A jet engine is a combustion engine and, due to this, it generates heat. To ensure that it works correctly, the heating must be controlled.
A jet engine is a combustion engine and due to this reason, it generates heat. And to ensure that it works correctly the heating must be controlled. Like any other engine, in a jet engine to cool and lubricate the moving parts oil is used.
There is, however, a very important assembly in a jet engine that is extremely sensitive to heat. The turbine assembly. The exposure of the turbine blades beyond their limit temperatures even for a few seconds can render them useless costing the operator millions. Thus, the combustion must be controlled in such a manner that the temperature does not rise above and beyond the capacity of the turbines.
The oil system of a jet engine is not very dissimilar to the oil system of your car. Just like your car, there is an oil tank and a pump that pumps oil around the engine. In a jet engine, the compressor and the turbines rotate on a shaft that is run by ball bearings. This reduces friction and makes construction easier. As these ball bearings carry huge loads, they can heat up and stress easily. So, to carry away the heat from the bearings, oil is routed through the ball bearing chamber. The oil is first pumped out from the oil reservoir or the tank to an oil filter. This filter ensures that no carbon particles or any other impurities make past it. From the filter, the oil goes from pressure feed lines into the bearing chambers.
The oil is then sprayed directly on the bearings by oil jets. This oil is then returned to the oil tank (after the cooling process) with the help of scavenging pumps. Before being passed into the tank, it goes through a Fuel Cooled Oil Cooler (FCOC). In the FCOC, the fuel is passed through pipes over which the oil is passed. This process adds heat to the fuel and carries away heat from the oil. It is a win-win situation because fuel needs heat while the oil needs it taken away.
The chemically correct (stoichiometric) ratio to burn air and fuel mixture is 15:1. Burning at this ratio can generate heat exceeding 2000 degrees Celsius, which is far too high for the turbine blades. Due to this reason the combustion is mechanically controlled to ensure that the burn is at the correct temperature.
In a jet engine combustion chamber, the air that comes from the compressor stages divides into three. One is called the Primary air, the second is called the Secondary air and the third is called the Tertiary air. Out of these, only the primary air enters directly into the chamber for combustion. Interestingly, only 20% of the air that comes from the compressor is this primary air. The primary air is mixed with fuel and burned at the stoichiometric ratio of 15:1.
The secondary air and the tertiary air are routed around the combustion chamber. The former is then slowly introduced into the combustion chamber through holes drilled on the casing of the chamber called the secondary air holes. This reduces the temperature of the burn and increases the molecules of air that gets mixed with the fuel. The secondary air is also a mere 20% of the total airflow. The secondary air also plays another important role. It flows into the combustion chamber through swirl vanes which helps it to form a vortex. When this secondary air mixes with the primary air, a toroidal vortex forms which anchor the flame near the fuel injecting nozzles and prevents it from going away.
The rest of the airflow, which is the remaining 60% is called the tertiary air. This air is also introduced into the combustion chamber and is used to further cool the burn. It also cools the casing of the chamber. When the secondary air and the tertiary air are mixed with the primary air, the air and fuel mixture become very weak. By the end of combustion, the mixture is in the range of 100:1, a far fetch from the chemically correct mixture ratio of 15:1. This ensures that the temperature of the air that gets passed to the turbines is cool enough so that they do not get damaged and stressed.
As previously discussed, the turbine blades do not like heat. When subject to heat they undergo ‘creep’. A creep is a form of stress which expands the blade. When creep is allowed to develop it can grow to the point where the blade can touch the casing of the engine which can cause a catastrophic failure. So, the turbine blades and their assembly need a cooling system.
The turbine blades are cooled by engine bleed air. Air from the compressor stages of the engine is bled and fed to the blades to cool them. There are holes drilled on the blades through which the air is passed.
In early jet engines, only Low Pressure (LP) air is passed through the blades. This type of cooling is known as Single pass with Internal Cooling. As engines started to get bigger and the requirement for more thrust came about, turbine blades had to have the capacity to withstand it. So came about the Single pass Multi-Feed Internal Cooling with Film Cooling. In this type of cooling, both LP and High Pressure (HP) air are passed through the blades which makes the cooling more efficient. Also, in this type of cooling system, the holes are drilled strategically such that a film of air is coated around the blades. This forms a boundary layer that itself cools down the blade.
This type of cooling was pretty good, but it was further developed. Engineers found out that passing air through the blade more than once could optimize the cooling a lot more. It was also realized that passing the air through the blade about five times could increase the efficiency of the cooling to a respectable level. So, it was decided to develop the Quintuple pass, Multi-Feed Internal Cooling with Extensive Film Cooling. In this system, HP air is passed through the blade four times, while the LP air is routed through it once. As in the previous type of cooling, the holes are drilled such that a film of air is blanketed around the blades, cooling the blades a lot more.
The turbine blades are attached to turbine discs and these discs need cooling as well. The turbine discs are cooled similarly to the turbine blades by passing HP air over them.
The limitation of the turbine blades to withstand high temperatures is one of the reasons why there is a limit on jet engine thrust. One way to overcome this is to use afterburners which are used by military fighters. In an afterburning jet engine, the air and fuel are ignited in the exhaust or the jet pipe which is directly behind the turbine blades. The air from the turbines is burned in the jet pipe by mixing it with fuel and lighting it up using a set of burners. This gives the aircraft an extra boost of power.
Journalist - An Airbus A320 pilot, Anas has over 4,000 hours of flying experience. He is excited to bring his operational and safety experience to Simple Flying as a member of the writing team. Based in The Maldives.