*2.2.2 Application of taper shape and grooves*

The grooves were used to improve the lubrication performances and heat transfer in various mechanical components. In addition, profiles (taper shape) were applied to cylindrical roller bearings to release the stress concentration at the both ends of the rollers. Previous study has suggested application of partial grooving, circumferential grooving, taper shape, and the design of optimal clearance to improve the lubrication characteristics of the fuel pump in marine diesel engines. The hydrodynamic lubrication analysis of the fuel pump performed by using two-dimensional Reynolds equation and Reynolds boundary condition to compare lubrication characteristics of the pump with variation of taper shape, groove condition, and viscosity.

**Figure 11(a)** shows a tapered plunger, with tapered applied to the plunger stem. In the axial and radial directions, the dimensionless taper lengths of the upper section of the plunger stem are A1 and B1, respectively. C1 and D1 are the dimensionless taper lengths of the lower section of the plunger stem, respectively. A grooved plunger is shown in **Figure 11(b)**. L1 is the dimensionless distance between the stem's edge and the first groove. Dimensionless groove width and depth are represented by L2 and H2, respectively. L3 is dimensionless distance between grooves, and N is number of grooves. The depth of groove is considered in film thickness equation in case shallow groove. On the other hand, the pressure in the deep groove calculated using the continuity of flow rate.


#### **Table 1.**

*Determination of optimal clearance [10].*

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

**Figure 11.** *Geometries of plunger with taper and groove [12]. (a) Taper shape (b) circumferential groove.*

The taper to the stem part of the plunger applied to improve lubrication characteristics of the pump by using a pressure generation by the wedge effect. The generated pressure helps to restore the plunger to the barrel center during reciprocating motion. **Figure 12(a)** and **(b)** show the effect of tapering the upper part of stem. The dimensionless minimum film thickness in the tapered plunger is greater than that of an untampered plunger. When A2 increased from 0.312 to 0.938, the dimensionless minimum film thickness increases by roughly 15%. However, the minimum film thickness does not change when the A1 increases beyond 0.938. Furthermore, the change in dimensionless minimum film thickness differs by less than 50% compared to the variation of B1. According to **Figure 12(c)**, tapering the bottom section of the stem does not affect lubricating properties. During reciprocating action, the lower section of the stem pops out of the barrel for a short period due to reduced fluid film pressure in the bottom part of the stem.

The imbalance in pressure can be alleviated by applying a circumferential groove, which allows passage in the circumferential direction. These grooves equalize these pressures and restore the plunger to the center of the barrel by facilitating flow around the periphery of the plunger from the high-pressure zone to the lowpressure zone. **Figure 13** shows the film parameter with dimensionless viscosity and groove type (shallow groove, deep groove). The film parameter in the case of shallow grooving is greater than that of deep grooving. The percentage of increase in the film parameter calculated based on the film parameter of no groove condition. A plunger with a shallow groove improves the lubrication properties more effectively in low-viscosity circumstances, since deep grooves are less efficient in highviscosity conditions because of increased viscous friction. However, it is difficult to

*Dimensionless minimum film thickness with taper geometries and crank angle [12] (a) variation of A1, (b) variation of B1, (c) effect of C1 and D1.*

#### **Figure 13.**

*Film parameter with groove types and dimensionless viscosity [12].*

apply circumferential grooves with shallow depth because of the economics of the product for processing, and the reduction in ability to trap wear particles.

The effect of application both circumferential groove and taper shaper to the plunger investigated. **Figure 14** shows various types of plunger. **Figure 15(a)** compared the lubrication characteristics of the plunger applied both taper and grooves with those cases where either are applied singly. A pump's film parameter is highest

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

**Figure 14.** *Types of plunger [12].*

when both taper and groove are used. In the low viscosity condition, the absence of groove and taper increases film parameters by about 390%. Furthermore, the use of a taper is more successful than the use of grooves in improving lubricating qualities. As shown in **Figure 15(b)**, the film parameter varies when the number of grooves is changed in the case of taper + groove. The difference in film parameters is less than 4% between N = 2 and N = 3. However, the lubricating properties of a pump with three or more grooves are not improved as the pressure imbalance occurs mainly in the head and upper part of the stem [12].

Besides, research on fuel pump with spiral grooves has performed in order to improve the durability of the fuel pump in marine engines. The application of spiral grooves is quite effective in the design of fuel pumps requiring high pressure for high power of the engines. This is because a spiral groove is one continuous groove and can effectively release uneven pressure distribution surrounding the plunger. In addition, spiral grooves are not machined to the both ends of the stem part because the grooves cause a pressure drop of compressed fuel [14].

## **2.3 Lacquering**

Environmental restrictions affect the composition of marine fuel oil and the design of marine diesel engines. To significantly limit the sulfur content of fuel oil and emission, there is a requirement to strengthen environmental laws. Low-sulfur fuel oil is used in most marine engines to meet international environmental requirements. However, the use of such fuel oils can cause unexpected concerns. These concerns include higher lubricant consumption due to lacquer build-up on the cylinder lining, as seen in **Figure 16**.

