Investigation Lubricity Performance of Lubricating Oil Used in Marine Diesel Engine—Fuel Injection Pump

Abstract

Diesel engines commonly suffer from oil contamination by fuel and other chemicals during operation and maintenance. This contamination alters the oil’s lubricating properties, leading to increased wear, corrosion, and other potential problems. Therefore, it is crucial to understand how oil degrades and becomes contaminated due to different replacement strategies is crucial for both engine operators and manufacturers. This study focuses on the impact of fuel dilution on specific properties of engine oil under real-world operating conditions in a marine diesel engine. Oil samples were collected regularly from the crankcase of the engine fuel injection pump and tribological tests were performed. These tests aimed to assess how marine gas oil affects the oil lubricity performance and how it maintains its useful life. The results confirm that diesel dilution primarily affects the oil lubricating abilities as well as overall performance.

1. Introduction

Energy conservation and environmental responsibility are becoming increasingly important across industries. Marine transportation, a significant contributor to global fuel consumption, is actively seeking solutions to reduce fuel usage and emissions. One approach focuses on improving engine performance to minimize fuel consumption and carbon footprints. Studies have shown that wear and friction between engine components are the main culprits behind energy loss [1]. One of the key strategies to enhance the tribological behavior of marine engines involves using high-quality fuels and lubricants.

Proper lubrication is essential to performance, durability, and reliability, especially for marine engines where it is not possible to easily call services or deliver necessary spare parts once a vessel or rig platform is at sea. Lubricating oil is a composite substance of hydrocarbon molecules containing numerous atoms of carbon, hydrogen, oxygen, and others. Due to this composition and atomic structure as well as special additives, the oil film is resistant to flow due to its adhesion to wetted surfaces. The viscosity of lubricating oil is an important indicator of its condition due to its rapid changes in this parameter in the event of lubricant failure [2,3]. If the viscosity is low, less energy is needed to shear the oil and cool it but, on the other hand, low viscosity causes the oil film to be thinner and therefore more easily and quickly ruptured, resulting in dry/mixed friction. A good understanding of the viscosity of engine oil is significant because it is one of the most important properties of lubricating oil and also diesel engine conditions. Moreover, another similarly important lubricating oil condition indicator is the content of any impurities that dissolve in oil or mix with it as a result of being flushed out by the oil during its flow through various parts of the engine.

The contaminants that leak into oil during operation constitute a critical factor that reduces the lubricating properties of engine oils. Many different contaminants can be present in oils that cause changes in oil viscosity, possible chemical changes in additives, and changes in oil lubricity. The two most common contaminants in engine oils are fuel and water [4,5]. In turbocharged and direct injection (DI) engines, unburned fuel can be attached to the cylinder walls. This fuel then leaks through the piston rings and contaminates the oil in the crankcase. Research has shown that this problem may be particularly severe in DI engines and the fuel content in the oil may reach up to 9%. As a result, the oil viscosity may drop significantly, even by 30% [6]. The concentration of fuel in oil at the TDC (top dead center) position of the piston ring on the wall of the cylinder liner is much higher than in the oil sump. Using gas chromatography and mass spectrometry, as much as 21.9% of fuel was found in the engine oil in the top dead center (TDC) zone after one hour of engine operation [7].

