Renewable sources for the production of diesel fuel are growing significantly. Animal fats, waste cooking oils, and vegetable oils are a few examples of renewables used in the production of biofuel. Whilst biofuel has reduced our dependence on fossil fuel, it comes at an expense for the automotive industry.
Types of biodiesel (EN 14214 standard) like FAME (Fatty Acid Methyl Esters) produced via vegetable oil transesterification are related to an increase in fuel consumption due to its low energy content, increase in the diesel NOx emission, and poor cold operability (high cloud point and pour point) which can lead to engine deterioration.
However, these disadvantages in FAME biodiesel have been overcome by the second-generation biodiesel produced by hydrotreatment of waste cooking oils (i.e., Hydrotreated Vegetable Oils – HVOs).
HVOs (EN 15940 standard) are straight-chain paraffinic hydrocarbons that do not contain aromatics or sulfur. HVOs deliver superior cold operability, high energy content, and low pollution that is attributed to its absence of oxygen, low carbon, and high hydrogen.
Hydrotreated Vegetable Oils Explained
HVOs are a class of renewable diesel that is similar to fossil fuel diesel (EN 590 standard), except for its density.
Despite the fact that the HVOs without oxygen content deliver better cold flow properties required for high performance of the fuel injection system, it shows poor lubricity behavior that can have negative impacts on engine components. The absence of sulfur can also attribute the poor lubricity of HVOs.
If HVOs without sufficient lubricity are introduced to injection equipment, it can result in noncompliance with ASTM D975 or EN 590.
HVOs are therefore blended with anti-wear (AW) additives, allowing compliance with the lubricity factor specified in D975 and EN 590. High-Frequency Reciprocating Rig (HFRR) is used for quality control of blended fossil fuel diesel and HVOs and to screen AW additives compatible with HVOs.
As Figure 1 shows, the HFRR is a benchtop tribometer consisting of a piezo sensor and flexure system for friction measurement and a linear deformation sensor for inline stroke length calibration, localized temperature control system and fluid heating, salt-free humidity control, voice coil actuator used for precision control of amplitude and frequency. This design complies with ISO 12156 and ASTM D6079.
Figure 1. Description of the Ducom High-Frequency Reciprocating Rig (HFRR 4.2) used in this study.
Ducom HFRR is regularly benchmarked against PCS HFRR in the ASTM proficiency testing program (PTP) or round-robin test program. Please contact Ducom for related reports.
Video. Environmental chamber in Ducom HFRR 4.2.
HFRR ball wear scar diameter (MWSD), measured according to fuel lubricity test standards like ASTM D6079 or ISO 12156-1, is used to check whether HVOs comply with D975 or EN 590. In this HFRR study, according to ISO 12156, we have tested ten HVO samples with AW additives and ten HVO samples without AW additives. Pacific Sensor donated specimens of HFRR ball and disks.
As Figure 2 shows, the average Ball MWSD for the HVOs with AW additives was 399 μm (N = 10, precision at 26 μm) and for HVOs without AW additives was 618 μm ( N = 10, precision at 33 μm). It should be noted that the ASTM relevant statistics described in this study were all automatically generated using MOOHA.
Figure 2. Graph representing the ball Mean Wear Scar Diameter (MWSD) for HVOs with and without AW additives. Ducom HFRR enabled with MOOHA was used for lubricity test according to ISO 12156. Total 20 tests were conducted, ten each for HVOs with or without AW additives. All the test data were automatically logged in a digital data log book provided by MOOHA.
When sold in the EU, fossil fuel diesel is required to comply with the MWSD maximum limit of 460 μm, according to EN 590. In this study, the AW additives used improved the lubricity of HVOs and have enabled HVO compliance with EN 590.
Figure 3 shows an interesting observation of the association between wear and friction for HVOs. For HVOs with AW additives, friction and wear have a linear relationship, but there is no such relationship for HVOs without AW additives.
Figure 3. Graph representing the relationship between ball Mean Wear Scar Diameter (MWSD) and friction coefficient for HVOs with and without AW additives. Ducom HFRR enabled with MOOHA was used for lubricity test according to ISO 12156. Total 20 tests were conducted, ten each for HVOs with or without AW additives. All the test data were automatically logged in a digital data log book provided by MOOHA.
This supports that AW additives used to lower the wear can also lower the friction. Overall power consumption will decrease from low friction in a fuel pump. Unexpectedly, the friction coefficient for some HVOs without AW is above 0.7, which represents the seizure of any lubricated system.
In conclusion, the AW additives compensate for poor lubricity by HVOs as a result of the absence of sulfur and oxygen. HVOs with AW additives are in compliance with EN 590 MWSD limit, as established using Ducom HFRR and MOOHA. Therefore, HVOs with AW additives in comparison to FAME biodiesel is more suitable as a drop-in fuel.
This information has been sourced, reviewed, and adapted from materials provided by Ducom.
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