Wednesday, 16 March 2016

Distributor Fuel Injection Pump

Fuel injection pumps play an important role in delivering fuel to the injectors at the required pressure and timing. The injection sequence should be faster, which requires the pump to be compact and light in weight. Distributor type fuel injection pump fits the criteria of light weight and compact design. It also goes by the name axial-piston distributor pump.

In the year 1962, Bosch introduced its first distributor type fuel injection pump and since then it has been widely used in almost all types of vehicles. It houses a compact governor and all together the pump’s size is pretty much smaller than the inline fuel injection pumps. The pump and governor has been continuously improved over a period of time to meet the low fuel consumption and low emission demands.




For an in-direct fuel injection, a distributor pump generates 350 bar of pressure. Whereas, for a direct fuel injection system, it generates pressure in a range of 900 bar to 1900 bar. The pressure generation depends on the speed of the engine. They can be used in engines having 3 to 6 cylinders.
There are two types of distributor pumps:

·         VE type pump: These are also known as axial piston distributor type pumps. The piston compresses the fuel by moving in an axial direction relative to the drive shaft.

·         VR type pump: It is also known as radial piston type distributor pump. These have multiple pistons arranged in a radial direction relative to the drive shaft motion. The pressure achieved in VR pumps is higher than that of VE pumps.

This article will concentrate on VE pumps alone. It relies on a single piston to distribute fuel to all the cylinders of an engine.

FUEL SYSTEM LAYOUT:




The fuel injection system consists of a fuel tank. Fuel from the tank is supplied to the VE type distributor fuel injection pump via a fuel filter. The fuel is supplied with the help of a pre-supply pump if the tank is located at a lower position compared to the fuel injection pump. Fuel is pressurized in the fuel injection pump and then delivered to the nozzles. In addition, there is a solenoid shut off valve to block the flow of fuel to the high pressure fuel injection pump when the ignition is switched OFF. The fuel flow is varied with the help of a mechanical governor.  A hydraulic timing device is used to vary the fuel injection timing.

FUEL SUPPLY STAGE:
In the fuel supply stage, fuel is supplied from the tank to the fuel injection pump at the required pressure. This stage comprises of the following components:
  •          Fuel tank
  •          Pre-supply pump in fuel tank (optional)
  •          Fuel filter
  •          Fuel lines (Low pressure)
  •          Vane pump (Low pressure pump which is integrated in the high pressure pump)
  •          Pressure Control Valve (PCV)

Fuel Tank:
It should be corrosion resistant and should prevent leaking of fuel even if the pressure goes beyond the operating pressure of at least 0.3 bar.

Fuel Lines:
The fuel lines are made of flame resistant metal tubing. It should be strong enough to prevent damage and should avoid leakage that can occur at twists and turns.

Fuel Filter:
It reduces the level of contamination by removing solid particles. To ensure that the solid particles not clog the filter, a separate storage is provided for the removed particles.

Vane type pump (Low pressure pump):

It sucks the fuel from the tank and supplies it to the high pressure distributor pump. For each rotation, it supplies a constant amount of fuel to the high pressure pump. As the speed increases, the amount of fuel supplied also increases.

Vane pump’s impeller is mounted on the inside of the drive shaft through a key and keyway arrangement. The drive shaft runs the impeller. Impeller is surrounded by an eccentric ring which is mounted in the pump housing. An impeller has 4 floating blades which float outwards against the eccentric ring.


As the drive shaft rotates the impeller, the floating blades are pressed outside against the eccentric ring as a result of centrifugal force. Fuel from the tank flows through the inlet passage provided in the housing and is collected in the chamber formed by the impeller, any 2 floating blades and eccentric ring. As the shaft keeps rotating, the fuel in the chamber is transferred to a constricted space. As a result of this, fuel is pressurized to a margin of 4 bar at idling speed and 10 bar at maximum speed of engine. The low pressure fuel then escapes out through the spill port.