For example, in truck vehicle diesel engines, incomplete filter regenerating cycles can result in significant amounts of unburnt fuel being mixed with the lubricating oil. This contamination significantly degrades the oil’s physical-chemical and lubricating properties. In marine diesel engines, the fuel dilution in the lubricating oil is mostly caused by malfunctioning fuel injectors. These injectors can malfunction in several ways, such as leaking fuel or failing to properly atomize it. Both scenarios lead to excessive amounts of fuel entering the oil. The dilution of fuel oil in lubricating oil in the trunk piston engines is a normal process, provided that the quantity of fuel does not exceed a few percent. But it is difficult or ambiguous to determine this value due to the different operating conditions of engines (i.e., different loads), especially in marine engines. Moreover, different oils and therefore different properties are used in these engines. There is no consensus among researchers on how fuel dilution affects the lubricity of engine oil. According to some research [8] carried out in specific engine operating conditions where the start–stop cycle protocol was used; fuel-to-oil content reached 6%; however, the lubricating oil presented only a minimal effect on friction during this time and an inconsistent effect on wear when compared to fresh oil. However, in contrast, other studies have found a reduction in the oil load-carrying capacity by increasing the fuel content, starting from the hypothesis that the effectiveness of high-pressure additives in the engine oil has been reduced [4]. Another study conducted on SAE 5W-50 engine oil, where 6% gasoline was added to it and tested under various load conditions, showed that friction and wear increased by 3–16% and 2–10%, respectively [9]. Other studies, in which up to 15% of diesel oil was added to the lubricating oil of a five-cylinder car engine with a capacity of 2700 cm³ [10], showed a significant impact on the viscosity of the engine oil (its significant reduction) but no visible increase in engine wear was observed. This research aimed to specify the impact of fuel oil on two key components of engine oil: the base oil and the antiwear additive. It is well established, for example, that fuel dilution in engine oil can lead to a multitude of detrimental effects on engine performance [11]. These include the occurrence of boundary lubrication, accelerated reduction of lubricating oil pressure, excessive wear of engine components like piston rings, cylinder liners, bearings, and crank-pins, a reduction in engine performance, and as a result of all the above, a reduction in engine lifetime.

Additionally, in the modern maritime industry sector, there are standards for marine fuels that must be met. These standards include the level of sulfur content in fuel, which in the case of many ports and sea areas cannot exceed 0.1%. To achieve such low sulfur content in marine fuel, it has to be removed during the refining process, which has been carried out for many years. But, to achieve such a low sulfur content, components that provide natural lubrication are also to be removed. However, low sulfur content in fuel and consequently in exhaust gases is also beneficial for the atmosphere and the marine natural environment. Nevertheless, diesel engine parts such as the fuel pump and fuel injectors require proper lubrication as they are susceptible to excessive and premature wear. A way to improve the lubricating properties of ultralow sulfur fuels is various additives that improve lubricity and restore lost wear protection. Due to their structure, the particles of these additives create a lubricating film on the surfaces of the working parts. As a result, friction between these surfaces is reduced and the service life of these parts is extended. The use of such additives has become an economical and necessary solution when using fuels with an ultralow sulfur content. However, these additives improve the lubricating properties only; they do not completely eliminate the deterioration of the fuel lubricating properties. In consequence, the diesel engine parts such as the injection pump and the injectors are still deprived of additional lubrication and are exposed to excessive wear.

This study’s aim was to analyze changes in the operational properties of lubricating oil used in the crankcase of the fuel injection pump of a marine engine. This paper focuses on oil lubricity, mainly on what changes are determined by conditions where there is no contact of lubricating oil with exhaust gases or unburned fuel during operation. Oil can be contaminated only by “fresh” fuel entering the oil through leaks in the precision pair (barrel–plunger) in this pump. The possibility of contamination by water is also very limited due to the lack of fuel injection pump cooling and small crankcase volume capacities where condensation could occur. It should also be remembered that the oil may be contaminated by solid particles as a result of frictional wear of pump or engine components. Such contamination will undoubtedly worsen the condition of the plunger–barrel system, and thus cause greater leakage of fuel into the lubricating oil and cause even faster degradation of the oil lubricating properties [12].

The following physical and chemical properties of oil were changed during the research: density, flash point, chemical composition, diluted fuel level in oil, total base number (TBN), viscosity, and acid number as well as the degree of sulphation, oxidation, and nitration, presented in [13,14,15,16]. The course of changes in these properties was presented in another study by the author of [5]. One of the strategies to ensure good properties of lubricating oil is to replace it periodically. In order to correctly determine the time between such replacements so that the operation is optimal in terms of engine operation and replacement costs, the actual physicochemical condition of the oil must be monitored [17], although tribological properties, such as lubricity, also have a huge impact on the lubrication process and wear of engine parts. This paper focuses on the changes in the oil tribological properties such as lubricity and friction coefficient, during the operation of marine engines in order to determine the optimal (minimum) interval between oil changes.

