Difference between car engine oil and aircraft engine oil?

Answer 1

While it is true that automotive oil is far more advanced than aviation oil, the answer lies in the fact that most aircraft engines are air-cooled while automotive engines are water-cooled. Air-cooled aircraft engines are built with greater clearances and are designed to consume (burn) some oil.

Water-cooled automotive engines are designed and built to much tighter tolerances, so they do not consume much oil. These differences in design tolerances are due to the large temperature differentials that are found in high-continuous-power-output, air-cooled, aircraft engines versus the low- and intermittent-power-output, water-cooled, auto engines.

There can be a 300 degree F temperature difference between the cylinder head and cylinder base in an operating aircraft engine. That kind of temperature differential causes a lot of distortion in the cylinder, necessitating the requirement for large clearances. Automotive engines, being water-cooled, have lower temperature differentials across the engine and thus suffer lower levels of distortion and can be designed and built to tighter tolerances.

Aircraft engines were designed before additives were available and have not really changed much over the years. When ashless dispersant oils were introduced for auto engines, they were also suitable for aircraft engines and eventually were adopted for aviation use.

However, when zinc antiwear and metallic detergents were formulated into auto oils, an important divergence occurred. Aircraft engines burn a fair amount of oil and, if these metal-containing detergents and antiwear compounds are present, they can form metallic ash deposits in the combustion chambers. These deposits can lead to destructive preignition, which could burn holes in the tops of pistons with obvious catastrophic results. For that reason, it was decided that aviation oils were to remain ashless to avoid the risk of metallic deposits.

The benefit of using ashless dispersant oils is, obviously, a cleaner engine. Aircraft engines would also benefit greatly from the addition of other automotive additives such as anti-wear, detergents, and corrosion inhibitors, but the downside is added cost. Ashless versions of these performance additives can cost up to 10 times more than standard ash-containing additives.

About the Author ...

Edward Kollin is the Technical Director of Aircraft Specialties Lubricants (ASL). Formerly, Kolin worked at Exxon, participating in the development of Exxon Elite aviation oil before striking out on his own with ASL.