Elsevier

Tribology International  摩擦学国际

IF 6.9SCIEJCR Q1工程技术1区TopEI
Volume 202, February 2025, 110333
第 202 卷，2025 年 2 月，110333
Tribology International

The degeneration mechanism of lubricating oil in the ammonia fuel engine
氨燃料发动机中润滑油的劣化机理

https://doi.org/10.1016/j.triboint.2024.110333Get rights and content  获取权限与内容
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Highlights  要点

  • •
    The changes in the physicochemical properties of lubricating oil are analyzed.
    分析了润滑油理化性质的变化。
  • •
    The influence mechanism of ammonia on lubricating oil is revealed.
    揭示了氨对润滑油的影响机制。
  • •
    Ammonia affects the viscosity and dispersion stability of the lubricating oil.
    氨会影响润滑油的粘度和分散稳定性。

Abstract  摘要

As a hydrogen-rich carbon-free fuel, ammonia has attracted great attention in the engine field. However, there are few studies on the lubricating oil of the ammonia engine, which hinders the development of the ammonia engine. To study the impact of ammonia on the physicochemical properties and the lubricating performances of the lubricating oil, water-lubricating oil, and lubricating oil-water-ammonia were prepared. By investigating dispersion stability, viscosity, thermal stability, and film-forming ability of different lubricating oils, the effect of ammonia on the physicochemical properties and lubricating properties of lubricating oil was analyzed. The effect mechanism of ammonia on the lubricating oil was also explored. This work can be a foundational contribution to the lubricating oil for the ammonia engine.
作为一种富氢无碳燃料，氨在发动机领域备受关注。然而针对氨燃料发动机润滑油的研究却十分匮乏，这制约了氨燃料发动机的发展。为探究氨对润滑油理化性能与润滑性能的影响，本研究分别制备了纯润滑油、水-润滑油混合液及润滑油-水-氨混合液。通过考察不同润滑油的分散稳定性、黏度特性、热稳定性和成膜能力，系统分析了氨对润滑油理化性能与润滑性能的影响规律，并揭示了氨影响润滑油性能的作用机制。本工作可为氨燃料发动机润滑油的研发提供理论基础。

Keywords  关键词

Lubricating properties
Lubricating oil
Physicochemical performances
Ammonia engine

润滑特性 润滑油 理化性能 氨燃料发动机

1. Introduction  1. 引言

To meet the need of greenhouse gas (GHG) emissions reduction, the transition of the engine fuel from fossil to low/zero-carbon fuels is necessary[1], [2]. Ammonia (NH3), as a hydrogen-rich and carbon-free fuel, only nitrogen and water are produced when it is fully burned[3], [4]. Ammonia fuel can effectively reduce the carbon emissions of engines, thus attracting significant attention to the ammonia fuel engine. As shown in Fig. 1(a), major classification societies are developing the ammonia fuel engine, and have indicated that the fuel, the combustion, and the exhaust gas aftertreatment in the ammonia fuel engine require major attention. Additionally, the effects of ammonia on the lubricating performance of the ammonia fuel engine also have been emphasized [5], [6]. For the engine, the application of advanced lubrication strategies can not only ensure the high reliability and long life of the ship, but also improve the performance of the equipment, reduce the power loss of the ship, and thus reduce the greenhouse gas emissions throughout the life cycle of the ship[7], [8], [9]. However, there are few reports on the tribological characteristics of the ammonia fuel engine. It is worth noting that the corrosion properties of ammonia fuel and the physicochemical properties of ammonia fuel are different from those of the traditional fuel (diesel, methanol, LNG, etc.), so the study of the tribological characteristics for the ammonia fuel engine is necessary[10], [11], [12].
为满足温室气体(GHG)减排需求，发动机燃料必须从化石燃料转向低碳/零碳燃料[1][2]。氨(NH₃)作为一种富氢无碳燃料，完全燃烧时仅产生氮气和水[3][4]。氨燃料能有效降低发动机碳排放，因此氨燃料发动机备受关注。如图 1(a)所示，主要船级社正在研发氨燃料发动机，并指出需要重点关注该发动机的燃料系统、燃烧过程及尾气后处理技术。此外，氨对发动机润滑性能的影响也受到特别强调[5][6]。对发动机而言，采用先进润滑策略不仅能保障船舶的高可靠性与长寿命，还能提升设备性能、降低船舶动力损耗，从而减少船舶全生命周期的温室气体排放[7][8][9]。但目前关于氨燃料发动机摩擦学特性的研究报道仍较为匮乏。 值得注意的是，氨燃料的腐蚀特性与理化性质均不同于传统燃料（柴油、甲醇、液化天然气等），因此研究氨燃料发动机的摩擦学特性十分必要[10], [11], [12]。
Fig. 1
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Fig. 1. (a) The key research area of ammonia engine, (b) Problems and research duration of the lubrication for different engine.
图 1. (a) 氨发动机的关键研究领域，(b) 不同发动机润滑的问题及研究持续时间。

