Marine diesel engine is equipped to drive propeller as the main propulsion or generator as auxiliary machinery. Diesel engine is using Diesel Oil (DO) or Heavy Fuel Oil (HFO) with poorer quality for working. With different kinds of fuel oil, the fuel oil system also needs the different structure requirements. Thus, the marine fuel oil system can be classified into two types:

- Diesel Oil System (DO System);

- Heavy fuel oil system (HFO System).

In general, DO system installed in small type, high speed marine diesel engine and FO system installed in large power, low speed ones operated with both of HFO and DO. Thus, DO system and HFO system always have some basic differences, in Figure 1, we can see one of typical HFO system:

Figure 1: 
HFO system. 1. Store tank (HFO); 2. Transfer pump; 3. HFO settling tank; 4. Supply pump for purifier; 5. Heater; 6. Purifier; 7. HFO service tank; 8. Flow meter; 9. Mixing tank; 10. Circulating pump; 11. Heater; 12. Viscometer; 13. Filter; 14. Fuel injection pump; 15. Return lines; 16. Air vent; 17. D.O tank; 18. Three way valve.

In comparison between HFO system and DO system, we can see the following basic differences:

- There is no Mixing tank (9) in DO system, because the diesel engine is only operating with diesel oil when ship sailing or maneuvering. On the contrary, in HFO system, there must be used DO when maneuvering, and thus, mixing tank is very important for transferring HFO to DO and vice versa;

- In many case, in DO system, fuel oil heater is not required.

In fact, to make sure the diesel engine can work safely and efficiently, the fuel oil system must be equipped the following component:

1. Tanks

When a ship sailing, necessitates the inclusion of the follows tanks in the fuel system:

Storage tank: To store fuel oil for ship;

Settling tank: Here the HFO is settled for few days so that water, sludge and other impurities can be separated and drained out…according to gravity separation;

Service tank: Purified oil is stored in service tanks for immediate use;

Mixing tank: Its purpose is to produce a gradual variation of fuel quality during transition period from diesel oil to heavy fuel oil or vice versa

2. Filter

There are two kinds of filter: fine filter and pre-filter. In some kinds of diesel engines, filter installed above fuel injector.

- Pre-filter is installed before the supply pump. Its core is made by net with 0,1 mm eye bore.

- Core of fine filter is made by laminated soft paper. Some types of cloth or felt, some types of fiber around perforated pipe. Fine filter installed after supply pump.

3. Supply pump

Its function is to transfer the fuel oil from service tank to fuel injection pump. Another name for this pump is charged pressure pump or circulating pump. In fact, this pump is driven by diesel engine or individual electric motor.

4. Fuel injection pump

Fuel injection pump is the main part of fuel oil system. There are two kinds of fuel injection pump: Bosch type and valve-controlled type. In-line pumps are used in the small-engine-size range, individual single barrel injection pumps are used in larger diesels. Fuel injection pump is connected with fuel injector by high pressure tube.

5. Fuel injector

Its function is to supply fuel oil into combustion chamber in timing and metering. Fuel oil is sprayed in the cylinder in the form of fine mist.

6. Piping

High pressure pipe is used for transferring fuel oil from injection pump to injector. Low pressure one is for transferring from service tank to fuel injection pump by transfer pump. Furthermore, low pressure one leads the returned oil from fuel injection pump and fuel injector to tank.

Among those devices, the fuel injection pump and fuel injector play an important role for warranting sprayed fuel oil on timing, metering and fine mist. Figure 2 shows the Bosch piston types of fuel injection pump with different adjustment methods of supplied fuel amount.

Figure 2: 
Adjustment methods on supplied fuel oil amount. a: Initial adjusting, b: Terminal adjusting, c: Combined time adjusting, 1: Helical groove, 2: Vertical groove, 3: Discharge hold

In Figure 3, it shows character of pump adjustable at the beginning of the delivery stroke. Hence, helical groove on the piston (1) will covering capacity Vertical groove (2) on the plunger when they move from bottom to top. (Figure 2-a).

Figure 3: 
Initial adjusting amount of fuel oil supply(pump adjusted by beginning of fuel delivery)

To adjust amount of fuel oil supply for the working process of diesel engine, we change relative and position between plunger and barrel. Thus, when piston move up, at (1) the piston stroke is maximum, and then, amount of fuel delivery is maximum, at (2), (3), the piston stroke is smaller, it resulting in a less amount of fuel oil supplied. (Figure 3).

Volume flow rate by pump is equivalent with volume of fuel injected into combustion chamber [2][3]:

Therefore, injecting pressure can be calculated:

In which:

ϕ_cn :flow coefficient; pc: fractions pressure at the moment of fuel injected into the cylinder [Pa]; ρ_nl: Fuel density at injection time [kg/m3]; F_i: injector nozzle bore area [m²];i: number of nozzle orifices; h_a:Piston stroke of fuel injection pump [m];ϕ: angle of crankshaft rotation [rad]; Fp:Cross area sectional of fuel injection pump [m²];n:revolution of engine [v/p].

Based on formula (2), it shows that:

- Fuel injected pressure is first – order function proportion with density at constant revolution of the engine;

- Fuel injected pressure is square-law proportion with revolution of the engine.

Injection process is the mobility of liquid through the fuel injector. If hydraulic loss in fuel injector, leakage loose are neglected, according to principle of Bernoulli [2]-[4]:

From formula (3), injection flow:

In which: C- constant; i – number of nozzle orifices; ρ_nl :specific gravity of fuel oil [kg/m3] P_ph.lt'; fuel injected pressure [Pa]; W_ph.lt': Fuel flow rate through nozzle[m/s].