Due to the shape of eccentric ring, the volume of the chamber in which the fuel is collected is reduced when it rotates to the fuel discharge side. This arrangement pressurizes the fuel.
Both the fuel inlet side and fuel discharge side has kidney shaped cells. The inlet side has the fuel inlet bore connected to the fuel inlet passage and the discharge side has the spill port which supplies fuel to the high pressure pump.





Pressure Control Valve (PCV):

As the speed of the drive shaft increases, the pressure generated by the vane pump also increases. This pressure governs the functioning of the hydraulic timing device. Therefore it is important that the pressure generated should not exceed the optimum pressure.




A pressure control valve is used to control the internal pressure. It consists of a spring loaded valve. When the internal pressure is beyond a set value, then the valve plunger is pushed against the force of the compression spring. As a result, the return line is exposed and the fuel escapes through the return line. This reduces the internal pressure. The return line is placed adjacent to the fuel discharge side of the vane pump.

The fuel that escaped through the return line is directed back to the fuel inlet side of the vane pump through an internal passage. The opening pressure of the spring loaded valve can be adjusted by varying the spring tension.

DISTRIBUTOR PUMP DESIGN:

The distributor pump has a compact body in which various parts are integrated together. A typical distributor pump is made of the following components:

  •    Vane pump (Low pressure pump)
  •     High pressure distributor pump
  •      Mechanical governor
  •      Hydraulic timing device
  •      Solenoid shut off valve

HIGH PRESSURE DISTRIBUTOR PUMP:

The high pressure pump has one plunger or piston that pressurizes the fuel and then distributes it to individual cylinders through high pressure fuel lines and nozzles. The fuel is delivered at the specified timing and quantity. The distributor pump consists of the following components:

·         Distributor Plunger/Piston:
The rotational motion from the drive shaft is transferred to the plunger via a roller ring assembly, cam plate and yoke assembly. So the entire unit rotates at the same speed. The cam plate provides the reciprocating motion to the plunger. A plunger has vertical grooves equal to the number of cylinders in an engine. The vertical grooves act as fuel inlet passage to the barrel during inlet stroke of the piston. The stroke movement of plunger is 2.2 to 3.5 mm depending on the type of pump.




The plunger moves to Top Dead Centre (TDC) and compresses the fuel. Two symmetrically arranged plunger return springs push the plunger back to Bottom Dead Centre (BDC) after fuel compression has taken place. The plunger has a fuel delivery line running through its length and this line is connected to the distributor port and spill ports.


·         Cam Plate:






Cam plate has cam profiles which help in plunger reciprocation. The number of cam profiles is equal to the number of cylinders in an engine. The design of cam profiles affects the injection pressure and the injection duration.

·         Distributor Body:





The plunger and barrel are precisely fitted into the distributor body. The plunger also has a control collar which covers and uncovers the spill port to vary fuel quantity. The barrel has distributor slots in its inner circumference which supply fuel to the respective injectors via delivery valve. The distributor body also has a electric shut off valve to block the supply of fuel to the barrel when engine is switched OFF.




FUEL METERING INSIDE THE DISTRIBUTOR BODY:
The distributor body generates the pressure required for fuel injection. There are several phases of plunger stroke for precise fuel metering to take place.

·         Intake stroke:

When the piston moves from top dead centre (TDC) to bottom dead centre (BDC), one of the vertical grooves match with the fuel inlet passage and thus the fuel enters the plunger barrel.

·         Pre-stroke:

As the plunger keeps rotating, it closes the inlet passage. Now the piston starts moving from BDC to TDC and some amount of fuel flows back to the inner chamber of the pump through a slot provided at the top of plunger (also known as pre-stroke groove). Pre-stroke is necessary to prevent slow rise in injection pressure.

·         Effective stroke:

As the plunger moves further up towards TDC, the pre-stroke groove is closed and the injection pressure increases rapidly due to compression. The fuel is delivered to the delivery slot and then supplied to the delivery valve. The delivery valve lifts from its seat and allows the fuel to escape to the injector.

