As more countries around the world continue to demand advancements in fuel economy and emission standards in modern engines, the automobile industry is leaning on high-performance engine oils to help accomplish that goal.
One property that is critical for engine performance is an oil’s viscosity at low temperatures. Oil that has thickened due to aging and oxidation can cause catastrophic engine failure when—especially in cold conditions—it is too thick to be pumped into critical parts of the engine.
The Romaszewski Oil Bench Oxidation test can be used to quickly and economically develop oil formulations that will maintain low-temperature performance over the oil drain interval. Adoption of the ROBO test into the API SN and ILSAC GF-5 passenger car engine oil specifications presented a significant cost savings to the engine oil industry. The $1,500, 40-hour bench test can be run in place of the Sequence IIIGA fired engine test, which cost $40,000 when GF-5 made its debut 10 years ago and takes 100 hours to run.
With recent concerns about protection dwindling against low-speed pre-ignition as oils age, the ROBO test may yet again prove its importance to the industry.
Concerns about low-temperature engine oil performance dramatically increased following a series of engine failures in the United States Upper Midwest during the winter of 1980 and 1981. At the time, engine oils were not equipped to deal with drastic cooling cycles, especially as they aged. The oil became gelated within the crankshaft, causing engine failure due to a lack of oil circulation.
These failures prompted the standardization of various cold-temperature engine oil performance properties. A few of the earliest methods for determining cold-temperature pumpability include the ASTM D3829 Mini-Rotary Viscometer (updated in 1987 to the ASTM D4684 MRV TP-1) and the ASTM D5133 Scanning Brookfield Technique tests.
The ROBO test was developed in 2003 by Raymond Romaszewski and others of RohMax Oil Additives (now Evonik Oil Additives), and the method was standardized as ASTM D7528. The test simulates an engine’s volatilizing and oxidation processes on the oil using laboratory conditions and equipment. It plays a significant role in the engine oil industry by allowing assessment of the performance of oil additives such as viscosity modifiers and pour point depressants as well as the effects of other formulation changes.
Running the ROBO
Before any tests can be run, the test equipment must first be calibrated according to the ASTM Test Monitoring Center’s parameters for use with standardized reference oils. Calibration is intended to produce comparable oil-aging characteristics to those obtained through the TMC’s Sequence IIIGA matrix reference oils.
To properly calibrate the equipment, the user must age the reference oils in the ROBO procedure and obtain a certain range of MRV viscosity values. The MRV, or mini-rotary viscometer test, measures the apparent viscosity at the engine’s oil pump at a specified low temperature after a specified cooling and resting cycle.
First, the test unit simulates the oil aging process. Two hundred grams of test oil is first combined with a ferrocene (iron) catalyst to stand in for wear debris in the engine. This mixture is stirred and heated at 170 degrees Celsius for 40 hours in a 1-liter reaction vessel. A feed of liquid nitrogen dioxide is evaporated, mixed with dry air and then introduced below the reaction surface while a feed of air flows across the surface of the liquid at negative pressure.
Nitrogen dioxide is a critical component of the ROBO test and is used to simulate the effects of blow-by gas within the engine. However, due to supply challenges that began in 2017, the ROBO Surveillance Panel, which operates with the ASTM Test Monitoring Center, advanced a draft proposal in March to replace the pure liquid NO2 feed with a dilute 1.13 percent NO2 air feed at 185 milliliters per minute. This change should also improve operational convenience and safety. The dilute NO2 setting produces similar relationships to the current pure NO2 feed for results from the MRV and percent kinematic viscosity increase at 100 C. The change could be adopted as early as June this year.
In the meantime, the American Petroleum Institute has invoked provisional licensing on April 1 for ILSAC GF-6 and API SP, citing a backlog of ROBO test availability.
After the 40-hour oxidation process, the concentrated oil undergoes low-temperature viscosity bench testing using the ASTM D4684 method (“Standard Test Method for Determination of Yield Stress and Apparent Viscosity of Engine Oils at Low Temperature”).
Last, the evaporated oil is condensed and weighed in order to calculate evaporative loss.
