Question

With reference to a main engine fuel system of the high-pressure common rail type:

(a) Sketch a common rail fuel injection system from booster pump inlet to cylinder head fuel valves, labelling the MAIN components. (6)

(b) Explain how the fuel pumps are operated and the common rail pressure is maintained. (5)

(c) Explain how fuel injection timing and quantity is regulated for the common rail fuel system sketched in part (a). (5)

Answer 1

Part (a)

Part (b)

In common rail system, a common rail is pressurised with fuel using a set of jerk type pumps driven by a three-lobe cam which is connected to the crankshaft via gear trains. The pumps are of variable delivery type, and maintain the rail pressure around 1000 bar, controlled by an electrically driven shaft linked to the engine computer module. The engine computer control system known as the 'Wartsila Engine Control System (WECS)controls the delivery from the common rail to the individual cylinders via the volumetric Injection control system.

Part (c)

To be able to time the fuel injection the control system must know the crank angle of the individual units. To do this two crank angle sensors are fitted at the free end of the engine. These sensors are accurate to 0.1°. Each cylinder has its own electronic control system comprising of a cylinder control module and a variable driver module. Each Cylinder Control Module calculates the correct injection start angle, taking into account dead time, VIT, and Fuel Quality Setting. It also controls the quantity of fuel injected and the sequence of injection (i.e. for low load running).

When the Rail Valves are energised for injection by the Valve Driver Module, oil from the Control Oil Rail opens the Injection Control Valves. The fuel injectors are pressurised and fuel oil pressure behind a Fuel Quantity Piston in the Volumetric Control Unit maintains this pressure at the injectors. As the Piston moves to the left a feedback signal is sent to the Cylinder Control Module

When the desired amount of fuel has been injected the Valve Driver Module energies the solenoids which move the Rail valves back to the return position. The Injection Control Valves interrupt the supply to the injectors, and the increase in pressure on the LH of the fuel Quantity Piston moves it back to its starting position.

✓ Verified Model Answer (Text Solution)