Diesel is a distillate of crude oil and is extensively used by transportation sector. According to U.S Energy Information Administration, the global diesel fuel consumption in transportation segment is projected to increase by 48% from 2012 to 2040, as shown in Figure 1. As a result, diesel will be a major factor to the mobility efficiency in the years to come.
Figure 1. Consumption of diesel fuel from 2010 to 2040 (source: www.eia.gov).
Diesel provides two crucial functions for an automotive vehicle. It serves as a fuel, a source of chemical energy that is converted into mechanical energy within an internal combustion engine. It also acts as a lubricant and protects the important moving parts, for instance, the gears used in a diesel fuel pump as illustrated in Figure 2. However, seasonality could influence the behavior of the diesel as a lubricant and engine fuel.
Figure 2. Gears in Bosch High Pressure Diesel Fuel Pump (Type CP3).
Flow Properties of Diesel Fuel
The flow properties of the diesel fuel govern the safety of an internal combustion engine. During the winter season, the low temperature environment causes the paraffin in the diesel fuel to crystallize, which clogs the filter and blocks the diesel flow. It can be properly explained by the cold filter plugging point (CFPP) of diesel fuel. Fuel suppliers treat the diesel (fuel blends and additives) to adhere to CFPP standard (EN 590) that assures diesel flow based on the season – summer, first transition period, winter and second transition period (refer Table 1). Season dependent modification of the ingredients present in the diesel fuel can also influence the diesel lubricity. However, this is yet to be investigated.
Table 1. Cold Filter Plugging Point (CFPP) specifications for diesel fuel during different seasons in Netherlands. The section “code” is specific to this study.
||15th April - 30th Sep
||1st Oct - 15th Nov
||16th Nov - 28th Feb
||1st March - 14th April
Friction Profiles of Diesel
At Ducom Instruments, testing laboratory began to monitor the lubricity behavior of diesel procured from a nearby gas station in the city of Groningen – Netherlands (Figure 3). The diesel fuel samples were taken from May 2016 till March 2017. It includes all the time frames described by EN 590 standard shown in Table 1.
Figure 3. Gas station in Groningen (A) and collection of diesel fuel using a safety container (B).
Ducom High Frequency Reciprocating Rig (HFRR), as illustrated in the Figure 4, is a ball on disc tribometer, used to investigate the lubricity of Diesel fuel, in accordance with the standard - ISO 12156 (refer Table 2). The two parameters used to compare the diesel samples are friction coefficient and mean wear scar diameter of the ball.
Figure 4. Image of Ducom High Frequency Reciprocating Rig (HFRR), including the description of the operating parameters.
Table 2. Diesel fuel lubricity test according to ISO 12156, using Ducom HFRR.
|EN 52100 ball ∅
|EN 52100 disk ∅
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Friction profiles of diesel were different according to the season, as shown in Figure 5. Friction coefficient of summer diesel (average of four samples from May 2016 till September 2016) was found to be higher than the diesel during the first transition period (October 2016). Overall, friction coefficient of the winter diesel (November 2016 till February 2017) was the highest.
The viscosity of diesel during the winter and transition periods should be lower compared to the summer diesel. Lower viscosity fuels are easy to shear which results in lower friction. This explains the friction behavior of diesel fuel from the first transition period (moderately hydrodynamic lubrication). Any additional decrease in the viscosity, as anticipated for the winter diesel, increases the contact between the rubbing surfaces. A mechanism referred to as boundary lubrication could be the reason for high friction of winter diesel.
Figure 5. Real time changes in the friction coefficient of summer diesel (S), diesel of the first transition period (T1), winter diesel (W) and diesel during the second transition period (T2). The error bars represent the standard deviation over 4 test. One test per diesel sample per month.
As shown in Figure 6, wear scar of the balls after the HFRR test is also different based on the season. In Figure 7, the mean wear scar diameter (MWSD) on these balls were measured and plotted as a function of the season. Average ball MWSD of summer diesel is 376 µm which is 75% more than the ball MWSD of diesel from the first transition period. The average ball MWSD of winter diesel is comparable to the summer diesel.
Figure 6. Wear scar on the ball after HFRR test of different diesel samples.
Figure 7. Mean wear scar diameter (MWSD) of the ball for summer diesel (S), diesel of the first transition period (T1), winter diesel (W) and diesel during the second transition period (T2). The error bars represent the standard deviation over 4 test. One test per diesel sample per month.
Ducom HFRR lubricity data (May 2016 to February 2017) indicate that the diesel in October (first transition period) has an ideal combination of anti-wear additives and low viscosity. During the winter season, the friction was higher but the MWSD was comparable to the summer diesel indicating that anti-wear additives in diesel fuel are adequate to offset the low viscosity of winter diesel fuel. All the MWSD reported in this research is within the highest MWSD (460 µm) limit for diesel fuel supplied in Netherlands (source: Infineum International Limited - “World Wide Diesel Fuel Quality Survey 2014”). Ducom’s explanation of the mechanism may not be exhaustive due to lack of knowledge on the composition of the diesel fuel.
This HFRR study performed by Ducom proves that the diesel fuel’s lubricity relies on the season. Moreover, the company states that the diesel supplied by the gas station close to the city of Groningen conforms to the national standards of HFRR MWSD for diesel fuel.
This information has been sourced, reviewed and adapted from materials provided by Ducom.
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