**2.4 Bearings**

The development of marine diesel engine was high-power, compact structure, and hence the bearings were required to work normally under smaller size, high load, and thinner oil film conditions. **Figure 19** shows big end bearings of connecting rod. In recent years, coupling simulations between elasto-hydrodynamic lubrication (EHL) and nonlinear multi-body dynamics (MBD) are carried out to dynamicallyloaded bearings of marine diesel engines, and the coupling analysis is an effective method to investigate the lubrication characteristics of marine bearings [23].

In a marine engine, the connecting rod bearing is a key friction component. It converts the reciprocating action to rotary motion by connecting the crankshaft and pistons, as well as the cross-head slide. The bearing of the connecting rod significantly affects engine performance by improving its reliability, durability, and strength. To analyze the EHL for the large end connecting rod bearing of a lowspeed two-stroke marine diesel engine, AVL Excite Power Unit software was used.

**Figure 19.** *Big end bearing of medium-speed diesel engine.*

#### *Tribology in Marine Diesel Engines DOI: http://dx.doi.org/10.5772/intechopen.100547*

This software considers nonlinear multibody dynamics. Bearing lubrication can be calculated more accurately if the following factors are taken into account: friction surface roughness, elastic deformation of the bearing and journal, oil supplying qualities, and the influence of the cavity on the oil film lubricant [24].

The wear caused by insufficient lubrication is the most general cause of endurance life issues. An absence of lubrication in the journal-bearing system leads to bearing seizure, and normally, to total destruction of the part. Insufficient lubrication caused by factors such as a machining error in the manufacture of the crank pin and the bearing leads to metal-to-metal contact between the crank pin and the bearing, which results in adhesional wear. The crank pin bearing which connects the connecting rod and crank arm, converting a reciprocating motion into a rotary motion plays an important role in a marine diesel engine. Through the motion analysis of the piston-connecting rod-crank arm system, the bearing loads and lubricant velocity were calculated. The numerical algorithm for the hydrodynamic lubrication analysis coupling with the motion analysis of the piston-connecting rod-crank arm system developed to investigate lubrication characteristics. The maximum film pressure decreased with decreasing clearance and lubricant temperature, and that film thickness increased with decreasing clearance and lubricant temperature. The lubricant temperature had a higher effect on the film thickness than the clearance [25].

Fretting is phenomenon that concerns mechanical components in contact that are designed to be fixed but undergo small relative displacement due to fluctuation loads. The fretting is one of main issues of connecting rod bearing in marine diesel engines. **Figure 20** shows fretting fatigue fracture between bearing bush and small end. The fretting damage begins with local adhesion between mating surfaces and processed when adhered particles are removed from the surface, they may react with air or other corrosive environments. Surface crack can be initiated by fretting, and led to catastrophic failure after crack propagation. The fretting influenced by contact pressure, friction coefficient and relative slip motion. **Figure 21** shows numerical results about contact pressure, tangential stress and slip amplitude at certain crank angle. The fretting severity on mating surface is evaluated as fretting damage parameter (FDP). The FDP is defines in Eq. (3). The τ is the frictional shear stress at the interface and δ is the absolute slip amplitude in the tangential stress direction in Eq. (3). The potential for fretting initiated fatigue fracture on the mating surface is also evaluated using the Ruiz criterion and defined as fretting fatigue damage parameter (FFDP) in Eq. (4). The σ is the tensile tangential stress on the contact surface in Eq. (4). The greater slip amplitude and tangential stress can increase the possibility of fretting fatigue damage. Moreover, the contact pressure at the mating surface is not an effective parameter to predict the fretting damage because the areas where contact pressure is high, the FDP and FFDP are close to zero.

