A common large diesel arrangement uses a jerk-type fuel pump with a variable injection timing mechanism. The pump has a plunger operated by a cam, but the point at which pressure starts to build can be altered by changing the effective spill/closing point.

Main parts:

• Fuel cam and roller tappet give the plunger its upward stroke.
• Plunger and barrel pressurise fuel.
• Suction/spill ports allow filling and spilling.
• Delivery valve prevents backflow and gives sharp pressure collapse.
• Fuel rack rotates the plunger to vary effective stroke and fuel quantity.
• VIT linkage or servo changes the relative timing between plunger movement and port closure, giving earlier or later injection.

The exact design differs between engine makers. Some alter the pump barrel position, some alter the roller guide or cam follower position, and modern engines may use an electronically controlled spill valve. The principle is the same: change when high pressure begins without upsetting the required fuel quantity control.

Changing injection timing

Injection begins when the pump plunger covers the spill/suction port and fuel pressure rises enough to open the injector needle.

With VIT, the mechanism changes the point at which this happens:

• If the port is covered earlier in the plunger stroke, pressure builds earlier, so injection is advanced.
• If the port is covered later in the plunger stroke, pressure builds later, so injection is retarded.
• This may be done by moving the pump barrel, changing the roller guide position, or controlling a spill valve electronically/hydraulically.

Thus VIT varies the start of injection according to engine load to obtain correct peak pressure and fuel economy.

Changing fuel quantity

Fuel quantity is changed by rotating the pump plunger with the fuel rack.

• At low fuel setting, the plunger helix uncovers the spill port early.
• Pressure collapses early, so injection ends early and less fuel is delivered.
• At high fuel setting, the helix uncovers the spill port later.
• Injection continues for longer, so more fuel is delivered.

So, in a conventional jerk pump, VIT mainly changes the start of injection, while the fuel rack changes the end of injection and therefore the quantity delivered.