to build a fuel efficient 0-60 in 3.5
seconds sportscar with this :-)
compressed air captured and released
http://www.halfbake...-air_20supercharger
very similar topic to this [alantimothycooper, Oct 04 2004]
Opel Eco-Speedster
http://www.0-100.it...s/frame_english.htm
Top speed of 155 mph, 93 mpg. Nice. [galukalock, Oct 04 2004, last modified Oct 21 2004]
The Air Car
http://www.theaircar.com/
MDI's compressed air car. [JephSullivan, Sep 15 2005]
Nitrogen powered bicycle
Nitrogen_20powered_20bicycle
[ldischler, Sep 16 2005]
[link]
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Mac yoda, thx for the feedback - but... |
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I don't see why an engine should be stronger - if it can
handle the force of the combustions/explosions it can also
handle the compressed air, right? The air is used to
propel the pistons directly via high pressure tubing - the
same tubes used to compress it into the tank are used to
release the high pressure. So it is not used to compress
air for combustion - like a turbocharger. The gaskets and
injectors don't receive the high pressure, only the
cylinder and valves.
It's a good idea to regulate and use the (remainder of the)
compressed air to "turbo charge" the engine (saves a
turbo charger). |
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Actually cars can run on compressed air - see:
http://www.bellwetherinteractive.com/mdi/
technology.html. There used to be a video of a running
engine on http://www.zeropollution.com/. |
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Would you have extra valves on the cylinders to feed the air tank, or some kind of switch off the normal exhaust valves? |
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This would encourage people to use engine braking; I'm not sure how I feel about that. |
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I was wondering once about how much energy compressed air actually stores. Its a fraction of what hydrocarbon fuels do but its more by weight of air than batteries in electric vehicles!
Correct me if i'm wrong with this but:
At 1 atmospheres a cubic metre of air has enough energy to generate 1.5kw for a minute assuming you have a generator with 75% efficiency.
So a 1000 litre tank at 300 atmospheres thats 7.5kwh or enough to push a very aerodynamic car 40-50 miles.
But ... thats about 600hp for 60 seconds, so yes you can have your fuel efficient sportscar! |
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You'd need one of those new BMW's with electro-actuated valves. Then, just hack the software in it, install a switch in (or tell it to actuate regeneration when you depress) the brake pedal, and tell it to use the air in the reservoir when you let off. Voila. |
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Course, BMW is already a sports car... |
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The additional engineering you propose would either degrade the performance of the engine under normal running, or add uneconomical weight and construction cost. |
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A compressed air system that does not use the car's pistons may be better, but an electrical regenerative braking system that has only a small electrical storage capacity (and therefore requires only a small battery and not a fuel cell system) is likely to be more energy, cost and space efficient, not to mention safer in case of an accident. |
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In short an electrical version (basically a starter system) beats reinventing the internal combustion engine. |
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Vote: Concept (+), Beaten by existing technology (-), net = neutral |
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Increase the power-out to power-in ratio of the regen braking by passing the compressed air through an exhaust-to-air heat exchanger on the way to the expansion device. Then it would be a brayton cycle with intermediate storage. |
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But, the more you pressurize the air, the higher the temperature, and the less heat received from the heat exchanger – thus low cycle efficiency. Low pressure means big tank (as discussed). An inter-cooler would be cool! |
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I would guess that it would be difficult to achieve complete expansion going back through the IC engine as an air motor. Admission valves would have to close early in the expansion stroke – difficult at high speeds. Perhaps a small turbine would be best to assist the IC engine during acceleration. Heat from the exhaust would be available then, with a slight lag. |
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Keep the spark plugs firing during breaking/compressing in case the controls did not stop gas flow. Then you can avoid a bang in the tank. |
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Using the engine as an air compressor for braking on bigrigs is called a jake brake. the compresed air is just wasted though(as far as i know). But i don't see any reason why one couldn't collect it and use it for some purpose.
http://www.straightdope.com/mailbag/mjake.html |
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A well-earned croissant! I also like Measured Disorder's idea of using the low temperature benefit of the expanding gas. |
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This idea will come a lot close to baked if they get electrically controlled valves. This way you could instantly change the valve overlap, etc to capture compressed air efficiently. |
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FloridaManatee, I agree that a compressed air system has some serious competition with the hybrid electrical...but it does has some advantages over electrical: sheer weight of storage device (air tanks vs batteries), speed of power release, evironmental impact, etc. |
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ALso, agreed that needing ducting to the cylinder probalby means compromises on the engine performance design...but OTOH designers may be able to escape emissions difficulty under extreme power requirements by relying on compressed air during those times. |
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I remember somebody figured a simple way some time ago to recycle the energy used in braking and re-use it for acceleration. Deceleration would stretch some industrial-strength rubber bands or bunggee cords under the vehicle. A gear would be shifted, and the wound-up energy would be transfered to propel the car forward again. Pretty simple. |
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Well, it looks like MDI baked your idea. I included the link above. From what I gather, the engine can run in two modes, compressed air for low speeds, and fossil fuel for higher speeds. Apparently, Mexico City will replace all their taxis with MDI air cars. |
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This idea goes a step further than yours by allowing you to fill the compressed air tank with an external source of compressed air instead of just with engine braking. |
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"The gaskets and injectors don't receive the high pressure," hello? The gaskets are for what, then? Direct injection means the fuel is injected directly where? |
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Valving would be finicky but after that it's a matter of whether the energy conserved offsets the added mass and complexity. |
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Knew about MDI, [Jeph] and I'm glad you added the link, since I couldn't remember their webaddress. |
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Could this be adapted for bicycles, using the frame as the air reservoir and a compressor instead of brakes? |
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I drew an idea for tech drawing class once, a clutch-actuated flywheel to absorb energy and decelerate a bicycle. Of course, with the gearing I envisioned and the resultant top speed, it would allow the bike to balance very nicely while waiting to go again. |
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Using a perfect mechanical system, I don't think you could get more than 25% of the pre-existing kinetic energy back from a dead stop. |
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