Engine Testing for Cold Temps
The Sequence IIIG test (ASTM D7320), first implemented in the API SM and ILSAC GF-4 specifications, utilizes a 1996-1997 GM 3.8L V6 engine to evaluate various properties of oil performance, including high-temperature viscosity increase and prevention of piston deposits, valve train wear and hot stuck rings. The volume of engine oil is measured after an initial run of 10 minutes and is periodically checked to maintain the same level throughout the test. The engine then takes 15 minutes to accelerate to the required speed and load conditions.
The engine runs for 100 hours at 3,600 revolutions per minute at 150 C. Oil samples are drawn and evaluated at 20-hour intervals. The aged, oxidized engine oil is then subjected to analysis including the cold cranking simulator, viscosity measurement at 40 C, oxidation measurement by Fourier-transform infrared spectroscopy and other such tests.
The Sequence IIIG contains two additional post-test measurements: The Sequence IIIGA evaluates the cold-temperature viscosity performance after high-temperature engine operation, while the Sequence IIIGB evaluates the phosphorus retention of a sample exposed to similar engine conditions.
In a similar fashion to the ROBO test, the aged reference oils are evaluated using MRV measurements.
For the upcoming ILSAC GF-6 specification, which has its first licensing date on May 1, the Sequence IIIG test has been replaced with the Sequence IIIH. The IIIH utilizes a more modern 2014 Chrysler Pentastar 3.6L V6 engine running for 90 hours instead of the previous 100 hours. This test has also been approved for use in the API SN, API SN Plus and API SP specifications, and costs about $50,000 to $60,000 to run.
The Sequence IIIH consists of a similar initial 8-minute lubricant leveling procedure to its predecessor, followed by the 15-minute acceleration to test conditions. The engine then operates at 137 brake horsepower at 3,900 rpm and 151 C. The 90-hour run time is split into four segments of 20 hours and one segment of 10 hours.
After a careful analysis of reference oils across both test methods, the ROBO has been shown to correlate well with the engine conditions of the Sequence IIIGA. According to an article published in the November 2017 Journal of ASTM International, data analysis of 34 reference oils revealed that ROBO successfully predicts the outcome of Sequence IIIGA pass/fail results 81 percent of the time.
Further linear regression analysis within the same study showed excellent statistical correlation. This analysis, along with the good MRV and pVis correlation, showed the ROBO to be a strong alternative to the Sequence IIIGA.
Like the Sequence IIIG, the Sequence IIIH also oxidizes and volatilizes the oil, leading to viscosity increase, but it tends to exhibit more oxidation and less volatilization than the Sequence IIIG.
ROBO is the Future
The ROBO test continues to be used as a critical research and development tool for oil formulations, especially in improving low-temperature performance and oil pumpability. And, as the industry recognizes that oil aging and oxidation affects how well engines are protected against low-speed pre-ignition, the ROBO may come into play in another arena.
Advancements in engine design for improved fuel efficiency have led to the rise of turbocharged direct-injection gasoline engines. These engines are vulnerable to LSPI, which occurs when the engine experiences premature combustion at high loads and low speeds. This causes excessive pressure buildup within the engine cylinders, creating heavy “knock” that can severely damage pistons and lead to complete engine failure. (See Page 32 for more on LSPI.)
Combating this phenomenon requires a holistic approach, as LSPI involves interactions between the engine design, operating conditions, oil formulation and fuel quality. The ILSAC GF-6 and API SP specifications aim to standardize engine oil formulations that minimize the likelihood of experiencing LSPI. For these specifications, the ROBO test can be used with or instead of the new Sequence IIIHA (ASTM D8111).
Alan Flamberg holds a Ph.D. in chemistry from Stanford University. He retired after a 34-year career at Rohm and Haas, RohMax, and Evonik.
Raj Shah, Ph.D., is a director at Koehler Instrument Co. in Long Island, New York, where he has been for 25 years. Contact him at rshah@koehlerinstrument.
Sarjeel Zaman is an undergraduate student in chemical engineering at Stony Brook University, where Shah is chair of the industrial advisory board.