Previous studies have indicated that changing the fuel can affect the working conditions of the engine, and the lubricating oils also need to be adjusted accordingly[13], [14], as shown in Fig. 1(b). For the diesel engine, the sulfur element in the fuel can cause tribocorrosion in the engine, especially for the cylinder liner-piston ring (CLPR). The disadvantage of the sulfur element can be eliminated by increasing the total base number of the lubricating oil. And the current research is focused on improving the performance of the lubricating oil[15], [16], [17]. For the LNG engine, the lubrication effect of LNG is poor, and the temperature of the LNG engine also is higher than that of the traditional engine. It is important to consider the ash content of gas fuel and thermal damage of material when developing lubricating oil. However, excessive ash can also lead to an increase of wear on the friction pair surface. For the methanol engine, the impact of methanol and its combustion products on the physicochemical properties of lubricating oil is significant, particularly in regard to the degradation of the lubricating oil [18]. As a result, methanol and its combustion products can lead to an increase in the mechanical wear of the engine. Due to the degradation of lubricating oil, methanol and its combustion products can also directly contact the friction pair of the engine, resulting in tribocorrosion of the friction pair. Although the lubricating oils of the engine have been studied for over 50 years[19], research on lubricating oils for the ammonia engine has just begun. The current researches of the ammonia fuel engine mainly focus on injection strategies, ignition behaviors, and exhaust gas aftertreatment. However, few researches about the tribological properties of the ammonia fuel engine have been reported. The effects of ammonia fuel and its combustion products on the performance of lubricating oil are unclear, which hinders the development of the ammonia fuel engine. Therefore, it is necessary to study the change of lubricating oil in the ammonia engine, which is beneficial to developing the specific lubricating oil for the ammonia engine.
先前的研究表明，改变燃料可能会影响发动机的工作条件，润滑油也需要相应调整[13],[14]，如图 1(b)所示。对于柴油发动机而言，燃料中的硫元素会引起发动机的摩擦腐蚀，特别是气缸套-活塞环（CLPR）部位。硫元素的不利影响可通过提高润滑油的总碱值来消除。目前的研究重点是提升润滑油的性能[15],[16],[17]。对于 LNG 发动机，LNG 的润滑效果较差，且 LNG 发动机的温度也高于传统发动机。在开发润滑油时，需要考虑气体燃料的灰分含量以及材料的热损伤问题。然而，过量的灰分也会导致摩擦副表面磨损增加。对于甲醇发动机而言，甲醇及其燃烧产物对润滑油理化性能的影响显著，特别是会导致润滑油的降解[18]。 因此，甲醇及其燃烧产物会导致发动机机械磨损加剧。由于润滑油性能退化，甲醇及其燃烧产物还可能直接接触发动机摩擦副，造成摩擦副的摩擦腐蚀。虽然针对发动机润滑油的研究已持续 50 余年[19]，但氨燃料发动机润滑油的研究才刚刚起步。目前关于氨燃料发动机的研究主要集中在喷射策略、点火特性和尾气后处理等方面，而涉及氨燃料发动机摩擦学特性的研究鲜有报道。氨燃料及其燃烧产物对润滑油性能的影响尚不明确，这制约了氨燃料发动机的发展。因此，有必要研究氨燃料发动机中润滑油的变化规律，这将有助于开发适用于氨燃料发动机的专用润滑油。
For ammonia fuel, it has a certain corrosiveness. The corrosion behavior of the metal in ammonia environments varies significantly due to factors such as ammonia concentration, temperature, and exposure duration[20]. Researches indicate that high ammonia concentrations can accelerate corrosion, particularly in the copper or the aluminum[21], [22], while steel shows a relatively better resistance[23]. The primary damage observed in steel within ammonia environments is stress corrosion cracking (SCC)[23], [24], [25], while the SCC can be mitigated by adding water (0.1 wt% − 0.5 wt%) into ammonia fuel[26]. However, ammonia can easily combine with water to form the ammonia solution, which can also corrode the metal. Besides, water can be introduced by multiple ways, including the ammonia fuel itself, its combustion products, and scavenging processes[15], [27], [28]. As a result, the effect of ammonia solution cannot be ignored as well. Based on the combustion performances of ammonia fuel, the inherent structural characteristics of the engine, and the multi-lubrication state of the friction pairs, the tribological performance of the engine may be influenced by ammonia fuel in two main ways, as illustrated in Fig. 2(a).
对于氨燃料而言，它具有一定的腐蚀性。金属在氨环境中的腐蚀行为会因氨的浓度、温度以及暴露时间等因素而产生显著差异[20]。研究表明，高浓度的氨会加速腐蚀，特别是在铜或铝材料中更为明显[21]，[22]，而钢材则表现出相对较好的耐腐蚀性[23]。在氨环境中，钢材主要的损伤形式是应力腐蚀开裂（SCC）[23]，[24]，[25]，而通过向氨燃料中添加少量水（0.1 wt% − 0.5 wt%）可以减轻这种应力腐蚀开裂现象[26]。然而，氨容易与水结合形成氨水溶液，同样也会腐蚀金属。此外，水可以通过多种途径引入，包括氨燃料本身、其燃烧产物以及扫气过程[15]，[27]，[28]。因此，氨水溶液的影响同样不容忽视。 根据氨燃料的燃烧特性、发动机的固有结构特征以及摩擦副的多润滑状态，氨燃料可能会通过两种主要方式影响发动机的摩擦学性能，如图 2(a)所示。
Fig. 2
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Fig. 2. (a) The potential effects of ammonia on the engine tribological performance, (b) the research strategy for the changes of lubricating oil in the ammonia engine.
图 2. (a)氨对发动机摩擦学性能的潜在影响，(b)氨发动机润滑油变化的研究策略

Firstly, due to the physicochemical properties of the ammonia and its combustion products, the electrochemical reactions could appear on the surface of the ammonia fuel engine. Under the action of load and frequency, the electrochemical reactions can also lead to the tribocorrosion within the engine[29].
首先，由于氨及其燃烧产物的物理化学特性，氨燃料发动机表面可能发生电化学反应。在载荷和频率作用下，这些电化学反应还会导致发动机内部产生摩擦腐蚀[29]。
Secondly, ammonia and its combustion products have the potential to accelerate the degradation of lubricating oil[11]. When the lubricating oil is degraded, the engine will have poor lubrication performance, resulting in an increase in the mechanical wear on the surface of the friction pairs [30].
其次，氨及其燃烧产物可能加速润滑油的劣化[11]。当润滑油发生劣化时，发动机润滑性能下降，将导致摩擦副表面机械磨损加剧[30]。
In this work, the changes of the lubricating oil of the ammonia fuel engine were investigated by a comparative study. The lubricating oil (P-O), the water-lubricating oil (W-O), and the ammonia-water-lubricating oil (N-O) were prepared (Fig. 2(b)), and the dispersion stability, thermal stability, viscosity-temperature characteristics, and lubrication characteristics of different lubricating oils were investigated respectively. The transformation law of ammonia fuel and its combustion products on the physicochemical properties of lubricating oil were proposed, and the effect mechanism of ammonia on the lubricating oil was explored.
本研究通过对比实验探究了氨燃料发动机润滑油的变化规律。分别制备了纯润滑油(P-O)、水-润滑油混合物(W-O)以及氨水-润滑油混合物(N-O)(图 2(b))，系统考察了不同润滑油的分散稳定性、热稳定性、粘温特性及润滑特性。研究提出了氨燃料及其燃烧产物对润滑油理化性能的转化规律，并揭示了氨对润滑油的作用机理。

2. Experimental  2. 实验部分

2.1. Material  2.1. 材料

The lubricating oil (15W-40) was one type of fully formulated oil, which was supplied by a commercial company. The key additive components in the lubricating oil are shown in Table 1. The distilled water was provided by the laboratory, and the ammonia solution (25 %, AR) was purchased from Shanghai Macklin Biochemical Technology Co., Ltd. The cylinder liner-piston ring (CLPR) was used as friction pairs to obtain the Stribeck curves. The cylinder liner was polished, and the CLPR underwent ultrasonic cleaning in the ethanol for 10 mins before the friction test.
所用润滑油（15W-40）为某商业公司提供的全配方油品，其关键添加剂成分如表 1 所示。实验用水为实验室提供的蒸馏水，氨水溶液（25%，分析纯）购自上海麦克林生化科技有限公司。采用缸套-活塞环（CLPR）作为摩擦副获取斯特里贝克曲线。缸套经抛光处理，CLPR 在摩擦测试前需用乙醇超声清洗 10 分钟。

