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Article

Impact of High-Concentration Biofuels on Cylinder Lubricating Oil Performance in Low-Speed Two-Stroke Marine Diesel Engines

by
Enrui Zhao
1,2,
Guichen Zhang
1,*,
Qiuyu Li
1 and
Saihao Zhu
1
1
Merchant Marine College, Shanghai Maritime University, Shanghai 201306, China
2
School of Navigation and Shipping, Shandong Jiaotong University, Weihai 264209, China
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2025, 13(6), 1189; https://doi.org/10.3390/jmse13061189
Submission received: 23 May 2025 / Revised: 14 June 2025 / Accepted: 16 June 2025 / Published: 18 June 2025
(This article belongs to the Section Ocean Engineering)
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Abstract

With the implementation of the ISO 8217-2024 marine fuel standard, the use of high-concentration biofuels in ships has become viable. However, relatively few studies have been conducted on the effects of biofuels on cylinder lubrication performance in low-speed, two-stroke marine diesel engines. In this study, catering waste oil was blended with 180 cSt low-sulfur fuel oil (LSFO) to prepare biofuels with volume fractions of 24% (B24) and 50% (B50). These biofuels were evaluated in a MAN marine diesel engine under load conditions of 25%, 50%, 75%, and 90%. The experimental results showed that, at the same engine load, the use of B50 biofuel led to lower kinematic viscosity and oxidation degree of the cylinder residual oil, but higher total base number (TBN), nitration level, PQ index, and concentrations of wear elements (Fe, Cu, Cr, Mo). These results indicate that the wear of the cylinder liner–piston ring interface was more severe when using B50 biofuel than when using B24 biofuel. For the same type of fuel, as the engine load increased, the kinematic viscosity and TBN of the residual oil decreased, while the PQ index and the concentrations of Fe, Cu, Cr, and Mo increased, reflecting the aggravated wear severity. Ferrographic analysis further revealed that ferromagnetic wear particles in the oil mainly consisted of normal wear debris. When using B50 biodiesel, a small amount of fatigue wear particles were detected. These findings offer crucial insights for optimizing biofuel utilization and improving cylinder lubrication systems in marine engines.

