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> The Economist. Oct 26, 1996
> Car engines - Not all hot air
>=20
> THERE is something silly about the four-stroke petrol engine.
> Although it is the workhorse of almost all passenger vehicles it
> suffers from some glaring inefficiencies. Only one of the four
> strokes of the piston provides power to thecar while during
> part of another stroke the piston is moving against the pressure
> of the ignited fuel. All of this is an extravagant waste of energy.
>=20
> Such compromises are necessary because of an oddity in the
> engine's design-namely that the initial compression of the
> combustible gas its ignition by the spark plug and the
> power-producing expansion that follows all occur in one and
> the same place at the top of the same cylinder. Over the years
> alternative designs have been proposed that ease this
> constraint. Guy Negre an engineer in Brignoles in France
> whose background is in Formula One engine designthinks he
> has hit on a particularly ingenious variant which can run far
> more efficiently and cleanly than a four-stroke engine on the
> same amount of petrol. As a bonus and without any
> modification the engine can also be made to run on nothing
> more than compressed air.
>=20
> At the heart of Mr Negre's engine is a small combustion
> chamber. A piston in a compression cylinder outside this
> chamber forces air through a valve into it compressing and thus
> heating a mixture of air and petrol in the process. With all
> valves to the chamber closed the hot mixture is ignited. The
> design of the engine allows the fuel to burn over a period up to
> four times longer than in a four-stroke engine. The longer burn
> time means that the fuel is consumed far more completely
> leading to less in the way of unwanted by-products such as
> carbon monoxide and nitrogen oxides.
>=20
> In the final part of the cycle the hot gas escapes through a valve
> into a separate expansion cylinder where it drives a piston
> cooling in the process. Oneof the benefits of having separate
> compression and expansion cylinders is that the expansion
> cylinder can be made several times larger than the compression
> cylinder. This takes maximum advantage of the stroke that
> provides the engine's power. Another benefit is that at the end
> of this expansion the gas is at atmospheric pressure and a
> temperature of only 100 degrees C. The output of
> conventionalengines by contrast is usually at a pressure of
> several atmospheres and a temperature of 500 degrees C
> which means that lots of potentially useful energy is
> beingthrown away and demands a complicated exhaust system
> to cool and depressurise the gases. As a result of these
> advantages Mr Negre's engine is some 50% more efficient than
> the four-stroke variety.
>=20
> The benefits of the design do not stop there. Because the
> expansion piston provides power at every second stroke rather
> than every fourth Mr Negre's engineprovides the increased
> mechanical efficiency of the two-stroke engines common
> onmotorcycles. Yet unlike such engines it pollutes much less
> and is a great deal quieter because the explosion of the fuel
> happens in the small combustion chamber rather than being
> allowed to propagate through the cylinder as in conventional
> motors. The design also allows the engine to be light: fully
> functioning test versions weigh only 34kg (75lb).
>=20
> But the most surprising aspect of Mr Negre's invention is that it
> does not necessarily need petrol at all. He has found that
> forcing a burst of cold compressed air into the combustion
> chamber can deliver sufficient power to the piston to drive a
> car. While engines running on compressed air are nothing new
> the novelty in this case is that cold compressed air at 40
> atmospheres of pressure is being added to that chamber when
> it has already been charged up withair by the compression
> stroke. Since compressing air also heats it the air in the
> chamber is at about 400 degrees C and 21 atmospheres of
> pressure. A hot gas expanding can provide a lot more useful
> mechanical energy than the same quantityof cold gas at the
> same pressure. Although less powerful than a petrol model
> thezero-emission version of Mr Negre's engine can power a
> car in city traffic for one hour on 15 litres of compressed air.
>=20
> This clean engine does however require a source of
> compressed air. One way to do this would be to have an
> electrically driven compressor on board. In other words to
> make the car an electric one in disguise. But rather than having
> to wait ages for batteries to recharge a pressure refill at the
> local petrol station would take only a minute. And with the
> savings made by having a smaller lighter engine and no exhaust
> a car could easily accommodate a compressor that could run
> off the energy produced by braking. In contrast to an ordinary
> electrically powered car though this one could easily be
> switched while driving from compressed-air injection to fuel
> injection giving it the necessary power tozip along a motorway.
>=20
> The first commercial prototype of Mr Negre's engine
> unmemorably named the MDI-Ev3 is currently being tested by
> ADMP a small engineering outfit that he hasset up for use in cars
> and buses. With the backing of MDI an engine developer
> inLuxembourg ADMP hopes to sell a licence for its technology
> to a big engine-manufacturer. Public transport should provide a
> convenient test-market for the engine and MDI has already
> begun discussions with the Parisian transportauthority RATP as
> well as with private bus firms to equip urban buses with the
> compressed-air engine. A Parisian bus could run an entire day
> on 1500 litres of compressed air which could be stored on
> board. If Mr Negre's brainchild takes off Parisian smog may
> one day be reduced to the fumes of Gauloises.
>=20
> =A9 1996 The Economist Newspaper Limited. All rights reserved
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