It doesn't get much cleaner than a car that runs on air, but is it the transportof the future or a passing novelty? Science Editor Dick Ahlstromreports.
A Luxembourg company promises the ultimate in low cost, pollution-free motoring - a car that runs on air. It needs no fuel other than a supply of compressed air and a specially designed engine that can deliver a top speed of 110 kmh and a range of 200 to 300 km.
It sounds fanciful but Moteur Developpment International (MDI) has developed just such a car. Inventor, Guy Négre, promises a four-seater that can get you there and back for just one cent per kilometre.
His company has already developed a model range including a three-seater mini car, family car, van, taxi, pick-up and also large-scale public transport vehicles. Curiously all but the buses have identical engineering aspects, weighing in at 750 kg, a max load of 500kg and a top speed of 110 kmh.
The features do vary a bit however, with offerings including air bags, air conditioning, ABS and anything from two to six seats depending on model. All, however, use the same power plant, an engine that runs on a "fuel" supply of compressed air.
Before the purists turn away in disbelief, the use of compressed air as a fuel source isn't completely without precedent. In fact compressed air energy storage is more than 20 years old as applied in energy production.
The idea is to use off-peak, "surplus" electricity at night to pressurise underground caverns to between 800 and 1,600 psi. This pressurised air is then used to drive electricity turbines during daytime peak demand and so deliver pollution-free electricity.
The first such working plant was built in Germany more than two decades ago and there is a 10-year-old plant in McIntosh, Alabama, both using large caverns created in salt deposits. A US Department of Energy lab is now studying the possibility of using this technology in a 2,200-foot deep inactive limestone mine in northeastern Ohio.
The ESB uses a similar approach at Turlough Hill, Co Wicklow. It pumps water to the top of the hill using off-peak power and then releases it back down through turbines to generate power when it's needed.
MDI uses compressed air instead of igniting petrol or diesel to get its motors running. The company, formed in 1991 in Luxembourg, began researching an engine which can run exclusively on compressed air, but can also switch between air and conventional fuels at higher speeds or to extend range.
The CATs 34 engine is one of its newest designs. It has four two-stage pistons with eight compression and/or expansion chambers. It also has a five kW motor/alternator, a key requirement given the way the engine is used.
When working exclusively on air, the engine remains idle when the car is stationary. It "starts" when compressed air is released from a long, narrow, high-pressure cylinder running nearly the length of the car. The electric motor takes over rather than air when doing parking manoeuvres and performs other tasks including compressing air when running on petrol, as a conventional alternator to recharge the battery and as an electric brake.
Once the car drops to 60 kmh in the city, it switches exclusively to compressed air.
The company has come up with a most unusual crank and piston arrangement in that the piston can be held at top dead centre for 70 degrees of the cycle. The company suggests this allows more time for pressure to build in the cylinder and deliver more torque.
MDI has moved well past "concept" car level and at the moment is building its first production factory in France. It employs 30 engineers and holds registered patents in 120 countries. MDI has developed a range of prototype engines and body designs. It will work off initial orders for 14,000 vehicles from Spain alone where it first exhibited the car in Barcelona in July 2000.
But does the whole thing make economic sense - and, more importantly, engineering sense? Probably not, according to Dr Will Smith of University College Dublin's department of mechanical engineering.
Smith, who is also programme director of UCD's Formula Student racing team, had a number of concerns about the compressed air approach. His main problem was with efficiency losses given the multi-step approach needed to make the car go.
If you start at the point where electricity is first used, a good electric compressor would work at about 90 per cent efficiency, converting electric power to stored air power. If you give the air car engine a high efficiency rating, then it might use the air power at, again, a 90 per cent efficiency.
Yet, if you used an electric car you could achieve 90 per cent efficiency in a single step. "You have a two-step process rather than a one-step," he says. "So you are starting with a disadvantage from the beginning." Every step causes a consequent loss of efficiency in converting the initial energy source.
He also raises questions about the compressed air cylinders. These would store only a small amount of air unless held at very high pressures, something that would have safety implications, he says.
Also the car is not pollution free. It uses either its petrol compressor to store air, causing emissions, or it uses electricity from home or a compressed air refuelling station which requires electricity, thus producing emissions.
Learn more at MDI's web site at www.theaircar.com