Moving Into the Future with 1930s Engine Technology

I recently wrote about the differences between auto oils and aviation piston engine oils (“Why can’t aviation oil be like automotive oil?”)

One of the differences is that auto oils contain zinc dithiol phosphate (ZDP) compounds that give the oil significantly greater load-carrying properties than aviation oils. The zinc additive is great for anti-wear, but is very corrosive to soft metals like silver and copper, which are used in aircraft engines.

This raises the question about the performance of oils like Aeroshell 15W/50 that contain an anti-wear phosphorus additive. The oil contains cresyl diphenyl phosphate (CDP), which is a good anti-wear additive, but not as effective as ZDP.

The main problem is that aviation oils do not contain ash-type detergents, which provide high basicity to neutralize any acid build-up from moisture and contaminants that result from low usage and low temperature operation. The low temperature operation and inactivity tends to promote rust on the lifter faces, which leads to pitting and eventual failure.

So are we stuck with ashless oils and poor performance forever in aviation piston engines or is there a way out?

All of the FAA-certified engines are certified on Mil-L-22851 (SAE-1899) oils that cannot be easily changed. But going forward, new engines could be certified on a new specification lubricant that the engine manufacturers develop.

This happened in the automotive world when the SAE specification did not limit the phosphorus level from ZDP in auto oils. The auto industry wrote its own spec with a lower phosphorus level because the high level of phosphorus was poisoning catalytic exhaust mufflers.

I know there are no catalytic mufflers on aircraft yet, but if the manufacturers improved their engines so that they would tolerate some level of ash-containing additives and wrote a new specification around it, most of their lubricant-related problems could be solved.

Again, if we follow the automotive world, it switched its engines over to roller lifters so that the engines would operate well on lower ZDP oils. The industry then wrote the specs with a lower ZDP level allowed.

If aircraft engine manufacturers switched to roller lifters and got rid of all soft metals in their engines, they might be able to allow a very low level of zinc in the new spec. With a low amount of zinc in the oil, they should be able to improve the oil consumption past the rings and improve engine efficiency, which would allow a low level of an ash-containing detergent to help neutralize the acid build-up in low usage engines.

Since the new engines will be operating on unleaded fuels, it will probably be necessary to convert to liquid cooling to save the exhaust valves and seats. This can be liquid cooled cylinders like the Continental Voyager cylinders or a whole new design like that used in converted automobile engines.

Unfortunately, the procedure for certifying an engine is based on the present engines used in general aviation, which were designed in the 1930s. These engines may not be easily adapted to new technology designs, such as dual spark plugs in every cylinder.

A final point is a new design engine will probably need electronic injection and controls. There are two reasons for this.

First is temperature control. If you want the exhaust valve and seats to last, you will need all the cylinders to operate at similar air/fuel ratios. You cannot operate an engine with one cylinder rich of peak and running cool while the other cylinder is very hot operating at peak temperature.

The second reason is fuel economy. To make flying more affordable with high-cost unleaded fuels, it will be necessary to lean out the engine and run all cylinders lean of peak.

Pilots are used to $100 hamburgers, but will they get used to $100 an hour fuel bills in a small single-engine aircraft?

I am not an engine designer and I definitely do not have all of the answers. But these are just some suggestions to help with the transition to unleaded fuel and make the future of general aviation better.