by Hank Hogan

It’s like having your cake and eating it too. According to Satoshi Hirano, project manager in the engine design and engineering division of Toyota Motor Corp., popular friction modifiers that lower viscosity do not contribute to coking deposit problems potentially seen with turbocharged, downsized engines. That is good news, because both turbocharging and lower viscosity lubricants are being pursued to boost vehicle fuel economy while preserving performance.

Hirano reported on the results of a Toyota study which showed this at the recent F+L Week conference in Singapore, an event organised by F&L Asia. Not all was good news, however. The study showed the dangers from coking, such as oil degradation and formation of performance-robbing deposits in the turbocharger.

“Temperature control is very important from an engine design perspective. At the same time, from the viewpoint of engine oil design, improvement of the antioxidant seems a effective way to prevent deposit formation,” Hirano said.

For gasoline engines, deposit formation begins at temperatures above 180^o C. Spikes to this temperature and higher can happen when the following sequence occurs: a high load engine operation followed by engine shut down after short idling. When that happens, heat from the exhaust system flows back into the turbocharger turbine, shaft, and bearing, which can lead to coking.

It is understood that a turbocharger coking test will need hundreds of hours of operation to cause the coking deposit formation. By taking pre-aged engine oil, the Toyota team successfully evaluated each given test condition in 40 hours and reproduced the coking deposit formation for certain test condition.

The Toyota team also conducted laboratory tests to look at when coking happens and under what conditions, evaluating both used oil and fresh oil. They found that key was the presence of insoluble materials, which in gasoline engine oils, were found to be polymerized hydrocarbons with carbonyl and ester functional groups. Lubricants with even small amounts of insolubles formed deposits relatively quickly at anything above 180^o C. Those free of these external materials did not coke, even at significantly higher temperatures. For instance, fresh oil free of insolubles started to coke at 280^o C, a full 100^o C higher than used oils with insolubles in them.

“Excessive accumulation of insolubles causes a variety of deposit issues in the engine,” Hirano said.

The source of insolubles is degraded fuel molecules, he said. A combustion product, the degraded fuel molecules end up accumulating in the engine oil. There they polymerize, forming long molecular chains. Its thermal stability is worse than that of engine oil. When heated sufficiently, the insolubles clump up into deposits. A hot tube test was used to investigate the starting temperature of the coking phenomenon, by exposing a small amount of oil to hot air in the heated glass tube. Used oil collected from the field, oil oxidized in the lab, as well as fresh oil were used in the experiment. Only used oil from the field exhibited the level of deposit formation that can lead to coking at the temperature range related to turbocharger bearing area. Fresh oil showed none. The oils that were oxidized in the lab went through an equipment called ISOT, which allows the oil to be degraded without insoluble formation. Since there was a big difference between the coking start temperatures with the used oil and the oil that was oxidised in-lab (coking temperature is well above 200° C), Toyota was able to determine that insolubles are the main driver of lower coking start temperature. Because the starting point is degraded fuel that’s been through combustion, fresh oil doesn’t exhibit anything like the temperature sensitivity and problem that used oil does.

It should be noted that Toyota tested both gasoline and diesel oils to investigate the coking start temperature. As mentioned, the insolubles produced by the combustion of a gasoline engine have poor thermal stability and are made of

The study also looked at the impact of MoDTC, a friction modifier that is popular among Japanese car makers.

Toyota researchers also looked at screening tests to detect turbocharger coking, such as ASTM D6335, also known as the TEOST 33C test, which has been used in some of the ILSAC engine oil specifications. As already mentioned, the Toyota researchers found that the presence of insolubles derived from degraded fuel molecules was the key factor to reproduce the turbocharger coking phenomena.  The TEOST 33C test does not involve it.  Therefore, they do not think it is a good engine oil-screening test for turbocharger coking.

Interestingly, they used 98 RON gasoline, which is a high octane fuel and in Japan, contains an elevated level of aromatics.

“They are prone to form more insolubles than other hydrocarbons. Actually by taking premium fuel in the Japanese market, we are increasing stress on coking. So from that viewpoint, fuel quality is very important when we take this technical assessment of this phenomena,” Hirano said.

The Toyota researchers also looked at what could be done to prevent the problem. They examined, for instance, two different methods for cooling the turbocharger, taking temperature measurements at different locations. These showed that dropping the bearing temperature from about 215^o C of an air-cooled turbocharger condition to less than 160^o C of the same engine equipped with a cooling circuit significantly reduced coking. Indeed, at around 140^o C, deposits were nearly eliminated. In contrast, the uncooled engine bearings from the hot engine were covered by black coke.

Beyond temperature control of the engine, another counter measure is to prevent polymerisation of the degraded fuel that accumulates in the engine oil. That requires steps to be taken to prevent oxidation. Hirano said that lubricant manufacturers will have to do their part to make this happen.

Hirano acknowledged that degraded fuel is unavoidable in real-life use of gasoline-fueled engine. However, reduction of  insolubles formation is possible. “In order to achieve better anti-oxidancy of the engine oil, there are two ways: better anti-oxidant formulations and higher quality base oils. These are very important to prevent coking in the engine,” he said.