杏吧原创

THE internal combustion engine took a giant leap forward last week when an
American scientist showed how to extract more energy from a car鈥檚 exhaust gases
than was ever thought possible. The trick? Turn the exhaust system into a
laser.

Since the four-stroke petrol engine was invented in the mid-19th century,
engineers have been striving to extract the last ounce of power from it, guided
by the laws of thermodynamics. These laws predict that an engine鈥檚 efficiency is
ultimately limited by the temperature of the gases as they burn.

As the hot gases expand, they drive a piston through a cylinder, generating
power that鈥檚 transmitted to the wheels. But once this expansion is over and the
gases begin to cool, you can never get more useful energy out鈥攐r so
physicists thought.

Now, by turning to the strange laws of quantum mechanics, physicist Marlan
Scully of Texas A&M University in College Station claims it is possible
after all to extract more useful energy from hot exhaust gases. The secret lies
in the workings of a new type of laser being developed in a handful of labs
around the world.

The revolutionary idea could lead to new generation of engines that extract
useful energy from pure heat. It could make cars of the future far more fuel
efficient, though Scully believes tiny nanoscale motors are likely to benefit
from the idea first.

The laser works by passing single atoms through a cavity containing a pair of
mirrors separated by a distance matching the wavelength of light that the atom
can emit. If the hot atom has enough energy, it gives up a photon as it passes
through the cavity. This photon then bounces between the mirrors, which
effectively stores it in the cavity. The process continues as other atoms pass
through, also giving up photons. If one mirror allows a small proportion of
photons to escape, the cavity will emit light of one wavelength鈥攖hat set
by the mirror gap鈥攋ust like a laser.

Scully鈥檚 idea is to apply this principle to car exhaust. Since the exhaust is
hot, the atoms it contains can be persuaded to give up this energy in the form
of coherent light by passing them through an appropriately sized mirrored
cavity. He calls his idea a 鈥渜uantum afterburner鈥�. 鈥淚t鈥檚 controversial stuff,鈥�
he admits. 鈥淏ut we should have completed a proof of principle experiment with
CO₂ molecules soon.鈥�

It鈥檚 an enormous surprise for physicists that the laser energy can be
extracted at all. But Scully鈥檚 work鈥攚hich is partly funded by the US Navy
and US Air Force鈥攄oes not run counter to the laws of thermodynamics.
Rather, his new limit for the energy available from the heat in waste gases
comes from exploiting quantum effects which operate outside the laws of
thermodynamics.

Muhammad Zubairy, a visiting professor at Texas A&M who has been studying
Scully鈥檚 work, plans to publish a paper with him explaining just what could be
done with this laser energy once it has been acquired from the exhaust. 鈥淚n
principle, you could use it for anything,鈥� says Zubairy.