Auxiliary power unit

A small engine helps big machines.

It makes power for lights and tools. This unit can even start the big engines.

Large planes and ships use them. Some tanks use them too. They help save fuel. A small engine can keep food cold in a truck. It is a very useful tool for many machines.

An auxiliary power unit, or APU, is a small engine. It provides power for things other than moving a vehicle.

You can often find them on large planes and ships.

In many planes, the APU is in the tail. It makes electricity to run lights and tools.

Some APUs also make air pressure. This helps the plane work. On the Boeing 787, the APU only gives electricity.

APUs help in many ways. They can start the main engines. In the past, some planes used small engines called "putt-putts."

These helped start the big engines.

Some tanks use an APU too. It gives power without using much fuel. This helps the tank stay quiet. Even space shuttles used them. They gave power to move parts of the shuttle.

They used a fuel called hydrazine. This helped the shuttle fly and land safely.

An auxiliary power unit, or APU, is a special device used on vehicles. It provides energy for tasks other than moving the vehicle forward.

You can find these units on large aircraft, naval ships, and some big land vehicles. On many planes, the APU is located in the tail. It can create electricity to run the plane's systems. Some APUs produce 115 V AC power at 400 Hz. Others might produce 28 V DC power instead.

This extra power is very important for many different jobs.

A typical gas-turbine APU has three main parts that work together. First, the power section mixes air and fuel to create hot gas. This gas spins a turbine to make shaft power.

Next, a load compressor section uses that power to provide air pressure. This part uses inlet guide vanes to control the flow of air. Finally, a gearbox section transfers the power to a generator. This generator creates the electricity needed for the vehicle. The gearbox also helps run other parts like fuel units or cooling fans. Some designs even use a starter motor to get the APU running.

People have used different kinds of APUs for a long time. During World War I, British Coastal class blimps used an ABC auxiliary engine. This engine powered a radio transmitter and an air blower. In World War II, American planes used small engines called "putt-putts."

The B-29 Superfortress bomber had a putt-putt in its rear section. German engineers also used a unique system for jet engines. An engineer named Norbert Riedel designed a two-stroke engine for these jets. It was hidden inside the nose cone of the engine. This helped rotate the compressor to start the jet engine.

Many famous machines rely on these helpful units to function. The Boeing 727 was the first jetliner to use a gas-turbine APU in 1963. This allowed the plane to work at smaller airports without ground help. Space Shuttles used three APUs powered by hydrazine fuel. These provided hydraulic pressure to move engine parts and landing wheels.

Even tanks like the M1 Abrams use them. A 100 kg APU can replace a heavy 230 kg battery. In the business jet market, Honeywell holds about 90% to 100% of the share. Other companies like Pratt & Whitney are also very important.

APUs are useful because they save energy and provide independence. For example, a refrigerated truck can use an APU to keep food cold. This works without needing a plug from a building. Some trucks are even testing fuel cell APUs to reduce pollution. These use diesel fuel more efficiently than old engines. In the past, some construction tools used a "pony engine" to start. This small gasoline engine helped the main diesel engine start in the cold. Whether in space or on a truck, APUs keep important systems running smoothly.

An auxiliary power unit, or APU, is a specialized device used to provide energy for functions other than propulsion. While a vehicle's main engine is designed to move it forward, the APU handles secondary tasks.

These units are essential on large aircraft, naval ships, and certain large land vehicles. In aviation, APUs are vital for maintaining electrical and pneumatic systems. They can produce 115 V AC at 400 Hz to run aircraft electrical systems. Other models may produce 28 V DC power instead. By providing this independent energy, the APU ensures that critical systems remain active even when the main engines are not running.

A modern gas-turbine APU typically consists of three distinct sections. The first is the power section, which acts as the gas-generator. In this section, air and fuel are mixed, compressed, and ignited to create hot, expanding gases. This highly energetic gas spins a turbine, which creates shaft power to drive other components.

The second part is the load compressor section. This section provides pneumatic power, which is air pressure used for various aircraft needs. It uses inlet guide vanes to regulate airflow and a surge control valve to keep the operation stable. The third part is the gearbox section. This section transfers power from the main shaft to an oil-cooled generator to create electricity. It also connects to accessories like fuel control units, lubrication modules, and cooling fans.

The history of the APU shows how technology has evolved to meet different needs. During World War I, British Coastal class blimps used an ABC auxiliary engine. This engine powered a radio transmitter and could run an auxiliary air blower during emergencies. In World War II, American military aircraft often used small engines known as "putt-putts." For example, the B-29 Superfortress carried a putt-putt in its unpressurized rear section. This engine drove a P2 DC generator rated at 28.5 volts and 200 amps. It was used to start the main engines and provided power during the descent to land.

German engineering also produced pioneering APU designs during the Second World War. Engineer Norbert Riedel designed a mechanical starting system for early German jet engines. This system used a two-stroke flat engine hidden inside the engine nose cone.

For the Junkers Jumo 004, a manual pull-handle in the nose cone started the piston engine. This engine then rotated the compressor to start the jet. The Riedel unit was an extreme short-stroke design to fit within the small space of the nose cone. It was used in several designs, including the BMW 003 and the Heinkel HeS 011 prototypes.

Significant milestones in APU development changed how vehicles operate. In 1963, the Boeing 727 became the first jetliner to feature a gas-turbine APU. This allowed the aircraft to operate at smaller airports without needing ground facilities for power. In space exploration, the Space Shuttle relied on three redundant APUs powered by hydrazine fuel. These units were used during ascent, re-entry, and landing. They provided the hydraulic pressure needed to move control surfaces and engine valves. During landing, the APUs also powered the brakes and nose-wheel steering.

The market for these units is highly specialized and dominated by a few major companies. Honeywell is a leader, holding a 65% share of the mainliner APU market. They are the sole supplier for several major aircraft, including the Boeing 777 and the Airbus A320neo. Pratt & Whitney is another major player, holding significant shares in regional aircraft and helicopters. In the business jet sector, Honeywell's market share is even higher, ranging from 90% to 100%. The production market for APUs was valued at $800 million in 2017, while the maintenance and repair market was worth $2.4 billion.

Beyond aircraft, APUs serve many practical roles in other industries. Some armored vehicles, like the M1 Abrams tank, use a 100 kg APU to replace heavy 230 kg batteries. This makes the vehicle more fuel-efficient and reduces its infrared signature. In the commercial sector, refrigerated semi-trailers use independent APUs to maintain low temperatures during transit. Researchers are also testing fuel cell APUs for heavy trucks. In 2008, a partnership demonstrated that a fuel cell could power a Peterbilt Model 386 for ten hours. These advancements aim to reduce emissions and increase fuel efficiency across many different transport systems.