## 2 Materials and Methods
### Engine Cooling System  
Each engine has three different cooling systems. The main system is comprised of the block, crankcase and cylinder cooling jacket and it is refrigerated by two circuits, the primary and the secondary circuit, separated by a plate heat exchanger. The secondary cooling system supplies the energy needed for preheating the boiler feed water (batch \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameters** & \multicolumn{4}{c}{**Degree of Loading**} \\ \hline  & 100 & 80 & 60 & 40 \\ n, rotation speed (rpm) & & & 1500 & \\ Effective power & & & & \\ MP\({}_{\text{eng}}\), mechanical power (kW) & 617 & 494 & 370 & 247 \\ _EP\({}_{\text{eng}}\)_, electrical power (cos(\(\varphi\)) = 1) (kW) & 598 & 478 & 357 & 236 \\ Effective average pressure (kPa) & 1370 & 1100 & 820 & 550 \\ Efficiency and fuel consumption & & & & \\ \(\eta_{\text{me-eng}}\), mechanical engine efficiency (\%) & 40.3 & 38.8 & 36.6 & 32.9 \\ \(\eta_{\text{el-eng}}\), electrical engine efficiency (cos\(\varphi\) = 1) (\%) & 39.1 & 37.6 & 35.3 & 31.4 \\ \(\widetilde{TP}_{\text{biogas}}\), biogas consumption (kW) & 1530 & 1272 & 1012 & 752 \\ Thermal losses & & & & & \\ Radiation power losses (kW) & 28 & 25 & 21 & 17 \\ Mechanical power losses (kW) & 19 & 16 & 13 & 11 \\ Exhaust gas power losses (25 \({}^{\circ}\)C) (kW) & 476 & 406 & 330 & 252 \\ Exhaust gas power losses (120 \({}^{\circ}\)C) (kW) & 375 & 323 & 264 & 203 \\ Thermal power losses in the main water circuit (kW) & 299 & 244 & 195 & 155 \\ Thermal power losses in the auxiliary water circuit (kW) & 110 & 103 & 96 & 81 \\ _TP\({}_{\text{IC}}\)_, thermal power losses in the intercooler (kW) & 36 & 33 & 30 & 20 \\ Thermal power losses in the oil exchanger (kW) & 74 & 70 & 66 & 61 \\ Combustion and emissions 1  & & & & 8.5 & 8.2 \\ Exhaust gases dry oxygen (\%) & & & 500 & \\ NOx (mg/m\({}^{3}\)) & & & & \\ CO (mg/m\({}^{3}\)) & & & \(<\)1000 & \\ NMHC 2 (mg/m\({}^{3}\)) & & & \(<\)300 & \\ Other parameters & & & & \\ \(T_{4}\), outlet temperature of the turbine exhaust gases (\({}^{\circ}\)C) & 474 & 488 & 503 & 518 \\ \(M_{\text{air}}\), inlet air mass flow (kg/h) & 3030 & 2510 & 1970 & 1460 \\ \(M_{\text{eg}}\), exhaust gases mass flow (kg/h) & 3320 & 2740 & 2160 & 1600 \\ \(p_{\text{bp\_max}}\), maximum exhaust back-pressure (kPa) & & & 4.5 & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Engine operating parameters obtained at different loading degrees. operation) and the surplus heat is dissipated in cooling towers. The second cooling system is known as Auxiliary. This circuit is composed of the intercooler and lubricating oil circuit, also having two internal circuits, as in the previous case. This cooling system keeps the air-fuel mixture temperature at 65 \({}^{\circ}\)C to limit the risk of detonation before combustion takes place in cylinders. The heat absorbed from the air-fuel mixture is dissipated in the cooling towers. The oil outlet temperature of the heat exchanger is between 75 and 90 \({}^{\circ}\)C.  
The third cooling system is the Emergency system. This system aids in cooling the block, crankcase and cylinder jackets, and it is deployed when the main circuit is not capable of removing all the heat. The primary circuit of this system is located in series with the main cooling system, and the secondary circuit is located in series with the auxiliary circuit. Since each engine has two lines of six V-cylinders, there are two blocks of cooling systems for the crankcase and cylinder jacket, and two others for the intercooler and oil exchangers. In calculations, they were considered to be arranged in parallel. The three circuits consist of plate heat exchangers, expansion vessels, three-way thermostatic valves (to allow a regulation of 8-10 \({}^{\circ}\)C temperature), temperature probes, manometers, and circulation pumps for the primary and secondary circuits. The fluid used in the heat exchanger of the primary circuit is a mixture of water and 20% glycol. Secondary cooling systems and cooling towers use water as fluid.