An engine running so hot that it vaporizes its lubricating oil seems a prelude to catastrophe. But a group of researchers at Pennsylvania State University theorize that it is possible to lubricate an engine with oil vapor successfully, opening the door to far higher operating temperatures than ever before imagined.
The discovery intrigues engine designers for several reasons. An engines radiator squanders a third of the energy and fuel, dissipating it into the atmosphere as waste heat rather than allowing it to be converted into power. But the radiator is necessary to keep the engine operating temperature under control.
This article was originally published in Popular Science, July 1991.
“The problem with lubricants that are available is that you put too much lubricants in the piston-ring-cylinder zone and the temperature is high – as high as is desired for, say, a low heat rejection engine – you form deposits in the piston-ring-cylinder zone,” says Dr. Irwin Klaus, professor emeritus of chemical engineering at Penn State. “High level of deposits generally leads to excessive wear and other problems,” such as engine failure.
Ever since the first energy crisis of the early ‘70s, low heat rejection (or more optimistically, adiabatic, or zero heat loss) engines have been the goal of many engineers working to reduce fuel consumption. However, conventional lubricants were only able to withstand temperatures in the range of 482 degrees F, inadequate for adiabatic engines.
The inspiration for a new technology of high temperature engine lubrication came from an unlikely source. Klaus and his associates at Penn State were working on ways to lubricate the dies used to cast stainless steel drill bits. Liquid lubricants were causing problems when sprayed on the hot die because of the rapid cooling, which caused heat fatigue.
A solution, first patented by Klaus and Chun W. Lai in 1976, was to apply the lubricants in a vapor phase. The lubricant was headed and carried in a gaseous state to the die, where it was deposited in 5 to 300 molecular monolayers. The high temperatures of the vapor phase lubricants did not cool the die, and Ashley worked better at higher temperatures, particularly the range of 900 to 1500 degrees F.
Lubricating with a vapor is fundamentally different from the most common form of lubrication, hydrodynamics, where an incompressible liquid between two surfaces causes them to “hydroplane” over each other. In another study, Penn State’s Steve Hsu, using a microscope, noted that during hydrodynamic lubrication disparities, or high points, opposing surfaces under pressure actually touched, causing flash temperatures that created chemical films. Where pressure increased to the point where the fluid thickness was than the surface roughness, these films became the primary lubricant.
“It is evident,” Klaus states, “that the temperatures at the contact point are very high, even though the temperature of the oil going in is room temperature and coming out is not much higher than room temperature. The temperature of that thin film appears to be about 350 [degrees C]” – about 660 degrees F.
“If you follow that notion,” says Klaus, “it takes very little lubricants on the surface to do a good job of lubrication. So why not use just the amount you need? The excess merely become sludge, varnish – undesirable things.”
Thus was born the philosophy of vapor-phase lubrication, the products of the simultaneous exploration into fields that are only loosely related. Vapor-phase lubrication essentially preempts the necessity for rubbing action to form the anti-wear layer of chemicals.
Because vapor-phase duplication needs high temperatures to work, it’s particularly attractive for low heat rejection engines. In fact, the anti-wear chemicals don’t form well under conventional engine temperatures and are produced best in very high temperatures.
It’s always a long way from theory to application, however, and Klaus notes that vapor-phase lubrication is most likely in diesel engines, as designers move toward higher temperatures to meet emission requirements and improve fuel economy.
The vapor-phase lubricant could be delivered to the cylinder wall in several ways: either by injection into the charge air and the combustion chamber through the intake valve, as an additive to the fuel, or below the piston compression ring through holes in the cylinder wall.
Hot nitrogen gas has served as a carrier for the vapor-phase lubricant in test rigs, but exhaust gas could be used in a production vapor-phase engine. It’s available, requires no additional apparatus on the engine and, says Klaus, is almost instantly up to the temperatures needed. Vapor-phase lubrication would most likely be limited to the upper cylinder wall and ring area. The rest of the engine could, and probably would in early applications, remain lubricated conventionally by liquid lubricants and a pressurized system. Extensive testing is required for such a concept to verify the results and optimize the system. But the promise is great.
Addendum: Does it work? More information from 1993 can be found in a 1993 article, Progress in Vapor Phase Lubrication, published in Although you haven’t seen vapor-phase lubrication in your favorite gasoline or diesel car or semi doesn’t mean that it still isn’t being research, even 30-plus years later. If you want to get very deep into the weeds and your math and engineering skills up to date, there’s Molecular Probing of the Stress Activation Volume in Vapor Phase Lubricated Friction published in 2023 by ASC Publications.
