Performance evaluation of nano-graphene lubricating oil with high dispersion and low viscosity used in diesel engines

Xin Kuang; Xiping Yang; Hua Bian; Rong Kuang; Nanrong Hu; Shengyong Li

doi:10.1371/journal.pone.0307394

. 2024 Aug 16;19(8):e0307394. doi: 10.1371/journal.pone.0307394

Performance evaluation of nano-graphene lubricating oil with high dispersion and low viscosity used in diesel engines

Xin Kuang ^1,^#, Xiping Yang ^2,^*,^#, Hua Bian ^2,^‡, Rong Kuang ^3,^‡, Nanrong Hu ^3,^‡, Shengyong Li ^1,^‡

Editor: Sameer Sheshrao Gajghate⁴

PMCID: PMC11329151 PMID: 39150954

Abstract

The basic tribological experiments have reported that nano-graphene lubricating oil has excellent anti-friction and anti-wear properties, which has been widely concerned. However, the real anti-friction effect of nano-graphene lubricating oil and its impact on engine power performance, economic performance and emission performance remain to be proved. This has seriously hindered the popularization and application of nano-graphene lubricating oil in the engine field. In this paper, nano-graphene powder was chemically grafted to prepare nano-graphene lubricating oil with high dispersion stability. The influence of nano-graphene on physicochemical properties of lubricating oil was studied, and the influence of nano-graphene on engine power performance, economic performance and emission performance was explored. The results show that after modification, the dispersion of nano-graphene in lubricating oil is improved. Compared with pure lubricating oil, the addition of nano-graphene makes the kinematic viscosity of lubricating oil slightly lower, and has little effect on the density, flash point, pour point and total acid value of lubricating oil. The reversed towing torque of nano-graphene lubricating oil is reduced by 1.82–5.53%, indicating that the friction loss decreases. The specific fuel consumption of the engine is reduced, which indicates that the fuel economic performance is improved. Engine HC+NO_X, CH₄, CO₂ emissions do not change much, but particulate matter (PM) emissions increase by 8.85%. The quantity concentration of nuclear particles, accumulated particles and total particles of nano-graphene lubricating oil are significantly higher than that of pure lubricating oil. And the increase of the quantity concentration of accumulated particles is more obvious than that of nuclear particles, and the larger the load, the more obvious this phenomenon. In order to apply nano-graphene lubricating oil to the engine, it is also necessary to further study its impact on the post-processing system, adjust the control strategy of the post-processing system and then test and calibrate.

1. Introduction

Industry experts believe that the engine will remain viable for a long time and will remain the most important power source for passenger cars in 30 years [1]. Lubricating oil is known as the "blood" of the engine and is one of the key factors affecting the mechanical efficiency, reliability and emissions of PM of the engine [2–4]. In recent years, the urgent needs of near zero emissions and low fuel consumption of engines have put forward higher requirements on the quality of lubricating oil, such as low ash content, low sulfur and phosphorus, low viscosity, long oil change cycle, anti-friction and anti-wear performance, etc. [5–8], especially excellent anti-friction and anti-wear properties. With the development of nanotechnology, nano-additives have been applied to the field of industrial lubrication, such as soft metals, metal compounds, organic compounds, graphite and its interlayer compounds [9–13]. Among them, nano-graphene, the thinnest two-dimensional nano-carbon material that only contains carbon elements, has attracted attention as a lubricating oil additive [14–17]. Nano-graphene has a unique nanolayered structure, forming a transfer film at the friction interface. Good tribological properties are obtained due to the self-lubrication of graphene by sliding between layers [17].

A large number of basic tribological experiments have found that the addition of an appropriate amount of nano-graphene as a general industrial lubricating oil additive can play a role in reducing friction and wear [18–23]. In recent years, researchers have begun to study the addition of nano-graphene to engine lubricating oil, mainly focusing on its dispersion stability and tribological properties.

Alqahtani et al. selected dimethylformamide as solvent, mixed nano-graphene into SAE 5W-30 lubricating oil, and studied the tribological effects of graphene nanosheets on engine SAE 5W-30 lubricating oil by using a four-ball friction tester. The results showed that the wear marks and friction coefficients were improved by 15% and 33% respectively after 0.12 wt.% nano-additive was added [15]. Nowduru et al. investigated the effects of nano-graphene on the tribological properties of engine oil 15W-40 using a four-ball test in accordance with ASTM D4172. The results showed that the mean coefficient of friction at room temperature and 75°C was reduced by 25.5% and 16%, respectively. And the wear resistance at room temperature and 75°C was increased by 13% and 16%, respectively [24]. Using a reciprocating tribometer to simulate the ring-liner interfaces of an engine, Ali et al. investigated the effects of graphene nanoparticles on the tribological properties of a fully formulated engine 5W-30 lubricating oil at different speeds and loads in accordance with ASTM G181-11. The experimental results showed that compared with 5W-30 lubricating oil, nano-additive can reduce the wear rate and friction coefficient by 25% ~ 30% and 26.5% ~ 32.6%, respectively [25]. Franzosi et al. investigated the tribological properties of functionalized graphene nanoplatelets added to fully formulated engine lubricating oil using a continuous sliding/rolling ball-on-disk tribometer. Friction tests showed that 0.2 wt% graphene lubricating oil reduced friction by 28% and 11% in the boundary and elastohydrodynamic lubrication regimes, respectively [26]. In the previous stage, Kuang et al. of our research group used oleic acid and stearic acid to chemically modify nanographene, and studied the tribological properties of the engine’s key friction pair, the cylinder liner and piston sample, under the action of lubricating oil on a reciprocating high-frequency friction test machine. The results showed that adding 25 ppm of nano-graphene to lubricating oil had the best frictional and wear reduction effect. Compared with pure lubricating oil, at 25°C and 100°C, the friction coefficient was reduced by 6.37% and 17.72%, and the wear amount was reduced by 8.18% and 25.19%, respectively.

Nano-graphene has broad application prospects. However, as an engine lubricating oil, different from industrial lubricating oil for general use, it is currently required to transition to Euro VI, national VI and above ultra-low emission regulations. The complicated and extremely harsh working conditions of the engine have brought higher challenges to the dispersion stability, physicochemical properties of nano-graphene lubricating oil and the anti-friction and anti-wear effect under real working conditions. At the same time, the impact of nano-graphene lubricating oil consumption on emissions and post-treatment system reliability deserves attention. There is little research on these areas. Ali et al. added graphene with a sheet diameter of 5–10 μm and a thickness of 3–10 nm to 5W-30 lubricating oil at a mass concentration of 0.4%. European driving cycle tests on graphene lubricating oil and pure lubricating oil was carried out respectively, and the characteristics of engine emissions at different speeds and loads under the same conditions were compared. The results showed that: under most working conditions, the emission performance of graphene lubricating oil was improved, CO₂ emission was reduced by 3.4–4.66%, NO_X was reduced by about 3–5%, and HC was reduced with the increase of engine load [27]. This shows that the addition of nano-graphene will have a certain impact on CO₂, HC, NO_X emissions, but there is a lack of attention to the dispersion of nano-graphene in lubricating oil, the physicochemical properties of nano-graphene lubricating oil, engine friction loss and PM emissions.

