• General
  • Tap Atomizer
  • Hole Atomizer
  • Two-stage atomizer
  • Electromagnetic Atomizer
  • peak and hold
  • Piezo Injector
  • Unit injector

Injectors inject the diesel fuel into the diesel engine; this can be in the swirl chamber with indirect injection engines, or directly in the cylinder with direct injection engines. The injector is mounted in the cylinder head. There are different types of atomizers. The different atomizers are described on this page.

Tap Atomizer:
Tap injectors are used in diesel engines with indirect fuel injection. The fuel is injected into a separate chamber; namely the anterior or vortex chamber. In this space, the fuel is mixed with the air, resulting in a combustible mixture. The tap atomizer has 1 injection opening. The injector needle is lifted at a certain fuel pressure, resulting in a fuel atomization from the injection opening. The opening pressure of dispensers is between 100 and 135 bar.

Hole Atomizer:
Hole injectors are used in direct fuel injection diesel engines. The injector has multiple injection ports that inject fuel directly into the cylinder. The combustion chamber of these engines is located in the piston bottom. The arrangement of the injection openings is precisely matched to the shape of the combustion chamber. Due to this construction, the atomizer can only be mounted in the cylinder head in one way. The opening pressure of this atomizer is between 180 and 250 bar.

Two-stage atomizer:
Diesel engines with direct fuel injection have a relatively hard diesel knock. This has to do with the large pressure increase in the cylinder due to the large amount of fuel that is injected in one go.
By using the so-called pre-injection, the combustion is started carefully. The diesel knock is therefore a lot less.
There are manufacturers who mount these so-called two-stage atomizers. These are hole atomizers, but with two springs; namely a strong spring and a weaker spring. Due to the weaker spring, the needle will be able to lift a little at a lower pressure. This injects a small amount of low-pressure fuel into the cylinder; the pre-injection. This initiates combustion. The needle of the atomizer runs against a stop that is locked in with the strong spring. The pressure will continue to rise to the normal opening pressure. The needle will open to its maximum at that moment: the main injection.

Electromagnetically Operated Atomizer:
Electromagnetically actuated injectors are used in common rail diesel engines. The atomizer is operated by an electromagnet. The engine management determines on the basis of the engine speed, throttle position, temperatuur, tax and the engine position when the atomizer should open and close and how long it should stay open. In rest position, the atomizer is not controlled. The common rail pressure of up to 1300 to 2000 bar (depending on diesel engine generation) is continuously applied to the inlet pipe of the injector.
When the engine management gives a signal to the injector, the coil is energized and the solenoid operated valve will be pulled upwards. The fuel pressure located above the control pin will be discharged through a small opening with the remaining return fuel. This causes the fuel pressure above the control pin to drop very quickly. The control pin will move upwards. This is facilitated by the fact that there is a conical part on the underside of the control pin. The control pin, including the injector needle, is moved upwards. At this point, the opening on the underside of the injector is released, allowing fuel to be injected into the combustion chamber at a common rail pressure of up to 1300 bar.

With solenoid-operated injectors, there is no need to use two springs to take care of the pre- and main injection. The electromagnetically operated atomizer can be operated several times in succession. Pre- and main injection times may also vary as engine operating conditions change. It is also possible that this atomizer provides two main injections. The advantage of multiple injections is a smoother combustion process.

peak and hold:
The electromagnetically operated atomizer is energized by the electromagnetic coil. When the current through the coil is high enough, the solenoid overcomes the spring force acting on the valve. A short current and voltage peak is required to get the injector needle moving. The on-board voltage of 14 volts is too little to lift the needle from its seat. In just 0,3 nanoseconds (10^-9 seconds) the coil is energized at 20 Amps at 80 Volts (peak). After the injector needle has been opened, it is held open with a 12 Ampere at 14 Volt (hold) until the actuation stops and the needle is pushed back into its seat by the spring force.

Piezo injector:
Piezo injectors are used in both petrol and diesel engines.

The element in a piezo injector lengthens or shortens as the applied voltage changes. The change in length only varies by thousands of millimeters. That is not enough to be able to open the injector needle sufficiently. That is why several piezo crystals are connected together, so that the injector needle can make a larger stroke.

The figure shows what happens to the piezo element when the applied voltage is increased.

The drive voltage of a piezo injector is between 100 and 160 volts. This voltage is obtained using capacitors in the motor control unit. The current is a few milliamps. With this voltage and current, the total change in length of the piezo element is approximately 0,08 millimeters. Closing of the atomizer needle is done by briefly reversing the flow direction.

The advantage of the piezo injector compared to an electromagnetic injector is that it switches about five times as fast. As a result, the system can be controlled more accurately, there is a faster reaction time and more injections per work cycle.

Pump atomizer:
Volkswagen has used unit injectors for a while. The operation is complex and is therefore described on a separate page. Click here to go to the unit injector page.

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