What's the difference between solenoid and piezo diesel injectors?

Diesel manufacturers are developing new fuel injectors to meet more stringent emission standards. Enter: the piezo injector.


Diagram of a solenoid versus piezo injector.
Photo: Bosch - Solenoid operated injector, left. Piezo injector, right.

Since the entire fuel injection event may only last a few milli-seconds, injector opening time is critical for precise fuel control. Opening time refers to the elapsed time between when the injector is commanded on by the ECU and when fuel begins to exit the nozzle.


First generation common rail fuel injection utilizes a solenoid operated fuel injector. The injector contains a solenoid which when energized creates a magnetic field. The magnetic field lifts an armature which in turns causes the nozzle needle to open, allowing fuel to exit the nozzle.





Enter: The Piezo Injector

A new style of injector has been developed which opens faster and provides more precise fuel control than the solenoid operated injector. The piezo (pee-ay-zo) injector uses the piezoelectric effect to open the injector.


Piezoelectric sensors have been used in automotive gasoline engines for engine knock detection (detonation) for many years. A piezo crystal will generate a voltage when mechanical pressure is exerted on the crystal. When detonation occurs sound waves are exerted on the crystal, which generate an electrical signal that is interpreted by the ECU as detonation. In this scenario, the piezo crystal is used as a sensor.


A piezo crystal can operate in reverse as well; if a voltage is applied to the surfaces of the crystal, the crystal will elongate. The physical change in the crystal is near instantaneous with the application of voltage. In this mode of operation the crystal is used as an actuator.


New piezo injectors utilize this effect to actuate the injector. The ECU applies an electrical voltage to the injector's piezo crystal. The crystal expands, closing a servo valve inside the injector which causes the injector's nozzle needle to lift, allowing fuel to exit the nozzle.


The piezo injector can open in as little as 150 micro-seconds (.00015 Seconds) resulting in more precise fuel control compared to solenoid operated injectors.



 

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