Bleed Air - Recent Developments in Civil Aircraft

Recent Developments in Civil Aircraft

Bleed air systems have been in use for several decades in passenger jets. Boeing announced that its new aircraft, the 787 would operate without use of bleed air (and the two engines proposed for the aircraft, the General Electric GEnx and the Rolls-Royce Trent 1000, are designed with this in mind). This represents a departure from traditional aircraft design, and proponents state that eliminating bleed air improves engine efficiency, as there is no loss of mass airflow and therefore energy from the engine, leading to lower fuel consumption.

In a bleed air system, air is compressed to several atmospheres by the engines, only to then be cooled and expanded (energy "lost") to a narrow pressure and temperature margin (requires complex systems to regulate temperature and pressure) to be piped around the plane, only then to be cooled or expanded again (energy "lost") to roughly one atmosphere of pressure. Additionally, eliminating bleed air reduces the aircraft's mass by removing a whole series of ducts, valves, heat exchangers and other heavy, maintenance intensive equipment.

The APU (auxiliary power unit) no longer needs to supply bleed air when the main engines are not operating. Aerodynamics are improved due to the lack of bleed air vent holes on the wings. To pressurize the cabin, electric air compressors are used. By driving cabin air supply compressors at the minimum required speed, no energy wasting modulating valves are required. High temperature, high-pressure air cycle machine (ACM) packs can be replaced with low temperature, low pressure packs to increase efficiency. At cruise altitude, where most aircraft spend the majority of their time, and burn the majority of their fuel, and where the outside air is typically very cold, the ACM packs can be bypassed entirely, saving even more energy. Since no bleed air is taken from the engines for the cabin, engine oil contamination of the cabin air supply is eliminated.

Lastly, advocates of the design say it improves safety as heated air is confined to the engine pod, as opposed to being pumped through pipes and heat exchangers in the wing and near the cabin, where a leak could damage surrounding systems.

Because the aircraft has no bleed air powered engine starters, and no requirement for bleed air for cabin pressurization during flight, there is no need to have the APU drive a load compressor to supply bleed air to the aircraft during a period of engine loss, or on the ground at the start of each flight. This simplifies aircraft design, eliminates another maintenance requirement and saves weight.

In contrast, eliminating bleed air creates a requirement for another source of energy for cabin pressurization, anti-ice/de-ice systems, and other functions previously covered by bleed air. The other source is electricity from large generators fitted to the main engines and APU. Therefore, from a systems point of view, this approach may potentially be less efficient. Rather than high pressure air from the engine (pneumatic energy) being used directly to pressurize the cabin, the pneumatic energy is converted to mechanical energy by the engine itself, and this shaft horsepower (mechanical energy) is taken from the engine to drive a generator (electrical energy) that is then used to drive a motor (mechanical energy) that is used to drive a compressor (pneumatic energy). Energy is lost at each conversion step, but despite all of these conversions, Boeing still expects a net energy savings. In the 787, the compressor motor is driven by the same power converters that are used to drive the electric engine starters for major weight savings.

While the 787 is considered a "bleedless" aircraft, a minor amount of bleed air is still used for engine pod intake de-ice. The amount of bleed air requirement for engine pod de-ice is so small, and since engine pod de-ice is not used for the entire flight, the engines are designed as if there were basically no typical bleed air requirements.

Airbus does not currently (as of November 2007) have any plans to eliminate bleed air from its 787 competitor, the A350, and is improving its technology by improving the quality of cabin air and reducing the amount of needed bleed air and therefore increasing efficiency. The use of improved bleed air technology improves air quality in an aircraft pressure cabin at the same time. Airbus has received many patents in the last two decades for improving the efficiency of bleed air and improving the quality and security of cabin air.