Fuel Injection Rate Control

FUEL INJECTION RATE CONTROL
With the present type of injection pump, the pre-stroke is fixed. However, with the variable injection rate injection pump, the operating condition of the engine and control condition of the driver are detected by various sensors, and the information signal is processed by the control unit. Based on the control signal from the control unit, the control sleeve installed in the fuel chamber of the barrel portion is moved up and down by the pre-stroke actuator to change the pre-stroke and control the fuel injection rate.

NOTE: Hereafter in this text, from the point of explaining the action with the operation diagrams, the expression "fuel injection rate" is not used. The term "fuel feed rate" is used instead.







Fuel Feed Rate (proportion of fuel supplied per unit of time)
When supplying a fixed amount of fuel, if the feed timing is long, a smaller amount of fuel is supplied per unit time, and this gives a low fuel feed rate. On the other hand, if the feed timing is short, a larger amount of fuel is supplied per unit time, and this gives a high fuel feed rate.







Pre-stroke
The amount of cam lift when the injection pump cam rotates and moves from the plunger lift starting point a to the fuel feed starting point A is called the pre-stroke.

PRE-STROKE CONTROL MECHANISM

Fuel Feed Mechanism
The operation of the fuel feed for the variable fuel feed rate injection pump is as follows.







1. Before fuel feed
Until the plunger rises and the suction port is blocked by the control sleeve, the fuel in the pressure chamber passes through the suction port and flows out to the fuel chamber. Therefore, the pressure in the pressure chamber does not rise.
2. Start of fuel feed
When the plunger rises further and the suction port is blocked by the bottom end of the control sleeve, the fuel feed operation starts.
3. During fuel feed
While the suction port is between the bottom of the control sleeve and the spill port, the fuel continues to be fed.
4. Completion of fuel feed
When the suction port and the notched groove (lead) of the plunger are connected with the spill port of the control sleeve, the high pressure fuel in the pressure chamber is drained through the spill port to the fuel chamber, and this completes the fuel feed.

Relation between Fuel Feed Rate and Position of Control Sleeve







When the control sleeve is at the top, the pre-stroke is at its maximum, and as the plunger lift speed is fast, the fuel feed time can be made shorter. This means that the amount of fuel fed per unit time is larger and the fuel feed rate is higher.

On the other hand, when the control sleeve is at the bottom, the pre-stroke is at its minimum, and as the plunger lift speed is slow, the fuel feed time is longer. This means that the amount of fuel fed per unit time is smaller and the fuel feed rate is lower.

Under normal operating conditions, for low speed, the control sleeve is at the top to give a high fuel feed rate, and at high speed, the control sleeve is moved to the bottom to prevent the fuel feed rate from becoming too high.

Pre-stroke Control Mechanism
The pre-stroke control mechanism consists of the following.







When the control sleeve goes up, the pre-stroke becomes larger, and when it goes down, the pre-stroke becomes smaller.

Pre-stroke Actuator







The pre-stroke actuator has a shape that forms three sides of a square, and consists of an iron core with a coil wound around it. A rotor is placed between the jaws of the actuator, and a magnetic force proportional to the control signal (current) from the control unit is generated by the rotary solenoid. The action of this magnetic force then causes the rotor to rotate. The control sleeve is interconnected, so it moves up or down.

Power Circuit
The power source for the pre-stroke actuator has a separate circuit. If there is any abnormality in the pre-stroke servo system, this circuit functions to stop the control, and at the same time it opens the pre-stroke cut relay to cut the power for the pre-stroke actuator.












Pre-stroke Sensor
The pre-stroke sensor compares the target position sent from the control unit and detects if the pre-stroke actuator is actually in the correct position.
Like the rack sensor, it is a non-contact displacement meter, and sends out an alternating wave current (sine wave) from the OSC terminal (oscillation terminal) of the control unit. It detects the position of the pre-stroke actuator (control sleeve) from the MDL (middle) signal generated by the change in the impedance (self-induction) between the two coils, and sends this to the control unit. The pre-stroke sensor harness is equipped with a shielding wire to prevent noise.

The shielding wire acts to prevent the influence of any outside interference, thereby protecting the signal detected by the sensor; it also acts to set the measurement standard. If there is improper connection of the shielding wire (particularly at the connector), or there is improper grounding of the control unit case, this may cause disconnections or resistance (increased or reduced) which will cause abnormalities in the pre-stroke control. To prevent this, please be sure to carry out the grounding connection of the control unit and the pre-stroke sensor wiring harness properly.