Air start system
Encyclopedia
An air-start system is a power source used to provide the initial rotation to start large diesel
and gas turbine engines.
Compared to a gasoline
("petrol") engine, diesel engines have very high compression ratio
s to provide for reliable and complete ignition of the fuel without spark plug
s. An electric starter powerful enough to turn a large diesel engine would itself be so large as to be impractical, thus the need for an alternative system. When starting the engine, compressed air
is admitted to whichever cylinder has a piston
just over top dead center, forcing it downward. As the engine starts to turn, the air-start valve on the next cylinder in line opens to continue the rotation. As this goes on, fuel is injected into the cylinders, the engine is then under way and the air is cut off. To further complicate matters, a large engine is usually "blown over" first with zero fuel settings and the indicator cocks open, to prove that the engine is clear of any water build up and that everything is free to turn. After a successful blow ahead and a blow astern, the indicator cocks are closed on all the cylinders, and then the engine can be started on fuel. Significant complexity is added to the engine by using an air-start system, as the cylinder head
must have an extra valve in each cylinder to admit the air in for starting, plus the required control systems. This added complexity and cost limits the use of air-starters to very large and expensive reciprocating engines.
Another method of air-starting an internal combustion engine is by using compressed air or gas to drive a fluid motor in place of an electric motor. They can be used to start engines from 5 to 320 liters in size and if more starting power is necessary two or more motors can be used. Starters of this type are used in place of electric motors because of their lighter weight and higher reliability. They can also outlast an electric starter by a factor of three and are easier to rebuild.
An air-starter on a turbine engine would typically consist of a radial inward flow turbine, or axial flow turbine, which is connected to the High Pressure compressor spool through the accessory gearbox, plus the associated piping and valves. Compressed air is provided to the system by bleed air
from the aircraft's auxiliary power unit
or from an air compressor
mounted on ground support equipment
.
Compared to electric starters, air-starters have a higher power-to-weight ratio
. Electric starters and their wiring can become excessively hot if it takes longer than expected to start the engine, while air-starters can be run as long as their air supply lasts. Turbine starters are much simpler and are a natural fit for turbine engines, and thus are used extensively on large turbofan
engines used on commercial and military aircraft.
The air starter industry has been known to be extremely competitive thus creating questionable, and unscrupulous behavior among many suppliers.
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
and gas turbine engines.
Compared to a gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...
("petrol") engine, diesel engines have very high compression ratio
Compression ratio
The 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...
s to provide for reliable and complete ignition of the fuel without spark plug
Spark plug
A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...
s. An electric starter powerful enough to turn a large diesel engine would itself be so large as to be impractical, thus the need for an alternative system. When starting the engine, compressed air
Compressed air
Compressed air is air which is kept under a certain pressure, usually greater than that of the atmosphere. In Europe, 10 percent of all electricity used by industry is used to produce compressed air, amounting to 80 terawatt hours consumption per year....
is admitted to whichever cylinder has a piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
just over top dead center, forcing it downward. As the engine starts to turn, the air-start valve on the next cylinder in line opens to continue the rotation. As this goes on, fuel is injected into the cylinders, the engine is then under way and the air is cut off. To further complicate matters, a large engine is usually "blown over" first with zero fuel settings and the indicator cocks open, to prove that the engine is clear of any water build up and that everything is free to turn. After a successful blow ahead and a blow astern, the indicator cocks are closed on all the cylinders, and then the engine can be started on fuel. Significant complexity is added to the engine by using an air-start system, as the cylinder head
Cylinder head
In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket...
must have an extra valve in each cylinder to admit the air in for starting, plus the required control systems. This added complexity and cost limits the use of air-starters to very large and expensive reciprocating engines.
An air-starter on a turbine engine would typically consist of a radial inward flow turbine, or axial flow turbine, which is connected to the High Pressure compressor spool through the accessory gearbox, plus the associated piping and valves. Compressed air is provided to the system by bleed air
Bleed air
Bleed air in gas turbine engines is compressed air taken from within the engine, after the compressor stage and before the fuel is injected in the burners. While in theory bleed air could be drawn in any gas turbine engine, its usage is generally restricted to jet engines used in aircraft...
from the aircraft's auxiliary power unit
Auxiliary power unit
An auxiliary power unit is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft, as well as some large land vehicles.-Function:...
or from an air compressor
Air compressor
An air compressor is a device that converts power into kinetic energy by compressing and pressurizing air, which, on command, can be released in quick bursts...
mounted on ground support equipment
Ground Support Equipment
Ground support equipment is the support equipment found at an airport, usually on the ramp, the servicing area by the terminal. This equipment is used to service the aircraft between flights. As its name implies, GSE is there to support the operations of aircraft on the ground...
.
Compared to electric starters, air-starters have a higher power-to-weight ratio
Power-to-weight ratio
Power-to-weight ratio is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measurement of actual performance of any engine or power sources...
. Electric starters and their wiring can become excessively hot if it takes longer than expected to start the engine, while air-starters can be run as long as their air supply lasts. Turbine starters are much simpler and are a natural fit for turbine engines, and thus are used extensively on large turbofan
Turbofan
The turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...
engines used on commercial and military aircraft.
The air starter industry has been known to be extremely competitive thus creating questionable, and unscrupulous behavior among many suppliers.
See also
- Coffman engine starterCoffman engine starterThe Coffman engine starter was a starting system used on many piston engines in aircraft and armored vehicles of the 1930s and 1940s. The Coffman system was one of the most common brands; another was the Breeze cartridge system, which was produced under Coffman patents...
- A similar system which uses an explosive cartridge to supply gas pressure.