In-line Fuel Injection Pump (Diesel Engine).

A(caps)n in-line fuel injection pump is a type of diesel fuel injection pump that is commonly used in large diesel engines, such as those found in heavy-duty trucks, buses, and construction equipment. It is called “inline” because the pump is located in a line with the engine’s cylinders, and each cylinder is supplied with fuel by a separate pump element.

In-line Fuel Injection Pump
In-line Fuel Injection Pump
In this article, you will read about in-line fuel injection pumps.

In-line fuel Injection pump

To deliver accurately measured amount of fuel at high pre sure to the injectors. The amount of fuel delivered should be controlled precisely concerning the timing, rate, and duration; and must satisfy the work the engine is required to do.
The basic design of an inline fuel injection pump consists of a high-pressure pump that is driven by the engine’s camshaft. The pump delivers fuel to a distribution block or manifold, which is connected to a series of fuel lines leading to each of the engine’s cylinders. Each cylinder has its pump element, which is controlled by a cam follower or plunger that is actuated by the engine’s camshaft.
In-line fuel injection pump elements.
Pump elements.


When the camshaft rotates, it pushes the cam followers or plungers, which then pressurize the fuel and inject it into the combustion chamber at the precise time needed for combustion. The injection timing and fuel delivery rate can be adjusted by changing the profile of the camshaft or by adjusting the position of the plunger.

Inline fuel injection pumps are known for their durability and reliability, and they are commonly used in high-performance diesel engines that require precise control of fuel delivery. They do require regular maintenance, including cleaning and adjustment of the pump elements and replacement of worn parts to ensure proper performance and efficiency.


In-line fuel Injection pumps general design

All our in-line Fuel injection pumps are cam-operated, spring return, plunger type, using one pumping unit for each engine cylinder; and incorporate their own camshaft and tappet gear.

Each pumping unit comprises the following essential components :

  1. Barrel and plunger.
  2. Delivery valve and seating. 

1. Barrel and plunger- 

Barrel with various plunger positions.
Barrel with various plunger positions.


Barrel and plunger, valve, and seating are highly ground steel-being finished to the finest limits and with the highest degree of precision to permit accurate operation at high speeds and pressures; each pair must, therefore, be regarded as inseparable and not interchangeable. To enable the pump to vary the quantity of fuel delivered per stroke, the plunger is provided with a vertical channel extending from its top edge to an annular groove, the upper edge of which is cut in the form of a helix. External means are provided whereby the plunger can be rotated in its barrel during operation.


The system of operation of the pump element, which is comprised of the plunger and barrel, is shown in Fig. When the plunger is at BDC as at (a), oil can enter through the barrel ports either by gravity flow from an overhead tank, or force feed from a fuel feed pump, the latter being the most usual arrangement. ln, a primed. the system, the barrel, and the pipes. leading from the pump to the injectors, are full of the pump plunger reaches the position (b), a certain amount of fuel is pushed back through the barrel ports until the plunger reaches the position (b) where the top land of the plunger has closed both ports. The fuel above the plunger is then trapped, and its only outlet is via the delivery valve which is mounted on top of the pump barrel. The pressure exerted by the rising plunger upon the oil causes this to lift the valve and enter the pipe which connects the pump to the injectors. As this is itself already full of oil, the extra oil that is being pumped in at the pump end causes a rise in pressure throughout the line and lifts the nozzle needle.

Here the lower edge of the control helix has uncovered the barrel port, thus allowing fuel to be bypassed back to the suction chamber (which is under very much lower pressure than the fuel oil above the plunger) by way of the vertical slot. This causes the delivery valve to shut under the action of this spring, and with the consequent collapse of pressure in the pipeline, the nozzle valve also shuts. The plunger stroke is always constant, but that part of it that is pumping is variable. Using the helical edge that runs around the plunger, which itself can be rotated within the barrel, it is possible to make this point of cut-off earlier, or later, in the stroke-compare positions shown at (c), (d) and (e) which show full load, half load and idling respectively. To stop the engine, the plunger is turned so that the verticle slot coincides with the barrel port during the whole of the plunger stroke shown, thus no fuel is delivered. The position of the plunger stroke at which the helical edge will uncover the port is adjustable by rotating the plunger axially using a toothed quadrant which is clamped to a sleeve having slots engaging the vanes of the plunger at its lower end.

The toothed quadrant meshes with a rack provided on the control rod which similarly actuates all the pump elements in the unit and is externally connected either to the governor or other controls by suitable linkage.


Control of output- 

The word “stop” and an arrow are engraved on the top of one end of the control rod indicating which way the control rod should be moved to stop the engine. A pump element at no output or “Engine Stopped” when the control rod(Fig) is in the “stop” position and the verticle channel of the pump plunger will be opposite the right-hand port in its pump barrel so that even if the engine is moved, no fuel will be pumped. 
To start the engine, the control rod should be moved over to the “Starting” position from the “Stop” so that the plunger will be in the position shown in Fig. At this point, the plungers are delivering more fuel than is required by the engine at full load, which condition is necessary to obtain easy starting. When the engine starts, the control rod should be released to the position giving the desired engine speed. In the fig, the pump is shown at the normal output, in which position the engine will be operating at normal load. The actual position of the control rod in these conditions can be found only by experiment on the particular engine concerned. The control rod can be connected to the governor at one end. In linking, however, care should be taken that no traverse or rotational forces are transmitted to the control rod which may result in the latter either jamming or becoming stiff in action with consequent faulty control of the engine.

