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In 2004, a field trial was initiated over a four-year period with tactical and combat vehicles. The spectrum included armored reconnaissance vehicles (ARV) as well as main battle tanks (MBT). Oil samples removed from the monitoring well were taken every 90 days with an oil sampling kit normally used for aviation components. To avoid cross-contamination with other functional fluids taken from the vehicle, every sample was taken with a new sampling kit.
Among the monitored vehicles were 40 main battle tanks (Figure 1), 25 armored infantry fighting vehicles (AIFV), 16 armored howitzers, 25 armored reconnaissance vehicles (ARV) and 10 mechanized infantry combat vehicles (MICV).
The evaluated oil parameters included:
Many challenges were encountered, such as sampling by non-professionals, rotation in the staff, decreased availability of the vehicles, different oil service quality and engine oil compositions, non-documented oil refill quantities and long-term used vehicles with unknown current engine conditions. There was also a need for continuous evaluation and suitability testing of gained results, as well as strong varying vehicle mission profiles and sophisticated diagnostic research to gain reliable information concerning engines with suspected impending failures. In addition, the oil condition data could not be correlated with fuel consumption since the last oil service, as some vehicles had been operated for several hours without refueling before sampling.
First, any correlation between kinematic viscosity and the operated kilometers since the last oil service was examined. For this, the data of one battalion equipped with main battle tanks was selected (Figure 2).
Concerning kinematic viscosity, there was no apparent trend related to the traveled distance. When evaluating oil oxidation with Fourier transmission infrared spectroscopy (FTIR), a similar result was obtained (Figure 3).
With several main battle tanks of a battalion, there were differences in the individual mission profile, engine condition and purity of the oil fill with regards to contamination by dust, water and refilled oil quantities of another oil supplier. Moreover, the oil service quality of armored vehicles was much more variable than that of commercial trucks because of the engine compartment.
The results showed no apparent trend concerning oil viscosity and oil oxidation. There was also no correlation between the traveled distance and the associated oil condition. However, a large variance in the results was evident.
Although no relationship between engine oil aging and the associated operating data could be derived, this seemed to be contrary to the results obtained in a former study with wheeled vehicles, shown in Figure 4.
In comparing these results gained from commercial on the shelf (COTS) trucks with the results derived from the current study, a few important differences were noted. For example, contrary to tactical vehicles, the mission profile of COTS trucks is dominated by on-road use. The operating conditions are also much more constant than the extreme varying mission profile of tactical vehicles off-road, as there are fewer cold starts and idle operations with a cold engine.
In the current study, the oil condition data could not be associated with fuel consumption since the last oil service. Depending on the intensity of use, some vehicles would not be refueled during a sampling interval.
In further investigation, the oil condition data was evaluated with respect to a single vehicle. For this, the entire amount of collected and plausibility proofed data was used as a guideline to assess the oil condition.
Next, the oil’s oxidation, nitration and sulfation were evaluated (Figure 5).
Within these parameters, a good correlation concerning oxidation and nitration was found. The correlation factor was between 0.92 and 0.95, while the firmness index was between 0.77 and 0.84.
In regards to oil aging, the results showed a low level because the accumulated fuel consumption over the observed distance was only about 4,500 liters (1,189 gallons) corresponding to 2,300 km (1,429 miles). The nearly sulphur-free diesel fuel (less than 10 ppm) was the reason for the very low sulfation level.
When analyzing the fuel content, soot content and kinematic viscosity at 40 degrees C, something conspicuous was observed (Figure 6).
Despite a nearly constant fuel content, the viscosity at 40 degrees C decreased from a fresh oil level of 104 mm² per second down to approximately 80 mm² per second. The slightly increasing soot content did not have any influence on this phenomenon.
The main reason for the observed viscosity loss was the biodiesel content of diesel fuel in Germany. Since the beginning of the current century, rapeseed methyl ester has been a permitted blend component of crude oil diesel. Up to a volumetric part of 5 percent (7 percent since 2010) is compliant with DIN EN 590.
The main disadvantage of biodiesel is its rather high boiling point of more than 300 degrees C. During partial-load operation of diesel engines, the oil fill will be diluted by non or only partly burned diesel fuel. Because of volatility reasons, the lower boiling components of crude oil diesel will be evaporated from the oil fill during longer full-load operation, but the biodiesel will remain.
The standard test method for fuel diluent in used automotive engine oils (DIN 51380) was not suitable to determine biodiesel components. With a modified GC method, a biodiesel content of about 6 percent was established as well as the fuel content according to DIN 51380. The additional biodiesel content is shown in Figure 7. According to this chart, the assumed fuel content of the oil fill meets 10 percent. Against this backdrop, the viscosity decrease is plausible.
Caused by the extreme high viscosity index of fuels, the viscosity decrease at 100 degrees C (from 14.2 mm² per second to 12 mm² per second) was much less significant.
On all investigated vehicles, the biodiesel content within the oil fill paused after a previous strong increase at a limit of about 7 percent.
