Multigrades Part 1
Shell Aviation product technical details
What are the advantage of using AeroShell Oil W 15W-50?
Many of you have noticed that we recommend the use of AeroShell Oil W 15W-50 in engines that are standing idle for periods of a week ore more, and have asked the question why?
To explain the reason for this it is probably useful to go into the background of what happens with an oil in use.
When an engine is running, acids form in the oil from a combination of combustion gases dissolving in the oil and the oil's own natural degradation. The combustion gases enter the crankcase by 'blowing' past the piston rings and, once in the crank, they dissolve in the oil. Oil degradation is the inevitable and unavoidable oxidation process which occurs when an oil is at high temperature and in contact with air.
These by-products do not cause any problems until you also have water present in the oil, which then 'activates' them by hydrolysing them to form, predominantly, Formic and Nitric acids. It is these acids, along with any free water which may be present, which cause corrosion in an engine.
Now water inevitably appears within the engine by condensing out of the atmosphere in much the same way as you will see moisture condensing on the outside of the aircraft structure; the air getting into the engine internals though the crankcase breather. As you will appreciate, this is a continual process and occurs if the aircraft is in use or not.
The usual and most effective way of eradicating problems arising from this acid and water attack is to fly the aircraft. This heats up the oil and drives off any moisture in the oil, thus eliminating the water problem and deactivating the acids. The only way to get the oil hot enough to do this is to fly the aircraft; ground running will not get the oil hot enough to drive off the moisture. We usually suggest a minimum of 30 minutes cruising flight every 2 weeks.
One of the advantages of using AeroShell Oil W 15W-50 comes when the aircraft is not flown this frequently as the oil contains both a corrosion inhibitor and an anti scuffing additive to help the occasional flyer.
If the aircraft cannot be flown with the frequency required to keep the oil 'dry', the corrosion inhibitor will suppress the formation of any corrosion during periods of inactivity, which would otherwise form due to the action of acids and water.
Furthermore, once the aircraft engine is started up after being inactive, the anti scuffing additive will have coated all the internal metallic surfaces with a molecular layer so that metal to metal contact is prevented if there is no oil present. This is particularly important during the first few seconds after start up as the oil pump will not pump oil to all the extremities of the engine immediately.
Another advantage of using the multigrade 15W-50 is that, not only does it pump quicker at low temperature than the monogrades - and even the 20W-50 multigrades on the market - but it is unusual in being a semi synthetic rather than a pure mineral oil.
We will expand on the significance of this, along with why multigrades are used and why they are not all the same in the next article.