Table 1. The components of lubricating oil.
表 1. 润滑油成分组成

Components  成分Content (wt%)  成分（重量百分比）CAS No.a  CAS 编号 a
Base oil  基础油> 86.00  > 86.00-
Zinc Dialkyldithiophosphates (ZDDP)
二烷基二硫代磷酸锌（ZDDP）		< 1.00  小于 1.0068649 −42 −3  68649-42-3
Overbased calcium phenate (OCP)
高碱值磺酸钙（OCP）		< 1.00  小于 1.0068855 −45 −8
Othersb  其他 b < 12.00-
a
CAS No. is the abbreviation of the Chemical Abstracts Service numbers.
CAS 编号是化学文摘社登记号的缩写。
b
Organic friction modifiers (OFM), detergent, dispersant, and antioxidant may be present in the lubricating oil.
润滑油中可能含有有机摩擦改进剂(OFM)、清净剂、分散剂和抗氧化剂。
Three types of samples were prepared to explore the effect of ammonia on the lubricating oil, as shown in Table 2. All lubricating oil was stirred at 300 r/min for 10 mins before being used.
为探究氨对润滑油的影响，制备了三种样品（如表 2 所示）。所有润滑油在使用前均以 300 转/分钟的转速搅拌 10 分钟。

Table 2. The ratio of different lubricating oil.
表 2. 不同润滑油的配比

Sample code  示例代码Lubricating oil (wt%)  润滑油（质量百分比）Water (wt%)  水（质量百分比）Ammonia solution (wt%)  氨溶液（质量百分比）
P-O100.00--
W-O95.005.00-
N-O95.00-5.00

2.2. Characterization  2.2. 表征

The macro and micro images of different lubricating oils were collected by the digital camera (a6000, Sony, Japan) and the optical microscope (DVM6A, Leica, Germany), respectively.
通过数码相机（a6000，索尼，日本）和光学显微镜（DVM6A，徕卡，德国）分别采集了不同润滑油的宏观与微观图像。
The elemental content of lubricating oils was measured by an inductively coupled plasma tandem mass spectrometer (ICP-OES/MS 7800, Agilent, USA) was used to measure.
采用电感耦合等离子体串联质谱仪（ICP-OES/MS 7800，安捷伦，美国）测定润滑油的元素含量。
The thermal stabilities of lubricating oils were performed on a thermogravimetric analyzer (449 C, STA, Germany) to obtain the degradation temperature from room temperature to 600 °C with a heating rate of 10 °C/min under nitrogen atmosphere.
使用热重分析仪（449 C，STA，德国）在氮气氛围下以 10°C/min 的升温速率从室温加热至 600°C，测定润滑油的热稳定性并获取其降解温度。
The rheology properties of different lubricating oils were obtained by a rotational rheometer (RSO, Brookfield, USA). The viscosity of the lubricating oil was measured using continuous flow sweeps from 25 °C to 100 °C at a shear rate of 10 s−1. The shear stress and the viscosity of lubricating oils were tested with the shear rate range from 0.1 s−1 to 1000−1 at 25 °C.
采用旋转流变仪（RSO，美国博勒飞）测定不同润滑油的流变特性。通过连续流动扫描测试润滑油在 10 s⁻¹剪切速率下从 25°C 升温至 100°C 时的粘度变化。在 25°C 条件下，测试了润滑油在 0.1 s⁻¹至 1000 s⁻¹剪切速率范围内的剪切应力与粘度特性。
The total base number (TBN) of different lubricating oils was tested according to ASTM D2896–21.
不同润滑油脂的总碱值（TBN）依据 ASTM D2896–21 标准进行测试。
The Stribeck curves of different lubricating oil were obtained by HFRR (UMT Tribolad, Bruker, USA). In all tests, the stroke length was fixed at 10 mm, the load was fixed at 50 N, and the frequency was changed from 0.01 Hz to 25.00 Hz. The interval of the frequency was set as 0.02 Hz, 0.20 Hz and 2.00 Hz, when the frequency was in the range from 0.01 Hz to 0.09 Hz, from 0.10 Hz to 0.90 Hz, and 1.00 Hz to 25.00 Hz, respectively. All tests were conducted at room temperature and atmospheric conditions. The cylinder liner and the piston ring were used as friction pairs in the HFRR test.
不同润滑油的 Stribeck 曲线通过高频往复试验机（HFRR，UMT Tribolad，Bruker，美国）获得。所有试验中，行程长度固定为 10 mm，载荷固定为 50 N，频率从 0.01 Hz 变化至 25.00 Hz。频率区间分别设置为：当频率范围为 0.01 Hz 至 0.09 Hz 时，间隔为 0.02 Hz；当频率范围为 0.10 Hz 至 0.90 Hz 时，间隔为 0.20 Hz；当频率范围为 1.00 Hz 至 25.00 Hz 时，间隔为 2.00 Hz。所有测试均在室温及大气条件下进行。HFRR 测试中使用的摩擦副为气缸套与活塞环。
The lubrication properties of different lubricating oils were also investigated by film-forming ability under elastohydrodynamic lubrication (EHL) regime with the oil film thickness tester (EHD281, PCS, UK). The liquid film test platform was a point contact photoelastic flow lubrication test bench with steel balls and glass discs. SRR was set as 0, the load was 10 N, and the rotational speed was 0–1000 mm/s. A rotating steel ball and a transparent flat disc were used in the EHL test. The ball specimens were made of standard AISI 52100 bearing steel, and discs made of both glass and sapphire were used, both of which were purchased from PCS.
不同润滑油的润滑性能也通过在弹性流体动力润滑（EHL）条件下形成的油膜能力进行了研究，实验使用油膜厚度测试仪（EHD281，PCS，英国）。液膜测试平台为一个点接触光弹流润滑试验台，采用钢球与玻璃盘的配置。实验设定滑滚比（SRR）为 0，载荷为 10 N，转速范围为 0–1000 mm/s。EHL 测试中使用了一个旋转钢球和一个透明平面圆盘。球体试样采用标准 AISI 52100 轴承钢制成，圆盘则采用玻璃和蓝宝石材料，均由 PCS 公司提供。
The chemical structures of lubricating oils were acquired by Fourier transform infrared (FTIR) spectroscopy (IR Tracer, Shimadzu, Japan), and the scan range was from 4000 cm−1 to 450 cm−1 (KBr disks).
利用傅里叶变换红外光谱仪（IR Tracer，日本岛津）获取润滑油化学结构信息，扫描范围为 4000 cm⁻¹至 450 cm⁻¹（溴化钾压片法）。
The interaction between different components in lubricating oil was characterized by 1H nuclear magnetic resonance (NMR) spectra. Samples were subjected to 1HNMR analysis by using a Bruker 400 MHz (AVANCE 400, Bruker, Germany).
通过氢核磁共振谱（¹H NMR）表征润滑油中各组分间的相互作用。使用布鲁克 400 MHz 核磁共振仪（AVANCE 400，德国布鲁克）进行¹HNMR 分析。

3. Results and discussion
3. 结果与讨论

3.1. The dispersion stability
3.1. 分散稳定性

As shown in Fig. 3, the appearance of the P-O remained clear and transparent without change over time. When water or ammonia solution was added to the lubricating oil, both the W-O and the N-O turned white and turbid at first. Over time, the W-O and the N-O began to stratify after standing for 120 h. The upper layer of the W-O gradually became transparent, while the lower layer of the N-O gradually became transparent.
如图 3 所示，P-O 的外观随时间推移保持清澈透明且无变化。当向润滑油中添加水或氨溶液时，W-O 和 N-O 最初均呈现白色浑浊状态。静置 120 小时后，W-O 和 N-O 开始出现分层现象。其中 W-O 的上层逐渐变得透明，而 N-O 的下层则逐渐澄清。
Fig. 3
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Fig. 3. The macro images of the P-O, the W-O and the N-O at different times and temperatures.
图 3. P-O、W-O 和 N-O 在不同时间与温度下的宏观形貌图