1. Introduction

As a primary mode of global trade, the shipping industry undertakes approximately 80% of the world’s trade volume, and pollution emissions from ships cannot be ignored. To reduce greenhouse gas (GHG) emissions from ships, the International Maritime Organization (IMO) revised and adopted the 2023 IMO Strategy on Reduction of GHG Emissions from Ships. This strategy mandates that international shipping achieve at least a 40% reduction in CO2 emissions by 2030 relative to the 2008 levels and attain net-zero GHG emissions by approximately 2050 [1,2]. To comply with these emission reduction targets, the shipping industry has predominantly implemented strategies encompassing alternative marine fuels/energy sources, energy efficiency improvements (including both technical and operational measures), and carbon capture, utilization, and storage (CCUS). Current viable alternative fuels and energy solutions for ships include biodiesel [3,4,5,6], liquefied natural gas (LNG) [7], methanol [8], ammonia [9,10,11], hydrogen [12], and solar energy [13]. To achieve these emission reduction targets, the IMO has introduced the energy efficiency design index (EEDI), energy efficiency existing ship index (EEXI), and carbon intensity indicator (CII) through the MARPOL convention, directly limiting the carbon emission intensity of ships [14,15]. Meanwhile, to mitigate sulfur oxide (SOx) and particulate matter (PM) emissions, the MARPOL convention specifies that since 2020, the sulfur content limit for fuels used by ships outside sulfur emission control areas (ECAs) is 0.50% (m/m), while within ECAs, the limit for ship fuel sulfur content is 0.10% (m/m) [16]. The European Union has included the shipping industry in its Emissions Trading System (EU ETS) since 2024, requiring all ships of 5000 gross tons or above entering and leaving EU ports to calculate carbon emissions based on voyage distances and procure quotas. Additionally, the FuelEU Maritime regulation mandates that the carbon intensity of ship fuels be reduced by 2% relative to 2020 by 2025 and by 80% by 2050 [17,18].
Biodiesel is predominantly produced from vegetable oils, animal fats, waste cooking oils, and other biological feedstocks via transesterification with methanol or ethanol to form fatty acid methyl esters (FAMEs) or through hydrodeoxygenation and hydroisomerization processes to generate hydrocarbon-based fuels (HVOs) [19,20,21]. Characterized by a low calorific value, low carbon-to-hydrogen ratio, and negligible sulfur content, biodiesel emerges as an ideal alternative marine fuel due to its abundant availability, renewability, and minimal environmental impact. Biodiesel performs similarly to conventional petroleum-based fuels and can be used directly in existing marine diesel engines without any modifications to the latter’s structure or fuel systems [22]. Additionally, biodiesel can be blended with conventional marine fuels at arbitrary ratios to produce biofuel blends of varying specifications. For instance, B24 biofuel denotes a 24% (v/v) FAME blend with 76% marine fuel oil. With the promulgation of ISO 8217:2024 (marine fuels), the previous 7% blending restriction for biodiesel in marine fuels has been eliminated, allowing both distillate and residual marine fuels to contain up to 100% FAME [23]. Consequently, investigating the impacts of high-concentration biodiesel, a promising alternative marine fuel, on the performance of large low-speed marine diesel engines holds significant scientific and engineering importance.
Large low-speed two-stroke diesel engines are widely employed in various large marine vessels owing to their high power output, superior fuel economy, and reliable operation. To mitigate SOx emissions and carbon intensity in marine applications, these engines can utilize residual or distillate marine fuels blended with biodiesel at varying concentrations [24,25]. Current domestic and international research on the impact of biodiesel blends on large marine diesel engines mainly focuses on combustion characteristics and emission performance. Studies have shown that biodiesel can significantly reduce the emissions of hydrocarbons (HCs), particulate matter (PM), carbon monoxide (CO), and SOx [26,27,28,29]. For instance, Nghia et al. [30] employed AVL-BOOST simulation software to investigate the impacts of varying biodiesel blend ratios on engine combustion and emissions. They found that increasing biodiesel content reduce CO and soot emissions but elevates specific fuel consumption and NOx emissions under the same load conditions. Wu et al. [22] utilized AVL FIRE software to simulate the combustion process in a low-speed two-stroke marine diesel engine fueled with pure diesel and three biodiesel blends (B5, B10, and B15). The results confirmed the good compatibility between marine diesel engines and biodiesel fuels, showing that exhaust gas recirculation (EGR) technology can simultaneously reduce NOx and soot emissions. Wei et al. [31] conducted comparative experiments on a low-speed two-stroke marine diesel engine using diesel, B50, and B100 biodiesel blends. Their findings revealed that increasing biodiesel content significantly decreases CO2, CO, and HC emissions while gradually increasing specific fuel consumption and NOx emissions at identical loads. In another study, Wei et al. [32] analyzed the effects of B10, B30, and B50 biodiesel blends on combustion and emission characteristics in low-speed two-stroke marine engines. They reported that biodiesel notably reduces exhaust gas temperature and black carbon emissions compared to conventional heavy fuel oil. Sagin et al. [3] evaluated the performance impacts of B10 and B30 biodiesel blends in a MAN B&W 5S60ME-C8 marine main engine (MAN Energy Solutions, Augsburg, Germany), demonstrating that biodiesel reduces NOx emissions by 14.71–25.13% but increases fuel consumption by 1.55–6.01%.
The cylinder liner–piston ring system represents a critical combustion chamber component in diesel engines, and maintaining optimal cylinder lubrication is essential for mitigating abnormal liner wear and enhancing engine performance. Current research on the effects of biofuels on the cylinder lubrication properties has predominantly focused on small-scale four-stroke diesel engines. For example, Dhar et al. [33] investigated the impacts of Karanja biodiesel on engine lubricant characteristics and found that after 200 h of biodiesel operation, the lubricant exhibited increased density, elevated carbon residue, and higher ash content, indicating severe lubricant degradation. Similarly, Gopal et al. [34] conducted a 256 h endurance test on a single-cylinder, four-stroke diesel engine and observed that PME20 biodiesel caused increases in lubricant density, ash content, water content, and insolubles, while reducing kinematic viscosity, flash point, and TBN. Temizer et al. [35] examined the influence of B10 rapeseed biodiesel and diesel on engine lubrication performance, revealing that the combustion of B10 biodiesel resulted in decreased total acid number (TAN), water content, viscosity, and flash point of the lubricant, along with increased TBN, density, and sulfated ash content. Additionally, biodiesel operation was associated with higher concentrations of metallic wear particles in the lubricating oil. However, these studies were predominantly conducted on land-based small-scale four-stroke engines, which are characterized by relatively low power output, stable operating conditions, and splash lubrication of the cylinder liner–piston ring system, allowing for a prolonged reuse of the cylinder lubricating oil.
Marine low-speed two-stroke diesel engines, characterized by high power output, variable load conditions, and harsh combustion environments when using inferior fuels, employ independent cylinder oil lubrication systems to ensure the optimal lubrication of the liner–piston ring assembly during operation with low-quality fuels [36,37]. The cylinder lubrication oil is injected onto the liner and piston ring surfaces via electronic or mechanical lubricators during the piston’s upward stroke, performing critical functions such as lubrication, friction reduction, neutralization of combustion byproducts, and heat transfer—all of which are vital for proper engine operation. After injection, part of the cylinder oil is consumed in the combustion chamber, while the remainder is discharged through the scavenge air box as drain oil; this is a fundamentally different operational mechanism from that in land-based small diesel engines. With the implementation of ISO 8217:2024 (marine fuels), marine engines are increasingly adopting higher concentration biofuel blends. These biofuels exhibit significant discrepancies from conventional low-sulfur fuels in key parameters such as viscosity, base number, and sulfur content—factors that critically influence the cylinder lubrication performance. Therefore, investigating the effects of biofuels on marine cylinder lubricant properties, analyzing cylinder oil degradation mechanisms, and studying liner–piston ring wear patterns are of utmost importance. Such research would enable the optimization of cylinder oil feed rates and base numbers, ultimately minimizing liner wear failures in marine engines.
This study systematically investigates the effects of B24 and B50 biodiesel blends on the operational performance of a marine low-speed two-stroke diesel engine (MAN 6S35MEB type) under variable load conditions (25%, 50%, 75%, and 90% loads). Through comprehensive analyses of the cylinder lubricating oil properties—including physicochemical parameters (e.g., kinematic viscosity, TBN, oxidation/nitration degrees), wear-related metallic element concentrations (Fe, Cu, Cr, Mo), and ferrographic morphology, this research evaluates how biodiesel concentration and engine load jointly influence lubricant degradation and cylinder liner–piston ring wear mechanisms.

