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An important energy storage method that is being given very little consideration is gas compression. The kinetic energy of the vehicle can drive an air pump or compressor that would compress air into a tank specially designed for pressures and heat of compressed air. Later the energy stored as compressed air can be released through a regulated nozzle to convert to electrical or mechanical energy.

The air pump would be connected mechanically directly to the drive train or the wheels or axle to have the braking effect. The storage process is not efficient as much heat is generated with the compression. However, the energy can be stored in the compression tank for a very long time with little loss. The capacity for energy storage is only limited by the volume of the storage tank and the maximum design pressure.

Special consideration has to be made for the lubricating oil used for the air pump/compressor. It has to be non-combustible other wise it will ignite with the oxygen in the heated compressed air. This would rule out most hydrocarbon based oils.

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    $\begingroup$ This technique is given little consideration because, as you say, it's inefficient. Electric regeneration is much more efficient, and you can easily vary the amount of regeneration/braking effect, while the braking effect of your air compressor is directly tied to compressor size and pressure, reducing the braking effect would mean wasting air by venting part of the air during the compression stroke. $\endgroup$
    – Hobbes
    Jun 12, 2017 at 13:40
  • $\begingroup$ @Hobbes It is inefficient but electric regeneration is only efficient depending on the charge cycle capacity of the batteries or capacitor bank. The charging current is limited. My understanding is the braking power or energy converted by the generator is the current being used for the load. The less the load, the less the current output, the less braking effect, and less efficiency. $\endgroup$
    – 0tyranny0poverty
    Jun 12, 2017 at 14:54
  • $\begingroup$ @Hobbes Granted the regulation of electrical regeneration is easier, it is not an insurmountable challenge to design an air compression storage system that would augment electrical regeneration to handle max pressures and temps for even the worst case braking situations. The non-combustible oil is one of the tougher challenges. $\endgroup$
    – 0tyranny0poverty
    Jun 12, 2017 at 14:55

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When I have looked at explosions in compressed air systems the pressures have been high like 2000 + psi.However , phosphate esters were synthetic oil that was recommended.

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You might find this link helpful : http://www.dailymail.co.uk/sciencetech/article-2568732/Car-runs-air-set-hit-streets-year-Peugeot-claims-new-hybrid-117mpg.html

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  • $\begingroup$ Thanks. I noticed it has a low pressure tank in the rear. This tells me it is a closed loop system with an expansion tank for the decompressed gas. I also noted the letter 'N' on the compression tank, which would mean they are using nitrogen instead of air. Nitrogen is referred to as 'dry air'. Nitrogen can be compressed without worries of igniting the lube oil. My system would be an open system using regular air with no expansion tank. The braking power of said closed loop system is limited to small cars due to limitation of expansion tank. $\endgroup$
    – 0tyranny0poverty
    Jun 12, 2017 at 15:21
  • $\begingroup$ So, as part of your open system you will also need to filter and condition the incoming air - wear caused by dust, corrosion caused by moisture. Why can't you increase the size of the expansion tank? $\endgroup$
    – Solar Mike
    Jun 12, 2017 at 15:29
  • $\begingroup$ If I were to use a closed loop, there are better choices besides nitrogen, CO2 for example. The expansion tank limits our storage capacity unless it is huge. It depends on the pressure differential between the compression and expansion tanks. We want the greatest pressure that the system can handle but still be safe in case of a collision. $\endgroup$
    – 0tyranny0poverty
    Jun 12, 2017 at 20:31

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