With the increase of temperature, the P-O did not change, while the W-O gradually transitioned from white and turbid to clear and transparent. Although the N-O also gradually changed from white and turbid to clear transparency, white insoluble precipitates appeared in the N-O, which was different from the W-O.
随着温度升高，P-O 未发生变化，而 W-O 逐渐由白色浑浊转变为清澈透明。虽然 N-O 同样逐渐从白色浑浊变为透明澄清，但 N-O 中出现了白色不溶沉淀物，这与 W-O 存在差异。
The micro images of different lubricating oils are shown in Fig. 4. The texture of platform was displayed in the image, as the P-O was clear and transparent. Numerous circular droplets appeared in the W-O, which was due to the separation characteristics of oil and water[31]. The water in the lubricating oil would be dispersed in the lubricating oil after mixing, resulting in the emulsification of the lubricating oil. This also led to the physical description of the W-O as white and turbid. Different from the W-O, even though water droplets were also presented in the N-O, N-O was still turbid from a microscopic perspective. This indicates that the action mechanism of ammonia solution on lubricating oil is different from that of water. It was difficult to explain the change of N-O based on the emulsification alone.
图 4 展示了不同润滑油的显微图像。由于 P-O 清澈透明，图像中可观察到平台纹理。W-O 中出现了大量圆形液滴，这是油水分离特性所致[31]。润滑油中的水分经混合后会分散在油液中，导致润滑油乳化。这也解释了 W-O 呈现白色浑浊的物理特性。与 W-O 不同，尽管 N-O 中也存在水滴，但从微观视角观察 N-O 仍保持浑浊状态。这表明氨溶液对润滑油的作用机理与水存在差异，仅凭乳化现象难以解释 N-O 的变化。
Fig. 4
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Fig. 4. The micro images of the P-O, the W-O and the N-O after stirring at 25 ℃.
图 4. 25℃搅拌处理后 P-O、W-O 与 N-O 的显微图像

To analyze the influence of the ammonia solution on the lubricating oil additives, the contents of Zn (ZDDP) and Ca (OCP) in the P-O, the N-O bottom and the N-O top were measured by the ICP-MS. And the samples before used were stood for 360 h at room temperature, and then heated at 100 ℃ for 2 h. As shown in Fig. 5(a), compared with the P-O, the Ca element content in the N-O bottom and the N-O top did not change significantly. This indicated that the ammonia solution did not impact the distribution of OCP in the lubricating oil. However, the Zn element content of different lubricating oils varied significantly. Compared to the P-O, the Zn element content of the N-O top was higher, while the Zn element content of the N-O bottom was lower. This indicated that the ammonia solution destroyed the dispersion stability of ZDDP in the lubricating oil.
为分析氨溶液对润滑油添加剂的影响，采用 ICP-MS 测定了 P-O、N-O 底部和 N-O 顶部中 Zn（ZDDP）和 Ca（OCP）的含量。样品在使用前于室温静置 360 小时后，再经 100℃加热 2 小时。如图 5(a)所示，与 P-O 相比，N-O 底部和顶部的 Ca 元素含量未发生显著变化，表明氨溶液未影响 OCP 在润滑油中的分布。然而不同润滑油中 Zn 元素含量差异显著：相较于 P-O，N-O 顶部的 Zn 元素含量升高，而底部含量降低，这说明氨溶液破坏了 ZDDP 在润滑油中的分散稳定性。
Fig. 5
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Fig. 5. The elements content of different lubricating oil, (a) room temperature, (b) 100 ℃.
图 5. 不同润滑油元素含量：(a)室温状态，(b)100℃状态

Fig. 5(b) shows the element content of different samples after 2 h with heat treatment at 100 ℃. The Zn element content and Ca element content of the P-O and the N-O top were equal, respectively. This indicated that the influence of ammonia solution on the ZDDP can be regulated by temperature. However, this also meant that the white insoluble precipitates in the bottom of N-O top were not the ZDDP, the OCP or their reactants, which may be caused by the interaction of the ammonia solution with other additives.
图(b)展示了各样品在 100℃热处理 2 小时后的元素含量情况。P-O 油样顶部与 N-O 油样顶部的锌元素含量和钙元素含量分别相等。这表明氨溶液对二烷基二硫代磷酸锌(ZDDP)的影响可通过温度进行调节。但这也意味着 N-O 油样底部出现的白色不溶沉淀物并非 ZDDP、OCP 或其反应产物，而可能是氨溶液与其他添加剂相互作用所致。

3.2. The thermal stability
3.2 热稳定性研究

Thermogravimetric Analysis (TGA) and Derivative Thermogravimetry (DTG) were carried out to analyze the thermal stability of lubricating oils. As shown in Fig. 6, the TGA curves of the P-O and the W-O were similar, and the difference of T5 %, T50 % and T95 % between the W-O and the P-O also were small. However, the T5 %, T50 % and T95 % of the N-O showed an improvement of 32.92 %, 18.14 %, and 33.35 % respectively, when compared to those of the P-O. The DTG results showed that the ammonia solution can make DTG curves shift to a higher temperature, and the temperature of the maximum weight loss rate increased, while the effect of water was the opposite.
采用热重分析(TGA)和微分热重法(DTG)对润滑油的热稳定性进行测试。如图 6 所示，P-O 与 W-O 的 TGA 曲线形态相似，且 W-O 与 P-O 在 T 5 % 、T 50 % 和 T 95 % 的差异较小。然而与 P-O 相比，N-O 在 T 5 % 、T 50 % 和 T 95 % 三个特征温度点分别提升了 32.92%、18.14%和 33.35%。DTG 结果表明氨溶液能使 DTG 曲线向高温方向偏移，最大失重速率对应的温度升高，而水溶液则呈现相反效果。
Fig. 6
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Fig. 6. The thermal stability of different lubricating oils, (a)the TGA curves, (b) the temperature at 5 % mass loss (T5 %), 50 % mass loss (T50 %) and 95 % mass loss(T95 %), respectively, (c) the DTG curves.
图 6. 不同润滑油的热稳定性对比：(a)热重分析曲线，(b)分别为质量损失 5%(T 5 % )、50%(T 50 % )和 95%(T 95 % )时的温度，(c)微分热重曲线

These results indicated that the effect of water on the T5 %, T50 % and T95 % of the lubricant was minor, and the temperature of the maximum weight loss rate of the lubricant can be reduced by adding water. However, the ammonia solution improved the thermal stability of the lubricating oil, which also means the action mechanism of the ammonia solution was different from that of the water.
实验结果表明，水分对润滑油 T 5 % 、T 50 % 和 T 95 % 温度的影响较小，但添加水分会降低润滑油的最大失重速率温度。而氨水溶液则能提高润滑油的热稳定性，这表明氨水溶液的作用机理与水不同。

