阅读精通， 补全短文， 完型填空各类题型上都有新增小说。
The APU is a small jet engine that is used to start the larger jet
engines. In airliners it’s usually at the very rear of the aircraft,
below the tail. Large engines are heavy and require a lot of torque to
spin up, more than a starter battery can generate. So, the battery spins
up a much smaller APU jet engine. High-speed bleed air from the APU is
used to spin up the main engine.
Once the engines are started, the APU is no longer required, but it does
provide a couple of secondary functions:
The Tiniest Electric Motor in the World
- It provides cabin air and electric power before the engines are
started (saving battery power).
- It provides an emergency source of electric power in the event of
- It can start the aircraft engines mid-flight in an emergency.
Scientists recently made public the tiniest electric motor ever
built. You could stuff hundreds of them into the period at the end of
this sentence. One day a similar engine might power a tiny mechanical
doctor that would travel through your body to remove your disease.
The motor works by shuffling（来回运动） atoms（原子） between two
molten metal droplets（小滴） in a carbon nanotube（微米管）. One
droplet is even smaller than the other. When a small electric current is
applied to the droplets, atoms slowly get out of the larger droplet and
join the smaller one. The small droplet grows – but never gets as big as
the other droplet – and eventually bumps into the large droplet. As they
touch, the large droplet rapidly sops up （吸入）the atoms it had
previously lost. This quick shift in energy produces a power
The technique exploits the fact that surface tension — the tendency
of atoms or molecules to resist separating — becomes more important at
small scales. Surface tension is the same thing that allows some insects
to walk on water.
Although the amount of energy produced is small — 20
microwatts（百十分之1瓦） — it is quite impressive（给人回想浓密的）
in relation to（与…相比较） the tiny scale of the motor. The whole setup
is less than 200 nanometers on a side, or hundreds of times smaller than
the width of a human hair. If it could be scaled up to the size of an
automobile engine, it would be 100 million times more powerful than a
丰田 MA奥迪Q3CH’s 225 horsepower V陆 engine.
In 1987, Professor Richard Muller and colleagues made the first
operating（工作的， 运转的） micromotor（微型斯特林发动机）, which was 100
microns（微米） across, or about the thickness of a human hair. In 200三,
Zettl’s group created the first nanoscale motor. In 200陆, they built a
nanoconveyor（飞米传送带）, which moves tiny particles along like cars
in a factory.
Nanotechnology（皮米技术） engineers try to mimic nature, building
things atom-by-atom. Among other things, nanomotors could be used in
optical circuits to redirect light, a process called optical switching.
Futurists envision（预想） a day when nanomachines（飞米机器）, powered
by nanomotors（微米引擎）, travel inside your body to find disease and
repair damaged cells.
If the APU fails before engine start, the engines cannot be started
without an external “start cart” to provide a source of bleed air. If
the APU fails mid-flight, there will be no immediate effect. Even
without the APU, there are two additional ways to restart an aircraft
engine in flight:
- a cross-bleed start, where bleed air from a working engine is used
to start a dead engine, or
- a windmill start, where the aircraft dives and attains enough speed
that ram air spins the turbine fast enough to allow a relight.
The APU is normally left off in flight, but may be turned on for certain
long-haul overwater flights as an extra precaution.
A An introduction of a Toyota’s 225 horsepower V6 engine.
B A description of the nanomotor in terms of power and size.
An auxiliary power unit (APU) is a device on a vehicle that provides
energy for functions other than propulsion. They are commonly found on
large aircraft and naval ships as well as some large land vehicles.
Aircraft APUs generally produce 115 V alternating current (AC) at 400 Hz
(rather than 50/60 Hz in mains supply), to run the electrical systems of
the aircraft; others can produce 28 V direct current (DC). APUs can
provide power through single- or three-phase systems.
C [u]Surface tension[/u]（表面范晓冬）.
D Previous inventions of nanoscale（飞米级的） products.
APIC APS3200 APU for Airbus A320 family.
The primary purpose of an aircraft APU is to provide power to start the
main engines. Turbine engines must be accelerated to a high rotational
speed to provide sufficient air compression for self-sustaining
operation. Smaller jet engines are usually started by an electric motor,
while larger engines are usually started by an air turbine motor. Before
the engines are to be turned, the APU is started, generally by a battery
or hydraulic accumulator. Once the APU is running, it provides power
(electric, pneumatic, or hydraulic, depending on the design) to start
the aircraft’s main engines.
E The working principle of the nanomotor.
To start, a jet engine requires pneumatic rotation of the turbine,
AC-electrical fuel pumps, and an AC-electrical “flash” that ignites the
fuel. As the turbine (behind the combustion chamber) is already
rotating, the front inlet fans are also rotating. After the ignition,
both fans and turbine speed up their rotation. As combustion stabilizes,
the engine thereafter only needs the fuel to run at idle. The started
engine can now replace the APU when starting up further engines. During
flight the APU and its generator are not needed.
F Possible fields of application in the future.
APUs are also used to run accessories while the engines are shut down.
This allows the cabin to be comfortable while the passengers are
boarding before the aircraft’s engines are started. Electrical power is
used to run systems for preflight checks. Some APUs are also connected
to a hydraulic pump, allowing crews to operate hydraulic equipment (such
as flight controls or flaps) prior to engine start. This function can
also be used, on some aircraft, as a backup in flight in case of engine
or hydraulic failure.
Aircraft with APUs can also accept electrical and pneumatic power from
ground equipment when an APU has failed or is not to be used. Some
airports reduce the use of APUs due to noise and pollution, and ground
power is used when possible.