Cooling the fuel
Excess fuel is returned to the tank at the common rail that is not injected into the cylinders. The return fuel is at high temperature as it absorbs heat from the injectors and the jacket of water. As it mixes in the tank with cooler air, the temperature of the fuel supply starts to slowly increase.
For every 10°F increase in fuel temperature above 100°F, the horsepower of the engine is reduced by about 1%. The high temperature of fuel also reduces its ability to lubricate components of the engine fuel system. If the fuel temperature to be supplied to the engine reaches a certain level (typically 140° to 150°F), the genset will shut down due to the safety cutoff. It is especially troublesome if the capacity of the tank is fairly low (e.g., auxiliary tanks), and the temperature of the return fuel is not decreased.