Ameer Randolph
Auxiliary power units (APUs) are devices on a vehicle that provide energy for functions other than propulsion. They are commonly found on large aircraft, naval ships, and some large land vehicles. The power section of the engine produces all the shaft power for the APU, which in turn powers other sections of the engine. The APU may use up to 2% of the fuel consumed during a typical flight. While some APUs have separate fuel tanks, others draw fuel from the main fuel tanks. Some truck owners have considered installing dedicated APU fuel tanks to save money on fuel taxes, but there are concerns about the legality of buying tax-free fuel for APUs.
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What You'll Learn
- APUs can be powered by hydrazine fuel, as seen in the Space Shuttle
- The Boeing 737 APU feeds from the #1 main tank
- APUs can be used in trucks to reduce idling and emissions
- Fuel cell APUs can be used to improve fuel efficiency and reduce emissions
- An APU is a device that provides energy for functions other than propulsion
APUs can be powered by hydrazine fuel, as seen in the Space Shuttle
APUs, or Auxiliary Power Units, are used in vehicles such as trucks and spaceships. They provide power to various systems, such as hydraulic pumps and control assemblies. In the context of the Space Shuttle, APUs played a crucial role in powering the hydraulic systems during launch and landing.
The Space Shuttle utilized hydrazine-powered APUs, which offered several advantages. Hydrazine is a highly toxic rocket fuel that, when exposed to a catalyst, decomposes energetically into hot gases at approximately 1700°F. This decomposition process can be easily controlled, making hydrazine a convenient choice for APUs. Additionally, hydrazine does not require oxygen, which is another benefit for spacecraft operating in the vacuum of space.
Each APU in the Space Shuttle produced about 135 horsepower, contributing to the overall functionality and performance of the spacecraft. The hydrazine fuel system in the APUs consisted of injectors, catalyst beds, decomposition chambers, and exhaust nozzles. This system ensured the efficient utilization of hydrazine fuel to generate the necessary power for the Shuttle's hydraulic systems.
However, there were concerns regarding the toxicity of hydrazine fuel. Efforts were made to replace the toxic hydrazine system with greener alternatives, such as high-pressure helium bottles. These alternatives aimed to mitigate the risks associated with hydrazine's highly toxic nature while still providing the necessary power for the APUs.
In terms of fuel tanks, some vehicles with APUs may have separate fuel tanks dedicated to the APUs. This setup allows for the use of untaxed fuel specifically for the APU, which can provide cost savings over time. However, the decision to install separate fuel tanks depends on various factors, including the availability of space, the complexity of the system, and the overall cost of implementation.
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The Boeing 737 APU feeds from the #1 main tank
The Auxiliary Power Unit (APU) on a Boeing 737 aircraft provides electrical power and pneumatics (compressed air) for starting the main engines and for use during ground operations. The APU is typically located in the tail cone of the aircraft, and its fuel source is an important consideration in the aircraft's design.
On the Boeing 737, the APU is fed by fuel from the #1 main tank. This is one of the two main fuel tanks located in the wings of the aircraft, each with a maximum fuel capacity of 10,643 lbs. The other main tank is labelled #2, and both main tanks feed fuel to the engines through a cross-feed valve.
The centre tank, located below the cabin floor, has a higher fuel capacity of 16,422 lbs. However, the fuel in this tank is not used to feed the APU directly. The Boeing 737 is designed so that the main tanks must be full if the centre tank contains 1,000 lbs or more of fuel. This is to ensure proper weight distribution and balance during flight.
While some aircraft may have separate fuel tanks dedicated to the APU, the Boeing 737 is not typically designed this way. A separate APU fuel tank would require additional fueling actions and considerations, such as the purchase of untaxed fuel ("reefer" fuel) and the loss of storage space for tools and supplies. However, Boeing does offer an additional DC fuel boost pump for the APU, which runs whenever the APU is running and the pressure from the AC pumps is low.
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APUs can be used in trucks to reduce idling and emissions
Auxiliary Power Units (APUs) are small engines that typically run on diesel fuel. They can also be electric, known as Electric Power Units (EPUs) or all-electric APUs, which are gaining popularity as the push for greener technology intensifies. These electric units do not require diesel fuel and are thus more environmentally friendly.
APUs are designed to reduce a truck's idle time. When a truck is idling, it burns excess fuel and puts unnecessary strain on the engine. APUs provide power to the cabin without idling the main engine, thus reducing fuel consumption and lowering a truck's carbon emissions. This is especially beneficial in urban areas where air quality is a concern.
