Introduction:
Injectors spray the diesel fuel into the diesel engine; that can be into the swirl chamber in indirectly injected engines, or directly into the cylinder in directly injected engines. The injector is mounted in the cylinder head. There are different types of injectors. On this page the different injectors are described.
Pintle injector:
Pintle injectors are used in diesel engines with indirect fuel injection. The fuel is injected into a separate chamber; namely the pre-chamber or swirl chamber. In this chamber the fuel is mixed with the air, creating a combustible mixture. The pintle injector has one injection opening. The injector needle is lifted at a certain fuel pressure, causing fuel atomisation from the injection opening. The opening pressure of pintle injectors lies between 100 and 135 bar.
Hole-type injector:
Hole-type injectors are used in diesel engines with direct fuel injection. The injector has multiple injection openings that spray the fuel directly into the cylinder. The combustion chamber of these engines is located in the piston crown. The positioning of the injection openings is matched exactly to the shape of the combustion chamber. Due to this construction, the injector can only be mounted in the cylinder head in one way. The opening pressure of this injector lies between 180 and 250 bar.
Two-stage injector:
Diesel engines with direct fuel injection have a relatively harsh diesel knock. This is related to the large pressure rise in the cylinder caused by the large quantity of fuel that is injected in one go.
By using so-called pilot injection, the combustion is initiated gently. The diesel knock is therefore much less.
There are manufacturers that fit these so-called two-stage injectors. These are hole-type injectors but with two springs; namely a strong spring and a weaker spring. Due to the weaker spring, the needle will already be able to lift slightly at a lower pressure. As a result, a small quantity of fuel is injected into the cylinder at a low pressure: the pilot injection. This initiates the combustion. The injector needle moves against a stop that is held by the strong spring. The pressure will continue to rise to the normal opening pressure. At that moment the needle will open fully: the main injection.
Electromagnetically actuated injector:
Electromagnetically actuated injectors are used in common-rail diesel engines. The injector is actuated by an electromagnet. Based on the engine speed, the accelerator pedal position, the temperature, the load and the engine position, the engine management system determines when the injector must open and close and how long it must remain open. In the rest position the injector is not actuated. The common-rail pressure of a maximum of 1300 to 2000 bar (depending on the diesel engine generation) is continuously present at the inlet pipe of the injector.
When the engine management sends a signal to the injector, the coil is energised and the electromagnetically actuated valve will be pulled upwards. The fuel pressure located above the control plunger is discharged via a small opening to the other return fuel. This causes the fuel pressure above the control plunger to drop very quickly. The control plunger will move upwards. This is facilitated by the conical section at the underside of the control plunger. The control plunger, and thus also the injector needle, is moved upwards. At that moment the opening at the underside of the injector is exposed, so that the fuel at the common-rail pressure of up to 1300 bar can be injected into the combustion chamber.
With electromagnetically actuated injectors, two springs do not need to be used to provide pilot and main injection. The electromagnetically actuated injector can in fact be actuated several times in succession. The duration of the pilot injection and the main injection can also vary as the engine operating conditions change. It is also possible for this injector to provide two main injections. The advantage of multiple injections is a smoother combustion process.
Peak and hold:
The electromagnetically actuated injector is energised via the electromagnetic coil. When the current through the coil is high enough, the electromagnet overcomes the spring force acting on the valve. To get the injector needle moving, a short current and voltage peak are required. The on-board voltage of 14 volts is too low to lift the needle from its seat. Within a time frame of only 0.3 nanoseconds (10^-9 seconds), the coil is energised with 20 Amps at 80 volts (peak). After the injector needle has opened, it is held open with 12 Amps at 14 volts (hold), until the actuation stops and the needle is pressed back onto its seat by the spring force.
Piezo injector:
Piezo injectors are used in both petrol and diesel engines.
The element in a piezo injector becomes longer or shorter as the applied voltage changes. The change in length is only a few thousandths of a millimetre. That is too little to open the injector needle sufficiently. Therefore multiple piezo crystals are connected in series so that the injector needle can achieve a greater stroke.
The image shows what happens to the piezo element when the applied voltage is increased.
The control voltage of a piezo injector is between 100 and 160 volts. This voltage is obtained with the aid of capacitors in the engine 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 millimetres. Closing of the injector needle takes place by briefly reversing the current direction.
The advantage of the piezo injector compared to an electromagnetic injector is that it switches about five times faster. As a result, the system can be controlled more precisely, there is a faster response time, and injection can take place more often per power stroke.
Unit injector:
Volkswagen has used unit injectors for a period of time. The operation is complex and is therefore described on a separate page. Click here to go to the page about the unit injector.