*5.1. Diesel Engine Performance*

Results are presented differentiating the performance of the control scheme of the shaft generator and the results of the fuel consumed by the engine when simulating the PTO and PTI conditions. The methodology considers the use of the IMOs EEDI and EEOI tools to show the benefits of the scheme, providing a reference to evaluate the design efficiency and the operational efficiency.

Results are plotted over the two simulation conditions considered but presented over a power range to simulate specific operational conditions such as slow steaming. This operational condition has been considered because represents the ability of the simulation to show the performance of the ship at low ship's speed, which can be used to evaluate a ship design over a higher range of options to ge<sup>t</sup> an efficient design. A more accurate evaluation of the EEDI could be necessary but still results are providing a grea<sup>t</sup> assertiveness of the methodology selected.

The data used to simulate the performance of the control scheme considers the use of a ship, which has specific information: its capacity, speed, cargo transported, distance navigated, power installed, and the type of fuel consumed. The type of ship considered was selected from a worldwide database of ships [26]. When analyzing the database and the specific information needed to evaluate the design and operational efficiencies of a ship, very large crude oil carriers were the type of ships more reluctant to be used because of the simplicity of their propulsion system and the significance of the amount and type of fuel consumed. The propulsion system consists of a diesel engine directly coupled to a fixed pitch propeller. The auxiliary power installed for this specific type of ship accounts for ~10% of the propulsion power installed [26]. Table 1 presents the open source data used for simulation that were fixed as the initial conditions. The range of data is only a reference of the type of ship found in the database and no consideration to the operational profile of them has been considered for simulation purposes.


**Table 1.** Operational parameters for simulation.

The engine selected to be modeled and evaluated is an engine from MAN [27]. The 7G80ME-C9.2-TII diesel engine was selected having a specified maximum continuous rating (SMCR) power of 33 MW at 72 rpms. The specific fuel oil consumptions (SFC) vary from 187.1 g/kWh at low engine load to 166 g/kWh at high engine load. The normal continuous rating of the engine was considered ~75% of the SMCR. The total power delivered by the auxiliary generators during the navigation has been considered ≈3% of the propulsion engine rated power. When consuming HFO, a carbon factor of 3.114 g CO2/g Fuel was used to estimate the emissions. With this information, the simulation of the control scheme was carried out and the results are presented next.

### *5.2. Control Scheme Performance at PTO Operating Region*

Results are presented considering the performance of the shaft generator operating as a PTO, including the power and fuel consumption performance and the evaluation of the EEDI and EEOI tools.

The control scheme was evaluated considering a navigation time period long enough to simulate a consistent increase of the brake power of the engine over a step time to reflect its performance and SFC variation to ge<sup>t</sup> a steady evaluation of the efficiencies. Blue curves in Figure 7 shows the results of a segmen<sup>t</sup> of the PTO operating region while simulating a period of navigation time of 5 h, where, in the left-hand side of the figure, it is possible to appreciate how the brake power lineally increases from 5 MW to 12 MW. The right-hand side of the figure shows the decrease of the SFC from a maximum value at low engine load, the power delivered as PTO is stable enough to allow for the ship to turn-off the generator set. The red curves are the results of not having a shaft generator installed. The increase in the brake power when using a shaft generator is ~5% of the SMCR.

**Figure 7.** PTO evaluation performance.

When evaluating the EEDI, a specific value of 27,412 g CO2/g Fuel was calculated. This value represents the amount of CO2 emissions by design, which is therefore a value that can be modified at design stages only when the main and auxiliary machinery are selected and allocated to the vessel. A better approach could be to install a less powerful engine, but that means a completely different approach of the design spiral of the new ship. Following this evaluation, EEOI has been considered because represents the ability to calculate the emissions of the ship when in service navigating different routes. EEOI allows to check the variations of the same parameters that EEDI uses to be evaluated, therefore provides with a more comprehensive form to understand the operational behavior of a well design ship.

EEDI provides a fixed value at the design conditions, ye<sup>t</sup> EEOI can be used to evaluate the performance at every variation of engine load. EEOI considers the total amount of fuel consumed and mass of cargo transported and distance navigated, the latests being just another representation of cargo capacity and ship's speed, respectively.