2. Delivery valve or anti-dribble device- 

When the helical edge of the pump plunger uncovers the port in the pump barrel near the end of the delivery stroke, the pressure of fuel is immediately reduced so that the delivery valve at once drops on its seating, thus cutting off communication between the pump and the nozzle until the next delivery stroke takes place. In coming to its seat to act as a non-return valve, the delivery valve is, however, made to perform the other highly important function of pressure unloading.
Delivery valve.
Delivery valve.


The double function is obtained using the novel, but entirely simple construction of the delivery valve unit; and reference to Fig will show that it is an ordinary miter-faced valve with a guide that has a circular groove cut in it, dividing the guide into two parts. The lower parts have four longitudinal grooves communicating with the circular groove. The upper part of the guide forms a small piston, which is an accurately ground plunger fit for the valve seating which is also internally ground. When the pump is on its delivery stroke, the pressure of the fuel rises and the delivery valve is pushed up until the fuel can escape through the longitudinal grooves over the valve face to the nozzles. Immediately after the pump releases the pressure in the barrel, the delivery valve resumes its seat, causing the small piston part of the guide to sweep down the valve seating with plunger action, thus increasing the space in the delivery pipe before the valve seats itself. The effect of this increase of volume in the delivery system is, of course, that of suddenly reducing the pressure of the fuel therein so that the nozzle valve in the nozzle can snap to its seat, thus instantaneously terminating the spray of fuel in the cylinder entirely without dribble.

Control rod stops for in-line fuel injection pump

The control rod stops. The control rod travel towards maximum delivery must, in most cases, be limited (smoke limit) by an adjustable stop. There are different types of control rod stops. The fixed stop shown in Fig is adjusted using a screw which is secured by a cotter pin.
Fixed control rod stop.
Fixed control rod stop.


The automatic control rod stop is essential in cases where the engine requires more fuel for starting than the normal full-load delivery. It is, therefore, frequently installed in place of the adjustable control rod stop. The essentials of the design and operation of such a stop are shown in Fig.
Automatic control rod stop.
Automatic control rod stop.


When the driver fully depresses the accelerator pedal whilst the engine is at a standstill, the spring in the yielding cap will relax. The control rod travel-and consequently the fuel delivery -is, therefore, greater than being full load performance.

As soon as the engine starts running, however, the control rod stop will no longer yield because the governor, with the aid of the automatic control rod, stops the spring, and retracts the control rod as far as its operating position, i.e. maximum delivery.

Phasing and calibration for In-line fuel injection pump

The essential functions of the injection pump, when fitted to the engine, are to ensure that an accurately metered quantity of fuel shall be injected into each cylinder and at an exact point in the stroke at which the engine requires it. The latter point is the first for which adjustment is made and this, of course, will affect the timing of the pump to the engine.
It is essential, therefore, during the adjustment of the pump, to ensure that the subsequent pumping elements commence injecting at exactly the correct interval in the camshaft degree after the No. 1 element. Assuming that the injection sequence of a 6-cylinder pump is 1, 5, 3, 6, 2, 4, then the No. 5 element must commence injection 60° after No. 1 (the pump works at half engine speed) and No. 3 at the same interval after No. 5 and so on. The interval on all types of pumps is 360° camshaft angle divided by the number of elements in the pump. This adjustment for the correct timing interval is known as “phasing ” or adjusting the phase angle of the pump.

Point of port closure- 

It is next necessary to determine the point to which the adjustment has to be made. This is generally referred to as “the point of port closure” which occurs shortly after the commencement of the plunger stroke when the rising plunger closes the ports through which the fuel has entered the element barrel. The actual commencement of injection occurs after this point of port closure, the interval depending mainly upon the plunger diameter, cam profile, pipe length, and the setting of the injector spring.

Finally, adjustment is made for the balance of fuel output, or in other words, the pump is calibrated.


Calibrating the injection pump- 

This is carried out by slackening the screw which clamps the quadrant to the sleeve and moving the sleeve with the plunger into the required position. This adjustment is accurately carried out at the factory when the pump is new, and a line is scribed across the quadrant and sleeve to indicate the correct setting. Wear on the elements etc., may necessitate some slight alteration to this setting after several hundred hours of running, but in no circumstances should the setting deviate more than a few millimeters from the original adjustment. It is consequently advisable before testing to see that these calibration markings are lined up as this will certainly reduce the amount of adjustment to be made. If, however, the pump has been overhauled and the sleeves and/or quadrants have been renewed, this guide will not be available, in which case the pump will have to be calibrated as shown in the figures.

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