Tracing Engine Wear
Radioactive Tracers Provide Quick Answers for Automotive Lubricant Tests
Even as an automotive engine is humming smoothly and producing power, critical parts are slowly wearing away, despite the best efforts of modern materials and lubricants.
Moving parts roll and slide against each other in an environment characterized by high speeds and pressures, extreme temperatures, and tight tolerances. The inevitable facts of engine life ultimately make themselves apparent through obvious symptoms of wear -- smoky exhaust due to increased oil consumption, or diminished power output due to loss of compression within the cylinders. However, this wearing-away can be measured, precisely and in real time, through the telltale presence of minute particles of metal abraded from wear-sensitive parts. This abrasion is too slight to be monitored to the desired degree of precision by direct observation as the wear occurs, and accurate measurement using physical inspection requires relatively long tests, then dismantling of the engine.
However, scientists at Southwest Research Institute (SwRI) can determine wear rates quickly, first by irradiating wear-sensitive parts to render them radioactive, and then measuring gamma ray emissions from abraded particles as they circulate within the engine's lubricating oil.
Much of this technology was pioneered at SwRI, perhaps the only facility in the world to use bulk neutron-activated tracers to measure engine wear in real time. SwRI is able to do this because of its extensive infrastructure that supports numerous other projects requiring the safe handling and use of radioactive materials, and because of the Institute's ongoing work for the nuclear industry.
Besides bulk activation, or irradiation of the entire part, SwRI also conducts engine wear studies using surface or thin layer activation, in which only atoms near the surface of the target part are bombarded with a high energy beam of charged particles. Using this method, wear can be measured by monitoring the decrease in activity of the irradiated part as it wears, or the increase in activity of wear debris as it collects on a filter in the fluid circuit.
Exploiting the ability of instruments to detect minute changes in radiation, SwRI scientists and engineers can meet manufacturers' demands for rapid results. At the same time, precise cause and effect information can be gathered under specific operating conditions in response to subtle changes due to modifications or adjustments in lubricant chemistry, or in response to a specific environmental condition. Thus, SwRI's radioactive tracer (RAT) technology can gather meaningful research information in a matter of minutes. Conventional test-and-measure procedures, used for qualification tests to meet prescribed industry standards, can require hundreds of hours.
Although the Institute has carried out radioactive tracer technology testing of automotive components for more than 40 years, new methodologies and advanced equipment have kept this technology in the forefront as a diagnostic tool for scientific examination of engine life and component wear. Beyond measuring component wear, the ability of RAT technology to measure oil consumption during engine operation provides another tool for evaluating engine performance with respect to hardware design, operating strategies, and lubricant characteristics. A major benefit of the radioactive tracer oil consumption method is that it provides reliable data for measurements taken over short periods of time. This reduces costs and shortens development time by allowing many parameters to be investigated quickly and accurately. This is especially timely, since oil consumption has been identified as a strong contributor to diesel engine particulate emissions, complicating the NOx-particulate emissions tradeoff. The tie-in to engine wear is also significant, since increases in ring wear almost always lead to increases in oil consumption.
RAT's usefulness is not confined to wear analysis within the engine. The same technology has been applied to studies involving pumps, transmission parts, valves and hydraulic components. It also has examined the effects of contaminants, including dust in engines and engine components such as fuel injectors.
This article will examine some of the primary methods of applying RAT technology, as well as some of the applications in which it can be brought to bear.
Benefits of RAT technology
Because RAT allows measurement in the parts-per-million to parts-per-billion range, very small changes in wear can be detected accurately and measured quantitatively as the engine is repetitively operated over specific test cycles and environmental conditions. This level of sensitivity provides significant cause-and-effect information under both transient and steady-state conditions and allows wear measurements to be routinely made with a high level of confidence.
In general, better data are obtained, more cost-effectively, than with conventional methods in which testing must be interrupted after several days or weeks so components can be removed for inspection and measurement, yielding individual data points. Since physical inspection is not required, RAT avoids possible changes in the way materials fit together, or in the engine's wear state; hence test-to-test continuity is maintained and sequential experiments yield more meaningful results.
With RAT, many test conditions and combinations of variables can be investigated simultaneously, in hours rather than weeks or months. This can reduce product development time for a manufacturer. RAT allows the use of off-the-shelf engine parts but requires that the components to be studied first be rendered radioactive. This requires that the components of interest be made of materials that are suitable for irradiation. In preparation for RAT analysis, wear-sensitive engine parts such as compression rings and connecting rod bearings are sent to a nuclear reactor, where they are exposed to thermal neutrons. Because the irradiation process produces artificial radionuclides that are characteristic of each target part, the wear of multiple components can be detected and measured separately.
The physics of RAT technology
Most radionuclides produced by neutron bombardment of stable materials decay by beta-minus emission (energetic electrons), as a neutron converts into a proton in each nucleus. Most beta decays populate excited states in the metastable product nucleus, which transition to lower energy states by emitting gamma-ray photons. The specific energies of these photons are characteristic of the parent element and serve to identify it. Intensity, meanwhile, is proportional to the amount of an element present at the time of measurement. In fact, these energetic gamma-ray photons are what is measured; they are the tracers. The beta particles are not used as tracers because they are readily attenuated by the walls of the test loop.
Because nuclear states within an atom have very well-defined energies, the energies associated with gamma-ray transitions between states are also well defined, such that the gamma rays for any given transition are nearly monoenergetic. This produces specific, well-defined, narrow peaks in the energy distribution spectra for given transitions, thereby allowing measurement of the wear of multiple components or different surfaces within the same component that have appropriately different metallurgies.