Therefore, from the perspective of engine application, the nano-graphene additive with the most potential and representative to reduce friction and wear was selected to be introduced into the engine low-viscosity lubricating oil. Based on engine performance test platform and multiple characterization methods, the dispersion stability, physicochemical properties, and real anti-friction effect of nano-graphene lubricating oil were studied. And the potential effects of nano-graphene additives on engine power performance, economic performance, emission performance, PM particle size distribution and post-treatment system were investigated, which laid a theoretical and scientific basis for the design, development and application of nano-graphene lubricating oil.

2 Experimental section

2.1 Experimental prototype and materials

The diesel engine model used in the experiment is KD192FW, and the relevant technical parameters are shown in Table 1. Commercially available 0# National 5 diesel was selected for the experiment, and its physicochemical properties are shown in Table 2. Nano-graphene was commercially available, and their transmission electron microscopy (TEM) photographs are shown in Fig 1. Through the measurement of Digital Micrograph software, it can be seen that the average thickness of nano-graphene powder is about 0.5–1 nm, and the number of nano-graphene layers is about 1–3 layers. The nano-graphene was chemically modified by oleic acid and stearic acid [28] as follows. First, 0.5 g of nano-graphene was dispersed into 100 mL of anhydrous ethanol. 2 g stearic acid and 3 g oleic acid were then added into them. Finally, the oil-soluble nano-graphene was obtained by centrifuge drying after stirring at 80°C for 4 hours. Previous experiment results [28] showed that the modified nano-graphene lubricating oil with a mass concentration of 25 ppm has the best tribological properties. The modified nano-graphene with a mass concentration of 25 ppm was added to the lubricating oil by weighing it in a precision balance. Then it was sufficiently intermittently dispersed at a low temperature after strong stirring by a magnetic mixer and a high-frequency ultrasonic disperser until it was steadily dispersed in the lubricating oil to obtain nano-graphene lubricating oil. It was recorded as MGL25. The pure lubricating oil used in the experiment is low-viscosity SN/CF 5W-30 diesel engine lubricating oil, labeled PLO, and its physicochemical properties parameters are shown in Table 3. The lubricating oils used in the experiment were PLO and MGL25 respectively.

Table 1. KD192FW diesel engine related technical parameters.

Parameter	Index
Form	Single cylinder, four stroke, air cooled, vertical
Cylinder diameter	92 mm
Piston stroke	75 mm
Rated power/speed	7.6 kW/3000 r·min^-1
Maximum torque/speed	25 N·m/2500 r·min^-1
Compression ratio	19.5

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Table 2. Main physicochemical properties of diesel for experiment.

Parameter	Index
Density (20°C, kg/m³)	0.836
Flash point (°C)	57
Low calorific value (MJ/kg)	42.90
Ash content (%)	0.001
Cetane number	51.5
Kinematic viscosity (20°C, mm²/s)	5.055

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Table 3. Physicochemical property parameters of PLO.

Density (20°C) (kg/m³)	Flash point (opening) (°C)	Pour point (°C)	Total acid number (in KOH) (mg/g)
0.893	225	-48	1.22

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2.2 Experiment of physicochemical properties of lubricating oil

The dispersion of freshly prepared lubricating oil with modified nano-graphene and unmodified nano-graphene after 1 week of placement were observed, respectively. Their absorbance was recorded periodically with the UV2600-204 UV-VIS spectrophotometer. Each measurement was repeated at least 3 times and the average was taken to get the final result. The data was converted to the corresponding relative concentration to quantitatively determine its dispersion stability. The upper layer of nano-graphene lubricating oil was taken for physicochemical properties measurement, such as density, flash point, pour point, total acid value, viscosity-temperature characteristics. The relevant measurement standards and instruments are shown in Table 4. The thermogravimetric (TG) curves of different lubricating oil samples were measured by TGA/DSC1 thermogravimetric analyzer. PLO and MGL25 samples were respectively weighed 2 mg. 80% N₂ and 20% O₂ was selected as the test reaction gas, and high-purity N₂ was used as the protection gas. The gas flow rate was 50 mL/min. The temperature was increased from room temperature to 400°C, and the heating rate was set at 10°C/min. The oxidation characteristics of lubricating oil were analyzed according to the TG curves.

Table 4. Lubricating oil test standards and instruments.

	Measuring standard	instrument
Density	GB/T 1884–2000	Anton Paar GmbHDMA 4200M
Flash point	GB/T 3536–2008	Closed flash testerSYD-3536-1
Pour point	GB/T 3535–2006	Automatic freezing and tipping point testerA1120
Total acid number	GB/T 7304、ASTMD664	Total acid number titratorAquamax
Viscosity-temperature characteristic	GB/T 265–88	Rotational viscometer brookfield DV-2 pro

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2.3 Engine reversed towing experiment

2.3.1. Experiment device and instrument

The schematic diagram of engine reversed towing experiment device is shown in Fig 2. The experiment bench system mainly includes experiment prototype, electric eddy current dynamometer, dynamometer control system, lubricating oil and exhaust temperature meter. The main experiment device and instrument are shown in Table 5. The start and stop of diesel engine and the regulation of operating conditions were completed by the control system EST2010.

Table 5. Main instruments and equipment for experiment.

Serial number	Name	Model	Manufacturer
1	Eddy current dynamometer	CWF75	Hangzhou Zhongcheng test Equipment Co., LTD
2	Dynamometer control system	EST2010	Hangzhou Zhongcheng test Equipment Co., LTD
3	Lubricating oil, exhaust temperature meter	DT-A	Shanghai Internal Combustion Engine Research Institute
4	barometer	YM3	Shanghai Mengde Instrument Co., LTD
5	hygrothermograph	LX8013	Guangzhou enjoy electronic Co., LTD

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2.3.2. Experiment scheme

Reversed towing experiment was carried out on the experimental prototype, and the reversed towing torque at different speeds was measured to compare the friction power consumption lubricated with two different lubricating oils. During the experiment, the throttle valve was kept fully open to reduce the error caused by the pump air loss, so that the friction between the piston assembly and the cylinder liner accounts for the main part of the reversed towing torque. The specific experiment process is as follows. After idling the engine for half an hour, when the lubricating oil temperature reached 80°C, the fuel supply to the engine was cut off and the reversed towing experiment was carried out. During the reversed towing experiment, the reversed towing torque of each lubricating oil was measured twice, and the speed increased from 2000 r/min to 3000 r/min, and then from 3000 r/min to 2000 r/min. The speed interval corresponding to each measurement point was 200 r/min, and the average value of the two measurements was the reversed towing torque. It should be noted that in order to exclude the influence of the viscosity-temperature effect of the lubricating oil on the friction force, the temperature of the lubricating oil was maintained between 80°C and 85°C. After the reversed towing experiment of PLO, the engine bench performance experiment of PLO was followed.