2. Materials and Methods

2.1. Experimental Equipment

This study utilized the MAN 6S35MEB two-stroke low-speed diesel engine (MAN Energy Solutions, Augsburg, Germany) in the Integrated Laboratory for Marine Power Plants at Shanghai Maritime University to investigate the effects of biofuels on cylinder lubricating oil performance. The engine features six cylinder liners, a rated speed of 152 rpm, and a rated output power of 3570 kW. The output load is regulated by a hydraulic dynamometer coupled to the engine’s power take-off end. Table 1 summarizes the main technical parameters of the diesel engine, while Figure 1 depicts the schematic layout of the test platform.

2.2. Experimental Materials

The biofuel utilized in this experiment was prepared by blending 180-grade low-sulfur fuel oil (LSFO, sulfur content: 0.47%) with waste cooking oil-derived biodiesel. Specifically, B24 biofuel consisted of a 24% (v/v) waste cooking oil blend with 76% LSFO. Table 2 summarizes the physicochemical properties of 180 LSFO and the biodiesel component. Notably, parameters such as density, kinematic viscosity, water content, ash content, and sulfur content decreased with an increasing biodiesel blend ratio. Additionally, the biofuel exhibited higher oxygen content, lower carbon-to-hydrogen ratio, and reduced calorific value compared to pure LSFO. At 40 °C, the viscosity of 180-grade LSFO is 168.6 mm2/s, while that of B100 biodiesel is 4.453 mm2/s, which is far lower than that of LSFO. With the increase in the mixing ratio, the viscosity of biodiesel gradually decreased. For example, the viscosities of B24 and B50 biodiesels are 47.40 and 13.51 mm2/s, respectively. The lubricating oil employed was Sinopec 5040 marine cylinder oil, classified as a Category II (Cat. II) cylinder lubricating oil, which is recommended for MAN B&W two-stroke engines of Mark 9 and higher specifications [38].

2.3. Experimental Methods

To assess the effects of B24 and B50 biofuels on the cylinder lubricating oil characteristics, experiments were conducted at four engine load levels (25%, 50%, 75%, and 90%), with each load test duration set to 10 h. Key thermodynamic parameters—including engine speed, scavenging air pressure, and combustion pressure—were continuously monitored via the MAN B&W-developed PMI system. Table 3 and Table 4 summarize the main engine operating parameters under B24 and B50 biofuels at different loads, while Figure 2 depicts the cylinder pressure profiles across these load conditions. It can be observed that at identical loads, the marine main engine’s rotational speed, scavenging air pressure, maximum compressed pressure, and output power showed minimal variation when using B24 and B50 biofuels. Notably, the main engine demonstrated a slightly higher overall output power with B24 biofuel, which may be attributed to the higher calorific value of B24 compared to B50.
During testing, residual oil samples were collected from each cylinder’s scavenge box under varying loads. Systematic analyses were performed on the physicochemical properties, wear element concentrations, and ferrographic morphology of the cylinder residual oil. Physicochemical property measurements included kinematic viscosity, TBN, water content, oxidation degree, and nitration degree. Wear characterization involved PQ index analysis and spectral emission spectroscopy for metallic element quantification.