Detection of these peaks, and resolution of peaks arising from closely spaced gamma-ray energies, is highly dependent on the energy resolution capabilities of the detector used in making the measurement.
Detecting and measuring radiation
Gamma-ray photons were typically measured by detectors known as scintillation counters, that usually incorporate materials, such as sodium iodide (NaI), containing atoms that are easily excited by incoming radiation and which emit visible light flashes when they return to their ground states. These flashes of light interact with a material which ejects electrons through the photoelectric effect. The ejected electrons are accelerated to electrodes of increasing positive voltage, with each electrode ejecting more electrons, so that the original signal is highly amplified. In this manner, a single electron may produce a million or more electrons, resulting in a detectable pulse of electrical current. The scintillation counter can detect the rate of incident photons and determine the activity levels of the radionuclides of interest.
More recently, germanium crystal detectors are used because of their improved resolution. Photon measurement in a germanium detector is more complex and relies on the generation and detection of electron-hole pairs leading to the appearance of an electrical pulse, which is proportional to the number of pairs present. This method of detection and measurement is well suited to SwRI's work because it provides excellent peak resolution which is not generally obtainable with ordinary sodium iodide scintillation detectors. High peak resolution provides a greater ability to discriminate between individual radionuclides. Better accuracy improves the potential for simultaneously interrogating various engine components having different chemical compositions.
Subatomic activity is governed by quantum rules that operate according to chance and probability. The decay of the nucleus of a radioactive atom, with the nucleus ejecting a beta particle and becoming the nucleus of an atom of a different element, is governed entirely by probability. For each particular type of radioactive element, there is a specific length of time during which 50 percent of the atoms will have decayed. This time is known as the half-life of the element and is used in this analysis to correct all data to "time zero" so that comparisons made from sequential testing will be meaningful.
Adapting RAT for oil consumption analysis
In a recent application, RAT technology has been used in the real-time measurement of oil consumption and the quantification of effects of oil consumption and oil composition on diesel engine emissions using different combustion technologies.
However, measurement of engine lube oil consumption differs significantly in methodology from the measurement of engine-parts wear. The oil consumption measurement is based on tritiation of the oil, followed by the measurement of radioactivity levels in continuous samples collected from the exhaust. Tritiation is the process of replacing some of the hydrogen atoms in the oil with radioactive tritium (3H) atoms through catalytic exchange. This is done in a sample of base stock which is then mixed with the fully formulated oil prior to testing. If all hydrocarbon in the consumed lube oil is burned to completion, all hydrogen -- including the tritium -- will be converted to water. Consequently, activity of the water collected in the exhaust sample will be directly proportional to the mass of oil consumed in the engine during the sampling period. During testing, a continuous exhaust sample is taken at each engine operating condition. The sample is processed to obtain the total amount of water available. The radioactivity of this water is directly related to the mass of lubricant consumed during the sampling period. This amount is calculated mathematically from the data.
Whether the objective is to measure metal lost to abrasion, oil lost to combustion, or component life lost or shortened due to contamination, the measurement problems are similar although the media and the methodologies differ greatly.
For each process, accurate measurement demands precision because of the gradual nature of the process being studied. And in each case, the demands of time and cost-efficiency may not allow for conventional procedures of repetitive testing, dismantling, and measuring by direct observation.
It is in this environment of compressed test intervals that RAT excels by offering the desired precision along with solid correlation between short-term tests and long-term trends inside the punishing environment of an operating engine. More importantly, RAT provides specific cause and effect information which is often unobtainable using long-term conventional methods.
1. M.B.Treuhaft, "Engine Wear Experiments," Southwest Research Institute Final Report 03-9500, February 1989.
2. J.P. Wisnewski and M.B. Treuhaft, "Lab Evaluation of Engine Wear as a Function of Dust," Technical Report No. 13468 for U.S. Army Tank-Automotive Command (03-2429), July 1989.
3. M.B. Treuhaft, F.A. Iddings, G.A. Boyd, and S.R. Sprague, "The Use of Radioactive Tracer Technology in Studying Lubricant Chemistry to Enhance Bearing and Ring Wear Control in an Operating Engine," SAE Technical Paper Series No. 941982, October 1994.
4. M.B. Treuhaft and X. Tao, "The Use of Radioactive Tracer Technology to Measure Real-Time Wear in Operating Engines," Proceedings of 1995 KSEA International Technical Conference: Part 1 Automotive Technology.
5. M.B. Treuhaft, "The Use of Radioactive Tracer Technology to Measure Engine Ring Wear in Response to Dust Ingestion," SAE Technical Paper Series No. 930019, March 1993.
6. G.F. Knoll, "Radiation Detection and Measurement," Second Edition, John Wiley & Sons, New York, New York, 1989.
7. "Texas Regulations for Control of Radiation (TRCR)," Texas Department of Health, Bureau of Radiation Control, Austin, Texas, January 1994.
8. "Radiation Safety Manual," Southwest Research Institute, San Antonio, Texas, 1993.
9. "Radiation Safety Procedure for Radioactive Tracer Engine Wear Studies," Southwest Research Institute, San Antonio, Texas, 1996.
10. R.C. Fernow, "Introduction to Experimental Particle Physics," Cambridge University Press, New York, New York, 1986.
11. H. Shaub, et al., "Engine Durability, Emissions and Fuel Economy Benefits of Special Boundary Lubricant Chemistry," SAE Paper 941983, October 1994.
12. M.B. Treuhaft and X. Tao, "The Use of Radioactive Tracer Technology to Measure Real-Time Wear and Oil Consumption in Operating Engines," First International Filtration Conference, "The Unknown Commodity," Southwest Research Institute, San Antonio, Texas, July 9, 1996.
Published in the Spring 1998 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.