Inline Fuel Injection Pump (Diesel)

Bosch In-line fuel injection pumps were first introduced in 1927. Since its introduction, it has kept countless number of diesel engines in function. In-line pumps are still widely used in large number of diesel engines, mainly because of its durability and ease of maintenance.

Requirements:

A fuel injection pump is used to supply fuel to the engine at a certain pressure. The pump generates the pressure and supplies the fuel with the right quantity at the desired timing. The pressurized fuel is delivered to the nozzle via a high pressure line. The nozzle injects the fuel inside the combustion chamber. There are various requirements to be met by a in-line pump, such as:

• Timing and duration of fuel injection


• The total volume of fuel to be injected


• The amount of pressure to be created

DESIGN:

The in-line fuel injection system consists of the following components:

• Fuel tank


• Feed pump to supply the fuel from fuel tank to the high pressure pump via a filter


• High pressure in-line pump to pressurize the fuel


• Nozzles to inject the fuel inside the combustion chamber


• Governor to vary the fuel quantity at varying speeds (usually a RSV governor)

In-line pumps can have a set of 2 to 12 cylinders. It is used in various commercial vehicles, agricultural and construction machineries. The maximum injection pressures can vary between 400 bar to 1350 bar based on the pump design.

Design of In-line pump:

It is an aluminium housing which has an internal camshaft. The camshaft is driven via a timing device or directly by the engine. The in-line pump camshaft rotates at the same speed as that of the engine camshaft (i.e) speed of camshaft is half the speed of the crankshaft.

Roller tappets sit over the cam lobes. The number of roller tappets equals the number of cylinders. Above each roller tappet, plunger return springs are placed to assist the plungers in returning to the bottom dead centre (BDC) after each stroke. The plunger is guided inside a barrel where the fuel is pressurized. Plunger has a vertical groove and a helical groove that assist in varying the fuel quantity. The plunger and barrel together are called plunger and barrel assembly.

Delivery valves are seated between the barrel-and-plunger assembly and the delivery valve holder. In the event of delivery stroke, delivery valve cone is lifted from the valve seat due to the high pressure created in the barrel. The delivery valve cone is pressed against the spring provided in the delivery valve holder. The fuel escapes through the holder to the nozzle via a fuel delivery line.

WORKING:

The fuel system layout consists of a feed pump which sucks fuel from the fuel tank and then delivers it to the high pressure inline pump at a low pressure. The camshaft is provided with a separate lobe that drives the feed pump. Diesel is then sent to a filter to remove the unwanted impurities such as dust, corroded particles, water, etc.

Fuel enters the fuel gallery provided in the in-line pump. The fuel gallery is directly connected to the fuel inlet ports in the barrels of all the cylinders.

Plunger stroke phases:

The position of the plunger results in various functions:

Intake Phase

Preliminary Phase

Delivery Phase

1. Intake phase: When the plunger is at bottom dead centre (BDC), the fuel inlet port in any one of the barrels is open and the fuel enters the barrel. This phase is called the intake phase.


2. Preliminary phase: When the plunger starts moving towards top dead centre (TDC), it closes the fuel inlet port and this is called preliminary phase. The fuel is now trapped inside the barrel.


3. Delivery phase: When the plunger continues to move further towards TDC, the trapped fuel is compressed. This increases the pressure inside the barrel-and-plunger assembly and the delivery valve cone is lifted from its seat to allow the pressurized fuel to escape through the delivery valve holder.

Fuel Delivery Variation:

The fuel quantity can be varied based on the position of the vertical and helical grooves. The position of these grooves can be varied with the help of a control rack and control sleeve assembly.

The control sleeve is meshed with the control rack. Translatory motion of the control rack is converted into rotary motion by the control sleeve. The plunger is seated in the groove of the sleeve, therefore the plunger rotates along with the sleeve.

Zero Delivery

Low Speed Delivery

High Speed Delivery

• Zero delivery: To achieve zero delivery, the vertical groove of the plunger should be in line with the inlet port of the barrel. In this position, the pressure chamber in the barrel is directly connected to the fuel gallery throughout the entire stroke from BDC to TDC. Therefore, the fuel in the barrel escapes back to the fuel gallery without getting delivered.


• Partial delivery: Partial delivery of fuel can be achieved by varying the helical groove position in line with the fuel inlet port. Different quantities can be achieved at different positions of the helical groove.


• Maximum delivery: Maximum delivery of fuel can be achieved if neither the vertical groove nor the helical groove is in line with the fuel inlet port.

GOVERNOR:

The top priorities of a fuel injection pump is to supply fuel to the engine at the right time under all operating conditions and at all operating loads. The governor has to constantly vary the control rack positions as the conditions keep changing. Some of the functions of a governor are:

• To precisely meter fuel quantities at various engine loads


• To supply the fuel to the nozzles at the right moment


• To supply fuel for a defined duration of time

Governor Requirements:

The basic function of a governor is to prevent the engine from exceeding its maximum revving speed. Diesel engines may over-rev due to excess amount of air and a governor can be used to cut the fuel supply till the engine speed goes below the maximum revving speed.

Variable Speed Governor (RSV):

RSV governor is used to control the fuel quantity at varying speeds between the idling speed and the maximum speed. It has a flyweight attached to one end of the camshaft. It also has a governor spring pivoted to a tensioning lever which acts against the force of the flyweights. When the speed of the engine is altered, the tension in the governor spring also alters accordingly such that the turning of the tensioning lever is kept in equilibrium with the opposing forces of the flyweights.

The variation in the control lever angle is transmitted to the control rack via fulcrum and guide lever linkages. This helps in varying the fuel quantity. Various speeds can be achieved by varying the control rack movement:

• Starting: The control rack is kept in the starting position by means of a starting spring whose one end is hooked to the control rack and the other end is hooked to the top end of the fulcrum lever. This sets the fuel injection pump to the starting quantity.


• Idle Speed: To achieve idle speed, the control lever is released and made to rest against the low idle stop screw. In this case, there is no tension on the governor spring and it sits in a vertical position. There is literally no force acting against the flyweights, therefore the flyweights start opening at low speed. The sliding bolt is forced to move outside in the right direction, as a result the guide lever is also pushed towards the right. The lever pivots the fulcrum lever to move towards the right and this pulls the control rack towards the idle speed stop. The tensioning lever comes in contact with the auxiliary idle speed spring and this regulates the idle speed of the engine.


• Low Speed: The control lever is pressed to a certain angle. This results in the increase in tension in the governor spring and thus acts against the flyweights, forcing the sliding bolt to slide to the left. This results in the control rack to move towards the left, increasing the fuel quantity, thereby increasing the engine speed. This happens for a brief moment, as the increase in engine speed results in the flyweight to rotate faster and generate a greater centrifugal to act against the governor spring. Equilibrium is achieved between the governor spring and the flyweights' force. The control rack moves towards the right again and the engine speed is kept in control.


• Maximum Speed: The control lever is compressed fully against the maximum speed stop screw.The working is similar to the one explained at low speed. At this point, there is maximum tension in the governor spring.