2. Materials and Methods

This research involved a series of tests on Marinol RG1240 oil. This is a trunk piston engine oil (TPEO) that adheres to the standards set by the American Petroleum Institute (API) [18]. This oil is dedicated for use in marine diesel engines powered by MGO (marine gas oil), which is a very low sulfur content light fuel oil. Marinol RG1240 oil is dedicated especially to heavily loaded engines, containing a selected set of additives that improve their properties such as antiwear, anticorrosion, washing, dispersing, and antioxidant properties. Oil for testing was taken after various periods of engine operation. The samples were taken from the crankcase of the fuel injection pump of a V-type marine diesel engine Pielstick PA4 V185 [19], fitted with two turbochargers and two intercoolers, which drive a 1.2 MW AC generator as shown in Figure 1.

The fuel amount was checked on the fuel injection pump by an electronic Woodward governor with a hydraulic actuator. A single, centrally located Bosch-type injection pump supplies light fuel (MGO) to this marine engine. This V-shaped pump with 16 plungers was positioned between the cylinder heads and turbochargers on the engine block. It received its drive power from (= was driven by) a toothed gear connected to the crankshaft. The pump crankcase contained approximately 1.8 L of lubricating oil, which required replacement every 100 operating hours as specified in the manufacturer’s manual [5].

2.1. Oil Samples

Oil samples were poured into 100 mL plastic containers from the pump crankcase with a syringe (20 mL) through the dipstick socket. Figure 2 presents some samples where it is possible to observe a difference in color. More contaminated oil takes on a color similar to pure fuel.

The sampling schedule consisted of taking samples every several hours during engine on load operation according to

Table 1. Oil samples for different times of oil operation in the engine (0–fresh oil).

Sample number	1	2	3	4	5	6	7	8	9	10	11
Intervals [h] for total oil replacement in pump crankcase	0	1	2	4	6	8	10	12	18	24	32

. The engine working hours counter indicated 67 h due to a reset after major maintenance according to the manufacturer’s manual. Due to the fuel injection pump’s crankcase holding a limited amount of oil (1.8 L), oil samples were collected at specified intervals (Table 1). Specifically, samples were taken only after the engine had been started and the oil had been completely replaced.

2.2. Laboratory Equipment Used for Oil Test

An oscillating type density meter DA-640 was used to determine the oil density of the samples. This device allows for the smooth detection of changes in the frequency of resonance vibrations and enables the measurement of the density of the tested liquid in the temperature range from 0 to 93 °C. The device meets ISO 12185 [20] and 15212 standards as well as the ASTM D1250, D1465, and D5002 standards. It is one of the most accurate measuring devices of this type as the obtained result reaches an accuracy of +/− 10 g/cm³. The viscosity of the oil samples was measured using a HAAKE MARS Rheometer [21]. This is a versatile instrument designed for the characterization of various liquids. It offers precise measurements, user-friendly operation, and a wide range of accessories for different materials, including polymers, paints, and food. For this research, two key features of the MARS Rheometer were crucial: the temperature module, which ensures accurate temperature control from −150 °C to 600 °C, and the application-specific measuring cells [5].

For testing the lubricating properties of Marionol RG1240, the T-02U Universal 4-Ball Tribometer was used. It was intended for the determination of extreme pressure and antiwear properties of lubricants as well as the determination of the tendency of lubricants and metal elements to produce surface fatigue failures. The tests made on the tribometer were performed using the standards of ASTM D 4172, ASTM D5183, and IP 300. During the tests, the tribometer consisted of the three bottom balls rotating in a special race (inside ball pot) and pressed against the top ball at the required load as presented in Figure 3a. The top ball was fixed in the ball chuck and rotated at the defined speed. During the tests, the following quantities were measured: friction torque, applied load, lubricant temperature, vibration level of the tribosystem, and time. The motor of the tribometer was automatically stopped when the preset time had elapsed, the preset friction torque was reached, or the preset vibration level was exceeded. The specific operating parameters of the tribometer are presented in [22].

