From independent transport operators in the aviation industry with a fleet to individuals privileged enough to own an aircraft of their own, those maintaining aeroplanes understand considerable costs are involved. Beyond the investment in the aircraft itself, owners and operators must contend with a multitude of expenses from fuel, hangar rental fees and general maintenance. As a result, it’s not surprising that many seek out ways to reduce running costs. Whether a business looking to improve their bottom line, or the owner of an aircraft wishes to reduce their spending, the question of whether an aircraft engine could be served by standard motor oil often pops up.
Aviation engine oils are well-known for coming at a higher price point than motor oil designed for the automotive industry. While this might seem like an economical option, the answer is that motor oil is unsuitable for aircraft piston engines. In fact, attempting to use it as a substitute can lead to far greater costs. Read on as we take a deep dive into why this approach is always best avoided.
Examining the design of automotive and aircraft piston engines
To get to grips with why motor oils us unsuitable for aircraft, it’s important to understand the difference between the engines installed in the two vehicle types. By design, automotive engines feature water cooled systems, while most types of aircraft engine are air cooled to help keep their overall weight as low as possible. Air-cooled engines count on the oil to take heat away from the engine cylinders, as well as to the cylinder head, and beyond to an oil cooler helping manage excess heat. With the aircraft engine, a temperature difference of 300 degrees can exist from cylinder vase and the oil-cooled cylinder head. For this reason, aircraft engines must have large clearances enabling the metal inside the cylinder to distort. These larger clearances by design ensure that oil can always pass through the cylinder and then be burned.
On the other hand, an automotive water-cooled engine carries heat away using a coolant system. The positive impact of this system allows for far better temperature regulation and much tighter tolerances in the engine. With tighter tolerances, the motor oil is far less likely to burn. While the motor oil used in a water-cooled engine does help in cooling the engine’s components by carrying heat away, that is not its primary function, which is to lubricate.
An air-cooled aircraft engine also tends to experience “shock cooling,”. This process happens as an aircraft descends, dropping from cruising altitude as its start its approach to landing. The aeroplane is throttled back, generating less heat from the aircraft piston engine, but gains airspeed which causes the engine to lower in temperature rapidly. Swift reduction in temperature can often damage cylinder heads, causing them to crack. However, by using the oil cooled cylinder heads which we mentioned above, this issue is reduced dramatically. The aircraft engine oil that passes through the cylinder head stays warm and this slows the rate of temperature loss within the cylinder head.
Understanding the different between aircraft and automotive engine oil
When aircraft engines were designed, ashless dispersants were not added to engine oil, and today this situation remains largely the same. The purpose of ashless dispersants is to help stop build-up of metallic ash and other disperse contaminants so that they become suspended within the engine oil. This prevents them from amalgamating and forming into sludge. These unwanted contaminants are then ferried through the oil filter and efficiently removed from the engine oil. The tangible benefits of adding ashless dispersants was originally recognised when used in automotive engine oils and was soon adopted by the aviation industry.
The core difference in aircraft and automotive engine oil lies in the additives included in formulation. The largest divide between the two oil types arose with the inclusion of detergents and zinc anti-wear additives containing metal which were not suitable for the extreme heat environment of an aircraft’s air-cooled piston engine. As discussed, air-cooled engines burn a vast amount of engine oil, and as a result, any detergents which contain metals along with anti-wear compounds such as zinc can easily form metallic ash deposits inside the aircraft engine’s combustion chamber. This activity can lead to pre-ignition in the chamber and even burn holes in the piston heads resulting in catastrophic consequences to the engine and the aircraft and its passengers and crew cruising at 15,000 feet. Above all, this is the key reason why an aviation engine oil will always be ashless.
Although specific additives are identified as extremely beneficial for automotive engine oils like zinc for its excellent anti-wear properties, they must not be included in aviation oil formulas. For safety and to protect an aircraft piston engine, only oils designed to be compatible should be used.