·         Residual stroke:

The effective stroke is complete when the spill port at the bottom of the plunger is exposed. This allows the fuel to escape to the pump’s internal chamber and thus the pressure inside the barrel is releases and there is no more fuel delivery to the injector.

VARIABLE SPEED GOVERNORS:
Variable speed governors are used to control the engine speeds from start to intermediate speed range and also controls it at high speeds. Speed variation is achieved by varying the fuel quantity.

Design:

The design is pretty much different compared to the one in Inline Fuel injection pumps. It consists of a flyweight housing with 4 flyweights. The flyweight housing has a gear at the bottom which is meshed with the drive shaft. It is mounted in its position with the help of a governor shaft. As the flyweights rotate, the movement is transferred to the sliding sleeve which slides up against the starting lever of the governor.




The governor mechanism consists of a starting lever, control lever and tensioning lever. At the end of the starting lever is a ball pin which engages with the control collar of the distributor plunger. A starting spring is attached to the top of the starting lever. There is an idle speed spring attached to the retaining stud at the top of the tensioning lever. A governor spring is attached to the retaining stud on one end, whereas the other end is connected to the rotational speed control lever via a linkage. The rotational speed control lever is linked to the accelerator pedal.



The governor spring tension and the flyweight force transfer the movement to the ball pin. The movement of the ball pin moves the control collar to vary the quantity of fuel delivered to the injectors.

The fuel quantity varies at different speeds. This is done with the help of a variable speed governor.

·         Starting speed:


When the engine is not running, the distributor pump doesn’t supply the fuel to the injectors and the flyweights and sliding sleeve of the governor rests at base position. At this point, the starting spring pushes the starting lever into its position and the movement is transferred to control collar which is brought to starting position. The effective stroke of plunger during position is higher. This allows maximum fuel to be delivered to the engine for starting. For starting the engine, the rotational speed control lever is pressed against the maximum speed screw.

·         Idle speed:

The starting lever force is overcome by a slight increase in engine speed. As the speed starts increasing, the flyweights’ radial movement results in the axial movement of the sliding sleeve which presses the starting lever against the force of starting spring. This results in the movement of control collar bringing it to idle speed position. The effective stroke is minimum for idling speed and this results in lesser fuel delivery to the injectors. The accelerator pedal is released and the rotational speed control lever rests against the idling speed screw.

The idle speed spring mounted in the retaining stud maintains a state of equilibrium with the flyweights’ force and maintains the starting lever in its position. This allows a steady amount of fuel to be delivered to the injector.

·         Operation under load:

When the accelerator pedal is pressed, the rotational speed control lever assumes a position between idle speed screw and maximum speed screw. When the speed of the engine goes beyond the idling speed, the starting spring and the idling speed spring are fully compressed and have no control over the movement fuel flow in this range.

The governor spring has the control over this speed range. When the accelerator pedal is pressed, the rotational speed control lever moves from its idle speed position to a position corresponding to the speed. This compresses the governor spring and the governor spring force exceeds the flyweights’ centrifugal force. As a result, the starting lever rotates and transfers the movement to the control collar. The effective stroke is increased and more fuel is delivered to the engine, thereby increasing the speed.

When the accelerator pedal is fully pressed (Wide open throttle), more amount of fuel is delivered as a result of governor spring’s control over the starting lever. As the speed increases, the flyweights’ centrifugal force increases and the sliding sleeve moves to oppose the spring force. The control collar remains in its wide open throttle position until the opposing forces between flyweights and governor spring achieve equilibrium.

If the speed of the engine further increases, the flyweights’ centrifugal force overcomes the governor spring force and reduces the effective stroke of plunger, thereby leading to speed reduction. Further increase of speed will lead to fuel being cut off

·         Engine Overrunning:
One of the features of a variable speed governor is to prevent overrunning of engines while descending a slope or a hill. The engine is driven by vehicle’s inertia. At this point the sliding sleeve presses against the starting lever and tensioning lever. The starting lever rotates in its axis to transfer the movement to control collar, wherein the collar brings the effective stroke to minimum or zero (in case engine is switched OFF).





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