3.3. The rheological properties
3.3. 流变特性

The rheological properties of different lubricating oils are shown in Fig. 7. There was a nonlinear relationship between the shear stress and the shear rate of the P-O, the W-O and the N-O. All lubricating oils had described a characteristic non-linear behavior of the non-Newtonian fluid. Although the shear stress of P-O, the W-O and N-O was similar at the same shear rate in the high-frequency region, the shear stress of N-O was significantly higher than that of the P-O and the W-O at the same shear rate in the low-frequency region. The viscosity changes of different lubricating oils under different shear rates are shown in the Fig. 7(b). Compared to the P-O and the W-O, the N-O showed a shear-thinning trend, which also corroborated the results of the shear stress curves.
不同润滑油的流变特性如图 7 所示。P-O、W-O 和 N-O 的剪切应力与剪切速率之间呈现非线性关系，所有润滑油均表现出非牛顿流体的典型非线性行为。虽然在高频区相同剪切速率下 P-O、W-O 和 N-O 的剪切应力相近，但在低频区相同剪切速率下 N-O 的剪切应力明显高于 P-O 和 W-O。图 7(b)展示了不同润滑油在不同剪切速率下的粘度变化。与 P-O 和 W-O 相比，N-O 表现出剪切稀化趋势，这也印证了剪切应力曲线的测试结果。
Fig. 7
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Fig. 7. The rheological properties of different lubricating oils, (a) the shear stress-shear rate curves, (b) the viscosity-shear rate curves, (c) the viscosity-temperature curves, (d) the average viscosity at different temperatures.
图 7. 不同润滑油的流变特性：(a)剪切应力-剪切速率曲线，(b)粘度-剪切速率曲线，(c)粘度-温度曲线，(d)不同温度下的平均粘度。

The effect of temperature on the viscosity of lubricating oils is shown in Fig. 7(c). The viscosity-temperature curve of W-O was smooth, which was similar to the P-O. However, the viscosity-temperature characteristic curve of the N-O can be divided into different stages as follows:
温度对润滑油黏度的影响如图 7(c)所示。W-O 的黏温曲线平滑，与 P-O 相似。而 N-O 的黏温特性曲线可分为以下不同阶段：
  • 1.
    Stage 1, as the temperature increased, the viscosity of the N-O decreased.
    第一阶段，随着温度升高，N-O 的黏度下降。
  • 2.
    Stage 2, as the temperature increased, the viscosity of N-O was almost constant.
    第二阶段，随着温度升高，N-O 的黏度几乎保持不变。
  • 3.
    Stage 3, as the temperature increased, the viscosity of N-O began to decline again.
    第三阶段，随着温度升高，N-O 的黏度再次开始下降。
  • 4.
    Stage 4, as the temperature increased, the viscosity of N-O slowly decreased.
    随着温度升高，N-O 润滑油的粘度缓慢下降。
As shown in Fig. 7(d), the viscosity of different lubricating oils decreased gradually with the increase of temperature. It is because that the higher temperature can promote the thermal movement of lubricating oil molecules, resulting in a smoother oil flow. The viscosity of W-O and N-O was higher than that of P-O at the same temperature. This indicated the water or the ammonia solution could increase the viscosity of the lubricating oil. It was noteworthy that the viscosity of W-O at 25 ℃ increased by 53.80 % compared to P-O, while the viscosity of N-O increased by 476.33 %. With the increase of temperature, the difference in viscosity of the P-O, the W-O and the N-O gradually became smaller. It was worth noting that the viscosity of N-O may pose challenges for the cold start of the engine.
如图 7(d)所示，不同润滑油粘度均随温度升高逐渐降低。这是因为较高温度会促进润滑油分子的热运动，使油液流动更为顺畅。在相同温度下，W-O 和 N-O 的粘度均高于 P-O，这表明水分或氨溶液会增大润滑油粘度。值得注意的是，25℃时 W-O 粘度较 P-O 增加了 53.80%，而 N-O 粘度增幅高达 476.33%。随着温度上升，P-O、W-O 与 N-O 之间的粘度差异逐渐缩小。需要特别指出的是，N-O 的高粘度特性可能对发动机冷启动带来挑战。
After the lubricating oil had been mixed with the water, a water-in-oil system was formed (Fig. 4). The interaction between the water droplets led to an increase in the viscosity of the lubricating oil[32]. Although the ammonia solution also contained water, the above results indicated that the change in viscosity of N-O cannot be directly attributable to the interaction of water droplets. The viscosity-temperature characteristic of the N-O was completely different from that of the P-O and the W-O, and the good viscosity-temperature characteristics of lubricating oil were affected by the ammonia solution medium.
润滑油与水混合后，形成了油包水体系（图 4）。水滴间的相互作用导致润滑油粘度上升[32]。虽然氨溶液也含有水分，但上述结果表明 N-O 的粘度变化不能直接归因于水滴的相互作用。N-O 的粘温特性与 P-O 和 W-O 完全不同，润滑油良好的粘温特性受到了氨溶液介质的影响。

3.4. The changes of TBN
3.4. 总碱值变化

The TBN additive in lubricating oil can neutralize acidic by-products from combustion, thus preventing corrosion of the friction surfaces. TBN serves as a crucial indicator of the base value of lubricating oil. As shown in Fig. 8, the TBN of W-O and P-O are identical, indicating that the addition of water does not affect the TBN. In contrast, the TBN of N-O increased by approximately 534.62 % compared to P-O and W-O, demonstrating that the ammonia solution significantly enhances the TBN of the lubricating oil.
润滑油中的 TBN 添加剂能够中和燃烧产生的酸性副产物，从而防止摩擦表面腐蚀。TBN 是衡量润滑油碱值的关键指标。如图 8 所示，W-O 与 P-O 的 TBN 数值相同，表明水分添加不会影响碱值。相比之下，N-O 的 TBN 较 P-O 和 W-O 提升了约 534.62%，证明氨水溶液能显著增强润滑油的碱值。
Fig. 8
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Fig. 8. The TBN of P-O, W-O, and N-O.
图 8. P-O、W-O 与 N-O 的碱值对比

The TBN of P-O is affected by the base number additive in the lubricating oil. More specifically, in this experiment, the TBN of P-O was determined by the content of the overbased calcium phenol. Calcium ions in the overbased calcium phenol can neutralize the acidic substances produced by combustion. However, the ammonia solution, being alkaline, also contributes to this effect. The hydrolysis of ammonia molecules generates hydroxide ions, which can also neutralize acidic substances. Therefore, the increase in TBN for N-O is likely due to the hydrolysis of the ammonia. However, it is uncertain that the ammonia solution can effectively neutralize acids which were generated during the operation of engine.
P-O 的总碱值受润滑油中碱性添加剂的影响。具体而言，在本实验中，P-O 的总碱值由高碱值酚钙的含量决定。高碱值酚钙中的钙离子能够中和燃烧产生的酸性物质。然而，氨溶液作为碱性物质同样具有中和作用——氨分子水解产生的氢氧根离子也能中和酸性物质。因此 N-O 总碱值的升高很可能是氨水解所致。但尚不确定氨溶液能否有效中和发动机运行过程中产生的酸性物质。