The test was carried out in dynamic conditions under a continuously increasing load. During this test, the friction torque values were recorded until the balls were seized. Seizure was defined as exceeding the limit value of the friction torque. The test conditions were as follows:

• load increase rate: 409 N/s;
• spindle speed: 500 rpm;
• initial load–maximum load: 0–7200 N;
• test time: 18 s;
• lubrication method: immersion.

The mean wear scar diameter of the steel balls was recorded to determine the relative wear preventive properties of the lubricant samples in sliding contact under the determined test conditions.

3. Results and Discussion

3.1. Density and Fuel Content in Oil

The oil samples were collected from the existing running engine operated in real conditions so there was no operational opportunity/possibility to stop the engine from taking the oil samples. Moreover, the design of the engine and fuel injection pump determined the method of extracting oil from the pump crankcase. That is why measuring the volume of the lubricating fluid (oil-fuel mixture) inside of the pump was almost impossible to be made precisely. For this reason, it was decided to collect oil samples each time after a complete oil change in the crankcase, maintaining the specified operating time of each sample. It was also impossible to determine the amount of fuel in the oil directly. The amount of fuel in oil was determined using the mass and volume ratio of clean and used oil, which is presented in Equation (1).

Based on the measurement of the oil density for each sample and the comparison with pure oil, mass balance (first line of system of Equation (1)) and volume balance (second line of system of Equation (1)) allowed to estimate the amount of fuel that leaked to the crankcase for a specific period of engine operation. As a result of the transformation of Equation (1)–the Equation (2) was obtained. Based on this transformation the amount of fuel for individual oil samples was calculated and the results are presented in Table 2.

$$\left\{\begin{array}{cc}{q}_{LO}\times {\rho }_{LO}+q_{FO}\times {\rho }_{FO}=q_{UO}\times {\rho }_{UO}\hfill & \hfill \left[\mathrm{k}\mathrm{g}\right]\\ q_{LO}+q_{FO}=q_{UO}\hfill & \hfill \left[\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{s}\right]\end{array}\right.$$

(1)

$$\begin{array}{cc}{q}_{FO}=1.8\frac{{\rho }_{LO}-{\rho }_{UO}}{{\rho }_{UO}-{\rho }_{FO}}\hfill & \hfill \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\left[\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{s}\right]\end{array}$$

(2)

where:

• q_LO—quantity of Lubricating Oil (Fresh Oil) = 1.8 in liters,
• q_FO—quantity of Fuel Oil in liters,
• q_UO—quantity of Used Oil in liters,
• ρ_LO—Lubricating Oil density = 0.8886 g/cm³ (the density of fresh oil measured in the laboratory, which is consistent with the oil manufacturer’s data),
• ρ_FO—Fuel Oil density = 0.8317 g/cm³ [23],
• ρ_UO—Used Oil density in g/cm³.

Table 2. Oil density measured for samples taken at specific operating times, including fresh oil (0 h).

Oil Working Period	Lubricating Oil Density at 15 °C	Fuel Oil Quantity	Fuel Oil Quantity (Volume)
	[g/cm3]	[L]	[%]
Fresh oil	0.8886	0	0.0%
after 1 h	0.8749	0.5708	13.7%
after 2 h	0.8744	0.5986	14.2%
after 4 h	0.8723	0.7227	15.3%
after 6 h	0.8708	0.8194	15.4%
after 8 h	0.8689	0.9532	18.1%
after 10 h	0.8645	1.3226	21.3%
after 18 h	0.8625	1.5253	35.5%
after 24 h	0.8614	1.6485	43.8%
after 32 h	0.8577	2.1392	54.3%

Table 2. Oil density measured for samples taken at specific operating times, including fresh oil (0 h).