2.4 Engine bench performance experiment

2.4.1. Experiment device and instrument

After the reversed towing experiment of PLO was completed, fuel-metering device and exhaust gas analyzer were added to measure fuel consumption and emissions. Their specific model parameters are shown in Table 6. The schematic diagram of the engine bench performance experiment instruments are shown in Fig 3.

Table 6. Main instruments for experiment.

Serial number	Name	Model	Manufacturer
1	Fuel-metering device	YHW-010	Hangzhou Zhongcheng test Equipment Co., LTD
2	Exhaust gas analyzer	HoribaMEXA-7200D	AVL, Austria

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2.4.2. Experiment scheme

The characteristics of effective fuel consumption and exhaust temperature change with the engine load lubricated with different lubricating oil at 1000 r/min, 1200 r/min, 1400 r/min, 1600 r/min, 1800 r/min, 2000 r/min and 2200 r/min were compared. Average values were measured 3 times at each working point. The emission performance comparison experiment was carried out in accordance with the standard, Emission Limits and Measurement Methods of Exhaust Pollutants of Diesel

Engines for Non-road Machinery (China III, IV Stage), mainly measuring CO, HC, NO_X and PM emission. After the engine bench experiment of PLO, the engine PM online measurement and collection test of PLO were carried out.

2.5 Online measurement and collection experiment of PM

2.5.1 Experiment device and instrument

The schematic diagram of PM online measurement and collection device is shown in Fig 4. The American EEPS3090 particle size spectrometer was used for PM online measurement. The sampling flow rate was 50 L/min. The position about 10 cm away from the exhaust pipe was used as the sampling point of the EEPS3090 particle size spectrometer. The pipe connected with the exhaust pipe was a stainless steel pipe with a diameter of 6 mm. The pipe wall was smooth without bending, minimizing resistance to PM. A metal mesh sampling device was used to collect PM samples.

2.5.2 Experiment scheme

The effect of different lubricating oil on PM particle size was studied after the reversed towing comparison experiment and engine bench performance comparison experiment. In 2003, China issued the standard "Automobile engine Performance Test Method" GB/T19055-2003, which clearly stipulates that the ratio of lubricating oil and fuel consumption at full load and rated speed shall not exceed 0.3%. In order to accelerate the generation and influence of lubricating oil on PM, the lubricating oil was added to the fuel at the mass ratio of 0.5% to burn together. The change of the PM particle size distribution of lubricating oil was measured online, and the appropriate amount of PM was collected for characterization, so as to study the influence of nano-graphene additives on the particle size of PM. The experimental fuel were two kinds of mixed diesel fuel, respectively, diesel added with 0.5% mass fraction of PLO and diesel added with 0.5% mass fraction of MGL25. The lubricating oil in the oil pan was the same as the lubricating oil added to the diesel. The fuel and lubricating oil in the oil pan used in the experiment are shown in Table 7. Lubricating oil and diesel are more easily miscible. When configuring the mixed fuel, the mixing rod was used to stir fully to achieve the effect of uniform mixing.

Table 7. Experimental oil product.

Test number	Fuel	Lubricating oil
1	Diesel added with 0.5% mass fraction of PLO	PLO
2	Diesel added with 0.5% mass fraction of MGL25	MGL25

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The specific experimental process was as follows. After the engine idled for half an hour, the formal experiment began when the lubricating oil temperature reached about 80°C. When heating the engine, the particle size spectrometer was opened and preheated for about half an hour. The specific experiment operation parameters are shown in Table 8 below. After the engine ran stably for 5 minutes, EEPS was used to sample and measure at each operating point. Three measurements were carried out, each with an interval of 30 seconds, and the average value was taken as the final measurement result.

Table 8. Experimental conditions.

Speed (r/min)	Load
3000	10%、25%、50%、75%、100%

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After the completion of the PM online measurement experiment, EEPS was removed, and the self-made sampling device metal mesh was used to collect PM samples. In each experiment, the engine was shut down after stable operation for 90 minutes at the calibration working point. After shutdown, the PM adsorbed in the PM sampling device were scraped off and stored in a clean glassware. JEM-2100(HR) field emission transmission electron microscope was used to capture the morphology of the basic carbon particles of PM. Before testing, the PM samples need to be pre-treated. The treatment method was as follows. A small amount of PM was placed in anhydrous ethanol and subjected to ultrasonic shock for 15 minutes. After standing for 5 minutes, a small amount of the upper layer solution was dropped onto the copper mesh microgrid by pipette. After drying, it was placed on the sample table of high-power transmission electron microscopy for measurement.

When the experiment of one oil product was completed, the fuel in the fuel pipe was released, the lubricating oil in the diesel oil pan was discharged, and fresh experimental fuel and lubricating oil were added. After running the engine, the lubricating oil in the diesel engine oil pan was discharged after running for about half an hour at the speed of 3000 r/min and the torque of 24.2 N·m, when the temperature of water and lubricating oil reached the appropriate value. In this way, the non-experimental lubricating oil in the diesel engine can be cleaned. And re-add the new lubricating oil for the experiment. At the same time, the fuel pipeline was cleaned by the new experimental fuel to ensure that the entire fuel line was filled with new experimental fuel to avoid the interference of the previous fuel on the experimental results.

3. Experimental results

3.1 Physicochemical properties of nano-graphene lubricating oil

One week later, the dispersion of the nano-graphene lubricating oil before and after modification is shown in Fig 5. It can be obviously seen that after a week, compared with the freshly prepared nano-graphene lubricating oil, the modified nano-graphene lubricating oil has a slightly lighter color and only a little sediment at the bottom of the test tube. While the unmodified nano-graphene lubricating oil has a significantly lighter color and more sediment at the bottom of the test tube. The relative concentration changes of nano-graphene lubricating oil before and after modification are shown in Fig 6. It can be seen that the dispersion stability of the modified nano-graphene lubricating oil is improved, which is consistent with the naked eye. At the same initial concentration, after standing for a week, the relative concentration of unmodified graphene lubricating oil was reduced to 0.262, while the relative concentration of modified graphene lubricating oil was reduced to 0.806. Therefore, the chemical grafting of graphene by oleic acid and stearic acid can significantly improve its dispersion stability in lubricating oil. This is due to the presence of hydroxyl groups on the surface of nanographene, which are esterified with the carboxyl groups of oleic acid and stearic acid. This results in the presence of long alkyl chains on the surface of nanographene, which are more soluble in lubricating oil.