2.4. Experimental Equipment

In this paper, a SYD-265 (Shanghai Changji Geological Instrument Co., Ltd., Shanghai, China) countercurrent capillary viscometer was used to detect the kinematic viscosity of the cylinder residual oil. A titration method was employed to determine the base number of the residual oil. A FluidScan 1000 multi-functional oil analyzer produced by Spectro Scientific, Inc. (Chelmsford, MA, USA) was utilized to detect the moisture, oxidation degree, and nitration degree of the cylinder residual oil. A SpectrOil 100 Series elemental analyzer (Spectro Scientific, Inc., Chelmsford, MA, USA) was used to measure the concentration of wear elements in the residual oil. A TTL-3 iron content meter and a YTF-8 analytical ferrograph produced by Yateks Optoelectronic Technology Co., Ltd. (Shenzhen, China) were applied to detect the PQ index and the abrasive types of ferromagnetic particles in the cylinder residual oil.

3. Results and Discussion

3.1. Physical and Chemical Analysis of the Lubricating Oil

3.1.1. Kinematic Viscosity

Kinematic viscosity serves as a critical parameter for evaluating the fluidity and viscous behavior of a lubricating oil, reflecting the magnitude of internal frictional forces during oil flow at a specified temperature. An optimal viscosity ensures the formation of a stable and continuous oil film on friction pair surfaces, thereby minimizing wear and friction. Deviations from the ideal viscosity—either excessively high or low—compromise oil film integrity and stability, ultimately reducing the lubrication efficiency.
In this study, a capillary viscometer was employed to measure the kinematic viscosity of the cylinder residual oil at 40 °C and 100 °C. To mitigate the influence of carbon deposits and wear particles on the viscosity measurements, the cylinder residual oil samples were filtered through a 0.45 µm organic membrane filter prior to testing. The viscosity test results for the cylinder residual oil are presented in Figure 3. Initially, the kinematic viscosity of the cylinder oil at 40 °C and 100 °C was 218.01 mm2/s and 21.83 mm2/s, respectively. The 100 °C kinematic viscosity ranged from 18.63 mm2/s to 21.83 mm2/s, conforming to the MAN diesel engine specifications for Category II cylinder lubricating oil (minimum viscosity: 18.5 mm2/s; maximum viscosity: 21.9 mm2/s) [49].
As engine load increased, the viscosity of the cylinder lubricating oil gradually decreased. At identical loads, the cylinder residual oil viscosity was higher for B24 biofuel than for B50 biofuel. This phenomenon may be attributed to increased fuel consumption at higher loads, which introduced more moisture from combustion byproducts and unburned biofuel into the cylinder oil, enhancing its dilution and reducing residual oil viscosity [50,51]. Additionally, the lower inherent viscosity of B50 biofuel (13.51 mm2/s) compared to B24 biofuel exacerbated the dilution effect on the cylinder oil, leading to further viscosity reduction under the same load conditions.

3.1.2. Total Base Number

The TBN of a cylinder oil is critical for neutralizing acidic species generated during diesel engine combustion, thereby reducing the risk of acid corrosion in the engine. The residual base number (RBN) is influenced by multiple factors, including the initial BN of the fresh oil, cylinder oil feed rate, fuel sulfur content, engine load, and engine modifications. Engine designers typically define residual BN thresholds to ensure a sufficient alkaline reserve for sulfur corrosion protection. However, prolonged operation under high RBN conditions may exacerbate ash deposition and bore polishing. According to the MAN service bulletins, when the main engine operates on low-sulfur fuel oil, the TBN of the cylinder residual oil should be no less than 80% of the initial TBN under normal operating conditions [38].
Figure 4 shows the TBN measurements of the cylinder residual oil. The data revealed that the TBN decreased with an increasing engine load, which can be attributed to the higher concentration of acidic species in the combustion products and enhanced dilution from unburned fuel. When using B24 biodiesel, the minimum TBN of the cylinder residual oil was 32.80 mg KOH/g, corresponding to 82.16% of the initial TBN. This value remained above 80% of the initial TBN, meeting MAN’s specifications for cylinder oil in main engines. Notably, at identical loads, the cylinder residual oil exhibited a higher TBN when using B50 biofuel. This phenomenon was likely due to B50 biofuel’s lower sulfur content, which reduced the formation of acidic combustion byproducts.
It should be noted that the base number (BN) of a cylinder residual oil theoretically represents the BN of the fresh oil minus the BN consumed in neutralizing acidic combustion products. However, since the residual oil contains incompletely burned biofuel with inherent acidity, the experimentally detected BN was slightly lower than the actual value. Considering the differences in sulfur content and acid value between B24 and B50 biofuels, as well as the variation in unburned biofuel content in the residual oil under different loads, we did not perform corrections on the actual BN of the cylinder residual oil.

