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http://www.cleanmpg.com/photos/data/2/European_Union_Flag.jpg The hybrid car that runs on air. (dailymail.co.uk/motoring/article-1162667/Emission-impossible-The-hybrid-car-runs-air.html)

http://www.cleanmpg.com/photos/data/501/2010_Prius-III_front_and_2009_Prius-II_behind.jpgRay Massey – Daily Mail (dailymail.co.uk) – Mar. 17, 2009

Prius-II and -III on NiMH. -IV on air?

For an HEV, the easiest solution of all by all appearances. -- Ed.

A car that runs on air is being developed by engineers at a top British university.

The team have achieved what could be a major breakthrough in the battle to create greener and cheaper motoring.

They have found a way to adapt a normal internal combustion engine to run on compressed air - generated within the vehicle - to give an extra boost to power the motor and considerably reduce the cost of running a car...

The new Brunel engine uses the same principle. But in this case, the engine and the braking generates its own compressed air to provide the added boost to the car...

'The means of achieving this type of energy management could be electric - such as with the Toyota Prius. But it is equally feasible to use mechanical means such as pneumatic - an air hybrid.

'The new air hybrid engine will be considerably cheaper to run and deliver significantly less carbon emissions.'

It also has advantages over petrol-electric hybrids like the Prius, say the engineers: 'Transforming an existing combustion engine in an electric hybrid is very expensive. It requires a complete redesign of the transmission system....

She added: 'The fuel consumption benefits [miles per gallon] you’d get from an air hybrid engine are the same as you’d get from an electric hybrid engine. The top speed you could achieve would be exactly the same as the standard engine you modify to make the air hybrid engine… http://www.dailymail.co.uk/motoring/article-1162667/Emission-impossible-The-hybrid-car-runs-air.html

On the back of an envelope, I figured out that a 2L pressure vessel at 1000psi contains roughly enough energy drive a small-ish car 1 to 1.5mi depending on what pressure the engine requires. That's a higher energy density than I expected, assuming I'm not off my rocker? I dare say that pneumatic hybrids might be a good low-cost option, with some trade-offs.

I wondered about the heat transfer and the brutality of the braking back through the transmission not to mention the passengers. Compressing the air will heat the engine and storage cylinder, is this used or lost? I know the batteries in the Prius heat up also, so it may be a similar loss.

Last I checked, regen braking is maybe 40% efficient by the time the energy returns to the front wheels. The rest is battery heat and conversion losses. You do encounter the same sort of thing when you compress a gas...much of the energy becomes heat. Some of it is lost to the surroundings immediately, some of it remains in the compressed gas until it dissipates. So efficiency of this system partially depends on how long energy is stored in the pressure vessel before it is sent back to the wheels.

Wait a second here. They're basically using a normal engine and during braking instead of using the brakes they compress the air using the engine. Problem is the compression ratio on a normal engine is only what? - 7 to 1, 8 to 1? 1 atm = ~15psi, so that means you reach 8*15=120psi. You're going to have to have a pretty big tank to store much energy at that pressure. They must be using a fancy adiabatic valve of some type to get the pressure up. How do they pressurize air while burning fuel in the engine? I'm not understanding their concept of operation.

Also to add to what brick said - when you pressurize gas you will see an increase in temperature. If you let the gas cool down then you will loose that energy and hence efficiency. However if you insulate the tank, or quickly depressurize the gas and use it's energy quickly as is the case during the combustion in a gasoline engine the losses are minimized.

Hi All:

___There is an additional heat of compression but some of that can be recovered if the energy is used in the vehicle relatively quick as a NiMH pack would do. Maintain a 3,500 optimal rate with a 5,000 psi max and a 1,500 psi min. No real worry about longevity, disposal, expense other than the volume and cap of the accumulator and attached sub-system to do the exact same thing our NiMH's and Inverter/Transverter do today.

___Regarding 120 psi, I suspect this system has a two-way compressor attached like an Motor/Generator and during or steady state, a small drag is induced to press up the accumulator for use later.

___The thing I like about this over a bladdered hydraulic tank with a precharge of N2 or air on top is the energy density can be scaled up and leaks will not be an issue. With a hydraulic system, you know a 10,000 psi hydraulic pressurized system is going to leak on the garage floor and when it does, the consumer is going to go nuts. Leaking air is not a problem as long as it is a small leak.

___Inexpensive and a production ready design remain to be seen however.

___Good Luck

___Wayne

I agree that scalability is a notable advantage for these systems. Not sure that 'small" vehicles (passenger cars) are they best initial platform. Maybe fewer of them in larger vehicles?

The metric of performance (for me) is fuel dollars saved vs. machinery dollars spent.

DAS