For a simple jerk type fuel pump, start of fuel injection is fixed at the point at which the top of the plunger covers the spill port. End of injection, which is variable takes place when the edge of the helix uncovers the spill port.
Variable injection timing or VIT is the term used to define a fuel injection pump which alters the timing of the start of fuel injection.
With jerk type injection pump this can be done by following methods:
- By altering the position of the barrel with respect to the plunger.
- Using a fuel pump with two plungers.
- Altering the fuel cam or cam follower position.
- By using specially designed fuel pumps such as sulzer fuel pumps.
VIT is employed to advance fuel injection timing so that maximum combustion pressure or Pmax is reached at about 85% load. This gives a more efficient engine by lowering the fuel consumption. See the diagram below.
The figure above shows variation of Pmax for a fuel pump with VIT (shown in red line) and without VIT (shown in dotted red line). Variation of fuel index and VIT index is also shown. Action of VIT starts at about 40% of engine load. From this load, start of fuel injection is advanced to increase the Pmax gradually. As the engine load reaches 85%, value of Pmax will be corresponding to the 100% load of the engine, as shown in the figure. From 85% load, start of fuel injection retards to keep the Pmax constant. From the figure, VIT index shows advance and retard of fuel injection.
The injection timing can also be manually adjusted to allow for more efficient combustion of fuels with different ignition qualities.
Hence the purpose of VIT on ships are:
- Improve the specific fuel consumption of the engine.
- Optimizing the combustion process for different quality fuels.
Fuel Pump with VIT
The sketch above shows the operating principle of the VIT in a fuel pump fitted to the Sulzer RTA engine. A plain cylinder plunger reciprocates inside the barrel. As the plunger moves up and down, two pivoted levers operate suction valve and spill valve push rods which open the suction and spill valves as shown. When the cam follower is on the base circle of the cam, the suction valve is open at the same time the spill valve is closed. As the plunger moves up in the barrel, the suction valve rod moves downwards and the suction valve closes. Fuel injection then starts and the fuel is delivered through a non return valve to the fuel injectors. As the plunger continues upwards, the spill valve push rod will open the spill valve, the pressure above plunger will fall and injection will cease.
The quantity of fuel delivered can be controlled by altering position of the eccentric pivot for the spill valve operating lever. This will cause the spill valve to open earlier or later.
By altering the position of the suction valve pivot, the start of injection can be controlled and therefore it can be seen that the pump utilizes VIT.