Optimization of Oil Bores on Connecting Rod Small End to Prevent Bushing Failures at Maximum Speed

Diesel engines are widely used in heavy-duty machinery, including engineering equipment, ships, and transportation vehicles [1–3]. The power density of diesel engines is developing toward less than 1 kg/kW and greater than 100 kW/L [4,5]. This leads to more severe operating conditions for friction pairs in diesel engines [6–9]. Meanwhile, many diesel engines adopt splash lubrication with oil sprayed from the piston cooling nozzle for small-end bearing to ensure the structural strength of connecting rod [10,11]. Thus, small-end bushings experience various failures such as scuffing, blackening, and loosening, which could stick the crank train [12–14].

Regarding the failures of connecting rod small-end bushing, numerous scholars have investigated the lubrication performance, wear resistance of materials, and mechanical properties of the bushing [15–19]. Yin et al. [20,21] established a lubrication model for the connecting rod small-end bushing based on multibody dynamics theory. Through bench tests, they obtained wear results and temperature data of the bushing under various operating conditions. The results indicated that the locations where the bushing surface turned black were consistent with positions of calculated high temperatures, suggesting that poor lubrication and excessive oil temperature were important factors leading to the lubrication failure of the bushing. Guo et al. [22] conducted lubrication analysis and modeling of the connecting rod small-end bushing using thermoelastic fluid dynamics and microasperity contact theory. They analyzed the heat generation, heat dissipation mechanism, and heat distribution for the bushing. Liu et al. [23] and Chen and Liao [24] researched the problem of the connecting rod small-end bushing loosening. Through bushing heating experiments and simulation analysis, they analyzed the extrusion stress of the bushing under various temperature rise curves and concluded that bushing loosening failure was caused by thermal deformation. Marmorini et al. [25] also studied the issue of bushing loosening. They provided numerical predictions of the tensile inertial forces that could lead to bushing loosening based on an elastic model, along with corresponding design reference data. Liu et al. [26] conducted friction torque, friction temperature, microstructure, and wear comparison experiments on connecting rod small-end bushings manufactured using different processes on a swinging friction pair wear test bench. The results indicated that the failure process of this friction pair is mainly adhesive wear, accompanied by mixed wear forms, including abrasive wear. Liu et al. [27] researched the abnormal wear of connecting rod small-end bushing in medium-to-high-speed diesel engines. The results showed that excessive total oil film pressure and rough contact pressure were the leading causes of abnormal bushing wear.

Through the analysis of existing research on bushing failures, it is evident that the primary reason is unstable splash lubrication. It results in starved lubrication of small-end bearing, coupled with the inability of oil to take away the heat generated by friction effectively. Consequently, localized overheating leads to lubrication failure of the bearing and then bushing failures. Most researchers are currently focusing on optimizing the structure of connecting rod small-end bushing in diesel engines to prevent frequent bushing failures. Due to the lack of an effective method to study splash lubrication, previous studies have not involved the optimization of oil bores on the connecting rod small end.

The smooth particle hydrodynamics (SPH) was first employed to simulate the splash lubrication of small-end bearing in our previous article [10]. It can be observed that a large amount of oil flows through the piston underside. But only a tiny portion of it enters the oil bores on the connecting rod small end, failing to provide powerful cooling and lubrication for the small-end bearing. The temperature of the small-end bearing is the highest at a maximum speed of 2500 r/min, but the oil flowrate and storage of the small-end bearing are minimal at this speed, resulting in poor cooling and lubrication effects on the bushing [10]. Therefore, the dangerous condition for bushing failures is maximum speed. It is necessary to optimize the two oil bores to collect more oil under this condition.

In this article, the optimization of oil bores on the connecting rod small end was carried out based on the established SPH model. There is limited space for enlarging the diameter of oil bores considering the size constraints of small end. Therefore, the optimization variable for the first step was the angle between the axes of two oil bores. A semicolumn baffle was added to the oil bore away from the piston cooling nozzle in the second step. And then, a semicolumn baffle was also added to the other oil bore. The oil flowrate and storage of small-end bearing were analyzed in each step to characterize the cooling intensity and oil supply stability.

