Engine oil oxidation is a major degradation mechanism, and tests for it have been incorporated into both original equipment manufacturer and industry lubricant specifications. Oil oxidation is typically evaluated in laboratory bench tests; however, investigations by Infineum have shown that the numerous bench oxidation tests currently in use are unable to mimic the mechanisms of real engines and may restrict future formulation developments.
Oxidation Testing
Oxidation is an inevitable consequence of the exposure of lubricants to high temperatures and pressures in highly reactive environments, such as those found in the piston zone and sump of modern internal combustion engines. Both engine and bench tests have been developed to determine the effects of lubricant oxidation.
Chassis dynamometer tests can stress the oil in a number of ways and run at a variety of operating conditions, including different fuel type and composition, engine design and duty cycle, to mimic real-world conditions. However, these tests are expensive to run – typically in excess of U.S. $50,000 – and, in many cases, they must be run under extreme conditions to replicate field effects in a short period of time.
This has led industry bodies such as the CEC as well as individual OEMs to develop a variety of oxidation bench tests that have now been incorporated into both OEM and industry lubricant specifications. This activity has been well intended and provides the key benefits of reduced test costs and less time required for lubricant performance evaluation. However, more than a dozen subtly different laboratory oxidation tests are currently in use, all with one common aim: to assess the oxidative resistance of lubricating oil in a standardized laboratory test.
The problem is that the mechanisms of real engine oil oxidation are fundamentally different from those of bench oxidation tests. The wear process in a real engine, for example, creates tiny iron particles. These fine particles react with combustion acids to form catalytically active Fe(III) iron oxide species, which can accelerate oxidation. Lubricants can be formulated to control the conversion of wear iron to active iron, thus limiting oxidation.
However, a key factor in comparing engine and bench tests is that the active Fe(III) – in the form Fe(AcAc)3 – is added early in the bench tests, rather than over time as a result of wear. In addition, bench tests can be run at unrealistically high temperatures, between 150 and 170 degrees C, and they incorporate a wide range of conditions, durations and fuel types.
Bench vs. Engine Testing
To better understand the value of the results produced in a laboratory, Infineum compared the oxidation mechanism and performance of lubricants in both real engines and bench oxidation tests. For the comparison, oils were tested in both a chassis dynamometer test and the CEC L-109, a severe bench oxidation test that will be used in ACEA specifications.
In an attempt to better match the conditions of the CEC L-109 test, the chassis dynamometer was set up to run at very harsh operating conditions. It replicates running a diesel van in the Infineum Mountain Drive Cycle, which simulates constant uphill driving on the Groglockner High Alpine Road in Austria, ascending to 2,504 meters. The test was run with high fatty acid methyl ester sump doping, and over the 30,000 kilometers, the cooling circuit was bypassed and no oil top up was carried out.
Despite this severe operation, it was not possible to mimic the bench test conditions in the engine, even with a prolonged test cycle. For example, the average oil temperature in the engine test was 123 degrees C, peaking at 140 degrees C, which is well below the 150 to 170 degrees C set by the laboratory tests.
Two oils were formulated for testing: Oil A controls the conversion of wear iron to active iron; Oil B does not. Oil B was run in the chassis dynamometer until the oxidation increase reached the same level as that oil in the CEC L-109 oxidation test.
When Oil A was run for the same duration, CEC L-109 oxidation increase was 40 percent higher than that in the chassis dynamometer test. This difference is a result of Fe(AcAc)3, a form of active iron added at the start of the CEC L-109 test, which disables the lubricants ability to control the conversion of wear iron to active iron.
A much bigger difference between the tests can be observed in the viscosity increase response. The CEC L-109 test generating a large viscosity increase compared to the chassis dynamometer. We attributed this difference to the mechanism of viscosity increase, which is disproportionally affected by the high temperatures and the Fe(AcAc)3 present in the bench test, compared with the chassis dynamometer.
These results led Infineum to the conclusion that the CEC L-109 bench test, and the many other similar bench tests, although well intended, have limitations. By oversimplifying the variety of complex reactions taking place in a real engine, the ability to predict real engine performance can be lost.
Effects on Formulation Flexibility
Currently, the main drivers for engine oil development are improving fuel economy, engine wear protection and cost reduction, as well as more specific issues including low speed pre-ignition. These drivers push oil formulations in one direction, but bench tests like the CEC L-109 unfortunately drive the formulation space in exactly the opposite direction and, thus, can hinder innovation.
Bench oxidation tests might end up defining the formulation space based on one part of the oxidation mechanism. However, this does not necessarily mean the formulation approach used is the best way to improve the consequences of oxidation in a real engine environment.
In addition, these tests have a number of impacts on the lubricant formulator, including limiting the ability to select components that provide proven real world performance; thus, making it harder to develop lubricants that can meet the needs of future hardware and increasing the cost of new formulations.
In addition, the proliferation of bench oxidation tests creates unnecessary complexity and actively works against engine oil innovation to meet real industry needs. A key concern is that when the severity of a bench test is set too high, the main objective for oil formulation is to pass the nonpredictive bench test. This could mean, for example, that too much additive is used or that the wrong type of additive solution is applied for truly relevant performance for an engine in the field.
It is clear that demonstrating field performance is now essential. And if bench tests remain at all, realistic limits must be set that are also commensurate with the accuracy of the measurement tool. Perhaps the time has come for industry to apply a high level of scrutiny and to conduct a cost/benefit analysis of bench and analytical tests to ensure they can model real field phenomena and are relevant to modern engines.
Conclusions
The results observed in this study highlight that lubricants designed to meet future hardware requirements must provide excellent oxidation control and wear protection by using traditional additive technology. In addition, by better understanding the underlying mechanisms involved, more innovative approaches can be used to tackle these problems. For example, additives can be developed to control the conversion of wear iron into active iron.
To achieve this balance, oil formulators need the freedom to select components and treat rates that can deliver optimum lubricant performance. In Infineums view, a holistic review of specifications is warranted to achieve a better balance of performance tests and to truly depict the necessary performance of lubricants.