Automotive turbochargers have changed significantly since they were first used over 60 years ago, Miiller, who is vice president of operations, told those gathered for the 21st International Colloquium Tribology at the Technische Akademie Esslingen in Ostfildern, Germany.

In engines, turbochargers push compressed air into the engine cylinders, increasing the air-to-fuel ratio and allowing a more efficient fuel-burn. While the technology has shown significant benefits in engine efficiency, original equipment manufacturers noticed in the 1980s and 1990s that European short-trip driving patterns were causing some engine oils to form coke-like deposits, said Miiller.

The industry identified high heat zones as the cause of the problem. Temperatures in an operating turbocharger could reach 300 degrees Celsius, but when the engine was turned off, some areas that were no longer being cooled by circulating air could reach up to 600 C-and the oil sat in those hot zones. The result, Miiller explained, was deposit formation in the turbocharger drive shaft bearings and oil inlet passages. The deposits interfered with oil flow to the bearings, causing equipment failure.

OEMs recognized the need for a bench test to prescreen engine oils before investing in expensive engine tests, and to provide ongoing evidence of engine oil quality. The problem became serious enough that Savant got a phone call from an OEM requesting help developing a screener oxidation bench test, Miiller recalled.

The original TEOST 33C bench test was developed in the mid-1990s and still works well under conditions in which the oil in the turbocharger is not sufficiently protected from heat after the engine is shut down, as in older turbos, Miiller was quick to point out.

For the test, a 116 milliliter oil sample is placed in a reservoir and heated to 100 C to simulate the engine environment that the oil endures. This includes the steady gaseous flow of 3.6 mL/min. each of moist air and nitrous oxide, simulating the combustion-generated exhaust gases to which the oil is continuously exposed. This helps to develop the chemistry of the deposit-forming precursors in both the turbocharger and engine. The sample of heated oil is then pumped through the test cell at a rate of 0.4 grams per minute. Iron naphthenate is added to the test sample at 100 parts per million to mimic the contamination that may come from engine hardware. This circulating test sample is passed over the outer surface of a hollow steel rod containing a temperature-controlling thermocouple, positioned 71 millimeters from the top of the rod.

To simulate the start-stop conditions often found in driving cycles that create temperature variations, especially during short runs, Savant designed a heating cycle for the depositor rod that varied between 200 C and 480 C for each 9-minute cycle, for a total of 12 cycles over a two-hour test. The weight of accumulated deposit on the heated rod and suspended in the circulated test sample is reported as the test result.

The sponsoring OEM supplied four oils for testing that were running primarily in engines in Europe. Two of the oils were routinely failing and two performed well. The test clearly differentiated between the oils, Miiller said, and was adopted by ASTM International as ASTM D6335, Standard Test Method for Determination of High Temperature Deposits by Thermo-oxidation Engine Oil Simulation Test.

The past two decades, in particular, have seen a lot of changes in turbocharger design, Miiller said.

There have also been considerable changes in engine oil formulations, he continued, including reduction of zinc dialkyldithiophosphate, increased levels of molybdenum and a number of variations in between.

Recently it became clear that the ASTM D6335 bench test was no longer adequately differentiating between oils, and the OEMs came back to Savant and requested an update. To show the diminished correlation, they suggested running the bench test using four oils that had been run in the General Motors Turbo­charger Deposit test, an updated gasoline engine test developed with new turbo designs and improved engine oil formulations. This confirmed the poor correlation, as the bench and engine tests returned the same result for only one of the four oils. Development of the new TEOST test began in early 2017.

Modern turbochargers seem to have reduced coking tendencies, thanks to improvements such as intercooling, turbo-idling, oil wicking and water injection. Turbo-idling, in particular, continues turbocharger cooling after the engine is shut down, reducing hot spots in the hardware. However, field reports indicate that redesigned turbos continue to form deposits at lower temperatures (250-300 C), Miiller observed.

We had to take a look at these conditions along with the new formulations to make sure what we were doing was really correlative, he said.