The kinematic viscosity changes of PLO and MGL25 with temperature are shown in Fig 7. It can be seen that the kinematic viscosity of MGL25 is reduced compared with that of PLO. And with temperatures reaching 60°C, the decline gradually slows. This is because graphene slips between layers to form self-lubrication, and when the temperature is below 60°C, the possibility of folding or agglomeration is small when the appropriate amount of graphene is added to the lubricating oil, which is conducive to reducing the internal friction when the lubricating oil fluid flows. When the temperature is higher than 60°C, some nano-graphene may fold or agglomerate, and the internal friction of lubricating oil decreases slowly.

The measurement results of density, flash point, pour point and total acid value are shown in Table 9. It can be seen from the table that compared with PLO, the density, flash point, pour point and total acid value of nano-graphene lubricating oil have almost no change. This shows that nano-graphene additives have no impact on the safety of lubricating oil. In addition, oleic acid and stearic acid did not affect the total acid value of the nano-graphene lubricating oil by chemical grafting modification of the surface of the nano-graphene additive. The TG curves of different lubricating oils are shown in Fig 8. It can be seen that the weight loss curves of PLO and MGL25 basically coincide. It shows that the addition of nano-graphene has little effect on the oxidation characteristics of lubricating oil. This is because the mass fraction of nano-graphene added is 25 ppm, which is very small and not enough to affect these parameters.

Table 9. Physicochemical properties parameters of different lubricating oil.

Lubricating oil	Density (20°C) (kg/m³)	Flash point (opening) (°C)	Pour point(°C)	Total acid number (in KOH) (mg/g)
PLO	0.893	225	-48	1.22
MGL25	0.894	225	-48	1.23

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3.2 Analysis of reversed towing experiment results

The reversed towing torque of the engine is shown in Fig 9. With the increase of speed, the engine reversed towing torque of different lubricating oil increases. At any speed, the reversed towing torque of nano-graphene lubricating oil is reduced compared with that of PLO. As the speed increases, the decline gradually slows down. This is because the higher the rotational speed, the higher the lubricating oil temperature, and the decreasing trend of lubricating oil viscosity gradually slows down with the increase of temperature. And the fluid friction also shows a similar trend. In the speed range of 2000–3000 r/min, the reversed towing torque reduction rate of MGL25 is 1.82–5.53% compared with that of PLO. This means less friction loss under nano-graphene lubricating oil. The trend of friction reduction in the reversed towing experiment is consistent with the conclusion of the basic tribological experiment [25].

3.3 Bench performance experiment

3.3.1 Comparative experiment of external characteristic

After nano-graphene was added to the original pure lubricating oil SN/CF 5W-30, the engine worked normally without abnormal phenomenon. The external characteristic curve of the engine lubricated with different lubricating oils is shown in Fig 10. As can be seen from the figure, with the increase of speed, effective fuel consumption first decreases and then increases, and the exhaust temperature increases. Compared with PLO, the effective fuel consumption of the prototype of MGL25 under external characteristic conditions is improved by 2.31–6.01% and the exhaust temperature is reduced by 4.91–15.63%. This shows that the fuel economy is improved after the addition of nano-graphene. The results are consistent with those of the reversed towing experiment. This is due to the reduction of engine friction losses lubricated with MGL25.

3.3.2 Load characteristic comparison test

The load characteristic curves of the engine of different lubricating oils are shown in Fig 11 Compared with PLO, the effective fuel consumption of the prototype of MGL25 has decreased. In the load characteristic experiment at 1000, 1200, 1400, 1600, 1800, 2000 and 2200 r/min, the effective fuel consumption is improved by 0.23~2.31% and 2.03~4.39%, 2.25~4.59%, 2.63~ 6.61%, 3.13~5.49%, 3.02 ~ 6.00% and 2.53 ~ 5.26%, respectively; Exhaust temperature decreases to 0.76~6.62%, 5.56~11.72%, 7.61~13.77% and 9.69~12.83%, 9.97~14.49%, 9.16~15.63%, 10.98~15.48%, respectively. This shows that the fuel economy is improved after the addition of nano-graphene. This is due to the fact that nano-graphene reduces friction losses and improves the mechanical efficiency of the engine. The results are consistent with those of the reversed towing experiment. This is due to the reduction of engine friction losses lubricated with nano-graphene lubricating oil.

3.3.3 Emission performance comparison test

The engine emission experiment results of different lubricating oils are shown in Table 10. It can be seen from the table that, compared with PLO, CO emission of the prototype of MGL25 is reduced by 11.68%, which is consistent with the research results in the references [29]. HC and CH₄ do not change. NO_X and CO₂ emissions are reduced by 2.19% and 4.59% respectively, which is similar to the reference results [27]. However, PM emission increases by 8.85%. This shows that the addition of nano-graphene leads to an increase in PM emissions. This is because when the engine is working, a small amount of lubricating oil will inevitably participate in the combustion. Compared with the pure lubricating oil, the viscosity of the nano-graphene lubricating oil is reduced to a certain extent, resulting in a lower mixture concentration of the local combustion chamber, a more complete combustion of the mixture, and a lower CO emission. When the nano-graphene lubricating oil is involved in combustion, the nanoparticles may self-nucleate, and these self-nucleating particles and their agglomeration will be directly adsorbed on the surface of carbon or further agglomerate with carbon particles, resulting in the increase of PM.

Table 10. Engine emissions.

	CO	THC	NO_X	CH₄	CO₂	NMHC	NMHC+NO_X	PM
	g/kW·h	g/kW·h	g/kW·h	g/kW·h	g/kW·h	g/kW·h	g/kW·h	g/kW·h
National III emission limits	5.5	——	——	——	——	——	7.5	0.6
PLO	1.37	0.12	6.39	0.01	854.15	0.11	6.50	0.305
MGL25	1.21	0.12	6.25	0.01	814.99	0.12	6.37	0.332

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3.4 Particle size distribution

The particle size distribution results under different torques and different rotational speeds were obtained through the experiment. The experimental results were expressed in the form of dN/dlogD_p(/cm³), where N is the particle number and D_p is the particle size.The quantity concentration and particle size of PM are related to the running condition of diesel engine, and the logarithmic form is advantageous for comparison. After statistical analysis of the test data, it is found that the particle size between 250–560 nm is almost zero, so the figures only show the particle size distribution between 5.6–250 nm.

