**2.2 Fuel injection pump**

Fuel injection pump is one of the main components of the marine diesel engines. The fuel pump is device that supplies compressed fuel into the cylinders of the diesel engines, and controls the amount of fuel oil needed to gain the desired power. Moreover, it operates with a timing that keeps the engine running smoothly. The mechanical reciprocating fuel pump which used in marine engines is referred to as a "Bosch type pump," and it consists of a barrel and a helical plunger as shown in **Figure 3** [11, 12].

Various factors, such as lacquer, foreign debris, and insufficient clearance, can block the plunger inside the barrel and cause a 'stick' problem in most fuel pumps. **Figure 4** shows how the development of the lacquer in the pump limits the space between the plunger and the barrel [11]. Several researches tried to solve and improve the stick problem as follows:


**Figure 2.** *Honing in cylinder liner.*

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

**Figure 3.** *Fuel injection pump [11].*

• Prevention of lacquering

In this section, researches on optimal design of clearance, and application of taper shape and grooves focused among several methods. Grooves applied various machine components have several functions such as reduction of friction loss, oil reservoir and trapping of wear particles [13]. Trap effect of groove investigated with

**Figure 4.** *Lacquer formation in fuel pump [15].*

#### *Tribology of Machine Elements - Fundamentals and Applications*

variation in cross-sectional shape and Reynolds number. The effect of groove is evaluated using computational fluid dynamics (CFD) analysis. The simulation based on the standard k-ε turbulence model and the discrete phase model (DPM). The simulation results are also capable of showing the particle trajectories in flow field. Various cross-sectional shapes of groove such as rectangular, triangle, U shaped, trapezoid, elliptical shape evaluated. Through the CFD analysis, it found that the cross-sectional shapes favorable to the creation of vortex and small eddy current are effective in terms of particle trapping effect [13]. In particular, the residual fuel used in marine diesel engines contain relatively many foreign materials or impurities, so the design of groove which have a good trap effect needed to prevent the three body abrasive wear and sticking. Prevention of lacquering explained in Section 2.3.

#### *2.2.1 Optimal design of clearance*

Regarding the lubrication characteristics of lubricating system, the influence of the clearance is significant, and many studies have been performed on the effect and design of clearance in terms of varieties of bearing and joints. The clearance between the plunger and barrel in fuel injection pump is also important to keep stable operation for marine diesel engines. In marine diesel engines, high viscosity fuel oils such as heavy fuel oil (HFO) and low viscosity fuel oils such as light diesel oil (LDO) have been used, and several lubrication techniques have been used depending on the fuel oil viscosity. The supplied fuel and lubricant oil lubricate the system when low-viscosity fuel oil is used, while only the fuel oil lubricates the system when high-viscosity fuel oil is used. Therefore, it is necessary to test the pump's lubrication characteristics at different viscosity levels. Furthermore, since the pump may work at high pressures of up to 1,500 bar, deformation should be taken into account. Due to the substantial effect of restriction circumstances on pump deformation, structural analysis is adequate for clearance design when maximum fuel oil supply pressure is applied. In addition, the highest reduction in clearance is used solely as a design limit.

The clearance in the fuel injection pump is estimated by structural and hydrodynamic lubrication studies. A structural study looks at structural changes in the plunger and barrel when they are subjected to maximum supply pressure due to fuel oil compression. Furthermore, the structural study also evaluates the maximum reduction in clearance caused by deformation. As viscosities and clearances vary, the hydrodynamic study of lubrication analyzes the lubrication properties. The clearance between the plunger and the barrel is divided into two sections, head and stem as shown in **Figure 5**. The lubrication characteristics are then compared by calculating the film parameters from the minimum film thickness and surface roughness, as shown in Eq. (1). The surface roughness is determined from the surface roughness of the plunger and barrel, as shown in Eq. (2).

**Figure 5.** *Dimensionless clearance of stem part (A) and head part (B) [11].*

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

$$\text{Film parameter} \left(\mathcal{A}\right) = \frac{\text{Minimum fillm thickness} \left(h\_w\right)}{\text{Surface roughness} \left(R\_q\right)}\tag{1}$$

$$R\_q = \sqrt{R\_{q1}^2 + R\_{q2}^2} \tag{2}$$

**Figure 6(a)** shows the positions of three sections where the deformations of the barrel and plunger have been evaluated. The deformation (enlarged 300 times) in the top, middle, and bottom sections is illustrated in **Figure 6(b)–(d)**. The ratio of displacement to clearance of the stem in the plunger is represented by a dimensionless displacement, which is a dimensionless value. Since the study is conducted in the absence of a clearance condition, overlapping regions indicate a decline in clearance. This is because the primary orientations for the barrel and the plunger are virtually perpendicular, and the spill port of a barrel has a significant impact on deformation. The clearance reduction is assessed in two parts. In the head, the barrel's and the plunger's deformations are studied. However, deformation of the plunger is investigated only in the stem as a low film pressure does not distort the plunger barrel. As shown in **Figure 7**, the deformation of the pump is studied quantitatively to determine the highest reduction in clearance. This figure also shows that the plunger's centerline and the barrel's line have a dimensionless displacement. The dashed line represents the displacement of the barrel, and the solid line represents that of the plunger. When deformation of the plunger is larger than that of the barrel, a reduction in clearance at the head occurs. **Figure 8** shows the dimensionless displacement at the red line of the plunger. In **Figure 8**, the region to the left of the pink dashed line is the area in which the plunger is inside the barrel during reciprocating motion. The maximum reduction in the clearance is the maximum displacement in the stem part. The structural analysis found that the clearance should be higher than the maximum clearance decrease to avoid metal-tometal contact between the barrel and the plunger due to deformation.

