# How good is compressed air power? **URL:** \ **Category:** Factual Questions\ **Created:** [February 4, 2001, 4:47am UTC](https://boards.straightdope.com/t/how-good-is-compressed-air-power/53080 "2001-02-04T04:47:45Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![rngadam](https://avatars.discourse-cdn.com/v4/letter/r/97f17d/32.png) [@rngadam](https://boards.straightdope.com/u/rngadam)\ **Post date:** [February 4, 2001, 4:47am UTC](https://boards.straightdope.com/t/how-good-is-compressed-air-power/53080/1 "2001-02-04T04:47:45Z")
[http://www.zeropollution.com/zeropollution/index.html](http://www.zeropollution.com/zeropollution/index.html) Yes, I know, it only moves the problem from polluting car engines to producing electricity - but hey, at least here in Quebec we get most of our electricity from hydroelectric barrage, which are a magnitude less polluting. I also know that the real solution involve a massive re-engineering of our car-based society, but… What I want to know is: how efficient and environmentally sound is this technology compared to other alternatives out there? ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [February 4, 2001, 5:34am UTC](https://boards.straightdope.com/t/how-good-is-compressed-air-power/53080/2 "2001-02-04T05:34:00Z")
I must not be understanding something. The site continuously trumpets “zero-emission compressed air power”, yet shows a schematic with a “constant volume combustion chanber”, and discusses running off of “petrol, LNP, and even diesel”. It is a somewhat confusing website, and seems to offer no real detailed explanation of how the cars work. I will suspend judgement, as a result, except to say that by definition any combstion source that emits a greenhouse gas should not really be termed a “zero emission vehicle” anymore, IMO. ---
**Author:** ![ScoobyTX](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@ScoobyTX](https://boards.straightdope.com/u/ScoobyTX)\ **Post date:** [February 4, 2001, 5:50am UTC](https://boards.straightdope.com/t/how-good-is-compressed-air-power/53080/3 "2001-02-04T05:50:08Z")
Every conversion of energy from one form to another brings inefficiency. For conventional internal combustion engines, we are converting fuel (gas/diesel/CNG/Propane/etc.) directly into motive power, with a big loss on that conversion. With electric cars, we go from burning fuel to motive power, then add: -the losses for generation (sizable ~50%)(less for fuel cells: direct fuel to electricity), -transmission (getting the power across the power lines; not nearly as sizeable, but reasonably big), -conversion to chemical energy (charging the battery, small loss), -conversion back to electrical power (discharging the battery, small loss) -transforming this electrical power back into motive power (about 10% loss, 5-6% with expensive, premium efficiency motors). You do gain a little energy back with regenerative braking, which turns motive power back into electricity, instead of motive power into heat, like conventional auto brakes. With this compressed air system, we convert fuel to motive power to electricity(big loss ), transmitted over power lines (more loss) to motive power (10% loss), to drive an air compressor (another sizeable loss), then to storage (negligible loss), and finally to motive power. Aside from the additional losses, consider the following: -The cited page says that the compressed air is 4500 psi (about 300 times normal atmospheric pressure, or atm), with a capacity of 3200 feet[sup]3[/sup] (I assume this is at standard atmosphere, ~15 ft[sup]3[/sup]). It takes serious equipment to compress air this much. Your garden variety air compressor that you can buy at your local hardware store generates about 150 psi (10 atm). -A standard SCUBA tank holds about 70 ft[sup]3[/sup] at 3000 psi. To store 3200 ft[sup]3[/sup] at 4500 psi, it would take about 30 regular SCUBA tanks (assuming that they could hold 4500 psi, which they can’t, for normal use). That’s a lot of SCUBA tanks for that little car to haul around. -There is a lot of energy stored in that much compressed air. When released all at once (like with a burst air tank), it could do a LOT of damage. In researching this response, I found this [site](http://www.evcom.net/~n4mwd/scubadag.htm). If a gas tank is defective, it will leak fuel, which may burn (over a relatively long period of time), if ignited. If a 4500 psi air tank is defective, it will burst, releasing all of its energy over a short time (small fraction of a second). This will send pieces of the tank all over at a high speed. The only advantage I see is that all of the pollution happens at the power plant, where it can be controlled a lot better than at the tailpipe. This system actually is less efficient than electric vehicles. It also suffers from a problem that I associate with electric vehicles: It makes people feel a lot better about using their vehicle, without actually reducing global pollution (much), and while using more energy than they would have if they would have if they would’ve driven a good, modern gasoline powered (or diesel, even better, efficiency-wise) vehicle. ---
**Author:** ![ScoobyTX](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@ScoobyTX](https://boards.straightdope.com/u/ScoobyTX)\ **Post date:** [February 4, 2001, 6:20am UTC](https://boards.straightdope.com/t/how-good-is-compressed-air-power/53080/4 "2001-02-04T06:20:34Z")
CLARIFICATION: Re-reading my response (preview, preview, preview), I lumped in the loss for fuel to motive power in the 50% loss for generation. This is the approximate loss for fuel to generated electricity. It is the jump from fuel to motive power that is the big loss (about 45%). The loss for generation is about 10%(of the remaining energy). This equals about 50% loss from fuel to electricity. ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [February 4, 2001, 4:22pm UTC](https://boards.straightdope.com/t/how-good-is-compressed-air-power/53080/5 "2001-02-04T16:22:33Z")
I agree with **Anthracite** that the web site is least misleading, possibly a snow job. It calls them “compressed air” powered vehicles. But click on “engine”, and you find: > [@](#): > > MDI engine is, in fact, a radically new internal combustion engine. Two chambers, one for intake and compression and one for expansion and exhaust, are separated from a spherically shaped combustion chamber. The retention time in the combustion chamber is 30% to 100 % longer than in comparable conventional engines, thus giving rise to a more complete combustion at constant volume, while the spherical shape helps eliminate knock (which as been unknown in this engine). The untreated exhaust pollutant levels are lower than those of a conventional engine fitted with a catalyst, and the engine can be modified to run on petrol, LGP or even diesel. It AIN’T running on compressed air, and untreated exhaust pollutant levels “lower than those of a conventional engine fitted with a catalyst” AIN’T zero. It may be a clever and praiseworthy idea (or may not), but the marketing job is misleading as hell. ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [February 4, 2001, 6:58pm UTC](https://boards.straightdope.com/t/how-good-is-compressed-air-power/53080/6 "2001-02-04T18:58:18Z")
We can also find this: > [@](#): > > A version of MDI engine can, in addition to air, also function with the use of traditional fuel; petrol, diesel, natural or town gas, at very low consumption levels. The change of source of energy is handled electronically based on the speed of the vehicle; e.g. below 60 km/h it runs on air and higher speeds it runs on fuel.