hello, lately i've been thinking how to roughly design a compressed air powered car. i'd take an existing commercial car, take out its IC engine, fuel tank and all the other stuff and mount my own machine and tanks in. i want it to travel 300 km (185 miles) at speed of 90 km/h (55 mph). here are some very rough estimates: - energy aerodynamically superior modern cars (audi a2, toyota prius, honda insight, ...) have drag coefficients around 0.26 and frontal area cca 2.5 m2, they weight around 1200 kg. good tires should have coefficient of rolling friction around 0.005 - 0.01. energy used in a 300 km constant-speed travel would be: E = (Ffriction + Fdrag) × s = (approx.) 120 MJ. this number seems fine to me, since an ordinary small car burns 6 liters of fuel per 100 km, every liter has 35 MJ of energy, assume 20% efficiency: 3 (300 km) × 6 (liters per 100 km) × 35 (MJ per liter) × 0.2 (eff) = 126 MJ. storing 120 MJ in an air tank would require quite a pressure (let's say 450 bar, i've heard 500 bar compressed air applications exist already), and under that pressure quite a huge tank (about 500 liters). i assumed expansion would be isothermal or at least close to that (multi-staged and inter-warmed ). - propulsion to use as much energy as possible, i'd need an air motor that would work on all pressures from 450 to 1 bar, torque should not be a problem. a CVT would definitely come in handy and fancy (i know what you think, see "efficiency" below). a lot of existing air-powered vehicles use a decompression valve so their motors work on constant pressure (10 bar or so). that's like throwing out 90% of available energy, only increasing entropy. that's probably because of air engines, but that's just not good enough. - efficiency i know compressing air is very close to brutally wasting energy, but i thought the heat generated during compression could be captured in a chemical way like this: http://en.wikipedia.org/wiki/Sodium_acetate the air would then expand a bit, cool down, go through some heat exchanger, first get heat from the surrounding atmosphere and then from this heat reservoir. i mentioned a CVT, these transmissions can get quite hot, this heat could also be used for warming air. and regenerative braking - i'd somehow cool the brakes with a liquid that would then warm the air. - ecology i know you'll start screaming the standard "where do the electricity come from? - fossil fuel..." yada yada. i'm not trying to create energy from nothing, i'm just thinking it would be fine to get rid of gasoline and diesel for running cars. imagine a city with clear air (no brown clouds, huh?) and to me, more important thing - no noise. i just hate it when i can't hear my own mp3 player despite there's not a single car moving in front of a red traffic light. exhausts from large power plants are quite clean nowadays, and in 20 years we'll have fusion power plants anyway, the problem is only in energy storage - chemical is the most dense by far, but can't be used in other way than burning. and we all know what are the efficiencies of heat engines. batteries could be the second, but (in my opinion) they are even less clean than fossil fuels - buying a new 200kg lithium pack every 4 years isn't very clean. in fact, composite pressure vessels can outreach batteries in sense of mass energy density. now my questions: - is it possible to put 500 liters of air tanks into a medium sized car? note that steel tanks would weight 2 or 3 tons, carbon tanks only 100 or 200 kilos. i can sacrifice the trunk, although without excess joy. - speaking of aerodynamics, small and big cars have very similar frontal area surface and drag coefficient. does that mean that it would be easier to put all this compressed air into a bigger car (more space for tanks), since a bit heavier car wouldn't consume that much more energy? - could i reduce drag by installing some weird panes on the back of the car (something like horizontally turned pyramid or a cone)? - since a lot of fresh air would be needed to warm cold, expanded air between expansion stages, i could make a long and huge pipe to the rear part of the car, ending as huge exhaust pipes somewhere in the middle of the back of the car (vertically). would that contribute to lower drag coefficient? - what's the efficiency of existing air motors? can they be connected in series, let's say each stage would expand air volume 3 times, so 6 stages would do the job. would the first motor (the one working on 450 bar) actually work with such pressure? - i could use a vane pump-like air motor. many motors with different radiuses could be connected to a single shaft, and a computer would decide which motors would the air pass through. i actually already have all the calculations for that motors made (dimensions and dependent torque), my main concern momentarily is sealing. a itzy-bitzy vane pump (a 10 cm diameter (4")) and to put in 450 bar... i'd be very glad to hear your comments on my thinking. i'm quite a n00b in mechanical engineering, but i'm totally into it, just need a few dozens of tons of experiences.