Monday, March 19, 2007

There's an old saying...

...Actually two. The first is "if it's stupid and it works; it isn't stupid", the second is "if it seems too good to be true, it probably is".

What's amazing, is that sometimes, the first, can contradict the second in some pretty fun and interesting ways... but most of the time, it doesn't. Let's talk about one of those times.

So, the first saying...

One of the first examples given to illustrate the operating principles of small engines, is usually that of an air compressor. An internal combustion engine sucks in low pressure air, mixes it with fuel and a spark at something approaching a 14:1 ratio (with the fuel, not the spark), blows it up; and high pressure air comes out the other end... along with water vapor, carbon, carbon monoxide, carbon dioxide, nitrogen dioxide, ozone, partially burned and unburned fuel, and other nasty bi-products of the combustion of hydrocarbons.

Like almost all other reductions of complex machines into simple models, it's a stupid comparison; but it works (therefore, according to the maxim, it isn't stupid).

Now, what would happen if you took the fuel out? Well nothing because there would be no release of energy to make the pistons turn... so lets not simplify things that much. What if we take the fuel out AND we reversed the flow of high pressure air?

Well, hopefully something like this:

Actually, I was hoping for something a little less ugly and stupid looking but... as the saying says...

So the basic concept is this: Instead of high pressure exhaust coming out, we pump VERY high pressure air IN, which pushes the pistons (or turbines if you're so inclined, but I'm pretty sure they're using pistons in this application), turns the crank (and thus the transmission, and the wheels) and then exhausts as low pressure air (and a little water vapor and lubricating oil); basically an air compressor in reverse.

Here's the company line:

"Many respected engineers have been trying for years to bring a compressed air car to market, believing strongly that compressed air can power a viable "zero pollution" car. Now the first commercial compressed air car is on the verge of production and beginning to attract a lot of attention, and with a recently signed partnership with Tata, India’s largest automotive manufacturer, the prospects of very cost-effective mass production are now a distinct possibility. The MiniC.A.T is a simple, light urban car, with a tubular chassis that is glued not welded and a body of fibreglass."


"Most importantly, it is incredibly cost-efficient to run – according to the designers, it costs less than one Euro per 100Km (about a tenth that of a petrol car). Its mileage is about double that of the most advanced electric car (200 to 300 km or 10 hours of driving), a factor which makes a perfect choice in cities where the 80% of motorists drive at less than 60Km. The car has a top speed of 68 mph.

Refilling the car will, once the market develops, take place at adapted petrol stations to administer compressed air. In two or three minutes, and at a cost of approximately 1.5 Euros, the car will be ready to go another 200-300 kilometres.

As a viable alternative, the car carries a small compressor which can be connected to the mains (220V or 380V) and refill the tank in 3-4 hours.

The temperature of the clean air expelled by the exhaust pipe is between 0 - 15 degrees below zero, which makes it suitable for use by the internal air conditioning system with no need for gases or loss of power.

How does it work?

"90m3 of compressed air is stored in fibre tanks. The expansion of this air pushes the pistons and creates movement. The atmospheric temperature is used to re-heat the engine and increase the road coverage. The air conditioning system makes use of the expelled cold air. Due to the absence of combustion and the fact there is no pollution, the oil change is only necessary every 31.000 miles.

At the moment, four models have been made: a car, a taxi (5 passengers), a Pick-Up truck and a van. The final selling price will be approximately 5.500 pounds."


The geek in me is saying "oh cool", the engineer in me is saying "OK, how the hell are they gonna make THIS work?"

Not that a compressed air car can't work, obviously it can. The principle is very simple; the problem, as usual, is in the details. Specifically that little detail of efficiency.

Let's assume they pressurize the tanks to about 6000psi (about 400 bar, the highest conventional cascade compressor sets can go, the highest a reasonable pressure tank can store, and about twice a scuba tanks pressure).

If I'm reading correctly, they are taking 90 cubic meters of compressed air, and pushing a 1400lb or so vehicle, up to 68mph (110kph), with a 300km range.

I say 1400lb because thats about the minimum possible for the smallest model to hold two passengers, carry the essential propulsion gear, and electronics, and still have enough strength and structure not to collapse, even with advanced composites.

