F.Y.I. Hot Air

The ICE (internal combustion engine) vehicle is clearly highly successful. It uses an engine to extract chemical energy from a fuel, usually liquid, and the widespread availablilty of this liquid fuel and the ease and quickness with which the vehicle can be refueled (recharged) are highly important for that success. However, since much of vehicle operation is in cities, and much of that for commuting, the ICE vehicle has at least two serious problems. 1) Air pollution. The combustion of a carbon containing fuel will of course produce carbon dioxide. Since the temperature and pressure in the combustion chamber are high, and the time short, all manner of other compounds are formed, such as nitrogen oxides and many combustion products, as well as unburnt hydrocarbons. In a city, with lots of people and lots of vehicles, this can be a real problem. 2) Poor operation. Although an ICE can be quite efficient cruising, it is unsuitable to stop and go city use. Lots of energy is wasted during frequent braking, and lots of fuel is wasted with the engines running while waiting for a light change or for traffic to clear. The ICE is unsuited for this mode of operation, yet much of the total use of ICE vehicles is under these unfavorable conditions.

The battery - electric is a much better choice for commuting as it can use regenerative braking and does not use any energy while sitting idle. In addition, electric motors have only one moving part, the rotor and shaft, and only two wearing parts, the bearings. Clean them up and replace the bearings and they are as good as new, can be recycled, and essentially last for ever and are fairly cheap. They do not even need lubricant as the ICE does (using sealed bearings). Such vehicle's total weight and complexity can be kept low, which can lead to low cost, even with batteries, and overnight recharging is quite practical for a range on the order of 100 - 150 miles, quite adequate for most commuting. They are not non polluting as the electricty has to be generated somewhere, but it can be done at a large power plant where pollution can be much more easily dealt with, and for one thing, be away from the downtown area where the real problem lies.

However, battery electrics have at least two serious problems of their own. Perhaps the worst is limited battery life; generally several hundred recharge cycles or maybe 2 or 3 years, after which, the expensive batteries must be replaced, adding significantly to operating cost. They are still cheaper for commuting use than an ICE vehicle. The other problem is lengthy recharge times, on the order of hours. This is acceptable for the typical commuter use as the batteries can be recharged overnight and always be ready, but for use in something like a taxi, and there are lots in a big city, or a bus, the lengthy recharge time means it will often be out of service. This problem is unlikely to be solved as the electric power needed at a recharge station would be prohibitively great and the spiking would not do.

This has led to interest in compressed air vehicles, using a pressure flask and some form of air motor. The flask can be refilled from a large tank in minutes instead of the hours needed by electric. The tank can be kept filled with compressed air by running compressors as needed, even around the clock. Air vehicles have long been used in mines where ICE is unsuited because of limited air, and electric is not too desireable because of the possibility of a spark igniting methane or coal dust. On the face of it, air vehicles would seem to be as likely as electric vehicles, but there are two problems to consider. The first is the tremendous drop in pressure during normal operation. In an electric, the voltage remains quite constant, as a matter of fact, it is a bit hard to measure the differenece between fully charged and almost discharged to indicate how much charge is left. This results in very efficient transfer of energy, and is quite desireable. For air, there is a tremendous drop in pressure as you start to refill a flask. This means loss of energy and efficiency. It can be reduced by maybe using three tanks, one at a low pressure, one at medium, and the other at high. You would fill the flask from the low first, then switch to the medium, and finally the high. It would probably be too cumbersome to use more than three. For the motor, to get the same power as the pressure in the flask falls, you can keep opening the throttle, using air faster and faster. You would also probably want to use a multistage expansion motor, or perhaps three in series, a small high pressure engine with its exhaust going to a larger medium pressure one, and then to a lage low pressure one. As the flask pressure falls, you could bypass first the high pressure motor, then it and the medium.

An even graeater problem is adiabatic. That is a thermodynamic term that simply means without transfer of heat. Anyone who has studied thermodynamics would thik of it at once, but those who have not, are likely to overlook it, and not surprizingly, it is hardly ever mentioned. Overly simplified, thermodynamics says for a gas, the volumn, pressure, and temperature are interrelated and proportional.

Consider a diesel engine with a compression ratio of 20. Reducing the volumn by a factor of 20, raises the pressure by a factor of 20. That is 20 bars (times standard pressure), about 300 psi. Since there is no time for heat to leave the air, the compression is adiabatic and the temperature goes up by a factor of 20. That is hot. Hot enough to ignite the diesel fuel when it is spayed in, and diesel fuel is not real easy to ignite.

For the air engines, they are talking about 200 bars, 3000 psi. Ideally you would compress it adiabatically, retain that heat to have it avaialable for adiabatic expansion in the motor to recover all your energy (heat is energy) for high efficiency. In practice, you could never store air at that temperature, it is hot, hot, hot, and would vapourize most materials, and you sure would not want to handle it to fill your flask. In practice, you have to do something about all that heat. In winter, you could heat buidings, but what would you do in summer? You probably would need cogeneration, which means next to your compressor plant you would have to build a power plant to use the heat to generate some electricity to help with the compression. Expense and complexity.

Adiabatic expansion of normal temperature air drops the temperature dramatically. I mean COLD. You can pick up heat out of the air to offset some of this, using a radiator for a heating system much like a vehicle's cooling system, or perhaps heating fins like the cooling fins on an air cooled engine like a motorcycle, but you are going to have trouble with frost build up, and rain water spashed on it could result in a thick coat of ice, either of which, will reduce the transfer of heat, reducing the efficiency.

As a sidelight, you could have all the air conditioning you could ever want, simply by running the exhaust into the passenger compartment. For heating, you would probably have to provide another air motor direct coupled to a compressor for a heat pump, which is just a reversed air conditioner, with the evaporator outside and the condensor inside. An electric will have much the same problem the other way, heat could be obtained from the motors for heating (at least until it gets bitterly cold), but you would need another electric motor driving the compressor for a heat pump to provide cooling. In either case, a substantial amount of your stored energy might have to be used for heating or cooling as these vehicles will be using little energy for movement, so you might have to cut your range by half to be comfortable. Your commuter might be good for 150 miles in mild weather, but only 75 on a really hot or cold day.

It could be that electric will be used in the north and air only in the south. India has recently announced the intention of building air motor taxis to cut down on air pollution in some of their cities. Quick recharge for taxis and their hot climate make air the choice over electric. For the US midwest, electric is probably much the better choice.

In fact, although quick recharge may not be practical, battery exchange could be. For general use, it would not be practical to swap your battery for one that may or may not be good, but for fleet operators it would be quite practical and computers could keep track of everything, even predicting when a particular battery needs replacing. A vehicle, say a taxi, with battery running low, would pull in to the recharge facility, probably with a front wheel guide to position it predictably. Automatic machinery could then quickly pull the discharged battery and insert a charged one and the taxi would be on its way, while the machinery moved the discharged battery to a recharge bay, and in about 6 hours it would be ready to put into a taxi needing it. All things considered, I still think electric is the way to go, maybe with a small ICE, generator, and fuel tank on board if needed to extend the range, turning a limited use vehicle into a full use one. At least get you home if you over extended.