The SPH was employed to simulate the splash lubrication of connecting rod small-end bearing in our previous article. The accuracy of the model was verified by comparing experimental and simulated oil pressure and distribution. The theoretical principle and boundary conditions are also introduced in detail [10].

The basis of the simulation model is a 6-cylinder V-type diesel engine with a cylinder angle of 90 deg. As shown in Fig. 1(a), a single-cylinder model was established considering identical structure and similar oil flow of all cylinders.

The small-end bearing of this engine has two oil bores to maximize oil collection with splash lubrication. The oil bore near piston cooling nozzle is defined as bore 1, and the other is bore 2. Additionally, circumferential oil groove and two symmetrical axial oil grooves are machined on the inner surface to store and supply oil to the bottom. The structure of the small-end bearing is shown in Fig. 1(b). The specific geometry parameters of the simulation model are listed in Table 1.

The oil temperature was set to 90 ℃ which is normal during the operation of diesel engines. At this time, the density and dynamic viscosity are 805.1 kg/m³ and 0.015 Pa·s, respectively.

Symmetrical oil bores can simply the process of connecting rod. Therefore, the design of symmetrical oil bores was still adopted. The angle varied from 60 deg to 105 deg with an interval of 15 deg. Only the inflow volume of oil bores is discussed in this section, given that the oil distribution is similar from different angles.

Figure 2 illustrates the cumulative inflow volume of oil bores with different angles between the axes of two oil bores. Two seconds later, the stable number of particles involved in the simulation indicates the flow field enters quasi-static state [10]. Therefore, the cumulative inflow volume between 2 s and 3 s was linearly fitted. The fit slope k represents the flowrate of this oil bore. The cumulative inflow volume of oil bores 1 and 2 is both fluctuating. There are inlet and outlet two states of one oil bore in one rotation due to the interaction between piston motion and oil inertia. Oil bore 1 is a net inlet whose inflow is greater than the outflow. Then oil bore 2 is a net outlet. The flowrate of the net inlet was defined as the oil flowrate of small-end bearing.

The flowrate of small-end bearing initially increases rapidly with an increase in the angle between the axes of two oil bores. It reaches a maximum of 0.094 mL/s at an angle of 90 deg, which is 161.1% higher than the original 0.036 mL/s. A larger angle contributes to more oil collected by oil bore 1 during the clockwise swing of the connecting rod. However, the flowrate slightly decreases then. A more horizontal axis of oil bore 1 during anticlockwise swing leads to more oil being thrown out because of inertia.

Figure 3 illustrates the net inflow volume of oil bores with different angles between the axes of two oil bores. The net inflow volume of oil bores is also the oil storage of small-end bearing. The oil storage also increases first and then decreases with an increase in the angle. The maximal storage of 0.045 mL is obtained at the angle of 90 deg, increasing a little compared to the original 0.04 mL. However, 90 deg is the optimal angle between the axes of two oil bores. The maximal oil flowrate and storage are beneficial to the cooling and oil supply of small-end bearing, respectively.

The oil flowrate of small-end bearing has increased by 161.1% after the optimization of the angle between the axes of two oil bores, but the change in oil storage is not significant as shown in Fig. 3. The oil bore near the piston cooling nozzle is defined as oil bore 1, while the other one is defined as oil bore 2. A large amount of lubricating oil returns to the oil sump from the left side of oil bore 2. Only a tiny portion of it enters the small-end bearing according to the oil distribution in Ref. [10]. Thus, a single-side baffle was added to oil bore 2 to achieve greater oil flowrate and storage. The influence of the double-side baffle was also discussed due to its symmetry. The two structures are depicted in Fig. 4.

The baffle added to the oil bores will impact the oil distribution of splash lubrication. Therefore, four typical positions (0 deg, 90 deg, 180 deg, and 270 deg crank angle) are selected to analyze the oil distribution for three different structures.

Figure 5 illustrates the oil distribution at 0-deg Crank Angle with different structures of oil bores. The velocity of the connecting rod small end gradually decreases as the piston approaches the top dead center. Oil in oil bore 2 with baffle is thrown out due to its inertia, forming an oil streak in region A. This indicates a substantial enhancement in collection efficiency following the addition of a baffle. The oil from the piston cooling nozzle impacts the inner side of piston, forming many oil droplets in region B. In Fig. 5(c), the density of droplets in region B significantly reduces. A few of them enter oil bore 1 on account of the baffle.