Oil Working Period	Lubricating Oil Density at 15 °C	Fuel Oil Quantity	Fuel Oil Quantity (Volume)
	[g/cm3]	[L]	[%]
Fresh oil	0.8886	0	0.0%
after 1 h	0.8749	0.5708	13.7%
after 2 h	0.8744	0.5986	14.2%
after 4 h	0.8723	0.7227	15.3%
after 6 h	0.8708	0.8194	15.4%
after 8 h	0.8689	0.9532	18.1%
after 10 h	0.8645	1.3226	21.3%
after 18 h	0.8625	1.5253	35.5%
after 24 h	0.8614	1.6485	43.8%
after 32 h	0.8577	2.1392	54.3%

Since the fuel density was provided as a standard value at 15 °C, all calculations were performed assuming this temperature. Table 2 presents calculated ratio [%] how much of fuel was leaking into the lubricating oil.

Based on the data from Table 2, it can be easily noticed that the amount of oil in the injection pump lubricating oil increases (as expected) over time to above 50% (Figure 4), which is a very large dilution of the oil causing a significant change in its chemical properties, as shown in work [5]. Additionally, it can be noticed that the dilution of oil with fuel changes significantly after the first hour and remains at a relatively stable level for several hours. Thereafter, the fuel content increases proportionally to time by approximately 1.5% per hour.

3.2. Viscosity and Viscosity Index

Viscosity and Viscosity Index similarly, to the measurement of the operating oil density and the estimation of the quantity of fuel leaking through the fuel pump into the oil lubricating it were presented in work [5]. To ensure data accuracy, each oil sample underwent multiple tests (at least five) for both: density and kinematic viscosity following the ASTM D445 [24]. The average value from these tests, as shown in Figure 4, was used for further analysis. It was noted that the oil from individual samples had the lowest permissible kinematic viscosity values in the range from 3.8 to 21, 9 cSt at 100 °C [25]. However, the viscosity values determined for samples with an operating time of 8 h and more are lower than the limit values specified by SAE (Society of Automotive Engineers) [26]. The results show a significant drop in the oil’s kinematic viscosity after just one hour of operation (Figure 5). This is probably caused by a substantial dilution of the oil with fuel. As the engine continues to run, the rate of fuel contamination and the corresponding decrease in viscosity slow down. Notably, the viscosity difference between the oil samples collected after 24 and 36 h of operation is relatively minor [5]. Some other oil samples also reached similar values of viscosity, which can be observed on the graph by overlapping the lines i.e.

The minimum lubricating oil viscosity requirements vary among marine engine manufacturers. Some specify absolute kinematic viscosity values (cSt) at a specific temperature, while others define limits based on the percentage decrease in viscosity. The minimum absolute viscosity for certain manufacturers is 5.6 cSt at 100 °C [5]. Considering this value as a general benchmark (optimistically), the tested oil falls below the limit after just one hour of operation. The average viscosity measured for the oil sample collected after the first hour is 5.6 cSt and it continues to decrease with further operation. However, it is important to note that this value remains above the SAE standard limit of 3.8 cSt as defined by the ASTM D445 standard.

The viscosity index (VI) is a dimensionless indicator that determines how the viscosity of a fluid changes with the temperature changes. It is mainly used for lubricating oils and greases. A higher viscosity index means a liquid is more stable and will keep a more consistent viscosity at both higher and lower temperatures. This is better for oil used in machinery. Viscosity index grades are generally taken as [27]:

• Low—under 3,
• Medium—35 to 80,
• High—80 to 110,
• Very high—above 110.