3.4.1. Particle size distribution at rated speed

Fig 12 respectively shows the changes in the cumulative distribution of PM corresponding to different lubricating oils under different load conditions at the rated speed of 3000 r/min. Particles with a particle size of less than 50 nm are called nuclear particles, which are generally formed by the adsorption of volatile organic matter by the particle core formed by the combination of sulfuric acid vapor and solid carbon particles. Particles in the particle size range of 50 nm to 1000 nm are called aggregated particles, which are composed of nuclear particles that further accumulate into clusters and condense part of HC and semi-volatile substances such as sulfuric acid. It can be clearly seen from the figure that the concentration of nuclear particles, accumulated particles and total particles of MGL25 is greater than that of PLO, which is consistent with the reference results [30]. And the increase of accumulated particles is larger. And this phenomenon is more obvious at large loads. Studies have shown that particles with a particle size of less than 100 nm can pass through the alveoli and enter the blood, which is very harmful to human health. This means that the disadvantage of nano-graphene lubricating oil is more prominent, and it is necessary to adjust the control strategy of the post-processing system then test and calibrate to reduce the discharge of ultrafine particles into the atmosphere.

Statistically, two important parameters, geometric mean diameter and total quantity concentration, are shown in Fig 13. At the rated speed of diesel engine, the total quantity concentration of particles corresponding to MGL25 increases with the increase of the load. The total quantity concentration of particles corresponding to PLO is larger at full load and small load, while the total quantity concentration of particles corresponding to PLO at intermediate load is smaller. The total quantity concentration of particles of MGL25 is significantly greater than that of PLO. And the larger the load, the more obvious this phenomenon. With the increase of load, the geometric mean diameter of the particles corresponding to the two lubricating oils increases. At different loads, the geometric mean diameter of MGL25 is larger than that of PLO.

Under rated conditions, the morphology of PM corresponding to the two lubricating oils at different multiples (46,000, 94000, 190,000 times) is shown in Fig 14. The PM corresponding to the two lubricating oils are composed of dozens to hundreds of basic carbon particles similar to balls, showing irregular shapes such as clusters, chains, branches, etc.. There is not much difference in the intuitive shape, and nano-graphene does not affect the intuitive shape of the particles. It is worth noting that at 94000 times, it is easy to see that the basic carbon particle size of particles of PLO is relatively uniform; However the number of basic carbon particles with smaller particle size of MGL25 increased. The results are consistent with PM online measurements. This is because when the nano-graphene lubricating oil is involved in combustion, the nano-particles may self-nucleate, resulting in the risk of increasing nuclear particles. In addition, these self-nucleating particles and their agglomeration will directly adsorb on the carbon surface or further agglomerate with carbon particles to form accumulated particles, resulting in the risk of increasing aggregated particles. And with the increase of load, the temperature increases, resulting in the increase of agglomeration and accumulation of particles.

4. Conclusions

The real anti-friction effect of nano-graphene lubricating oil and its influence on engine power performance, economic performance and emission performance remain to be investigated. In this paper, nano-graphene powder was chemically modified to prepare nano-graphene lubricating oil, and the effect of nano-graphene on the physicochemical properties of lubricating oil was studied. The effects of nano-graphene on engine power performance, economic performance and emission performance were investigated by engine bench test. The main conclusions are as follows:

The dispersion stability of modified nano-graphene lubricating oil is improved.
The addition of nano-graphene makes the kinematic viscosity of lubricating oil slightly lower, and has little effect on the density, flash point, pour point and total acid value of lubricating oil.
Compared with PLO, the reversed towing torque of nano-graphene lubricating oil is reduced, and the friction loss is reduced.
After adding nano-graphene to lubricating oil, the engine fuel economy is improved.
Lubricated with MGL25, the engine emission of HC, NO_X, CH₄ and CO₂ of the engine has little change, but the emission of PM increases by 8.85%.
The quantity concentration of nuclear particles and accumulated particles of nano-graphene lubricating oil are significantly higher than that of PLO, and the increase amplitude of the quantity concentration of accumulated particles is more obvious than that of nuclear particles, and the larger the load, the more obvious this phenomenon.
The addition of graphene has little effect on the visual morphology of PM. The particle size of basic carbon particles of PM of PLO is relatively uniform, while the number of basic carbon particles with smaller particle size of PM of nano-graphene lubricating oil increases. It confirms that when nano-graphene lubricating oil participates in combustion, the nanoparticles will self-nucleate, increasing the risk of generating more nuclear particles.

In summary, the use of nano-graphene lubricating oil reduces engine friction losses, improves fuel economy, but the number of PM increases, and the nuclear particles increase. Nano-graphene lubricating oil may have adverse effects on post-processing, and further study of its effects on post-processing is needed, as well as re-control strategy adjustment and testing and calibration of the post-processing system. The research has laid a theoretical and scientific basis for the design, development and application of nano-graphene lubricating oil.

Data Availability

All relevant data are within the manuscript.

Funding Statement

This work was supported by Natural Science Research of Jiangsu Higher Education Institutions of China [grant number 23KJB470006; 23KJB460009]; Science and Technology Project of Nantong City [grant number JC22022066]; Natural Science Foundation of Jiangsu Province [grant number BK20231227]; High-level Talents Research Start-up Fund supported by Jiangsu Shipping College [grant number HYRC/202407; HYRC/202411]; Qinglan Project of Jiangsu Province of China.

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PLoS One. doi: 10.1371/journal.pone.0307394.r001

Decision Letter 0

Sameer Sheshrao Gajghate

26 Apr 2024

PONE-D-24-11357Performance evaluation of nano-graphene lubricating oil with high dispersion and low viscosity used in diesel enginesPLOS ONE

Dear Dr. Xiping Yang,

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Reviewer #1: This study presents a well-designed experiment investigating the effects of nano-graphene lubricating oil on engine performance. The research team utilized a controlled environment with varied operating conditions to gather robust data. The findings are particularly encouraging, demonstrating a reduction in friction and improvement in fuel economy with the use of this lubricant. The analysis is further strengthened by the application of advanced techniques for examining particle size distribution. This research paves the way for the development of more efficient and environmentally friendly lubricants.

Reviewer #2: The paper presents valuable experimental data and insights into the use of nano-graphene as a lubricating oil additive in engines. However, further clarification and discussion in certain areas, particularly regarding the implications of engine emissions would enhance the impact and significance of the study. Additionally, suggestions for future research in the direction of tribology could be included to guide further exploration in this field.

Reviewer #3: 1-The research results were not clearly reflected in the abstract, your abstract should clearly state the essence of the problem you are addressing, what you did and what you found and recommend. In addition, please provide final recommendation of your study.

2-1In the introduction, follow the literature review by a little more detailed state of the art analysis.

3-This should clearly show the knowledge gaps identified and link them to your paper goals.

4-Please emphasize both the novelty and the relevance of your paper goals. Correct this.

5-The addition of nano-graphene has little effect on the physicochemical properties

of lubricating oil; explain the reasons.

6-Authors should explicitly specify the novelty of their work.

8-What progress against the most recent state-of-the-art similar studies was made in this study? Mention this in the revised manuscript.

9-The discussion is poor; there is no deep discussion and compared with previous studies.