3.1.3. Water Content

Water content is a critical factor affecting the integrity of cylinder lubrication oil films, as it can exacerbate cylinder wear. This parameter is influenced by ambient air humidity and the moisture generated during combustion. Elevated water levels may indicate operational issues such as malfunctioning water mist catchers, leaking charge air coolers, or steam valve leaks from scavenge fire extinguisher systems into the under-piston space.
Figure 5 illustrates the water concentration in the residual cylinder lubricating oil. The results showed that the water content in the cylinder oil increased with the engine load, while biofuel concentration had a negligible effect on the cylinder residual oil water content. This phenomenon can be primarily attributed to the fact that, with an increasing load, the diesel engine consumed more oxygen for combustion. Consequently, moisture from the compressed air entered the cylinder oil through the scavenging air box. Moreover, higher loads resulted in increased water production during combustion. Both mechanisms contributed to the rise in water concentration in the cylinder residual oil.

3.1.4. Oxidation

When a lubricating oil reacts with oxygen in the air, a large number of oxidation products are generated, such as aldehydes, ketones, esters, and carboxylic acids containing carboxyl functional groups. These substances may cause an increase in the viscosity of the lubricating oil, an increase in its acid value, and the formation of sludge, all of which lead to a decline in the performance of the lubricating oil. Fourier transform infrared spectroscopy (FT-IR) is a commonly used method for measuring the degree of oxidation in a cylinder lubricating oil by detecting the characteristic absorption of carbonyl groups in the range of 1800 cm−1 to 1670 cm−1.
Figure 6 illustrates the trend of the oxidation degree of the cylinder residual oil. It is evident that as the load increased, the oxidation degree gradually rose. Additionally, for the same load, the oxidation degree was higher when using B24 biofuel. The main reason for this is that as the load increased, the combustion chamber temperature rose, leading to a greater degree of oxidation of the lubricating oil. Furthermore, under the same load, B24 biofuel had a higher calorific value, resulting in more heat being generated during combustion, which further promoted the oxidation of the lubricating oil.

3.1.5. Nitration

Nitration, similar to oxidation, occurs when a lubricating oil reacts with gaseous nitrates during the combustion process in engines. This reaction produces various nitration products, which can lead to increased viscosity, a higher acid value, and the formation of insoluble compounds within the lubricating oil. Additionally, an increase in the degree of nitration contributes to a decline in the TBN of the engine oil. The nitration degree of a lubricating oil can be measured using FT-IR, specifically by analyzing the absorption around 1630 cm−1.
Figure 7 illustrates the trend of the nitration degree of the cylinder residual oil. It can be seen that as the load increased, the nitration degree also rose gradually. Under the same load, the use of B50 biofuel resulted in a higher nitration degree. The main reason is likely that as the load increased, the combustion chamber temperature rose, leading to a greater degree of nitration of the lubricating oil. Additionally, under the same load, B50 biofuel produced higher levels of NOx [30], and NOx promoted the nitration reaction of the lubricating oil.