Or another formula:

In Which: F_i – nozzle area [m²];CD – injected coefficient; -density of fuel oil [kg/m³]; Δp-pressure drop when fuel injected. If Δp, F_i are the constant, amount of injected fuel:

In which: Δθ-period of fuel injected [oCA], n-revolution[rpm].

Moment of fuel injection is the moment when the nozzle is opened by pressure of fuel injection pump. Theoretically, the time when the fuel injection pump starts to work and moment of fuel injected is the same, there is no delay time. However, because of expanding of high pressure tube, leakage of piston-plunger of fuel injection pump, and, leakage of nozzle, thus, there is delay time between starting time of fuel injection pump and moment of fuel injected. Therefore, when we use different kinds of fuel oil for diesel engine, a moment of fuel injection depends on pressure and viscosity of fuel oil. In case of replacement traditional diesel oil by bio-diesel oil but keeping viscosity, the moment of fuel injection with bio-fuel only depends on their density. However, this difference is neglected for working process of diesel engine.

Since the ignition characteristics of the fuel effect the ignition delay, this property of fuel is very important in determining diesel engine operating characteristics, Blended biodiesel fuel with difference ratio: B5, B10, B15, B20 have smaller Cetane number in comparison with DO, in which, the ignition quality of a fuel is defined by its Cetane number. According to experimental, the ignition delay [2]:

In Which: τ_i - ignition delay [ms]; E_A- an apparent activation energy for the fuel autoignition process [J/mol]; R - universal gas constant [8,3143 J/mol.°K ]; p-pressure at the end of compression stroke [bar]; T–temperature at the end of compression stroke [°K ]; A,n-constants depends on fuel oil.

Formula (7) was built basing on experimental of closing space without parameters: mixed burning air in the combustion chamber. This formula is used for all kinds of fuel oil, thus, low accuracy. To make higher accuracy, we can calculate by Arrhenius adjustment formula:

In which: k- experimental coefficient; ϕ- ratio of fuel oil – charging air: 0,116.

Based on formula (7) and (8), The Ghent University, Belgium was found the formula:

The research team also carried out test in the impacts of mixed fuel (biodiesel oil – DO) on injected pressure. The testing method of research is showed in Figure 4 as follows:

- Test fuel injector on DO at ambient temperature and injection pressure of 280 KG/cm²(It is the standard pressure which is stipulated by maker) and measuring on the diameter of fuel spray which is recorded on A4 paper placed in 40mm far from fuel injector.

Figure 4: 
Measuring and evaluation on injected pressure of fuel oil

- Test in turn on the different mixed fuel B5, B10, B15 and B20 at the injected pressure 280kG/cm2and similar viscosity;

- The research team carried out take diameter of DO spray as a standard; Adjust tension of fuel injector spring so that diameter of the other fuel sprayed is similar to the diameter of DO sprayed; Test injected pressure on meter and record the results in Figure 2.

The data on the Table 2 shows that: Regarding to the other fuels, diameter of fuel sprayed is also different as its increasing from 32.20 mm (DO) to 32.28 mm (B20). This shows that the injected fuel spray of the high density fuel (B20: 0,8668 kg/dm³)is farer than the injected fuel spray of the low density fuel at the constant injected pressure.

The research team measure the injected pressure of fuel oil for the engine 6LU32 by the high accuracy pressure sensors and the results are shown in the On the basis of data on the fuel injected pressure as shown in Figure 5, we were able to separate data on the highest stuffing pressure for all kind of fuel and injection time of fuel as in Table 3.

Figure 5: 
injected pressure for the different fuels

Table 4 shows the supplied fuels vary on the other fuels at the similar rotation mode which is calculated for engine 6UL32, maker is Hanshin (JAPAN).

Table 4 is the result of the theoretical calculation of fuel amount injected into the engine when changing the kind of fuel.

In this case, the research team applied the formula (2) to calculate on engine 6LU32 of Hanshin manufacturer at nominal power. The amount of injected fuel calculated according to time [g/s] and according to cycle [g/cl]. The calculation results show that: depending on the density of the fuel, the amount of fuel injected into the engine will be changed

Table 5 shows the measurement data of the injection fuel of nozzle in experiment. The research team used fuel injection pump testing equipment at laboratory of Faculty of Marine Engineering – Vietnam Maritime University for evaluating the injection fuel over each fuel according to the process.

Figure 6: 
Impact of kind of fuel on amount of fuel injected

Comparison between the cycle fuel in theoretical calculation and that in experimental calculation is shown in Figure 6. According to theoretical calculation, the difference of the amount of fuel injected into the engine is not much on different fuels (from DO to B20). On the contrast, the difference becomes greater and reaches +15.01% (B20/DO) in experimental measurement.

The ignition delay of fuel is defined by its Cetane number and working conditions of the engine, such as: load regime, speed, temperature at the end of the compression process... The research team used the different fuels for diesel engine and used the modern equipment for analyzing combustion process of AVL manufacturer (Republic of Austria) for providing and collecting data on delay combustion of different fuels as shown in Figure 7 and Table 6.

The data in Table 6 is made when the diesel engine is running at load regime of 400 kW and rotation 238 rpm, using alternative fuels from DO to B20. At the last column of Table 6, the value is measured in rotation angle of crankshaft [°crank angle] ,here it should be understood as the period from the beginning of combustion of fuel to top dead center (TDC) of DO fuel 2,101 [°crank angle] ,TDC of B5 is 1.8447, while TDC of B20 is –0.6733.

Figure 7: 
The ignition delay of Blended bio-diesel fuels

In case of Blended bio-diesel fuel B20, the begin ignition after top dead center of the engine which is 0.6733 [°crank angle].

Thus, the fuel have Cetane value number much smaller than Cetane value of DO, then the ignition delay is greater, it is entirely comply with the actual rules.