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Forget batteries, how about a capacitor car?

Post in 'The Green Room' started by Sprinter, Nov 12, 2012.

  1. Sprinter

    Sprinter Minister of Fire

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  2. begreen

    begreen Mooderator Staff Member

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    The idea has been around for a while. There is a firm in Texas(?) that is working on the supercapacitor. There also are groups working on a superflywheel. Both are to provide for rapid release of energy that can quickly deplete a battery.
  3. peakbagger

    peakbagger Minister of Fire

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    It all comes down to KWhrs per pound. Ultra capacitors dont beat batteries for KWhr/ton. They have been used on fuel cell vehicles as fuel cells dont like to be ramped up and down quickly. The capacitors dampen things out.
  4. semipro

    semipro Minister of Fire

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    The main benefit of supercaps is how quickly they can be recharged; basically as fast as your wiring will allow.
  5. Sprinter

    Sprinter Minister of Fire

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    Thanks. I don't usually follow this kind of thing that much. I suppose I should. It is interesting. It sounds like the infernal combustion engine will be around for a long time to come, though, what with the high energy density and ease of storage of gasoline, not to mention the existing infrastructure. What is the consensus regarding the ultimate efficiency of ICE's? They certainly have improved a lot (say, over my '69 Camaro). What is the technical limit for them?
  6. pdf27

    pdf27 Member

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    Technical limit is the Carnot efficiency (ratio of temperature difference between peak combustion temp and exhaust divided between the ratio of peak combustion temp and inlet). That in turn is set by the pressure ratio you can achieve, which dictates the ratio of peak combustion to exhaust temperatures.

    Pressure ratio hasn't changed much over time for Gasoline engines (set by the knock/preignition limit of the fuel), but has improved a little for diesel engines. The real improvements have been incremental in getting closer to the Carnot limit. For practical temperatures and pressure ratios, you're looking at about 40% efficiency as a limit. That's for an engine at full throttle - Gasoline engines in particular are hugely inefficient at part load, which is where many of the improvements in fuel efficiency are coming from (use a smaller engine to get it working harder, then add a turbocharger to improve the specific power for the few times you really need it).
  7. Sprinter

    Sprinter Minister of Fire

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    That's what I was wondering. Thanks. Maybe a Tokamak-like design for the combustion chamber would improve the pressure ratio problem:)
  8. pdf27

    pdf27 Member

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    For diesels it isn't a big deal - they can put the pressure ratio as high as they like, the only limits are to do with air quality/clean combustion (sets an upper limit on temperature) and the fact that an engine with a higher pressure ratio will be a lot heavier for a little extra efficiency. At some point the cost of lugging all that extra engine around will outweigh the fuel efficiency gains.

    For petrol (gasoline) engines, you can't get the pressure ratio higher than about 8:1 or something like that (can't quite remember off the top of my head). If the compression ratio goes above this, then as you compress the fuel mix it will ignite before you reach the peak compression. That means for the last part of the stroke you're pushing against the hot gases - so don't get any benefit from the increased compression ratio. Different designs of engine don't really help with that.
  9. jharkin

    jharkin Minister of Fire

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    Max compression ratio for gas is a lot higher than 8:1... I know the engine in my Acura hatchback runs 11:1, and I've read that in the Mazda SkyActive designs they managed to pull off 14:1 somehow (usually diesel territory) but they had to do some real funky stuff to prevent or work around detonation.

    Another interesting approach to improving efficiency are systems like BMW valvetronic (also done by NIssan, Toyota and others now) that use infinitely variable intake valve lift to throttle the engine and eliminate the throttle butterfly. This reduces the pumping losses at part throttle getting around that limitation somewhat.

    But overall the biggest problem for fuel economy I think is the weight that modern cars have to carry around. The 1982 Honda Accord I drove in high school got nearly 40 mpg (with a carburetor! ) because it was light. Just Imagine what you could do with a tiny direct injection turbo diesel in a car < 2000lb.
  10. pdf27

    pdf27 Member

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    You're quite right - when I actually checked it 10:1 is fine for most cheap and simple engines, with the most exotic engines (i.e. Formula 1) reaching 17:1.
    The benefits of a diesel engine are less obvious in a small car - a 100 lb increase on a 2000 lb car is a much bigger impact than 200 lb on a 6000 lb car.

    Example of this is the Toyota Aygo/Peugeot 107/Citroen C1:
    Diesel - 1.4 litre engine - 57.4 mpg (US) - 1190kg - 54bhp/96 lbs-ft
    Petrol - 1 litre engine - 51.1 mpg (US) - 1180kg - 67bhp/67 lbs-ft

    The petrol engined variant is apparently a fair bit nicer to drive, and will be cheaper to build/maintain. Given this, a 10% improvement may not worth the extra cost - the effort may be better spent elsewhere.

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