3.2. Wear Element Analysis of the Lubricating Oil

3.2.1. PQ Index

The PQ index is a dimensionless quantitative measurement that indicates the concentration of ferromagnetic particles present in an oil. It operates based on the principle of electromagnetic induction. When ferromagnetic wear particles pass through a sensor coil during PQ index detection, they cause a change in the magnetic field of the coil. The extent of this change is directly related to the quantity and size of the iron particles in the sample. Measuring the PQ index does not require any pre-treatment of the oil sample and is particularly sensitive to wear particles that are larger than 5 μm.
Figure 8 presents the PQ index measurements of the cylinder residual oil. The data show that the PQ index for each cylinder increased with the rising engine load. At identical loads, the PQ index of the cylinder residual oil with B50 biodiesel was higher than that with B24 biodiesel, indicating more severe cylinder wear. This phenomenon can be attributed to two mechanisms: first, B50 biodiesel’s lower viscosity reduced the thickness of the cylinder lubricating film under the same load, deteriorating the lubrication conditions; second, the higher fuel sulfur content promoted the formation of lubricating combustion byproducts that mitigated the friction between the cylinder wall and the piston rings [52]. The lower sulfur content in B50 biodiesel weakened this self-lubricating effect, increasing the frictional resistance and exacerbating the wear of the cylinder liner–piston ring interface. Additionally, Figure 9 reveals that under identical test conditions, the PQ index value of the cylinder residual oil in Cylinder 2 was relatively low, while those of the residual oil in Cylinders 5 and 6 were higher. This discrepancy may correlate with variations in the in-cylinder average pressure during combustion: higher pressures enhanced the frictional forces between the cylinder liner and the piston rings, leading to more pronounced wear.

3.2.2. Spectral Analysis

In this study, theoil spectrometer (Spectro Scientific, Inc., Chelmsford, MA, USA) was utilized to analyze the composition and concentration of wear elements in the cylinder residual oil. This instrument employs atomic emission spectroscopy (AES) with a rotating disc electrode to generate an arc discharge that excites the atoms of wear elements in the lubricating oil to emit characteristic spectra [53]. This method is capable of detecting ionic elements and offers high precision for particles smaller than 5 μm. The spectrometer is known for its rapid detection capabilities, the simultaneous analysis of multiple elements, and detection accuracy. To enhance the equipment’s detection accuracy, the Q100 spectral analyzer was calibrated prior to measuring the wear element concentrations in the cylinder residual oil. Calibration was performed using 0 ppm, 100 ppm, and 900 ppm standard samples, in strict accordance with the operational specifications detailed in the manual. This process ensured that the detection precision for elements including Fe, Cu, Cr, and Mo complied with the standards defined in ASTM D6595 [54]. To improve the representativeness of the residual cylinder oil samples, ultrasonic vibration was employed during testing to thoroughly homogenize the oil, and multiple replicate trials were conducted to minimize sampling errors.
Figure 10 illustrates the correlation between the concentrations of four typical wear particle elements in cylinder residual oil, engine load, and biofuel. The data show that each element’s concentration increased with a rising load, and B50 biofuel consistently resulted in higher wear particle concentrations under identical conditions. This trend aligns with the PQ index findings. In this experiment, the maximum Fe concentration reached 142.76 ppm, which is below the 200 ppm threshold and complies with MAN’s wear iron concentration specifications for cylinder residual oil in main engines [32]. Iron primarily originates from the cylinder liner, uncoated piston rings, and the substrate of coated piston rings. The highest Cu concentration measured was 7.89 ppm, predominantly derived from stuffing box bronze components and piston wear rings. Chromium reached a maximum of 4.26 ppm, originating from piston ring grooves and piston rings. Finally, molybdenum peaked at 7.28 ppm, mainly sourced from the cermet coating on the piston rings.

3.3. Ferrography Analysis of the Lubricating Oil

Ferrography is a diagnostic technique that uses a high-gradient magnetic field to separate wear particles from a lubricating oil [55,56]. Through microscopic examination of the size, quantity, and morphological features of these particles, it enables the assessment of mechanical equipment’s wear condition and wear mechanisms, thus facilitating the maintenance of equipment safety.
In this experiment, the quantity and morphological characteristics of ferrographic wear particles in the cylinder residual oil after using B24 and B50 biodiesel at 90% load were compared. To obtain clear particle images, the tetrachloroethylene solvent was used to dilute the cylinder residual oil, reducing the oil sample’s viscosity and promoting magnetic metal particle deposition. The oil sample-to-tetrachloroethylene volume ratio during dilution was 3:1. The optical microscopy images of the ferrograms at 100× magnification are shown in Figure 11a,b, revealing that combustion-generated black carbon deposits coexisted with metal wear particles, with the deposited particles distributed in strip-like patterns. Figure 11c,d show 500× magnification results, indicating higher metal wear particle deposition concentrations for B50 biodiesel, with significant particle overlap.
To mitigate particle overlap and improve imaging clarity, the cylinder residual oil was first diluted 10-fold with fresh cylinder oil of the same type, homogenized via ultrasonic vibration, and then further diluted with tetrachloroethylene. The corresponding ferrograms are presented in Figure 11e,f. Most metal wear particles in the cylinder residual oil under both fuel conditions were normal wear particles with a size < 5 μm. Notably, B50 biodiesel use introduced a small fraction of fatigue wear particles (>15 μm), though no abnormal wear particles (e.g., cutting or severe sliding particles) were detected. These results indicate that the diesel engine cylinder liners exhibited acceptable lubrication conditions with both biodiesels in this study, while cylinder wear severity increased with biodiesel concentration. Figure 12 presents the wear morphologies of the No. 1 piston and cylinder liner of the main engine. The images show that after the test, the piston ring grooves remained clean, the piston ring surfaces exhibited no cracks or abnormal wear, and the cylinder liner surface was smooth without abnormal scratches, indicating that both the piston rings and the cylinder liner were in a state of normal wear. However, a comparison between diagrams (a) and (b) revealed that the piston head using B50 biofuel had more pronounced black carbon deposits, which contributed to increased cylinder wear under identical conditions.