That car is going to need something like 20hp minimum, maybe 30hp (about 20kw); just to be able to get up to that speed (and more importantly, to be able to pull away from a stop sign on a hill).

Looking at a similarly specced vehicle, the French 2cv (or technically the 3cv model, which had 33hp -24kw-, and a 68mph top speed - that it took three minutes to accelerate to ) weighed about the same, and had about the same horsepower, and was dimensionally quite a bit smaller, so I think it's a reasonable estimate given newer more efficient materials end technology.

You really just can't move that much weight without pretty close to that much horsepower, and you really can't have a car that size with much less weight. Laws of physics being what they are and all.

So, to the second saying...

The rest of this will be in metric because those are the units easiest to work with here.

Given the range they've specified of 300km, and the power needed; we're looking at about 72KW/Hours of power; or 2,600,000,000 joules of total energy. Either 24watts for three hours, or 7 watts for 10 hours, the total energy is going to be about the same.

That has to come out of 90 cubic meters of high pressure air. At 400 bar, 90 cubic meters of compressed air, has about one and a half times that amount of potential energy.

Now remember, you can't get out more kinetic energy, than you have in potential energy. In fact, you can't even get as much out (that's called unity); because all systems are inefficient at converting potential energy to kinetic energy, to some degree.

So, it's good that there's 1.5 times as much potential energy as the engine needs right?

Here's the problem though, there is no way in hell that the thing is even 50% energy efficient. In fact, it would be an engineering miracle if they got out much more than 25% of what they put in.

Not only that; but you've actually only got about 2/3 of that energy as usable, because even at 100% efficiency you're going to need to flow about 200bar of pressure to get that 24kw (and given the volume of air available and 3 hour range).

Ok, so once again, 90 cubic meters; only presuming a minimum 200bar operating pressure, and 25% efficiency: You'd need to pressurize the tank to at least 1200 bar, or almost 20,000 psi. That's 4 times the highest pressure scuba tank (some HPBA systems are at 300 bar) or six times a standard tank.

Now these are all just really rough numbers, and who knows, maybe there IS an engineering miracle here. Also, its entirely possible they ARE pressurizing the thing to 1200bar; there are plenty of industrial applications that use pressures that high. In fact, there are common industrial applications (and equipment) for low density fluids (very few substances remain a gas at such high pressures, and nitrogen which air is mostly made up of, isn't one of them) at up to 150,000 PSI. It's just that such high pressures are not easy (and not cheap) to deal with, to store, to distribute, and to create in the first place.

Speaking thereof, the energy required to pump the high pressure air is going to be tremendous; because typical air compressor systems are terrifically inefficient; never mind the transportation and distribution infrastructure, which at least double that energy cost for production.

So yeah, your three minute fill up may have no direct emissions; but you're using probably 1440 KWh of energy total (inc. manufacture and distribution of the high pressure gas). At bout 8 cents per KWh (American national average. In India, where the car will be made, the national average is about 6 cents per Kwh) that would be $115.

$115 ??? but the article says about $2.80 (1.50 euro, at about $1.87 per euro at the moment). At $0.05 per Kwh in India, they're saying that they are using 56kwh of power total.

Honestly, I don't see how that's possible. For one thing, there's no way you could get 700kg to 110kph on 18.5kw; even assuming 100% efficiency (which is impossible).

But wait, theres more: they say that the car will come with a compressor that will reach a full charge in 3 to 4 hours, off a 240v Indian standard socket (240v at 15amp, for 3.6Kwh per hour, 4 hours, total energy about 15Kwh). Once again, assuming 100% efficiency, thats only 5Kw of power to move that 700kg, 110Kph.

That is absolutely impossible. Even at 500kg, thats not possible... hell it's maybe just barely possible at 250kg, and thats lighter than any but the lightest motorcycles and riders.

Hell, it's not even possible for a small compressor operating off standard mains current to achieve anything close to the operating pressures required.

The best portable commercial systems can flow about 15cfm at 6000psi (about 4 hours to fill the cars tank to 6000psi), and require either a 40amp 240v circuit; or a 25hp diesel motor. They are also about the size of a desk, weigh several hundred pounds, and start at about $30,000.