Figure 6 illustrates the oil distribution at 90-deg CA with different structures of oil bores. The relative velocity between piston and oil from piston cooling nozzle is maximal at this position. The size of oil droplets generated by collision is smaller, but the quantity is greater. Oil bore 1 is located in region C with the highest droplet density after adjusting the angle between the axes of two oil bores to 90 deg. The oil droplets can naturally fall into oil bore 1 whether there is a baffle or not. Another part of oil droplets move and accumulate in region D under the guidance of the piston underside.

The number of oil droplets falling into oil bore 2 increases after adding the baffle.

Figure 7 illustrates the oil distribution at 180-deg CA with different structures of oil bores. The distance between the piston underside and the cooling nozzle is the shortest at this position. Almost all oil from the piston cooling nozzle enters the piston cooling gallery. The oil initially adhering to the piston underside almost entirely detaches due to its inertia and gravity. Only a small amount of oil falls into oil bores without the baffle.

The amount of oil accumulated in region D is significantly more than in region C as shown in Fig. 6. Thus, the increase in inflow volume of oil bore 2 is much greater than that of oil bore 1 after adding the baffle. Additionally, a thick oil film can be observed on the surface of the piston pin in region E. This film results from the detachment of oil from the upper surface of the connecting rod.

Figure 8 illustrates the oil distribution at 270-deg CA with different structures of oil bores. The oil collected in oil bore 2 accelerates to fill the oil grooves as the axis of this bore approaches the vertical direction. Oil slides off the piston pin to both sides, resulting in a thinning of the oil film attached to the exposed surface of the piston pin. Only a minimal amount of oil droplets scattered from the collision in region F enter oil bore 1 without the baffle.

After analyzing four typical positions, the oil distribution beyond the vicinity of the oil bores is generally consistent with the prototype. This indicates that the height design of the baffle is reasonable and will not weaken the cooling effect of the piston cooling nozzle.

Figure 9 illustrates the cumulative inflow volume of oil bores with different structures. The cumulative inflow volume of oil bore 2 becomes positive after adding the baffle, indicating it is the net inlet. The flowrate of it represents the oil flowrate of small-end bearing.

The oil flowrate of small-end bearing increases from 0.094 mL/s to 3.564 mL/s with a single-sided baffle. It further increases to 3.703 mL/s after adding a symmetrical baffle to oil bore 1. It should be noted that the oil entering small-end bearing without the baffle has not passed through piston cooling gallery. However, most of it is the oil discharged from piston cooling gallery after adding the baffle.

The net inflow volume of oil bores, also known as the oil storage of small-end bearing, is illustrated in Fig. 10. The oil storage increases from 0.045 mL to 0.8 mL with a single-sided baffle. With the significant increase in inflow volume per rotation, the fluctuation of oil storage is greater compared to the prototype. However, a symmetrical baffle on oil bore 1 cannot further increase the oil storage of small-end bearing.

1. The oil flowrate and storage of small-end bearing both increase first and then decrease with the increase of angle between the axes of two oil bores. 90 deg is the optimal angle for this diesel engine, where the flowrate and storage increase from 0.036 mL/s and 0.04 mL in the original engine (with 60-deg angle) to 0.094 mL/s and 0.045 mL, respectively. Optimizing the angle improves the cooling intensity of small-end bearing, but the change in oil storage, which characterizes the stability of oil supply to the contact zone, is not significant.

2. A semicolumn baffle above oil bore away from the piston cooling nozzle makes this bore a net inlet and the other bore a net outlet. The oil flowrate and storage of small-end bearing increase from 0.094 mL/s and 0.045 mL to 3.564 mL/s and 0.8 mL, respectively. It further greatly enhances the cooling intensity and oil supply stability of small-end bearing, conducing to prevent bushing failures.

3. Further adding a symmetrical baffle to the other bore, the oil flowrate of small-end bearing increases slightly from 3.564 mL/s to 3.703 mL/s with no change in oil storage. However, the symmetrical structure contributes to simplify the process of connecting rod and better its dynamic balance.

• The National Natural Science Foundation of China (Grant No. 52306038).

There are no conflicts of interest.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.