One of the main objectives of this work was to evaluate, using EEDI and EEOI, the influence of a shaft generator when applying a control scheme of its operation. The purpose is looking for reduction into the SFC at different engine loads. Also, to prove that the reduction of the SFC compared to the increase into the necessary power to be developed, to overcome the extra necessary brake power to be produced, to propel the ship and to use the shaft generator as PTO and PTI, respectively.

Figure 8 shows the results of the SFC variation at PTO operating region when applying the proposed control scheme, red curve, and the blue curve shows the SFC variation when not having a shaft generator. The SFC differences are between 0.1% to 2% over the whole engine load range plotted, the difference even though can be considered small is quite significant when evaluating the EEOI, having the maximum SFC reduction between 6000 kW and 7000 kW. The difference is barely noticed because of the scale of the plotted results.

**Figure 8.** Specific Fuel oil Consumption (SFC) difference at PTO and without shaft generator.

The EEOI values are presented in Figure 9 together with the SFC obtained when using the shaft generator control scheme. The SFC decreases while the engine load increases. The fuel consumed provides EEOI values in accordance with its consumption over the navigation period and the navigated distance, as was described and established before as the initial conditions for simulation. The EEOI increases, but at a low rate over the engine load, which is a consequence of the decrease in the SFC. Although the decrease of the SFC is not quite significant allows for a low increment of the EEOI value, which leads to a overall reduction of the amount of operational CO2 emissions.

Figure 10 shows the results of comparing the EEOI values of the applied control scheme, red curve, against not having a shaft generator installed, blue curve. Results are showing that the control scheme applied makes the ship to reduce the amount of operational CO2 emissions even though an extra amount of brake power is necessary to be generated providing grea<sup>t</sup> assertiveness of the methodology and the SFC reduction over the period when operating at the PTO operating region.

**Figure 10.** EEOI comparison.

### *5.3. Control Scheme Performance at PTI Operating Region*

Results are presented considering the performance of the shaft generator operating as a PTI including the power and fuel consumption performance and the evaluation of the EEDI and EEOI tools. Following the same procedure to describe the results of the PTO operating region, the PTI operating region results are shown.

The blue curves in Figure 11 show the results of a segmen<sup>t</sup> of the PTI operating region while simulating a period of navigation time of 2.5 h, where, in the left hand side of the figure, it is possible to appreciate how the brake power lineally increases from 24 MW to 27 MW when not having a shaft generator, red curve, which also means an increase in the SFC as can be seen in the right-hand side of the figure. When applying the control scheme to operate the shaft generator, the PTI reduces the brake power, blue curve on the left-hand side of the figure therefore, a reduction of the SFC as can be seen in the right-hand side of the figure. The PTI reduces the brake power ~5% of the SMCR.

Figure 12 shows the results of the SFC variation at PTI operating region when applying the proposed control scheme, blue curve, and the red curve shows the SFC variation when not having a shaft generator.

The SFC differences are between −0.4% to 1.5% over the whole engine load range plotted. The difference reflects that even though the engine load increases, for PTI operation, the SFC decreases because the engine goes back to the high efficiency region operation or MEOP described before. On the other hand, the engine without a shaft generator increases the SFC, as expected, because of the operation away of the MEOP.

**Figure 11.** PTI evaluation performance.

**Figure 12.** SFC difference at PTI and without shaft generator.

EEOI values are presented in Figure 13 against the SFC obtained when applying the shaft generator control scheme. The SFC decreases while the engine load increases, which gives a set of EEOI values in accordance with the amount of fuel consumed over the navigation period and the navigated distance. The lineal increment of the EEOI is considered low over the engine load and reflects the decrease of the SFC because of the control scheme applied. The operational emissions are reduced in accordance with the reduction of the SFC.

**Figure 13.** EEOI against SFC at PTI condition.

Figure 14 shows the results of comparing the EEOI values of the applied control scheme, blue curve, against not having a shaft generator installed, red curve.

**Figure 14.** EEOI comparison.

Results are showing that the control scheme applied makes the ship to reduce the amount of operational CO2 emissions because is making the engine to work closer to MEOP providing grea<sup>t</sup> assertiveness of the methodology.