10-The author should introduce the advantages of graphene. Why its addition will affect the tribological properties and improve the friction?

11-Please proof read again the paper to improve the English standard for the entire manuscript.

12-The authors mention that they achieved good dispersion, but this is not shown. The authors must add microscopic photos showing that they achieved good dispersion and avoided agglomeration.

13-The authors should measure the tribology properties and take the TEM image of the samples after and before the test .

**********

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Attachment

Submitted filename: Comments.docx

pone.0307394.s001.docx^{(13.6KB, docx)}

PLoS One. 2024 Aug 16;19(8):e0307394. doi: 10.1371/journal.pone.0307394.r002

Author response to Decision Letter 0

20 May 2024

Dear Editor,

Many thanks for your email of 26 April 2024, and for your significant efforts to review our manuscript ID PONE-D-24-11357 entitled “Performance evaluation of nano-graphene lubricating oil with high dispersion and low viscosity used in diesel engines”. We would like to thank the reviewers for giving us constructive suggestions which would help us both in English and in depth to improve the quality of the paper. Here we submit a new version, which has been modified according to the reviewers’ suggestions. Efforts were also made to correct the mistakes of the manuscript.

We appreciate your fair consideration of our manuscript and the opportunity to improve our manuscript. We believe that we have modified the manuscript in accordance with all additional issues raised by the reviewers. We hope the manuscript will be acceptable for publication in “Plos One”.

Yours sincerely,

Xiping Yang

The following is a point-to-point response to comments from two reviewers and the academic editor.

Response to reviewer #2:

Reply: Thanks for your significant efforts to review our manuscript and we have carefully studied your comments. According to the review comments you have given,

the manuscript has been revised. The following are responses to your comments.

1.The paper presents valuable experimental data and insights into the use of nano-graphene as a lubricating oil additive in engines. However, further clarification and discussion in certain areas, particularly regarding the implications of engine emissions would enhance the impact and significance of the study. Additionally, suggestions for future research in the direction of tribology could be included to guide further exploration in this field.

We have further clarified and discussed the impact of nano-graphene on engine emissions that will enhance the impact and significance of this study (highlighted in yellow color in line 39-40, 117-119, 123-137, 497-498 of the manuscript). Some suggestions for future tribology research are also put forward (highlighted in yellow color in line 496-499 of the manuscript).

We appreciate for your warm work earnestly, and hope that the correction will meet

with approval.

Once again, thank you very much for your comments and suggestions.

Yours sincerely,

Xiping Yang

Response to reviewer #3:

Reply: Thank you for careful and thorough reading of this manuscript and for the thoughtful comments and constructive suggestions, which help to improve the quality

of this manuscript. We welcome the opportunity to address and clarify the issues raised in the referee report. According to the review comments you have given, the manuscript has been revised. The following are point-to-point responses to your comments.

1. The research results were not clearly reflected in the abstract, your abstract should clearly state the essence of the problem you are addressing, what you did and what you found and recommend. In addition, please provide final recommendation of your study.

We have revised the abstract accordingly (highlighted in blue color in line 48-49, 52-53, 60-63 of the manuscript) .

2. In the introduction, follow the literature review by a little more detailed state of the art analysis.

In the introduction, we have added a more detailed statement of the analysis (highlighted in yellow color in line 85-115, 117-119, 123-137 of the manuscript).

3. This should clearly show the knowledge gaps identified and link them to your paper goals.

We have added the corresponding statement (highlighted in yellow color in line 125, 134-137 of the manuscript).

4. Please emphasize both the novelty and the relevance of your paper goals. Correct this.

We have added the corresponding statement (highlighted in yellow color in line 85-87, 108-109, 117-119, 123-125 of the manuscript).

5. The addition of nano-graphene has little effect on the physicochemical properties

of lubricating oil; explain the reasons.

The addition of nano-graphene has a certain effect on the physicochemical properties of lubricating oil. The addition of nano-graphene reduces the kinematic viscosity of lubricating oil at 25-100°C, which is described in line 331-336 of the manuscript. However, the addition of nano-graphene has little effect on the density, flash point, pour point, total acid value and oxidation characteristics of lubricating oil, which is described in line 339-348 of the manuscript. This is because the mass fraction of nano-graphene added is 25 ppm, which is very small and not enough to affect these parameters. We have revised the relevant statements (highlighted in purple color in line 49-51, 472-474 of the manuscript) and added relevant explanations in the manuscript (highlighted in blue color in line 348-350 of the manuscript).

6. Authors should explicitly specify the novelty of their work.

We have added the corresponding statement (highlighted in yellow color in line 85-87, 117-119, 123-125, 134-137 of the manuscript) to explicitly specify the novelty of their work .

8. What progress against the most recent state-of-the-art similar studies was made in this study? Mention this in the revised manuscript.

We have added the corresponding statement (highlighted in yellow color in line 85-87, 117-119, 123, 125, 134-137 of the manuscript) to explicitly specify the novelty of their work .

9. The discussion is poor; there is no deep discussion and compared with previous studies.

We have added in-depth discussions and comparisons (highlighted in blue color in line 317-323, 348-350, 388-389, 395-397, 412-418, 421-426 of the manuscript) to explicitly specify the novelty of their work .

10. The author should introduce the advantages of graphene. Why its addition will affect the tribological properties and improve the friction?

We have added relevant statements to introduce the advantages of graphene (highlighted in blue color in line 80-82 of the manuscript).

11. Please proof read again the paper to improve the English standard for the entire manuscript.

We have proofread the paper to improve the English standard for the entire manuscript.

12. The authors mention that they achieved good dispersion, but this is not shown. The authors must add microscopic photos showing that they achieved good dispersion and avoided agglomeration.

The dispersion of graphene in lubricating oil after one week was presented respectively before and after modification in line 327 of the manuscript. In order to quantify the dispersion effect of graphene in lubricating oil before and after modification, we made up the test to measure the absorbance change of graphene in lubricating oil before and after modification in a week. (highlighted in blue color in line 184-188, 317-323 of the manuscript). The absorbance change of graphene in lubricating oil can reflect the dispersion problem better than the microscopic photos.

13. The authors should measure the tribology properties and take the TEM image of the samples after and before the test .

In the early stage, our research group has carried out tribological performance testing, TEM image acquisition and related analysis of nano-graphene lubricating oil, and the relevant results have been published in a journal, which is described in the introduction (highlighted in yellow color in line 107-115 of the manuscript).

We appreciate for your warm work earnestly, and hope that the correction will meet with approval.

Once again, thank you very much for your comments and suggestions.

Yours sincerely,

Xiping Yang

Response to the academic editor:

Reply: Thanks for your significant efforts to review our manuscript and we have carefully studied your comments. According to the review comments you have given,

the manuscript has been revised. The following are responses to your comments.