Let's just go over this one more time.

1. It is impossible to get more energy out, than you put in (you can't reach over unity).

2. In fact, its impossible to get even as much energy out, as you put in (you can't reach unity).

3. The best possible energy efficiency for systems of this type might be 25%, or assuming a technological miracle let's say 50%.

4. Even at 100% efficiency (which is impossible), 20-24kw (28-33hp) would be required for the vehicle to reach 110kph.

5. Even at 100% efficiency (which is impossible) 20-24kw for three hours (the specified range) would require 60-72Kwh (or 5-7kw for 10 hours, requiring pretty much the s

6. Even at 100% efficiency for both the engine, and the compressor system, the MOST possible energy that could be stored by the compressor system specified (230v at 15amp) in 4 hours would be 15Kwh; or 5Kw per hour for the three hour range specified.

7. It is impossible that such a vehicle would weigh less than 500kg. In fact it is nearly impossible it would weigh less than 700kg.

9. 5Kw will not move even 500kg to 110kph, never mind 700kg. In fact, that power would only move such a weight to about 20kph.

So, what they are describing is flat out impossible. Either the company promoting this wonder care are outright lying (possible, but seems unlikely), shading the truth to a ridiculous degree (highly likely, but still not enough), the reporter is a credulous idiot (almost certain, but also not enough in and of itself), or all three.

Personally, I'm thinking all three.


Reader Smitty gives us a link to the actual designer of the vehicles in question.

Thank you very much sir, I appreciate that. I must have been stupid and not seen that among all the clutter on the article page.; and believe me I was looking.

At any rate, the first thing to note is, the vehicle is not in fact exclusively powered by compressed air. Apparently the motor is a hybrid diesel electric, with a low speed circuit powered by compressed air. This low speed circuit can be boosted by the electric portion of the drivetrain.

The low speed circuit operates at up to about 36kph generally. Above that speed the diesel circuit kicks in, up to about 60kph, when the compressed air circuit kicks out and the diesel goes it on it's own until top speed of 110kph. In addition to primary power, the diesel circuit is used to recharge the compressed air tanks when the pressure gets too low, or there is extra capacity the engine doesn't need for propulsion.

The waste heat from the diesel circuit is also used to maintain operating temperature of the engine, and reheat the super cooled exhaust air (very high pressure fluid, being released into very low pressure, means vapor phase change refrigeration whether you want it to or not). It is unclear what the range is when traveling exclusively on the air circuit.

Honestly, that's great. It's a good idea for using the waste energy of a vehicle to do something useful. Of course, the marketing page for the vehicles, and the article, make no mention of any of this. They refer to the car exclusively as a zero emissions vehicle, say it produces zero pollution, and explicitly say it doesn't burn any fuel, therefore never requires an oil change.

Lesse, shading the truth just a bit hard, a little bit of outright lying, and a credulous reporter?

Yeah I think so.

Now, as to my engineering assumptions; I haven't been able to find specific details as to how much HP the engine produces while on compressed air circuit vs diesel circuit, but this page does give some basic tech specs (in spanish):

Empty Weight: 550kg
Horsepower: 25bhp
Range without refueling in city driving: 150km

Stick in two small people and you get, 700kg or so, which is right in line with what I was saying, and with 25hp they are getting their top speed of 110kph... but their range is half what the article said, and that's at their projected "city speeds" of 36kph.

That's OK, seriously it's a significant engineering achievement. That's a VERY light, comparatively roomy city car that's incredibly efficient. It uses every possible means of increasing that efficiency, and apparently does a damn good job of it.

...just don't advertise it as an absolute zero emissions, pollution free wonder car that runs on compressed air, because it isn't.

What it is, is a more efficient and better conceived hybrid... and for once a hybrid that isn't a net environmental detriment (look it up, the electrical systems in hybrids cost more energy to produce than they save, and they are terribly harsh on the environment to dispose of).

That's great; more power to them, and I hope the car is a huge success.

Just one thing though: please stop trying to convince us you can do the impossible. We engineer types really don't like it when you do that; it makes us all twitchy.