What is jerk type fuel injection pump?
The jerk type fuel injection pump is a type of positive displacement pump that uses a helix shaped plunger to inject fuel. It is used to pump fuel (diesel, heavy fuel oil etc.) to an internal combustion (IC) engine's cylinders (the combustion space) via its fuel injectors. This type of pump is used for large marine two stroke diesel engine applications, although it has other uses also.
Cam Operated Jerk Pump
The jerk type fuel pump represents a reliable and efficient way to deliver fuel to an engine and forms a critical part of a fuel injection system.
What are the main parts of a jerk type fuel injection pump?
The main parts of a jerk fuel pump are the suction port, discharge port, barrel, plunger, relief valve (delivery valve), quadrant/pinion, spill port, and control rack; these parts are collectively referred to as a fuel pump assembly.
The suction port is where fuel enters the pump, and the discharge port is where fuel is discharged. Suction and discharge ports are also referred to as inlet and outlet ports respectively.
The control rack connects to the plunger via the quadrant; it controls the volume of fuel injected. The control rack moves linearly, whilst the quadrant moves radially.
The helix shaped plunger controls when the spill port is uncovered and thus the volume of fuel delivered (a detailed working explanation is given in the next section).
A spring loaded delivery valve installed above the plunger ensures that no fuel is delivered to the injectors once the fuel pressure reduces beyond a certain point.
Fuel is discharged from the fuel pump to one or more fuel injectors. Each fuel injector is essentially a spring loaded needle valve, which opens and closes depending upon the fuel system pressure.
How do jerk type fuel injection pumps work?
The jerk type fuel pump works by drawing fuel from a feed line or fuel tank, then delivering it to the engine cylinders via the engine's fuel injectors. The pump is driven by the engine camshaft i.e. it is a cam driven pump, which is connected via gears or a chain to the engine crankshaft via the flywheel.
The fuel injection pump regulates the amount of fuel that is delivered to the engine in order to regulate the engine's speed. When the engine is running at a higher speed, more fuel is being injected into the cylinders. When the engine is running at a lower speed, less fuel is being injected into the cylinders. Adjusting the control rack causes the quadrant to rotate, which causes the plunger to rotate, and more or less fuel to be injected into the engine; it is thus possible to change the engine speed by adjusting the control rack linearly. Because the volume of fuel delivered can be adjusted by the pump, these types of pump are often called variable injection timing pumps.
The Importance of the Helix Shaped Plunger
The helix shape of the plunger controls when fuel is bled/spilt from the cylinder, and thus when fuel is injected into the associated cylinder and for how long. Once the spill port is uncovered, the fuel pressure in the cylinder drops rapidly and the delivery valve begins to close. Covering the spill port for a longer period, causes more fuel to be injected, and the engine speed to increase. Covering the spill port for a shorter period, causes less fuel to be injected, and the engine speed to decrease. The duration of time for which the spill port is covered depends upon the position of the plunger's helix, which is rotated by the rack and quadrant.
Due to the helix shape of the plunger, this type of pump is also referred to as a helix type fuel pump.
Additional Resources
https://www.marinesite.info/2021/06/fuel-pump-in-marine-diesel-engine.html
https://marineengineeringonline.com/tag/fuel-pump-with-vit/
https://dieselpro.com/blog/types-of-marine-engine-fuel-pumps/