3.5. The lubricating properties
3.5 润滑性能

Fig. 9 illustrates the Stribeck curves of different lubricating oils. In general, all friction coefficients (COFs) experienced a rapid decrease with increasing frequency at phase 1, followed by a slower decrease in the second stage, and ultimately a gradual increase as frequency continues to rise. For the P-O, the lowest COF occurred at a frequency of 3 Hz, with a value of 0.047. For the N-O, the lowest COF was found at a frequency of 9 Hz, with a value of 0.029. This discrepancy can be linked to variations in sample viscosity. For the W-O, the change of COF in phase 2 was different from that of the P-O and the N-O, possibly due to the instability of the oil-water system.
图 9 展示了不同润滑油的斯特里贝克曲线。总体而言，所有摩擦系数（COF）在第一阶段均随频率增加而快速下降，第二阶段下降速度减缓，最终随着频率持续上升而逐渐增大。对于 P-O 油样，最低摩擦系数出现在 3 赫兹频率处，数值为 0.047；而 N-O 油样的最低摩擦系数则出现在 9 赫兹频率，数值为 0.029。这种差异可能与样品黏度变化有关。值得注意的是，W-O 油样在第二阶段的摩擦系数变化趋势与 P-O 和 N-O 不同，这可能是由于油水体系的不稳定性所致。
Fig. 9
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Fig. 9. The Stribeck curves of the P-O, the W-O, and the N-O.
图 9. P-O、W-O 和 N-O 三种油样的斯特里贝克曲线

According to the classical lubrication theory, the surface of the friction pair will be affected by the bearing capacity and the viscous force of the lubricating oil[33]. Both the frequency of the friction pair and the viscosity of the lubricating oil can influence the bearing capacity and the viscous force of the lubricating oil [34]. Increasing the frequency and the viscosity had both positive and negative effects. On the one hand, they can enhance the hydrodynamic lubrication effect and reduce the COF. On the other hand, they also can lead to an increase in the viscous force, and increase the COF[35]. Compared with the P-O, the viscosity of the N-O was significantly higher. This meant that in the process of balancing the viscous force and the bearing capacity, the N-O necessitated a greater frequency to exhibit a lower COF. This also indicated that even after adding the water or the ammonia solution, all lubricating oils still retained the lubricating property.
根据经典润滑理论，摩擦副表面会受到润滑油承载力和粘滞力的双重影响[33]。摩擦副运动频率与润滑油粘度均能影响其承载力和粘滞力[34]。提高频率与粘度存在双重效应：一方面可增强流体动压润滑效果降低摩擦系数，另一方面也会导致粘滞力增大反而提升摩擦系数[35]。与 P-O 油相比，N-O 油粘度显著更高，这意味着在平衡粘滞力与承载力的过程中，N-O 油需要更大运动频率才能呈现较低摩擦系数。该现象也表明即使添加水或氨溶液后，所有润滑油仍保持着润滑性能。
In order to further study the lubricating properties of different lubricating oil, a point contact photoelastic flow lubrication test bench was used to evaluate the film-forming ability of different lubricating oils, and the interference liquid film of the whole contact was recorded directly. Interferometric images of the contact at selected speeds are shown in Fig. 10 (a). In most images, the central region of the contact shows a uniform color, indicating a consistent film thickness. This uniformity results from the high and relatively stable pressure in this area, which keeps the lubricant film nearly constant until it reaches the contact's outlet, where a constriction in the film thickness occurs. This constriction, also evident at the contact's sides, is a result of the rapid decrease of the pressure at the outskirts of the contact. This suggests that P-O, W-O, and N-O all possess film-forming properties. However, the images of different lubricating oils show variations, particularly at speeds of 50 mm/s or 100 mm/s. This suggests differing film-forming properties among the lubricating oils.
为深入研究不同润滑油的润滑性能，采用点接触光弹流润滑试验台评估了各类润滑油的成膜能力，并直接记录了整个接触区域的干涉液膜图像。选定速度下的接触干涉图像如图 10(a)所示。多数图像中接触区中心呈现均匀色泽，表明该区域膜厚一致。这种均匀性源于该区域较高且相对稳定的压力，使得润滑油膜在抵达接触区出口前保持近乎恒定，随后出现膜厚收缩现象。这种收缩在接触区两侧同样明显，是由接触边缘压力急剧下降所致。这表明 P-O、W-O 和 N-O 均具备成膜特性。然而不同润滑油的图像存在差异，尤其在 50mm/s 或 100mm/s 速度下，反映出各润滑油成膜性能的差别。
Fig. 10
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Fig. 10. The film-forming ability of different lubricating oils under the load of 10 N: (a) Liquid film images at 25 ℃ for P-O, W-O and N-O with different rolling speeds (10, 50, 100, and 500 mm/s), (b) film thickness curves at the contact point and (c) average film thickness of P-O, W-O and N-O at different temperatures and rolling speeds.
图 10. 不同润滑油在 10 N 载荷下的成膜能力：(a) 25 ℃时 P-O、W-O 和 N-O 在不同滚动速度（10、50、100 和 500 mm/s）下的油膜图像；(b) 接触点处的油膜厚度曲线；(c) P-O、W-O 和 N-O 在不同温度和滚动速度下的平均油膜厚度。

As shown in Fig. 10 (b), the film thickness of different lubricating oils decreases with increasing temperature at a constant speed. And the film thickness initially increases with speed before stabilizing at the same temperature. This is mainly because the increase in speed enhances the hydrodynamic lubrication effect, which raises film thickness, while higher temperatures reduce the oil’s viscosity, potentially decreasing film thickness. However, starvation may occur when the speed becomes so high that the rate of formation of the pool of lubricant in the inlet region becomes insufficient to balance the rate at which it passes through and around the contact, which preventing further increases in film thickness.
如图 10(b)所示，在相同速度下，不同润滑油的膜厚均随温度升高而减小；而在相同温度下，膜厚起初随着速度增加而增加，随后趋于稳定。这主要是因为速度的提高增强了流体动压润滑效应，从而增加了油膜厚度，而温度升高则会降低润滑油的黏度，从而可能减少膜厚。然而，当速度过高时，入口区润滑油积聚的速度不足以弥补其通过和绕过接触区的速度，就会发生供油不足现象，从而阻止油膜厚度的进一步增加。
It was worth noting that at 25 °C and 40 °C, the film thickness of N-O at both 10 mm/s and 100 mm/s was higher than that of P-O, as shown in Fig. 10(c). This may be due to the higher viscosity of N-O compared to P-O at both temperatures. At 80 °C, the difference in viscosity between N-O and P-O was small, resulting in similar film thicknesses for both. Additionally, the effects of changes in speed and temperature on W-O are relatively small compared to P-O and N-O. This may be because the water in W-O disrupts the stability of the lubricating oil, leading to differences in film formation performance between W-O and the other oils. The above results indicate that the ammonia solution does not alter the lubricating properties of the lubricating oil. And the N-O demonstrates the typical behavior of liquid lubricants, with its lubrication effect being influenced by viscosity, temperature, and speed.
值得注意的是，在 25°C 和 40°C 时，如图 10(c)所示，N-O 在 10 mm/s 和 100 mm/s 下的油膜厚度均高于 P-O。这可能是由于 N-O 在这两种温度下的黏度高于 P-O 所致。在 80°C 时，N-O 与 P-O 之间的黏度差异较小，因此两者的油膜厚度相近。此外，与 P-O 和 N-O 相比，速度和温度变化对 W-O 的影响相对较小。这可能是因为 W-O 中的水破坏了润滑油的稳定性，导致 W-O 与其他油类在成膜性能上存在差异。上述结果表明，氨水溶液并未改变润滑油的润滑性能。N-O 表现出典型液体润滑剂的特性，其润滑效果受黏度、温度和速度的影响。