The standard ASTM D2270 [28] practice for calculating viscosity index from kinematic viscosity at 40 and 100 °C was used to determine the viscosity index of the tested samples. The calculation was made by using an open viscosity index calculator [29]. As shown in Figure 6, where the VI decreases considerably to about 25% of the initial value. However, the oil from almost all samples maintained a very high or high VI despite the decline. Only the last samples with a working time of 32 h went down to the VI medium level.

3.3. Four-Ball Rolling Contact Fatigue Test

The lubricity performance of oil samples was investigated based on their tribological behavior in terms of wear and friction. Damages due to rolling contact fatigue are predominant in mechanical applications. A good lubricant should protect the mating surfaces from sliding wear and protect them from damage occurring due to contact fatigue. The fourth ball was fitted to the ball chuck—Figure 3b, assembled to the tribometer spindle—Figure 3c, rotated at 500 rpm at room temperature of about 25 °C. The load was applied gradually from 0 to 7262 N. The examples of reports generated by the tribometer are presented in Figure 7: (a) for fresh oil and for comparison (b) for oil with 32 working hours.

In these reports, changes in several parameters can be observed: friction torque, friction coefficient, and load on the measuring node (four balls). Figure 7 shows examples of the variability trends of selected tests for oil samples (for spec. oil operating time) generated automatically by tribometer application.

However, reading the detailed values from the graph generated automatically by the tribometer software proves to be difficult due to its low accuracy. Therefore, the measurement data were exported to an Excel file where they were analyzed and a more accurate graph was prepared and presented in Figure 8. Since the entire range of data recorded by the tribometer is not needed to analyze the parameters considered important for the tested oil, the range was narrowed to the values from 4.5 to 7.5 s of the test, as shown by the green rectangle in Figure 7b. Thus, Figure 8, based on exported data, shows examples of the friction torque between balls immersed in the tested oil (for different oil samples). The graph also presents the trend line of the load for each presented sample (black line) and points for which LSL was estimated (red points). Additionally, the following points were highlighted: LSL for fresh oil as a reference (1) and LSI for fresh oil (2).

There were no changes in measured parameters such as the friction torque, load seizure limit (LSL), load seizure index (LSI), load seizure pressure (LSP), etc., in relation to the changes in the amount of fuel in the oil, change in viscosity, or density. Observed changes are rather accidental. For example, the load seizure limit (F_oz) for clean oil did not reach the highest values (what was expected) compared to the other samples—oil samples after 2 and 8 h of operation reached LSL later and oil samples after 2, 24 and 32 h of operation reached higher values of friction torque. The straight line that represents the load trend shows a proportional and constant increase for different samples.

3.4. Four-Ball Antiwear Test

The antiwear test of the lubricants was performed to investigate the wear-resisting properties of the lubricants according to the ASTM 4172 standard. The tests were carried out as per the standards at a specific temperature (75 degrees Celsius) and a specific load (392 N). The length of the wear scars of all three bottom balls was measured and the average of these wear scars was recorded. The frictional force was recorded and analyzed to find any sharp peaks during the test duration. The test was selected as a primary evaluation of the antiwear properties of the lubricant in sliding contact. Figure 9 presents examples of wear scar photos.

The wear scar diameters of the bottom steel balls were analyzed and the images were captured with a digital camera installed on the optical microscope. Some of the examples are presented in Figure 9. The wear scar diameters were measured using an image acquisition system and their size was determined as the average value of the horizontal and vertical measurements. Then, this value was used for the calculation of the wear scar diameter presented in

Table 3. Tests result of load limits (average values) obtained on a four-ball tribometer and average measured wear scar dimension values obtained from microscope measurements.

Oil Working Period	Trace Diameter	Load Seizure Index Ft	Load Seizure Limit Foz	Limit Seizure Pressure poz
	[mm]	[N]	[N]	[N/mm2]
Fresh oil	2.762	2106	3484	203
After 1 h	2.923	1571	3445	203
After 2 h	2.464	2165	3529	264
After 4 h	2.673	1939	3616	233
After 6 h	2.777	1568	3405	222
After 8 h	2.707	2054	3882	275
After 10 h	2.617	1703	3412	261
After 12 h	2.982	2194	4113	239
After 18 h	2.634	2015	3385	254
After 24 h	2.760	2078	4168	264
After 32 h	2.096	1618	3118	360

.