**Figure 20.** *Fretting fatigue failure of connecting rod [26].*

**Figure 21.**

*Numerical results with AVL software [26]. (a) Contact pressure distribution at crank angle 490o (b) tangential stress distribution at crank angle 110o (c) slip amplitude distribution at crank angle 310o .*

Therefore, the possibility of fretting damage at the upper split in connecting rod bearing was investigated using the Excite software from AVL and Ruiz criterion [26]. The Ruiz criteria is an effective empirical approach for evaluation of fretting fatigue damage parameter and has been demonstrated in two dimensional fretting studies of a typical dovetail interface problem. Moreover, this criterion is suitable for predicting the fretting damage of the connection rod bearing in marine diesel engines.

$$\text{FDP} = \texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\texttt{\mathbf{\texttt{\mathbf{\texttt{\mathbf{\texttt{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\bullet}}}}}}}}}}}}}}}}}}}}}}}}}}}}}\tag{\mathbf}}}\tag{\mathbf{\mathbf{\cdot}}}$$

$$\text{FFDP} = \sigma \bullet \tau \bullet \tag{4}$$

Aside from crank pin bearings and connecting rod bearings, various bearings are used in marine diesel engines. Since the size of most bearings are large, it is difficult to carry out experimental studies due to cost and volume, so most researches on bearings are focusing on analytical studies. However, in order to improve practically lubrication characteristics of bearings in marine diesel engines, more experimental studies should be conducted simultaneously.

#### **2.5 Condition monitoring with oil analysis**

Maintenance strategies play a crucial role in reducing the cost of down time and improving system reliability. Consequently, machine condition monitoring plays an important role in maintaining operation stability and extending the period of usage for various machines. Machine condition monitoring through oil analysis is an effective method for assessing machine's condition and providing early warnings regarding a machine's breakdown or failure as shown in **Figure 22**.

**Figure 22.** *Advantage of machine condition monitoring with oil analysis.*

The three main methods of oil analysis are off-line, in-line, and on-line techniques as shown in **Figure 23**. The method of analyzing lubricants through oil sampling is an off-line and has been mainly utilized in the past. The in-line is the method to analyze directly where the main flow of lubricant oil occurs, and the online is the method to analyze the lubricant oil in the by-pass. The in-line can interfere with the flow of lubricant during the measurement process and can be difficult to measure under conditions such as high temperature and high pressure. On-line analysis is the most effective method of the three methods used for analyzing lubricant oils. This is because it can monitor the machine condition effectively using oil sensors in real time without requiring excellent analysis skills and eliminates human errors. Determining the oil quality usually requires complex laboratory equipment for measuring factors such as density, viscosity, base number (BN), acid number (AN), water content, additive and wear debris. Real-time monitoring with oil analysis is also utilized in various industries, such as manufacturing, aerospace, power plants, construction equipment, wind-turbine and marine diesel engines.

**Figure 23.** *Methods of oil analysis.*

It is well known that faults and failures in marine diesel engines are always caused by wear in the tribo-systems. Vibration source complexity, multiphasic interference, and lower frequencies are the factors that make wear monitoring challenging. This has led to the use of oil analysis as a primary means of monitoring the status of marine diesel engines [27]. Even if the oil analysis has been applied in condition monitoring for marine diesel engines until now, there are some dissatisfactory circumstances in the oil analysis for them. This is because oil analysis takes in off-line mode, and it is not real-time. However, the on-line monitoring system with lubricant sensors is efficient to diagnose condition of marine diesel engines. Among oil properties, wear particle, viscosity, capacitance, base number, dielectric constant, water contents (relative humidity) are commonly measured [27]. The viscosity of the lubricant is its resistance to flow, and the condition of marine engines are normally diagnosed by monitoring the increment or decrement of it. However, if other lubricants with different viscosity or a large amount of fuel are not mixed, a sudden change in the viscosity of the lubricant does not occur easily in a short time.