According to a study of the General German Car Club (ADAC), a total fuel content of up to 15 percent within the oil fill should cause no detrimental effects on engine components. Therefore, an extension of the fuel and soot content guidelines should be considered, as they are based on experiences collected in former studies using diesel fuel without biodiesel content.
In contrast to oil degradation in truck drivetrain components, there were no visible trends over all oil samples of a single vehicle type. This was due to the large differences in mission profile as well as in wear, care and maintenance status of each vehicle. Refilled and not documented oil quantities were another disturbing factor.
The general extension of operating limits (i.e., fuel consumption, operating hours or kilometers since the last oil service) was not feasible. For this, an oil analysis program like the U.S. Army’s oil analysis program would be required. Taking into account the specific circumstances within the German army, such a program requires too much investment.
Atomic emission spectroscopy (AES) was used to determine the element content. With this efficient analysis method, all results for a 90-day sample series were achieved within a few days.
The same approach from the oil analysis was employed. First, the results of the wear element and debris analysis were collected. For further evaluation, the data from a 90-day sample series (vehicle-type specific) was placed in a diagram (Figure 8).
As mentioned in context with oil oxidation and viscosity, there was no apparent trend. This was not surprising because every vehicle had a specific operating history of its own. Differences in mission profile and mechanical stress, as well as the specific engine condition, caused differences in abrasion and contaminants.
One start-up at low ambient temperatures combined with on-going high speed and high load generated much more abrasion than several hundred kilometers of driving at a medium speed on the road.
To identify impending failures caused by abnormal wear, several evaluation methods were used, including the operated distance of the vehicle and the wear element content of the current sample in comparison with that of an earlier sample. This is the classic procedure of trend analysis derived from aircraft component monitoring. In addition, the ratio of wear elements within a single sample were controlled (i.e., iron vs. aluminum), as significant differences from earlier samples taken from the same vehicle could indicate an impending failure.
Three battle tank engines were identified with very high wear element content within the oil fill (Figure 9).
With regards to vehicles 2 and 3, it was found that they had been operated on a very dusty training range under heavy-duty conditions. For these vehicles, an additional oil service was recommended as soon as possible to remove abrasion and dirt from the engine. For vehicle 3, a check of the air filter was also proposed. This seemed necessary because of the high silicon content in the sample.
After receiving the results for vehicle 1, it was initially thought that the sample had been taken before the oil service and not afterward. However, the motor sergeant of the related unit confirmed sampling after the oil service, although he was not sure whether the sample had been taken in the correct manner because of a fluctuation in the staff. Therefore, an additional sample was taken after a distance of about 50 km in order to obtain the trend related to wear elements (Figure 10).
The results showed further increasing of wear elements. Due to doubts concerning the quality of the last oil service, an additional one was recommended. This was carried out with the utmost care. Immediately after the oil exchange, the engine was driven for about 1 to 3 km to homogenize the oil fill. Then, another oil sample was taken, as well as a subsequent one after the engine was driven about 100 km.
Inquiries were also made into performance irregularities of the vehicle. The responsible motor sergeant assured there were none. For security reasons, the driver of the vehicle was instructed to pay attention to signs concerning abnormal engine behavior.
The analysis result of the fresh oil sample taken directly after the oil replacement indicated a good service execution. However, the following sample showed another sharp increase in wear elements related to the driven distance and operating conditions. Therefore, the responsible motor sergeant was contacted again to obtain information about the condition of the battle tank engine. He confirmed that everything was in order and that none of the drivers had noticed anything conspicuous.
After further evaluation of the analysis results, the replacement of the concerned battle tank engine was proposed. The investigation report of the maintenance unit showed that a catastrophic failure could be avoided (Figures 11 and 12).
Figure 11 Figure 12
Bore polishing at an advanced stage and abrasive wear at the lower turning point of the piston rings were detected in 11 of 12 cylinder liners (Figure 11). One cylinder liner had been damaged by piston seizure (Figure 12). This kind of damage is typically caused by hard particles (e.g., sand dust) and deposits on the piston crown. The latter ones mainly consist of fuel and oil oxidation products that arise during long-term operation with idle speed and a cold engine. Subsequent rapid acceleration and driving with high speed are able to provoke the observed damage. The driver has no chance to detect this type of impending engine failure because there is no significant loss in power or excessive oil and coolant temperatures.
During the study’s four-year timeframe, approximately 2,000 engine lube oil samples were analyzed. The 116 monitored vehicles were driven a total of more than 550,000 km. Two impending engine failures were detected in time. One additional engine broke down because of a fire due to a broken fuel line inside the engine compartment. This, of course, could not be recognized by means of condition monitoring. The avoided cost for fully overhauling the two MBT engines was estimated at $392,220.
Among the conclusions derived from this study were that a catastrophic engine failure is a rare event (statistically not before 180,000 km) and that the detection of impending failures is possible with condition monitoring. In addition, for the selection of single vehicles needed for peace-keeping or out-of-area missions, condition monitoring is quite suitable. Vehicles with conspicuous metal debris in the drivetrain can be sorted out or repaired in time.