4. Conclusions

This paper systematically investigated the effects of B24 and B50 biodiesels on cylinder lubrication performance in large low-speed two-stroke marine diesel engines. By imposing four engine load conditions (25%, 50%, 75%, and 90%), the study compared how biodiesel concentration influenced the physicochemical properties, wear element concentrations, and wear particle morphological characteristics of the residual cylinder oil. The key research findings are summarized as follows:
(1)
Physicochemical properties: With an increasing engine load, the viscosity and TBN of the cylinder residual oil gradually decreased, while water content, oxidation degree, and nitration degree increased. Under identical conditions, B50 biofuel induced a more pronounced viscosity reduction for the residual cylinder oil, whereas B24 biodiesel led to a more significant TBN decline. Notably, both viscosity and TBN of the cylinder residual oil remained within the MAN diesel engine technical specifications.
(2)
Wear elements concentrations: As the diesel engine load increased, the PQ index and concentrations of wear elements (Fe, Cu, Cr, and Mo) in the cylinder residual oil rose progressively. At the same load, B50 biodiesel showed exacerbated wear severity of the cylinder liner–piston ring interface compared to B24.
(3)
Wear particle analysis: The cylinder residual oil contained a significant amount of ferromagnetic wear particles and black carbon deposits from combustion products. The size of these wear particles was mainly that of normal wear particles with a diameter of less than 5 μm. When using B50 biodiesel, the cylinder residual oil also contained a small amount of fatigue wear particles with a particle size greater than 15 μm.
In summary, this study investigated the effects of high-concentration biofuels on the lubrication performance of marine low-speed diesel engine cylinder oil under varying loads and analyzed the primary wear mechanisms in the cylinder lubrication system. In this manuscript, we prepared two types of biofuels, B24 and B50, using waste cooking oil and low-sulfur fuel oil (LSFO). We tested the impact of these biofuels on cylinder lubricating oil performance at four different load levels on a main engine test bench. In future work, our research will focus on onboard inline blending methods of biofuels with heavy fuel oil for marine engines to prepare biofuels of different concentrations. We will also investigate the effects of using biofuels of different concentrations on the main engine cylinder liner–piston ring wear, as well as adjustment methods for cylinder oil injection rate and base number, through the operation of vessels. We aim to provide more comprehensive guidance for the shipping industry.