1.Though introduction section clearly highlights the current study's novelty. however, a justification for using graphene as an additive over other nano particles has not been provided.1.

We have added the relevant statement introduction section (highlighted in yellow color in line 80-82 of the manuscript).

2.In Line 95, what is “0#” represents?

0# refers to the diesel label, which is divided according to the freezing point of diesel. 0# diesel is corresponding to the loss of diesel fluidity at no higher than 0°C.

3.In Line 100, Author claims that “The nano-graphene was chemically modified by oleic acid and stearic acid”. Whereas nano-graphene should not be reacted with acids.

There are oxygen-containing groups, carboxyl groups and hydroxyl groups on the surface of graphene or on the surface of graphene oxide during the preparation of graphene. The oxygen-containing groups on the surface of graphene can undergo ring-opening reactions with stearic acid under the action of catalysts. The hydroxyl group on the surface of graphene and the carboxyl group of oleic acid can be esterified under the condition of catalyst. Oil-soluble graphene can be obtained by grafting long-chain alkanes onto the surface of graphene.

4.In Line 102, Author claims that “2g stearic acid and 3 g oleic acid were then added into them” It is not cleared about the purpose of adding the acids. Again, why only these two particular types of the acids are added?

The purpose of adding stearic acid and oleic acid is to graft oleic acid and oleic acid onto the surface of nano-graphene to achieve oil solubility. This is stated in the first line 157-158 of the manuscript, with relevant references.

5.Comparing Table 2 and 3: Density of Diesel is 836 kg/m3 and density of PLO is 0.893 kg/ m3. If so, then how the flash point of PLO is significantly higher than diesel.

We are very sorry that we made a mistake. The density of diesel should be 0.836 kg/m3, not 836 kg/m3. We have corrected the mistake in line 173 of the manuscript (highlighted in blue color).

6.Line 208: “The experimental fuel were two kinds of mixed diesel fuel, respectively, diesel added with 0.5% mass fraction of PLO and diesel added with 0.5% mass fraction of MGL25.” Elaborate the process if possible.

This process was described in line 263-265, 271-274, 306 of the manuscript.

7.Line 215: “After the engine idled for half an hour, the formal experiment began when the lubricating oil temperature reached about 80°C.” already given in line Line:157.

Here are two experiments. They are the engine reversed towing experiment and the on-line measurement experiment of particulate matter. Both experiments require the oil to reach 80°C before starting formal experiments.

8.Table 10 represents the engine emissions where as the unit given in g/KW-h. How that can be possible?

According to the objects, contents and detection methods tested in different standards, the emission units are not the same. This study was in accordance with the "Non-road machinery diesel engine exhaust pollutant emission limits and measurement methods (China III, IV)" standard, the emission unit is g/kWh. It refers to the quality of pollutants emitted by the engine per unit time per unit power, that is a specific emission.

9.Moreover, From the table 10 it is found NOx emission is significantly lower for MGL25, whereas Fig. 10 represents fuel consumption for MGL25 is lower throughout the speed variation, that means higher efficiency of the engine. Why this controversy occurs?

MGL corresponds to a 2.19% reduction in NOX emissions compared to pure lubricating oil, a result consistent with lower exhaust temperature. The addition of nanographene has little effect on NOX. And this result is consistent with the reference [27]. The MGL corresponding fuel consumption is reduced by 0.23% to 6.61%, which is due to the nano-graphene reducing friction losses and improving the mechanical efficiency of the engine. We have added relevant descriptions in line 388-389, 395-397 of the manuscript (highlighted in blue color).

[27]Ali MKA, Xianjun H, Abdelkareem MA, Gulzar M, Elsheikh A. Novel approach of the graphene nanolubricant for energy saving via anti-friction/wear in automobile engines. Tribology International. 2018;124:209-29.

10.The quality of Fig.11 should be improved.

We have already improved the quality of Fig. 11.

11.In conclusion claimed about the good antifriction effect of nano-graphene lubricant over generally used lubricant. Whereas no comparison of tribological behaviours between these two lubricants have not found.

Our research group has done basic tribological experiments in the early stage, compared the tribological properties of pure lubricating oil and nano-graphene lubricating oil, and published a paper. We have added relevant descriptions in 107-115 lines of the manuscript (highlighted in yellow color). In this study, on the basis of the results of the basic tribological experiment, we studied the friction power consumption under the lubrication of the two lubricating oils by the reversed towing test. The results show that the antifriction effect of nano-graphene lubricating oil is better than that of pure lubricating oil, which is consistent with the results of basic tribological experimental.

We appreciate for your warm work earnestly, and hope that the correction will meet

with approval.

Once again, thank you very much for your comments and suggestions.

Yours sincerely,

Xiping Yang

Attachment

Submitted filename: Response to reviewers -20240508.docx

pone.0307394.s002.docx^{(25.4KB, docx)}

PLoS One. doi: 10.1371/journal.pone.0307394.r003

Decision Letter 1

Sameer Sheshrao Gajghate

24 Jun 2024

PONE-D-24-11357R1Performance evaluation of nano-graphene lubricating oil with high dispersion and low viscosity used in diesel enginesPLOS ONE

Dear Dr. Yang,

The uncertainty Analysis section is missing throughout the manuscript.
Nomenclature section is missing.

Add the physio-chemical properties of the lubricants

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Academic Editor

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Reviewers' comments:

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Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: All comments have been addressed

Reviewer #4: All comments have been addressed

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2. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #2: Yes

Reviewer #4: No

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

Reviewer #4: No

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

Reviewer #2: Yes

Reviewer #4: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

Reviewer #2: Yes

Reviewer #4: No

**********

6. Review Comments to the Author

Reviewer #2: (No Response)

Reviewer #4: The language should be improved

The authors should clearly correlate The experimental results with basic theory

A reduction of 1 to 5 percent was found in the reverse towing torque. I doubt The accuracy of experiment.

The authors must include uncertainty analysis

Most important property for lubricants. Viscosity index was omitted in physico chemical properties

**********

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Reviewer #4: Yes: Dr. Vadapalli Srinivas

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PLoS One. 2024 Aug 16;19(8):e0307394. doi: 10.1371/journal.pone.0307394.r004

Author response to Decision Letter 1

27 Jun 2024

Dear Editor,

Many thanks for your email of 25 June 2024, and for your significant efforts to review our manuscript ID PONE-D-24-11357R1 entitled “Performance evaluation of nano-graphene lubricating oil with high dispersion and low viscosity used in diesel engines”. We would like to thank the reviewers for giving us constructive suggestions which would help us both in English and in depth to improve the quality of the paper. Here we submit a new version, which has been modified according to the reviewers’ suggestions. Efforts were also made to correct the mistakes of the manuscript.

Yours sincerely,

Xiping Yang

The following is a point-to-point response to comments from two reviewers and the academic editor.

