2. Necessity of adjusting nozzle opening pressure
For the injection nozzle, the nozzle opening timing varies according to the nozzle needle opening pressure. If the injection nozzle does not open or close correctly, it affects the timing and volume of the fuel injection. Therefore, the nozzle opening pressure must always be adjusted to be at the specified value.
Nozzle opening pressure and injection timing volume
Opening pressure Low High
Injection timing Advanced Retarded
Injection volume Large Small
The nozzle opening pressure is adjusted by changing the thickness of the adjusting shims and adjusting the force of the pressure spring.
Reduce thickness of shimInjection pressure is low
Increase thickness of shimInjection pressure is high
3. Two-stage injection nozzle
In some diesel engines, the twostage injection nozzles are used. Use of two-stage injection nozzles makes it possible to lower the nozzle opening pressure. Therefore, injection stability in the low load range or at idling is better than that of normal injection nozzles. Also, diesel knock that occurs with small injection volumes is decreased.
(1) Construction
Two pressure springs and two pressure pins are built into the nozzle holder body. Clearance is provided between the needle tip and the pressure pin because of the two stages of the fuel injection. This clearance is called pre-lift. In order to adjust the fuel injection pressure at the 1st stage and 2nd stage, replace each pressure spring washer.
(2) Operation
<1> 1st stage operation
When the fuel pressure of the injection pump reaches approx. 18MPa (18kgf/cm2), the needle tip pushes the pressure pin up through the No.3 pressure spring washer, exceeding the force of the No.1 spring pressure. At this time, the fuel is injected. The amount of lift changes until the needle tip makes contact with the No.2 pressure spring seat. After the No.3 pressure spring washer makes contact with the No.2 pressure spring seat, the amount of the needle tip lift does not change until the fuel pressure reaches approx. 23 MPa (23kgf/cm2).
<2> 2nd stage operation
When the fuel pressure reaches approx. 23 MPa (23kgf/cm2), it overcomes the force of both the No.1 and No.2 pressure springs. And the needle tip pushes the No.2 pressure spring seat through the No.3 pressure spring washer. The amount of the needle tip lift does not change when it reaches the maximum amount of lift even if the fuel pressure changes. For this reason, when the engine is running at low load, small amounts of fuel are injected in the low-lift range only. When a load is applied to the engine, small amounts of fuel are injected in the pre-lift range, and then larger amounts are injected in the high-lift range. For this reason, when the engine is running at low load, small amounts of fuel are injected in the low-lift range only. When a load is applied to the engine, small amounts of fuel are injected in the pre-lift range, and then larger amounts are injected in the high-lift range.
Diesel EngineDelivery Valves