#### **Figure 6.**

*Dimensionless displacement on the three sections [11]. (a) Position of three sections in the axial direction (b) displacement of top section (c) displacement of middle section (d) displacement of bottom section.*

**Figure 7.** *Dimensionless displacement of barrel and plunger in the middle section [11].*

**Figure 8.** *Dimensionless displacement of plunger in stem part [11].*

In hydrodynamic lubrication analysis, an unsteady-state two-dimensional Reynolds equation is used to model the fluid film between the barrel and the plunger, and the Reynolds boundary condition is applied. Moreover, the equilibrium equation of the moment at fixed point and the equilibrium of the forces in the vertical and horizontal directions are used. The lubrication characteristics of the pump with variation of clearance are investigated in two parts, head and stem. When the dimensionless viscosity is 0.57, **Figure 9(a)** shows the dimensionless minimum film thickness with a change in the dimensionless clearance of the head. Furthermore, when the clearance in the stem is constant, the film parameters do not change to be more than a specific clearance in the head. The film parameters obtained according to the results of the minimum film thickness are shown in **Figure 9(b)**. **Figure 10** shows the changes in dimensionless viscosity and dimensionless clearance of the film parameter. Increments in the dimensionless viscosity raise the film parameter. If a film parameter of 3 or higher shows good lubrication properties, the pump's lubrication qualities are satisfactory throughout a wide range of viscosities. While in a low viscosity state, the film parameter is less than 3 when the dimensionless clearance of the stem is 0.25, 0.32 and 0.37. This means that, at low viscosity, metal-to-metal contact between the plunger and the barrel is possible. However, a low viscosity situation results in a parameter of 4.25 for a dimensionless

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

#### **Figure 9.**

*Dimensionless minimum film thickness and film parameter with dimensionless clearance (B) [11] (a) dimensionless minimum film thickness (b) film parameter.*

**Figure 10.**

*Film parameter with dimension viscosity and clearance (A, B) [11].*

clearance of 0.20. Since the stem of the reciprocating fuel pump system has a relatively large lubrication area, the dimensionless clearance of the stem must be small, and the clearance in the stem had a significant impact on the lubrication properties of the system. According to the hydrodynamic lubrication study of marine diesel engines, metal-to-metal contact does not occur when the stem clearance is modest and the head clearance is more than a specific clearance.

The tolerance and machining limit of the clearance are also required to establish the pump clearance. Since the tolerance of the clearance must be caused by the machining process, and the clearance is decided by two surfaces, the clearance in a genuine mechanical system is not a single number but rather a range of values. Furthermore, the manufacturer's processing capability is also taken into account while determining the clearance. Therefore, in this chapter, this concept is referred to as the "machining limit of the clearance".

In the scenario where the dimensionless clearance machining limit is 0.17 and the dimensionless tolerance maximum is 0.05, the dimensionless clearance of the pump is determined. To prevent metal-to-metal contact, the ideal dimensionless clearance in the stem and head should be greater than o.18 and o.20, respectively.

Assuming that the hydrodynamic lubrication analysis shows that the hydrodynamic lubrication characteristics are good when A = 0.20 and A = 0.30, the dimensionless clearance of the stem (A = 0.20) will be inadequate because the maximum dimensionless reduction in clearance induced by deformation is greater than the dimensionless clearance. Therefore, to prevent metal-to-metal contact despite having good lubrication characteristics of the pump for the hydrodynamic lubrication analysis, the dimensionless clearance must be greater than 0.20 for the stem part. However, since the dimensionless clearance is larger than the maximum dimensionless reduction in the clearance, the dimensionless of the head (B = 30) is correct. Therefore, the dimensionless clearance is determined while considering the maximum dimensionless tolerance, as shown in **Table 1**. However, there are a few limitations to this method. The disturbance generated by the cam was not taken into account when calculating the reciprocating motion. Moreover, a variety of fuel oils are used in marine diesel engines, and changing fuel oil causes a stick in the fuel pump. In this approach, the changes in fuel oil were not taken into account. Furthermore, this method is more accurate than the fluid–structure interaction (FSI) method even though structural and hydrodynamic lubrication analyses are used to estimate the proper pump clearance. Although this design method of clearance has some significant drawbacks, it improved the fuel pump durability and made it feasible to create engines with a 20% increase in power [11].
