How Aircraft engine oil system works?

The Oil System of an Aircraft Engine

What is lubrication 

In a chilly morning of the month of January you would find yourself rubbing your hands to keep yourself warm, Won't you?

Now just imagine if I put some oil in between your hands would you be able to keep yourself warm? You would answer in a shivering voice, "NO". What exactly happened ? Where has the heat absconded?

Initially, when you were rubbing your hands without any oil layer, you were actually generating heat due to friction and that heat was helping you to keep warm. Heat generation by friction in simple words can be attributed to a phenomenon that at a very microscopic level of magnification it can be seen that surfaces of both the hands were having some irregular bumps and grooves on their surfaces. Now imagine bump of one surface gets interlocked in the groove of the other surface, then it would require a greater amount of force to break that interlocking. This interlocking prevents the motion between two hands which is called friction. Now when these bumps and grooves come in contact particles of both the surfaces get bonded to each other. When the bonds between these particles are broken a considerable amount of heat is generated which was helping you to keep yourself warm.

What happened when there was a layer of oil present in your hands,? The fluidity of oil helps the layers of oil to easily slide over each other, due to which it acted as a lubricant and prevented the formation of a bond between the particles of the hands which further reduced the amount of heat generated. Though here we have talked only about the liquid form of lubricant but lubricants can be solid, liquid and even gases. 

With this, we have learned a very important concept of lubrication

Informal engineering words lubrication is defined as the action of applying a substance such as oil or grease to any machine component so as to minimize the friction and allow smooth movement. 

Oil System 

The oil system is very important for any machine may it be a small sewing machine used at home to the huge aircraft engine. You may easily develop a confusion between oil system and fuel system but they are way different from each other. The fuel system is used to supply the fuel inside the combustion chamber to increase the pressure and temperature of the incoming air inside the engine whereas the oil system is used to supply oil in various parts of the engine. The main parts which are to be taken care by the oil system are the bearing compartments and the gearboxes.

The oil system performs five basic functions :

1. Lubrication
2. Cooling
3. Cleaning
4. Vibration reduction

To summarize the first function of oil system that is lubrication, we can say in simple language that the process of reducing friction between two machine components which may be any components
inside the engine say a shaft rotating in the journal bearing as seen in the figure where oil acts as lubricant and prevents the contact between the two surfaces reducing friction between them which further reduces the wear and tear of these components increasing their life. 

The second function of cooling which also has been discussed earlier may be summarized as the function of oil in which it reduces the friction between the mating parts which reduces the heat generation. It also takes away the heat generated despite reduced friction. 

The third and a very important function which the oil system performs in cleaning, while relative movement of machine components against each other they tend to produce some chips and burs. The lubricant carries away these particles with it which helps in the cleaning process. 

Vibration Reduction is also done by the lubricant by absorbing the vibration produced as the components of an aircraft engine move at very high rpm,

You would be able to understand the above functions of the oil system more clearly once you come across the functioning of the oil system.

Firstly we would study the working of the oil system at the specific level and then we would take it as a broader concept.

Working of the oil system

Looking at the image above you can easily relate this to a shaft rotating between the balls of a ball bearing the yellow color signifies the lubricant. The lubricant enters the bearing compartment at a very high pressure and low temperature. After coming into the bearing compartment the high-pressure lubricant pushes away the old lubricant present in the compartment. The outgoing lubricant takes away the heat generated inside the bearing compartment along with any chips or debris that are produced due to the relative motion between the shaft and the balls of the bearing. After that, the outgoing oil is filtered and cooled down and again fed at high pressure to the bearing compartment. 

The complete oil system

Basic Components

1. Oil Tank 

The oil tank usually found in most common engines ranges from a capacity of 20-30 liters. Hot tank system and cold system are the two technologies used to store the oil inside the tank. In hot oil system as the name suggests the oil present inside the oil tank is having a considerably high temperature and vice versa for the cold tank system.

2. Pressure pump and scavenge pump with filter

As discussed earlier the oil sent in the bearing compartment is in a pressurized state and this pressurization is done with the help of pressure pumps. Generally used pumps are the gear type pumps or the vane pumps. The pressure produced ranges about 150 psi. The reason behind pressurization is to send the clean oil at high pressure so that the old and used oil present inside the compartment can be removed easily and completely. It is just like to throw a ball at another ball so that the first ball can displace the second ball from its position. 

After the pressurized oil has entered the compartment the dirty oil has to be removed and cleaned for removing the dirty oil we use similar type pumps as we used for pressurization but this type their function is to scavenge the dirty oil and the cleaning of dirty oil is done with help microparticle filters.

A very interesting fact is that in an oil system we have a single pressure pump and 5+ scavenge pumps. This is because as the dirty and heated oil would deteriorate the moving components and would increase their wear rate hence the used oil needs to be removed as quickly as possible hence more scavenge pumps are used so that scavenging can be done quickly.

3. Deoiler

Deoiler is also a very important component of the oil system. The oil used as a lubricant is aircraft engines is very costly and every single drop of it is precious hence efforts are made to prevent its wastage. For doing this a device called deoiler is used which separates out oil from the air that is used for breathing of the bearing compartments and the gearboxes.

4. Fuel Cooled Oil Cooler (FCOC) and Air Cooled Oil Cooler (ACOC)

When the heated oil is scavenged from the bearing compartment and gearboxes is required to be cooled before reusing it. This cooling of oil is done by either giving its heat to the fuel being used in the engine or the air passing through the engine. FCOC and ACOC are simple counter flow heat exchangers.

The Complete Oil system

Let's sum up the oil system by talking about its flow chart, the cool and clean fuel is taken from the oil tank with help of pressure pumps after which it is then taken to the oil filter where any impurity which may be present in the oil is removed. After this, the clean oil is then sent to the three bearing compartments, angle gearbox, and the main gearbox. A deoiler is mounted on the AGB which separates out any oil is present in the air being breathed out. Then the heated and dirty oil from the bearing compartments and the gearboxes is taken away with the help of scavenge pumps and then it is filtered in the scavenge filter after which the oil is cooled in the FCOC and ACOC and then the cool and clean oil is sent to the oil tank.

Thanks for reading!

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