3.6. Comprehensive analysis
3.6. 综合分析

Previous research demonstrated that the influence of water on lubricating oil primarily depended on the ratio of the water to the lubricating oil. However, the aforementioned results showed that the impact of the ammonia solution on the lubricating oil differed from that of the water. The lubricating oil underwent new changes when ammonia was added.
先前的研究表明，水对润滑油的影响主要取决于水与润滑油的比例。然而，上述结果表明，氨水溶液对润滑油的影响与水不同。当加入氨水后，润滑油发生了新的变化。
The FTIR of different lubricating oils is shown in Fig. 11 (a). Compared with the P-O, the FTIR of W-O exhibited notable distinctions, primarily characterized by the emergence of new peaks at approximately 3400 cm−1 and 1630 cm−1. However, the characteristic peaks of water were also located at 3400 cm−1 and 1630 cm−1, suggesting that the difference between the W-O and P-O was primarily due to the presence of water[36]. The water contains hydrogen-bond (H-bond), and the combination of H-bonds leads to a peak at 3400 cm−1. The peak around 1630 cm−1 was attributed to the shear bending vibration occurring within the O-H bond plane.
不同润滑油的 FTIR 图谱如图 11(a)所示。与 P-O 相比，W-O 表现出显著差异，主要表现为在约 3400 cm⁻¹和 1630 cm⁻¹处出现了新的峰。然而，水的特征峰也位于 3400 cm⁻¹和 1630 cm⁻¹处，这表明 W-O 与 P-O 之间的差异主要归因于水分的存在[36]。水含有氢键（H 键），H 键的结合导致在 3400 cm⁻¹处出现峰。1630 cm⁻¹附近的峰归因于 O-H 键平面内的剪切弯曲振动。
Fig. 11
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Fig. 11. (a) FTIR of the P-O, the W-O, and the N-O at room temperature, (b) the diagram of H-bonds in the ammonia solution, (c) FTIR of the N-O at room temperature, 50 ℃ and 75 ℃.
图 11. (a) 室温下 P-O、W-O 和 N-O 的 FTIR 图谱，(b) 氨水溶液中氢键的示意图，(c) N-O 在室温、50℃和 75℃下的 FTIR 图谱。

Ammonia solution also contained the H-bond, previous research has identified four types of H-bonds in the ammonia solution[37], as depicted in Fig. 11 (b). In this case, two characteristic peaks were exhibited in the FTIR of the ammonia solution. The absorption peak at 3400 cm−1 was caused by the symmetric and asymmetric stretching vibrations or the angle-changing vibration of hydrogen atoms. The absorption peak at 1630 cm−1 was attributed to the angular vibration of ammonia molecules[38]. These explain why there were obvious differences between the physicochemical properties of the N-O and the W-O, while the FTIR of them was similar. As depicted in Fig. 11 (c), the FTIR of the N-O at various temperatures provided additional evidence for the action of H-bond. With an increase in temperature, the peak of N-O shifted towards higher wave numbers. This phenomenon can be attributed to the disruption of H-bonds in the N-O caused by elevated temperature. Furthermore, this observation helps to explain the temperature sensitivity of the N-O's viscosity.
氨溶液同样含有氢键，先前研究已识别出氨溶液中存在的四种氢键类型[37]，如图 11(b)所示。该情况下氨溶液的傅里叶变换红外光谱呈现出两个特征峰：3400 cm⁻¹处的吸收峰源于氢原子的对称/非对称伸缩振动或角度变化振动，1630 cm⁻¹处的吸收峰则归因于氨分子的角度振动[38]。这解释了为何 N-O 与 W-O 的物理化学性质存在显著差异，而两者的红外光谱却较为相似。图 11(c)展示了不同温度下 N-O 的红外光谱，为氢键作用提供了进一步证据。随着温度升高，N-O 的吸收峰向高波数方向移动，此现象可归因于温度升高导致 N-O 中氢键的断裂。该发现也有助于解释 N-O 粘度的温度敏感性特征。
Since the electronegativity of O is higher than that of N, resulting in a more regular and tighter arrangement of H-bonds between water molecules compared to ammonia solution[39]. On the one hand, the H-bonds of the ammonia solution were easily sheared and dispersed during the mixing process. On the other hand, -NH might also form H-bond with other groups in the lubricating oil resulting in the ammonia solution building the H-bond network in N-O, and these groups could be provided by other additives.
由于氧(O)的电负性高于氮(N)，导致水分子间的氢键排列比氨溶液更为规整紧密[39]。一方面，氨溶液的氢键在混合过程中易被剪切分散；另一方面，-NH 也可能与润滑油中的其他基团形成氢键，使得氨溶液在 N-O 间构建氢键网络，这些基团可能源自其他添加剂。
Further FTIR and 1H NMR spectra of the N-O also indicated the presence of H-bonds in the lubricating oil. As mentioned earlier, the N-O was stratified after heating treatment, and the FTIR at different parts of the N-O was shown in Fig. 12 (a). Although the FTIR of N-O's supernatant liquid showed similarities to that of P-O, there were still absorption peaks observed at 3400 cm−1 and 1630 cm−1 in the FTIR of N-O's bottom sediment, which would be attributed to -NH group. The 1H NMR of the P-O and the N-O at room temperature are shown in Fig. 12 (b) and (c). In comparison to the P-O, the 1H NMR of the N-O exhibited a shift from high field to low field. This result also suggested that the ammonia solution would establish H-bond network in the lubricating oil. As shown in Fig. 12 (d), the 1H NMR of the N-O gradually shifted towards a higher field with increasing temperature. This indicated that increasing the temperature would destroy the H-bond structure in the N-O, which was also consistent with the previous findings.
进一步的 FTIR 和 1 H NMR 谱图也表明润滑油中存在氢键。如前所述，N-O 经热处理后出现分层，其不同部位的 FTIR 光谱如图 12(a)所示。虽然 N-O 上清液的 FTIR 与 P-O 相似，但在 N-O 底部沉淀物的 FTIR 中仍观察到 3400 cm −1 和 1630 cm −1 处的吸收峰，这应归属于-NH 基团。图 12(b)和(c)分别展示了常温下 P-O 与 N-O 的 1 H NMR 谱图。相较于 P-O，N-O 的 1 H NMR 谱显示出从高场向低场的位移，这一结果同样表明氨溶液会在润滑油中形成氢键网络。如图 12(d)所示，随着温度升高，N-O 的 1 H NMR 谱逐渐向高场移动，说明升温会破坏 N-O 中的氢键结构，这与先前的研究结果一致。
Fig. 12
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Fig. 12. (a) the FTIR of different lubricating oils, (b) 1H NMP spectra of the P-O and the N-O at 25 ℃ and (c) enlarged images, (d) 1H NMP spectra of the N-O at different temperatures.
图 12. (a)不同润滑油的 FTIR 光谱，(b)25℃下 P-O 和 N-O 的 1 H NMP 谱图及(c)局部放大图像，(d)不同温度下 N-O 的 1 H NMP 谱图。