**Figure 15.** *Film parameter with dimensionless viscosity (a) variation of type (b) variation of N [12].*

**Figure 16.** *Oil consumption increase in engines [15].*

Lacquer forming (lacquering) in the cylinder liners of marine diesel engines has been a matter of concern for at least 20 years. Lacquer development increases lubricating oil consumption, sticks the injection pump, and causes scuffing in the cylinder liner. Cylinder liners are the most essential engine components when it comes to oil consumption and friction losses. According to studies, friction between the cylinder liner and the piston ring is responsible for up to 40% of engine friction losses. The surface of the cylinder liner consists of a mixture of deep enough valleys and smooth plateaus, which is called honing, in order for the liner to hold a satisfactory amount of lubricant oil and to reduce friction. **Figure 17** shows that cylinder liner lacquer results in deposits in the grooves. Such deposits reduce clearance to the point of contact between plunger and barrel. Sticking can occur due to reduced clearance, the lubrication characteristics of the pump are deteriorated [15, 17].

Previous studies have speculated on lacquer producing methods. Shell [18] and Alberola [19] have presented two proposals.

According to Alberola, lubricant oils are liquid polymers with low molecular weight; therefore, deposit formation due to thermal and oxidative degradation of these oils can be considered a thermosetting process. In this process, a polymeric liquid undergoes two macroscopic phase transformations, gelation and vitrification, which turn the liquid into a solid. Gelation is the production of branched molecules with a potentially infinite molecular structure that occur at a critical point in a chemical reaction. First, a paste-like gel-like coating is initially formed from the lubricating oils. Then, a vitrified or glassy solid is formed from thermosetting

**Figure 17.** *Deposits in the groove of cylinder liner [15, 16].*

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

polymers in the thermosets. During this phase, the polymeric network becomes tighter due to the chemical cross-linking processes that continue to take place. Because the thermoset structure has transformed to a vitrified glassy state, molecular segment motions are no longer feasible. Finally, the pasty properties are lost in the glassy deposit, which is commonly referred to as a lacquer or varnish [18].

According to Shell, condensation of partially combusted and cracked fuel components on the surface forms lacquer (**Figure 3**). To form the layer, these components oxidize and polymerize before mixing with the calcium and zinc salts of lubricant oil. These metal components act as catalysts in the oxidation of the surface. The layer turns into a hard glaze under high temperatures. This process results in the formation of hard and glassy layers on the surface [19].

However, the two proposals have yet to be confirmed by detained chemical analysis of the lacquer [15, 17].

There are a variety of reasons for the formation of lacquer. Lacquer can be formed due to the use of only fuel oil, or a mixture of fuel oil and lubricant oil. The boiling point and aromatic content of fuel oils also affect the formation of lacquer. Compared to fuel oils that do not form lacquer, lacquer-forming fuel oils have higher aromatics and paraffinic contents [20, 21]. A higher than normal final boiling point may indicate higher than normal content of polycyclic aromatic hydrocarbons (PAHs) in the fuel oils. Past work has also suggested that distillate fuels containing heavier ends are more prone to form lacquer [22]. The base number (BN) level of lubricant oils and sulfur content of fuel oils are directly related to lacquer formation [21]. The marine diesel engines are normally designed to burn residual fuel oils containing high-level sulfurs, and need lubricant oils with an appropriate level of BN to neutralize the corrosive combustion acids. However, higher BN and sulphated ash indicated a higher deposit risk. In addition, engine tests that the lacquer increases when either the liner temperatures or inlet air temperature are too low. This is because the low temperatures favor conditions for condensation of partially combusted and/or heavier fuel ends on the surface. Operation with a lot of idle, part-load, or combined full load (or over load) operation seems to be the most lacquer-prone.

There are many variations in the appearance of the lacquer under different conditions. Normally, amber and brown, lacquer appears darker when viewed from an angle, probably because more light is reflected from the surface where most of the deposit is located. Moreover, the term "glazing" is used to describe the appearance of lacquer. The lacquer has a strong bonding force with the surface, so it is not easily physically removed. The degree of bonding force has been evaluated through pull-off test. The allowable criteria of marine paints specified in ISO 4624 is about 3 ~ 4 MPa. The pull-off pressure of lacquer were over 9 MPa, and the values are two or more times larger than the allowable criteria of marine paints.

Anthranequinone and other quinones are also insoluble in most solvents, but they are soluble in acids, such as sulfuric acid and acetic acid as shown in **Figure 18**. The presence of quinones in the lacquer would explain why the lacquer dissolves readily in a weak organic acid. Through this phenomenon, acids are effective in removing the lacquer.

The lacquer can form due to a variety of causes, so there are various measures that can be taken to prevent or minimize the problem. The maintenances of fuel injectors, turbocharger and cooling around the liners are effective factors in preventing incomplete combustion, and also influence the prevention of lacquer formation. To prevent lacquer formation in the fuel pump, systematic control of the lubricant oil flow and periodic inspection of the pump were suggested, in order to ensure replacement of the sealing ring, and oil sediment removal. Other countermeasures are use of alternative lubricants, and of multifunctional fuel additives.

#### **Figure 18.**

*Liner lacquer after partial cleaning with acetic acid [17].*

Several companies have proposed an "advised range" for the BN depending on the sulfur content of fuel oil to prevent lacquer formation [17].