He also explained that the company aims to develop correlation with two OEM engine tests: one for gasoline engines and the second for diesel passenger car engines. This is a difficult task because any difference in engine operating temperatures and piston blowby gases may produce somewhat different deposit-forming tendencies. Test developers are trying to find the sweet spot for specifying engine oils for either form of engine power.

During development, oils with GM turbocharger engine test data were used, including a Japan Automobile Manufacturers Association-approved SAE 5W-30 oil with high molybdenum content, four oils formulated by an additive company, and a GMTC01-1 SAE 5W-20 oil. Some oils without GM engine test data were also used: an SAE 5W-40 viscosity oil from a North American OEM, two JAMA SAE 0W-20s, a Nissan API SN 0W-20 and a Subaru API SN 0W-20.

Initial experiments lowered peak cycle temperature to 400 C and 430 C to reflect the lower, more constant temperatures in modern turbos, while keeping other test parameters the same. This reduced deposits for all oils but did not improve correlation or lead to discrimination between passing and failing oils. The failing oils were performing better than the passing oils, Miiller reported. Just lowering the temperature on the current test was not enough. We really had to dive in and understand what was going on.

Continued experimentation led to additional changes such as reducing the sample volume and changing the oil flow down the depositor rod so that all circulated oil contacted the rod. This parameter is similar to newer turbos with oil wicking design. Savants engineers developing the new test began using a wire-wound depositor rod like the one used in the TEOST MHT, a test that simulates conditions found in the ring belt and piston ring areas of the engine. Oil moves more evenly down this type of rod, which was kept at a lower, constant temperature of 285 C.

The controlling thermocouples sensor was moved further (97mm) from the top of the rod and the pumping rate was changed to 0.25 g/min. Moist air flow was increased to 10 mL/min. and nitrous oxide was eliminated. Iron naphthenate was increased to 0.44 mg/mL of oil and total test time was increased to 24 hours.

Another significant change was the redesigned glass mantle for controlling movement of volatile compounds within the test. If everything is being volatized, then thats a problem because now you dont have sufficient sample to complete the test, Miiller elaborated. But if you retain the entire sample, thats really not whats happening in the engine.

Instead of just letting the injected air come back out, which would tend to sweep your volatiles out, we didnt want to do that. We wanted to try to contain some of those volatiles. Some can escape like in a regular engine, but they cant just be pulled out automatically like in a regular [non-turbocharged] engine, he explained. And that worked really well, right off the bat.

At this point, the engineers started to see separation between the oils, but needed to get closer to field conditions for a successful test. A member of Savants marketing team suggested that they take a video of the process, which enabled them to get to the next step, Miiller chuckled. The video showed that at 18 hours, all of the oils stopped forming additional deposits, which meant the test could be stopped earlier than 24 hours.

Finally, test parameters were further refined, including increasing the oil reservoir to 20 mL to keep it from drying up, and increasing the test temperature to 290 C. The test was run on three different instruments with multiple operators in two separate labs, and results showed a clear separation between the tested oils and good repeatability between instruments.

At the time Miiller gave his presentation, deposits formed during the test were being analyzed to ensure they are the same types that form during engine tests. In the past, deposits contained significant amounts of nitrous compounds, but this has not been the case more recently. Miiller chalks up the change to evolving oil formulations, though deposits are dependent on both the oil and engine temperatures. So far, he said, the test deposit formations look very similar to what OEMs expect to see.

Interestingly, while molybdenum shows high deposits in the ASTM D6335 test, it has no unusual ramifications on the results of this test, Miiller revealed. This is significant, and we were able to show the response in any tests of oils carrying molybdenum.

Savant is also considering making slight modifications to the new bench test to include diesel turbocharged automotive engines, which may form turbocharger deposits at somewhat different temperatures than gasoline engines.

Before launching the updated TEOST Turbo Bench Test, the engineers will ensure that it matches engine test results and will examine additional oil formulations. The new test will be made available for ASTM evaluation in an upcoming round-robin study.