VIT (Variable Injection Timing) was introduced to keep the maximum cylinder pressure (Pmax) near its design maximum over a wide power range, so the engine can be optimised, reducing specific fuel oil consumption (SFOC). Without VIT, as load falls, the fuel charge per cycle falls and Pmax falls, and the engine runs with a lower Pmax (below optimum) at part load, so combustion efficiency (and the "compression/spread" of the cycle) suffers and SFOC rises. With VIT, the injection is advanced as load is reduced so that Pmax is held high at part load, giving better thermodynamic conversion and a flatter, lower SFOC curve.

Graph (Pmax vs engine power/load): [Describe] Without VIT: Pmax rises with load from low at idle to its design maximum at about 85% load then roughly constant; i.e. a sloping line. With VIT: Pmax is held roughly constant at the design maximum from about 50% to 85% load, then the timing is retarded above ~85% load so Pmax stays at the design limit (flat top) rather than climbing. The VIT curve is higher at part load (50-85% where Pmax is kept constant) than the no-VIT curve.

How fuel consumption is reduced: holding Pmax at a high value over the part-load range improves the thermal (thermodynamic) efficiency - the mean effective pressure for the same fuel is higher (e.g. more of the fuel energy is converted to work because the pressure is raised earlier and a higher compression/expansion ratio beneficially), so for the same work less fuel is burned, i.e. lower g/kWh. The advance of injection by VIT effectively raises the pressure rise point and makes the cycle operate more efficiently, directly lowering SFOC at the important cruising/part-load points where a merchant ship operates.

How VIT and FQS (Fuel Quantity Shaping / injection quantity setting) are achieved in camshaft-less engines:

In camshaft-less (electronically controlled) engines (MAN ME, WinGD X), there is no camshaft and no mechanical VIT device. Instead the engine's electronic control unit (ECU) controls the fuel injection timing directly by energizing the hydraulic-actuated fuel injection systems at the commanded crank angle. The injection quantity (fuel per stroke) is set by the duration for which the fuel valve is open (the injection period commanded by the ECU) and by the injection pressure profile, giving FQS - the ECU shapes the injection (e.g. pilot + main) and can change injection timing with load, i.e. VIT is software-controlled. Because the ECU knows the load (fuel index) it advances injection as load falls and retards it at high load to keep Pmax at the optimum/safe limit, without the mechanical cam/VIT arrangement.