Seizure resistance was determined based on the LSI (F_t) and the LSL (F_oz), which correspond to the nominal pressure on the surface of the wear mark at the node seizure as limit seizure pressure (p_oz) and is calculated from Formula (3):

$$p_{oz}=0.52\frac{F_{oz}}{d^2}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}[\mathrm{N}/{\mathrm{m}\mathrm{m}}^2]$$

(3)

where:

• d—average diameter of the wear scar on the balls [mm];
• 0.52—coefficient resulting from the distribution of forces in the four-ball friction node;
• F_oz—load seizure limit for each sample.

Table 3 depicts calculation results for the average values of parameters measured during the tests. The trace diameters represent the average wear scar size measured under a microscope in two directions (length and width). Similarly, the load seizure index and load seizure limit in Table 3 are the average values from various measurements for each oil working time. The limit seizure pressure p_oz is the value calculated with Equation (3) from the average trace diameter and average load seizure limit.

The load seizure limit values recorded by the tribometer for each oil sample are shown in Figure 10. This graph shows that for all seizure samples presented, the LSL limit was reached at a similar load level, as shown by the trend line for these values (black line). This can be understood as follows: the amount of fuel diluted in oil has no effect on LSL, despite increasingly larger, visible deviations from the values designated as the trend line.

The values of the limiting seizure pressure recorded by the tribometer are shown in Figure 11. This graph shows the trend of the pressure values at the moment of initiation of seizure that occurred during lubrication with different oil samples. In this case, it is observed that the value of LSP increases with increasing oil operating time and dilution with fuel.

4. Conclusions

The lubricating oil in marine engines works in harsh conditions. These depend on factors such as operating load and its changes, fuel quality, ambient temperature, and how the engine is used. The rate at which the oil degrades is heavily influenced by these combined factors. To prevent engine failure, it is crucial to change the oil before it loses its ability to protect the engine. However, frequent, unnecessary oil changes can be wasteful for both environmental and economic reasons. This research investigated the impact of fuel oil dilution on lubricating oil within a marine fuel injection pump through laboratory testing. Based on the experimental results, the key findings are the following:

• Due to the dilution of oil with fuel, the viscosity index decreased significantly up to 50% for 32 oil operating hours but it remained at a very high or high level (according to ISO grades) for most of the samples. Only oil that worked for 32 h reached the VI medium level.
• The increasing amount of fuel oil in the base lubricating oil reduced the antiwear and antifriction performance by decreasing viscosity to 16% at 50 °C and 29% at 100 °C if compared to fresh oil.
• At the same time, there is no visible effect of diluting the oil with fuel, even on the load seizure limit, where the average value of approximately 3600 N does not change with the amount of fuel in the oil (Figure 10).
• To ensure proper reliability of the lubricating properties of the oil, it is necessary to determine the optimal oil change interval. This can be achieved by closely monitoring the actual physical-chemical condition of the oil.
• The condition of the fuel injection pump significantly impacts the rate of degradation of the lubricating oil.
• In this specific case, due to the observation of lubricating oil being diluted by fuel, it is strictly recommended to adjust the oil change intervals from the manufacturer’s recommendation of 100 h to a more frequent schedule (e.g., every 24 h or less).
• Alternatively, the oil could be replaced with the one of higher viscosity. However, this could require a re-examination of the oil properties.
• Because the design of this pump is a special solution used currently in marine engines, the results of these tests cannot be applied directly to all marine engines. However, the general conclusions will apply to any situation in which the lubricating oil is diluted with fuel.

In summary, this study suggests that more frequent oil changes might be necessary for this particular engine given its operating conditions. To determine the correct oil change interval, further testing is recommended.