Author Contributions

E.Z.: writing the manuscript, carrying out experiments, data analyzing; G.Z.: conceptualization, methodology, supervision. Q.L. and S.Z.: data processing, editing figures. All authors reviewed the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (NSFC51779136).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors gratefully acknowledge the helpful comments and suggestions of the reviewers, which have improved the presentation of this paper.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The test platform of the main engine.
Figure 1. The test platform of the main engine.
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Figure 2. Cylinder pressure at different engine loads. (a) B24, (b) B50.
Figure 2. Cylinder pressure at different engine loads. (a) B24, (b) B50.
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Figure 3. Kinematic viscosity of cylinder residual oil. (a) 40 °C, (b) 100 °C.
Figure 3. Kinematic viscosity of cylinder residual oil. (a) 40 °C, (b) 100 °C.
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Figure 4. Residual base number of cylinder residual oil.
Figure 4. Residual base number of cylinder residual oil.
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Figure 5. Water content of cylinder residual oil.
Figure 5. Water content of cylinder residual oil.
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Figure 6. Oxidation of cylinder residual oil.
Figure 6. Oxidation of cylinder residual oil.
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Figure 7. Nitration of cylinder residual oil.
Figure 7. Nitration of cylinder residual oil.
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Figure 8. PQ index of cylinder residual oil. (a) B24, (b) B50.
Figure 8. PQ index of cylinder residual oil. (a) B24, (b) B50.
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Figure 9. Mean pressure of the cylinder under (a) 25% load, (b) 50% load, (c) 75% load, (d) 90% load.
Figure 9. Mean pressure of the cylinder under (a) 25% load, (b) 50% load, (c) 75% load, (d) 90% load.
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Figure 10. Wear elements of cylinder residual oil. (a) Fe, (b) Cu, (c) Cr, (d) Mo.
Figure 10. Wear elements of cylinder residual oil. (a) Fe, (b) Cu, (c) Cr, (d) Mo.
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Figure 11. Ferrography analysis of cylinder residual oil.
Figure 11. Ferrography analysis of cylinder residual oil.
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Figure 12. The wear morphology of the No. 1 piston and cylinder liner.
Figure 12. The wear morphology of the No. 1 piston and cylinder liner.
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Table 1. Technical specification of the main engine.
Table 1. Technical specification of the main engine.
NO.ProjectParameter
1Engine typeMAN 6S35MEB
2Engine stroke2-stroke
3Cylinder bore (mm)350
4Engine speed (r/min)142
5Engine power (kW)3570
6Piston stroke (mm)1500
7Torque (kN)240
8Firing order1-5-3-4-2-6
Table 2. Physical and chemical properties of biofuels.
Table 2. Physical and chemical properties of biofuels.
Property180LSFOB24B50B100Methods
Density (kg/m3) @15 °C958.0936.2911.2895.7ISO 12185 [39]
Kinematic viscosity (mm2/s) @40 °C168.647.4013.514.453ISO 3104 [40]
Flash point (°C)108131146170ISO 2719 [41]
Pour point (°C)161496ISO 3016 [42]
Acid number (mg KOH/g)1.581.370.520.39ASTM D664 [43]
Ash (%, m/m)0.040.0290.0080.011ISO 6245 [44]
Water content (%, v/v)0.210.120.060.04ISO 3733 [45]
Net heat of combustion (MJ/kg)40.8740.0039.5938.36ASTM D240 [46]
Carbon (%, m/m)86.584.682.077.34ASTM D6728 [47]
Hydrogen (%, m/m)11.111.011.211.36ASTM D6728
Nitrogen (%, m/m)0.960.710.510.41ASTM D6728
Oxygen (%, m/m)0.83.306.110.89ASTM D6728
Sulphur (%, m/m)0.470.3780.2530ISO 8754 [48]
Table 3. Operating parameters of the main engine using B24 biofuel.
Table 3. Operating parameters of the main engine using B24 biofuel.
Property25% Load50% Load75% Load90% Load
Engine speed (rpm)89.1112.7129.5138.4
Scavenging air pressure (bar)0.360.951.491.84
Max. compressed air pressure (bar)73.1112.7150.0166.1
Output power (kW)908180126203202
Table 4. Operating parameters of the main engine using B50 biofuel.
Table 4. Operating parameters of the main engine using B50 biofuel.
Property25% Load50% Load75% Load90% Load
Engine speed (rpm)89.7112.1129.9138.1
Scavenging air pressure (bar)0.320.891.351.85
Max. compressed air pressure (bar)73.3110.6147.1166.4
Output power (kW)883176925463272
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MDPI and ACS Style

Zhao, E.; Zhang, G.; Li, Q.; Zhu, S. Impact of High-Concentration Biofuels on Cylinder Lubricating Oil Performance in Low-Speed Two-Stroke Marine Diesel Engines. J. Mar. Sci. Eng. 2025, 13, 1189. https://doi.org/10.3390/jmse13061189

AMA Style

Zhao E, Zhang G, Li Q, Zhu S. Impact of High-Concentration Biofuels on Cylinder Lubricating Oil Performance in Low-Speed Two-Stroke Marine Diesel Engines. Journal of Marine Science and Engineering. 2025; 13(6):1189. https://doi.org/10.3390/jmse13061189

Chicago/Turabian Style

Zhao, Enrui, Guichen Zhang, Qiuyu Li, and Saihao Zhu. 2025. "Impact of High-Concentration Biofuels on Cylinder Lubricating Oil Performance in Low-Speed Two-Stroke Marine Diesel Engines" Journal of Marine Science and Engineering 13, no. 6: 1189. https://doi.org/10.3390/jmse13061189

APA Style

Zhao, E., Zhang, G., Li, Q., & Zhu, S. (2025). Impact of High-Concentration Biofuels on Cylinder Lubricating Oil Performance in Low-Speed Two-Stroke Marine Diesel Engines. Journal of Marine Science and Engineering, 13(6), 1189. https://doi.org/10.3390/jmse13061189

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