The use of APUs can also help fleet operators comply with increasingly stringent anti-idling regulations in certain states across the U.S. For example, some states have strict limits on how long a truck can idle, with fines for violations. By using APUs, trucks can stay compliant while still meeting driver needs for cabin power and comfort.
In addition to the environmental and regulatory benefits, APUs can also lead to reduced maintenance costs. By minimizing engine idling, APUs decrease wear and tear on the main engine, resulting in lower maintenance expenses over the truck's lifespan. This also extends the engine's life, reducing the need for costly repairs or replacements.
While APUs offer several advantages, there are some considerations to keep in mind. For example, some truck drivers have discussed the installation of dedicated APU fuel tanks to use untaxed fuel. However, this adds to the overall setup cost and requires separate fuelling actions at the pump. Additionally, the choice between installing an APU fuel tank or having a lockable storage box for tools and supplies is a concern for some truckers.
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Fuel cell APUs can be used to improve fuel efficiency and reduce emissions
The aviation industry is a challenging sector to abate in terms of emissions, and there is a lot of room for improvement in terms of powertrain designs and electrification architectures to reduce emissions. The use of fuel cell APUs is a promising solution to improve fuel efficiency and reduce emissions.
Fuel cells have several benefits over traditional combustion-based technologies used in power plants and vehicles. They can operate at higher efficiencies than combustion engines and can convert chemical energy directly into electrical energy with efficiencies exceeding 60%. This is because fuel cells have fewer moving parts and work similarly to batteries, producing electricity and heat as long as they are supplied with fuel.
Fuel cells also have lower or zero emissions compared to combustion engines. For example, hydrogen fuel cells only emit water and have no carbon dioxide or air pollutant emissions, which helps address critical climate challenges. The integration of fuel cell systems within aircraft applications can also result in weight savings, as the electrochemical device replaces the combustion engine, reducing the weight of heavy onboard aircraft components.
The replacement of conventional aircraft onboard power supply systems with fuel cell systems promises higher efficiencies, lower emissions, and weight savings. The challenge lies in integrating fuel cell systems within aircraft due to the specific volume and weight requirements, as well as the part load and transient behaviour of the fuel cell system under different ambient conditions. However, the Hamburg University of Applied Sciences (HAW) has conducted research on different types of fuel cells and their system integration, with efficiencies of initial designs varying between 25% and 66%.
The use of fuel cell APUs in aircraft can also improve passenger satisfaction. For instance, proton exchange membrane fuel cells (PEMFC) generate by-products such as heat for ice prevention, deoxygenated air for fire retardation, and drinkable water for onboard use. Additionally, the high price premiums associated with fuel cells can be offset by marketing them as solutions to multiple problems, not just power providers, thereby increasing their appeal and market penetration.
In summary, fuel cell APUs offer a promising approach to improving fuel efficiency and reducing emissions in the aviation industry. They provide higher efficiencies, lower emissions, weight savings, and additional benefits such as improved passenger satisfaction. While integration challenges exist, ongoing research and development are working towards overcoming these obstacles and maximizing the potential of fuel cell APU technology.
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An APU is a device that provides energy for functions other than propulsion
An Auxiliary Power Unit (APU) is a device that provides energy for functions other than propulsion. It is a completely independent system that can operate on the ground and in flight. Its main function is to provide energy when necessary, but it does not provide propulsion. Instead, it powers other sections of the engine, such as auxiliary gearboxes, pumps, and electrical generators.
APUs are used in a variety of vehicles, including aircraft, trucks, boats, ships, locomotives, and even military vehicles. They are particularly useful for vehicles with high on-board electrical energy demands, such as leisure yachts, planes, and cars. In aircraft, the APU allows the main engines to be started up and provides electrical energy and air conditioning when the engines are shut down. This reduces fuel consumption and maintenance costs for airlines, as the aircraft can obtain energy without running its main engines.
The use of APUs can also lead to fuel savings during taxiing, as the aircraft can taxi using energy from the APU without starting its main engines. This also reduces polluting emissions. In addition, APUs can provide emergency energy in the event of an engine failure, supplying electricity and air conditioning to the aircraft.
Some truck owners have considered installing dedicated APU fuel tanks to take advantage of the ability to buy untaxed fuel for their APUs. This could lead to cost savings, especially if fuel taxes increase. However, there are also concerns about the legality of buying tax-free fuel and the potential hassle of separate fueling processes.
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Frequently asked questions
No, APUs do not have separate fuel tanks. They use fuel from the main fuel tanks.
APU stands for Auxiliary Power Unit. It 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.
APUs use the same type of fuel as the vehicle they are installed in. This can include diesel, aviation, or logistic fuel.
