Will We All Be Airheads One Day?

This car runs on compressed air, and will be built and and sold in a revolutionary way. Peter Dron investigates French innovation and apostrophe abuse

Published: 31-Dec-2004


As oil resources gradually dwindle, what will power our cars in 20 or 30 years? Despite continued development, the pure electric car is at best a distant dream, while hybrids are costly to build and hydrogen still gives headaches to advanced engineers. MDI's answer is compressed air.

If the French company succeeds, that will not be its only revolutionary contribution to the car industry. Cars have been built in much the same way since Henry Ford adopted mass production from the small-arms industry (which copied the idea from the Connecticut clock trade). MDI's combined factory/sales outlet is an entirely new idea.

MDI (Moteur Développement International) was founded by French motor racing engineer Guy Nègre. He runs the company with his son, Cyril, who worked for Bugatti in the pre-VW/Audi era. Before turning his attention to alternative technology, Nègre senior headed a company called MGN (Moteurs Guy Nègre). One of his most interesting projects is on show in MDI's foyer at Carros, near Nice: a "broad-arrow" Formula One engine (with three banks of four cylinders). It was due to be tested by AGS when that small team went bankrupt.

MDI is not alone in working with compressed-air engines, but it has been developing the technology for a decade and expects to be the first into production, with factories worldwide and almost 40 industrial partners.

MDI's first planned production model is MiniCAT'S (the French being at the cutting edge of apostrophe abuse; to avoid offence, I shall drop the misguided punctuation), a three-door commuting car. The feline-sounding part stands for Compressed Air Technology System. Lightness and simplicity are paramount; the chassis consists of round-section, light-alloy tubes and frames, glued together, and the two-part floorpan/bodywork is foam-injected glass-fibre; other lightweight materials, such as hemp, are being considered. As Cyril Nègre explains: "A chassis can be glued in about 15 minutes. The diameter of the tube is larger than that of the original [welded] steel tubes we used and the result is lighter and torsionally stiffer."

The horizontally opposed, four-cylinder engine is rear-mounted, driving the rear wheels (to eliminate friction losses caused by transmitting power to steered wheels) via a multi-function "moto-alternator" and a gearbox with two or three ratios (no more are needed). The air tanks, under the floor, are carbon-fibre with a thermoplastic lining.

Only 6in longer than a Smart Fortwo, MiniCATS is more practical: it has three seats, a central driving position and more luggage space; the volume depends on how tall the driver is and how far back the bench seat is set. A range of vehicles is envisaged, using the same principles. CityCATS is a larger model; the various prototypes have included a roomy taxi and a pickup. MultiCATS is a bus that can have one or more trailer units, each with its own air tank and power unit controlled from the cab. MDI considers it suitable for urban use and airports, and has drawn up an attractive-sounding leasing system for municipal clients.

The tank of the production MiniCATS will contain 100 litres of air, sufficient for about 90 miles. This can be extended to 240-300 miles in the "dual-energy" version, involving a device similar to a domestic gas heater. As Cyril explains, "At less than 50kph [30mph] around town we will run on compressed air; outside town there will be help from the heater, with continuous combustion at 600-800C. There are no nitrogen oxide emissions and very low consumption, but the range will be greatly extended. Range depends on the volume of the tank, so the basic version is just for urban use, but with this solution you can go anywhere. Top speed will be 110kph [about 70mph] with compressed air, a bit more with the heater."

MDI's engines are horizontally opposed. Although there are four cylinders, the engine runs like a two-cylinder because the pistons on each side of the "boxer" are joined. The piston has a regular movement, stopping at Top Dead Centre for 70 degrees of crankshaft rotation before descending. It is not an Atkinson cycle engine, but works in a similar way. There are three connecting rods, but with a normal crankshaft.

The current experimental engine capacity is 1,200cc, fed compressed air at 200bar (2,900lb/sq in). Says Cyril, "Our air supply partner suggested we should start at 450bar, which is feasible but would make the car more expensive. We plan to start with 300bar. When people see that you can drive with that pressure without problems and that it is very safe, then we can raise the pressure."

Cyril is realistic about car-buying behaviour: "Our car has to be simple because it has to be cheap. If you produce even a very clean car but it is much more expensive than a normal car, people won't buy it. So the car must be cheap to buy and cheap to run. Using just compressed air in town, you spend €1·5 for 200km [about £1 for 124 miles]. That's about a tenth of the cost of a petrol car."

How does MiniCATS compare with an electric car? "In the CityCATs, we have 52 megajoules, which is not a lot of energy, but it is a bit more than the electric version of the Peugeot 106/Citroën Saxo, which produces about 37-39 megajoules. And that is a lot heavier than ours, so it uses a lot more energy. Filling our tank uses more energy than charging batteries. But globally, the efficiency of this kind of car is better than an electric car, because of the lower weight." A MiniCATS weighs only 550kg (10·8cwt). Other advantages over "pure" electric vehicles are lower build and running costs, improved range (especially with the external combustion option) and refills that are quicker than recharges; a refill at an "air station" will take about as long as a refill of a petrol car, while overnight refills can be achieved by plugging into the mains.

If you think this is all hot air, you will be surprised when you put your hand over the exhaust pipe on a hot day to find that the expelled air is at a much lower than ambient temperature: -20C, in fact. This allows a "free" air-conditioning system - free, that is, except for the power consumed by the fan.

MDI's contention is that compressed air is the most sustainable use of energy for urban commuting vehicles. The prototype van in which I had a brief ride was not fully developed and sounded a bit like a diesel 2CV but cold, clean air came out of the exhaust, and it is still early days. The compressed-air engine might not solve all our transport problems, but it looks worthy of development. That rogue apostrophe could be a "delete option".

The engine development story

Development of the project that has culminated in the MiniCATS began 10 years ago. Initial prototypes were petrol-powered, and MDI was investigating methods of reducing friction losses and improving efficiency in small engines. The earliest motor, in 1994, had a double combustion chamber.

The first compressed-air engine was developed in 1998. In MDI's prototype green taxi it covered many development miles around Brignoles (north of Toulon), where the company was then based. This engine retained conventional crankshaft motion, and Guy Nègre and his engineers soon discovered the drawback that torque output fell dramatically as engine rpm rose.

This led to a major redesign of the engine internals, and late in 1998 MDI bench-tested a new engine in which the piston stops at Top Dead Centre during 70 degrees of the rotation of the crankshaft; this is similar in principle to the 19th-century Atkinson cycle, an elegant design dismissed by many engineers as impractical in conventional engines.

This was still a vertical engine, but then the advantages of horizontally opposed construction began to be appreciated, and the first "boxer", a 600cc twin, ran on the test bench in 2000. Its successor, again with two cylinders but with double the capacity, incorporated low-pressure "marginal" lubrication, with very low-viscosity oil, further reducing friction losses.

The current MDI engine is the 800cc, four-cylinder P34. Between it and the gearbox, which may contain two or three ratios in production versions, is a multi-function "moto-alternator".

As well as doing the job of an alternator, this acts as a starter, assists in parking manoeuvres, boosts acceleration on demand, recuperates energy during braking and, plugged into the mains, turns the engine into a compressor to refill the tank in five and a half hours with 220 volts, or four hours with 380v.



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