Response to reviewer #4:

Reply: Thanks for your significant efforts to review our manuscript and we have carefully studied your comments. According to the review comments you have given,

the manuscript has been revised. The following are responses to your comments.

1.The language should be improved.

We have revised the language (highlighted in purple color in line 55-56, 70-71, 80, 84-87, 94-98, 125, 129, 135, 160, 164, 179, 191-192, 196, 198, 205, 209-210, 214, 216, 224, 228, 258-259, 263-264, 276, 279, 294, 323, 363, 365-366, 371, 374, 389, 404, 473 of the manuscript).

2. The authors should clearly correlate The experimental results with basic theory

We have added the corresponding description and analysis (highlighted in green color in line 325-329, 337-339, 342-344, 367-371, 388-390, 403-405, 414-422 of the manuscript).

3. A reduction of 1 to 5 percent was found in the reverse towing torque. I doubt The accuracy of experiment.

The authors must include uncertainty analysis

We have added the specific description of the measurement of the reversed towing torque in the experiment scheme. (highlighted in yellow color in line 239 of the manuscript). And an error bar was added to the reversed towing torque variation diagram. (highlighted in yellow color in line 377 of the manuscript).

4. Most important property for lubricants. Viscosity index was omitted in physico chemical properties

In the last revised manuscript, the relevant detection and analysis of lubricating oil viscosity index have been described. (highlighted in blue color in line 183-186, 335-336, 346 of the manuscript).

We appreciate for your warm work earnestly, and hope that the correction will meet

with approval.

Once again, thank you very much for your comments and suggestions.

Yours sincerely,

Xiping Yang

Response to the academic editor:

Reply: Thank you for careful and thorough reading of this manuscript and for the thoughtful comments and constructive suggestions, which help to improve the quality

1.The uncertainty Analysis section is missing throughout the manuscript.

We have added the specific description (highlighted in yellow color in line 239 of the manuscript). And error bars were added to diagrams (highlighted in yellow color in line 333, 377 of the manuscript).

2.Nomenclature section is missing

The nomenclature section for PLO and MGL25 is described in line 167-168 (highlighted in blue color of the manuscript)

3.Add the physio-chemical properties of the lubricants.

The physio-chemical properties of the lubricants, including viscosity index, were described and analyzed in the original manuscript (highlighted in blue color in line 183-193, 335-336, 346 of the manuscript).

We appreciate for your warm work earnestly, and hope that the correction will meet

with approval.

Once again, thank you very much for your comments and suggestions.

Yours sincerely,

Xiping Yang

PLoS One. doi: 10.1371/journal.pone.0307394.r005

Decision Letter 2

Sameer Sheshrao Gajghate

4 Jul 2024

Performance evaluation of nano-graphene lubricating oil with high dispersion and low viscosity used in diesel engines

PONE-D-24-11357R2

Dear Dr. Xiping Yang,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

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PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

Reviewer #2: All comments have been addressed

Reviewer #4: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #2: Yes

Reviewer #4: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

Reviewer #4: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

Reviewer #2: Yes

Reviewer #4: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

Reviewer #2: Yes

Reviewer #4: Yes

**********

6. Review Comments to the Author

Reviewer #2: (No Response)

Reviewer #4: the authors have adequately addressed the comments raised in a previous round of review. the manuscript technically sound, and the data support the conclusions

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Reviewer #2: No

Reviewer #4: Yes: v srinivas

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PLoS One. doi: 10.1371/journal.pone.0307394.r006

Acceptance letter

Sameer Sheshrao Gajghate

7 Aug 2024

PONE-D-24-11357R2

PLOS ONE

Dear Dr. Yang,

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This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

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Submitted filename: Comments.docx

pone.0307394.s001.docx^{(13.6KB, docx)}

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Data Availability Statement

All relevant data are within the manuscript.

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PERMALINK

Performance evaluation of nano-graphene lubricating oil with high dispersion and low viscosity used in diesel engines

Xin Kuang

Xiping Yang

Hua Bian

Rong Kuang

Nanrong Hu

Shengyong Li

Roles

Abstract

1. Introduction

2 Experimental section

2.1 Experimental prototype and materials

Table 1. KD192FW diesel engine related technical parameters.

Table 2. Main physicochemical properties of diesel for experiment.

Fig 1. TEM photographs of nano-graphene.

Table 3. Physicochemical property parameters of PLO.

2.2 Experiment of physicochemical properties of lubricating oil

Table 4. Lubricating oil test standards and instruments.

2.3 Engine reversed towing experiment

2.3.1. Experiment device and instrument

Fig 2. Schematic diagram of engine reversed towing experiment device.

Table 5. Main instruments and equipment for experiment.

2.3.2. Experiment scheme

2.4 Engine bench performance experiment

2.4.1. Experiment device and instrument

Table 6. Main instruments for experiment.

Fig 3. Schematic diagram of engine bench performance experiment device.

2.4.2. Experiment scheme

2.5 Online measurement and collection experiment of PM

2.5.1 Experiment device and instrument

Fig 4. Schematic diagram of PM experiment device.

2.5.2 Experiment scheme

Table 7. Experimental oil product.

Table 8. Experimental conditions.

3. Experimental results

3.1 Physicochemical properties of nano-graphene lubricating oil

Fig 5. Settlement of nano-graphene lubricating oil after one week.

Fig 6. Relative concentration changes of nano-graphene lubricating oil before and after modification.

Fig 7. Variation of kinematic viscosity of different lubricating oils with temperature.

Table 9. Physicochemical properties parameters of different lubricating oil.

Fig 8. TG diagram of different lubricating oil.

3.2 Analysis of reversed towing experiment results

Fig 9. Reversed towing torque variation diagram.

3.3 Bench performance experiment

3.3.1 Comparative experiment of external characteristic

Fig 10. External characteristic experimental curve.

3.3.2 Load characteristic comparison test

Fig 11. Load characteristic experiment curve at different speed.

3.3.3 Emission performance comparison test

Table 10. Engine emissions.

3.4 Particle size distribution

3.4.1. Particle size distribution at rated speed

Fig 12. Cumulative distribution of quantity of PM quantity corresponding to different lubricating oil.

Fig 13. The geometric mean diameter and total quantity concentration of particulate matter corresponding to different lubricating oils.

Fig 14. PM morphologies corresponding to the two lubricating oils at different multiples.

4. Conclusions

Data Availability

Funding Statement

References

Decision Letter 0

Sameer Sheshrao Gajghate

Roles

Author response to Decision Letter 0

Decision Letter 1

Sameer Sheshrao Gajghate

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Author response to Decision Letter 1

Decision Letter 2

Sameer Sheshrao Gajghate

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Acceptance letter

Sameer Sheshrao Gajghate

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Associated Data

Supplementary Materials

Data Availability Statement

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