Unit injection is widely used in many electronically controlled engines. Describe any one such system and its advantages compared with conventional systems.

Unit injection system (e.g. the MAN B&W ME engine's fuel injection system):

In the MAN B&W ME (camshaftless) engine, the fuel injection is by a unit injector system. Each cylinder has a fuel injection valve (injector) which is opened hydraulically. The system consists of:

1. A high-pressure fuel supply: fuel is supplied to the injector at a high pressure (e.g. 600-900 bar) by a fuel pump (or by a common rail in some designs).
2. A fuel injection valve (injector) in each cylinder: the injector has a needle valve which is opened by hydraulic pressure (from a servo oil system) or by a solenoid valve.
3. An electronic control unit (ECU): the ECU commands the injection timing and duration by energizing a solenoid valve (or a hydraulic valve) that controls the opening of the injector needle.
4. A hydraulic (servo) oil system: high-pressure servo oil (e.g. 200-300 bar) is used to actuate the injector and the exhaust valve.

Working: The ECU determines the required injection timing and quantity (based on the engine speed, load, and other parameters). At the commanded crank angle, the ECU energizes the solenoid valve, which admits servo oil to the injector, opening the needle and injecting fuel into the cylinder. The injection duration (and hence the quantity) is controlled by the time the solenoid is energized. The injection timing can be varied (VIT) and the injection profile can be shaped (e.g. pilot injection) by the ECU.

Advantages compared with conventional (cam-driven) systems:

1. Variable injection timing (VIT): the injection timing can be advanced/retarded with load to keep Pmax at the optimum, improving fuel economy and reducing emissions.
2. Precise control of injection quantity and timing: the ECU controls the injection accurately, giving better combustion and lower emissions.
3. Injection profile shaping: the ECU can provide pilot injection (a small pre-injection) to reduce NOx and noise, and can shape the injection for better combustion.
4. No camshaft: the mechanical camshaft and its drive are eliminated, reducing weight, wear, and maintenance, and allowing faster reversal.
5. Better part-load performance and lower fuel consumption.
6. Reduced emissions (NOx, smoke) through precise control.
7. Faster response and better manoeuvring (crash astern) because there is no mechanical camshaft to shift.
8. Individual cylinder control: each cylinder's injection can be adjusted independently for load balancing.

The unit injection system thus gives better fuel economy, lower emissions, and improved performance compared with conventional cam-driven injection.