The moisture is easily observed in lubricant oil contamination and it causes to increase the acid number, multiply microorganism and deteriorate the lubricant quality. The contamination of moisture is caused by water leakage during the operation of mechanical system. Water is a typical polar substance, and the presence of water in the lubricant oil increases the permittivity. The permittivity is a measure of the electric polarizability of a dielectric. The relative permittivity is called the dielectric constant. The dielectric constant of water is almost 80, whereas lubricant oil is about 2. A small increase in water contents of the lubricant oil caused as a sharp increase in permittivity. Therefore, it is possible to measure the amount of moisture in the lubricant oil by the dielectric constant. The Karl Fischer is a representative method for measuring moisture, but it requires a complex experimental device and skilled skill for the analyzer. Therefore, the measurement of water contents with moisture sensor or dielectric constant sensor is effective in the lubricant oil analysis [28, 29]. The oil analysis method with dielectric constant sensor is applied in marine diesel engines [27].

The base number (BN) is a property that is more associated with engine oils than industrial oils. It can be defined as the lubricant's ability to neutralize acids that are produced during use. The lubricant with proper BN should be used according to sulfur contents of marine fuels to prevent deposit and corrosion in shown in **Figure 24**.

**Figure 24.** *BN guideline according to fuel sulpher in marine diesel engines.*

*Tribology in Marine Diesel Engines DOI: http://dx.doi.org/10.5772/intechopen.100547*

**Figure 25.** *CaCO3 deposits on the piston top.*

#### **Figure 26.**

*Ferrogram photomicrograph of marine engine oil [30].*

**Figure 27.** *Relationships of wear debris size, concentration, and machine conditions [31].*

When a fuel with a high sulfur content is used and a lubricant oil with a low BN is used, there is a risk of corrosion. Conversely, when a fuel with a low sulfur content is used and a lubricant oil with a high BN is applied, deposit problem occur as shown in **Figure 25**. This is because the lubricant oil with a high BN partially neutralized sulfuric acid, and the remaining additive such as detergent react chemically, resulting in deposits. The tracking the BN of engine oil can determine how much life is remaining. The most reasons for a drop in the BN are related to low quality fuel such as residual fuels and oil oxidation. Therefore, the BN of lubricant in the marine diesel engines must be measured in order to monitor the condition of them.

Condition monitoring of machinery through analysis of wear debris is now an extensively applied as a tool in diagnostic technology. Wear debris analysis or analytical ferrography is a method of predicting the health of equipment in a nonintrusive manner by studying wear particles in lubricant oils. **Figure 26** show the ferrogram photomicrographs of marine engine oil. The shape and length of ferrous wear particles in engine oil evaluated by ferrography as shown in **Figure 26**. The correlation between wear debris, time and wear particles concentration is shown in **Figure 27**. During initial or normal operation of new engines, the wear size is normally between 1 μm and 10 μm. However, in abnormal condition, larger wear particles between 20 μm and 100 μm are detected. Thus, wear particles larger than 20 μm should be monitored in order to provide an early warning of the machine condition [28, 29]. The analysis of wear debris is important to detect critical stages of accelerated wear that precedes costly and dangerous components failures. Therefore, application of wear particle analysis and ferrography by oil sensors is essential means to keep good maintenance in marine diesel engines [32].

## **3. Conclusions**

This chapter explained the tribology of marine diesel engines, which are the heart of a marine system. Modern marine diesel engines must satisfy stringent reliability requirement. Various researches on tribological issues in the marine diesel engines were performed, the lubrication characteristics of machine components such as bearings, cylinder liners, fuel injection pump were improved. Besides, the phenomenon of lacquer is explained in terms of generating mechanism, causes, physical and chemical properties, and prevention or removal methods. Furthermore, condition monitoring with oil analysis is introduced to keep maintenance and to reduce the downtime cost of the marine diesel engines. A variety of tribological researches are needed in the future in order to improve the reliability of the marine diesel engines.

### **Acknowledgements**

This work supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (No. 20214000000010).

#### **Appendices and nomenclature**


## *Tribology in Marine Diesel Engines DOI: http://dx.doi.org/10.5772/intechopen.100547*