As shown in Fig. 13, when water was added to the lubricating oil, a water-in-oil system was formed, which caused changes in the physicochemical properties of the lubricating oil. After the ammonia solution was added to lubricating oil, the H-bond structure would be introduced in the N-O. The H-bond network in the N-O not only results in a considerable increase in the viscosity of the lubricating oil, but also disrupts the dispersion stability of the lubricating oil. Additionally, the H-bond network also can capture the lubricating oil additives. Compared with that of P-O or W-O, the better thermal stability of N-O can also be attributed to the formation of H-bond. The fracture of H-bonds is endothermic, which may improve the thermal stability of lubricating oil. Despite the N-O still demonstrating lubricating properties in controlled laboratory conditions, the physicochemical characteristics of the N-O change a lot. These changes suggest potential risks when using existing lubricating oils for the ammonia fuel engine, particularly due to the rise in viscosity and the potential failure of additives.
如图 13 所示，当水混入润滑油时，会形成油包水体系，导致润滑油的物理化学性质发生变化。氨溶液加入润滑油后，会在 N-O 结构中引入氢键。这种 N-O 结构中的氢键网络不仅会显著增加润滑油粘度，还会破坏润滑油的分散稳定性。此外，氢键网络还会捕获润滑油添加剂。与 P-O 或 W-O 相比，N-O 更优异的热稳定性同样可归因于氢键的形成——氢键断裂是吸热过程，这可能会提升润滑油的热稳定性。尽管在受控实验室条件下 N-O 仍表现出润滑性能，但其物理化学特性已发生显著改变。这些变化表明，现有润滑油用于氨燃料发动机时存在潜在风险，尤其是粘度上升和添加剂可能失效带来的问题。
Fig. 13
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Fig. 13. The influence mechanism of water and ammonia solution on the lubricating oil.
图 13. 水与氨溶液对润滑油的作用机理

4. Conclusion  4. 结论

In this study, by configuring different components of lubricating oil, a comparative study was conducted to explore the change rules in the physicochemical properties of lubricating oil for the ammonia engine. The action mechanism of ammonia on the lubricating oil was also investigated. The main conclusions were drawn as follows:
本研究通过配置不同成分的润滑油，开展对比研究以探究氨发动机润滑油理化性质的变化规律，同时考察了氨对润滑油的作用机制。主要结论如下：
  • 1.
    Although the ammonia solution can improve the thermal stability of lubricating oil, it can also affect the dispersion stability of the lubricating oil and capture additive components, which may have an influence on the anti-wear effect of the lubricating oil.
    虽然氨溶液能够提升润滑油的热稳定性，但也会影响润滑油的分散稳定性并捕获添加剂成分，这可能对润滑油的抗磨效果产生影响。
  • 2.
    The viscosity of lubricating oil significantly increased with the addition of ammonia solution, and the viscosity-temperature characteristics of the lubricating oil can also be disrupted by adding ammonia solution.
    添加氨溶液后润滑油粘度显著上升，同时氨溶液的加入还会破坏润滑油的粘温特性。
  • 3.
    The lubricating properties of the lubricating oil were influenced by the ammonia solution, which is mainly reflected in the viscosity of the lubricating oil.
    润滑油的润滑性能受到氨溶液的影响，主要体现在润滑油的粘度变化上。
  • 4.
    Different from that by adding water, the H-bond network can be generated within the lubricating oil by adding ammonia solution, resulting in a change on the physicochemical properties of the lubricating oil.
    与单纯加水不同，氨溶液的加入会在润滑油内部形成氢键网络，从而导致润滑油的物理化学性质发生改变。
  • 5.
    The existing lubricating oil is difficult to meet the needs of the ammonia fuel engine, and the lubricating oil formula needs to be adjusted.
    现有的润滑油难以满足氨燃料发动机的需求，需要调整润滑油配方。
In summary, the dispersion stability and viscosity-temperature characteristics of lubricating oil need to be further improved, which may affect the anti-wear properties of the lubricating oil. Besides, whether the TBN can be used to classify the lubricating oil in the ammonia engine still need to be confirmed.
综上所述，润滑油的分散稳定性和黏温特性仍需进一步提升，这可能影响其抗磨性能。此外，总碱值（TBN）能否用于氨燃料发动机润滑油的分类仍有待验证。

CRediT authorship contribution statement
作者贡献声明

Xiqun Lu: Writing – review & editing, Resources, Conceptualization. Rui Guo: Writing – review & editing, Methodology. Chang Ge: Investigation. Xing Xu: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Xuan Ma: Writing – review & editing, Investigation, Funding acquisition, Conceptualization. Baofeng Zhang: Investigation.
卢锡群：撰写-审阅与编辑，资源，概念提出。 郭瑞：撰写-审阅与编辑，方法论。 葛畅：调查研究。 许星：撰写-初稿，方法论，调查研究，正式分析，数据整理，概念提出。 马璇：撰写-审阅与编辑，调查研究，资金获取，概念提出。 张宝峰：调查研究。

Declaration of Competing Interest
利益冲突声明

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
作者声明，对于本论文所报道的研究工作，不存在任何可能被视为影响研究结果的已知竞争性经济利益或个人关系。

Acknowledgments  致谢

This work was supported by the Study on Tribology and Lubrication Technology of the Marine Low-Speed Engine (No. CBG5N21–1-2), the Tribological Design and Experimental Verification of Key Moving Parts (No. DE0305), the Study on the Key Technologies of Green Ammonia Synthesis and Engine Application, the Fundamental Research Funds for the Central Universities.
本研究得到船用低速发动机摩擦学与润滑技术研究（编号 CBG5N21–1-2）、关键运动部件摩擦学设计与试验验证（编号 DE0305）、绿色氨合成及发动机应用关键技术研究、中央高校基本科研业务费专项资金等项目资助。

Statement of Originality  原创性声明

  • 1.
    The work described here has not been submitted elsewhere for publication, and all the authors listed have approved the manuscript and the submission.
    本文所述工作尚未提交至其他刊物发表，所有列出的作者均已认可稿件内容并同意投稿。
  • 2.
    I have read and have abided by the statement of ethical standards for manuscripts submitted to Tribology International.
    我已阅读并遵守《Tribology International》投稿伦理准则声明。

Data Availability  数据可用性

Data will be made available on request.
数据可根据要求提供。

References

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