Show with the aid of a sketch how fuel can be continuously circulated through fuel-injection valves on large engines while the engine is operating or under standby conditions.

[Sketch notes: The fuel injection system has a fuel supply line and a fuel return line. The fuel is circulated continuously from the fuel supply (via the fuel pump) through the injectors and back to the fuel return line. Each injector has a fuel inlet and a fuel outlet (leak-off/return). The fuel is kept circulating so that it stays at the correct temperature and viscosity, and any air/vapour is removed.]

On large engines, the fuel is continuously circulated through the fuel injection valves to keep the fuel at the correct temperature and viscosity, and to prevent the fuel from cooling and solidifying (for heavy fuel oil) or from forming deposits. The arrangement:

1. Fuel supply: the fuel is supplied from the fuel service tank through the fuel pump (or a circulating pump) to the fuel injection system.
2. Fuel injection valves: each injector has a fuel inlet (from the supply line) and a fuel outlet (return line). The fuel flows through the injector body, keeping it warm and free of air.
3. Fuel return: the fuel returns from the injectors to the fuel return line, which leads back to the fuel service tank (or to the fuel pump suction).
4. Continuous circulation: the fuel is circulated continuously (even when the engine is stopped, under standby conditions) by a circulating pump, so the fuel in the injectors and the lines is kept at the correct temperature and viscosity.
5. Temperature control: the fuel is heated (by a fuel heater) to the correct temperature, and the circulation keeps the temperature uniform.
6. Pressure: the fuel is maintained at a suitable pressure (by a pressure-regulating valve on the return line) so that the injectors are always primed and ready.

The continuous circulation ensures that, when the engine is started, the fuel is at the correct temperature and viscosity for immediate injection, and it prevents the fuel from cooling, solidifying, or forming deposits in the injectors and lines. This is particularly important for heavy fuel oil, which must be kept hot and circulating to remain pumpable.