Overview

A solenoid type injector used in engines opens a valve and injects fuel by moving a plunger with magnetic force created by an electromagnet. Injectors in engines need to respond quickly for applied voltage to improve fuel consumption by maintaining the amount of fuel flow.
In solenoid injectors, one of the reasons that the response is delayed is eddy currents, which are produced when the magnetic flux generated by current flow undergoes time variations. The eddy currents are generated in a direction that inhibits changes in the magnetic flux, causing a delay in the initial rise of the attraction force when the current begins to flow. This reduces the injector’s responsiveness. Also, the reflection of the plunger at collision during open/close inhibits the open/close motion.
By running a transient response analysis in JMAG, the responsiveness of the injector accounting for the effect of the collision of the plunger and the eddy current can be obtained. Identifying the places where eddy currents are generated enables a designer to study whether or not responsiveness can be improved.
For this case, direct current voltage is applied to the solenoid type injector and the response characteristics is obtained after accounting for the effect of the collision and eddy current.

Position of Plunger and Coil Current

When accounting for collision and eddy current, the comparison of coil current and position of the plunger for each time is shown in Fig. 1. The position of the plunger is positive in the direction the valve opens, and the valve is fully open at position 100 (μm) and closed at position 0 (μm). The coil current value continues to rise to keep applying supply voltage to the coil until the time is 4 (ms). To keep the current at a minimum while the valve is fully open during 4ms to 6ms, the voltage is controlled to maintain the coil current at 0.25 (A). The coil current is controlled so that it does not flow from 6 (ms) and the valve will start to close due to fuel pressure, spring, and plunger weight.

Current Density Distribution

Fig. 2 shows the current density distribution when current starts to flow at 0.05 (ms), when the plunger moves in the direction the valve opens at 2.2 (ms), when the plunger moves in the direction valve closes at 7.1 (ms). The translation direction of the plunger is the direction the valve opens. By running current through the coil, eddy current will occur in the core, york and plunger. Eddy current will focus on the surface of each part since magnetic flux flow will suddenly increase at the start of current flow and because the power supply is direct current, eddy current will flow inside the parts as time passes. These eddy currents become the cause of the decrease in responsiveness.

Comparison of Response Characteristics

Depending on whether you account for eddy current and collision, the comparison of response characteristics of the plunger for each time is shown in Fig. 3 The attraction force characteristics is shown in Fig. 4, and current characteristics is in Fig. 5. The position of the plunger is positive in the direction the valve opens and the valvle is full open at position 100(μm) and the valve is closed at position 0 (μm).
Fig. 3 shows that the plunger is bouncing near 3.2 (ms) due to the collision, when eddy current and collision are accounted for. Also, the responsiveness is slower compared to when they are not accounted for.
The slow responsiveness is caused by the reduced attraction force. In Fig. 4, the lower attraction force can be verified when eddy current and collision are considered compared to when they are not considered.
As for the current characteristics, when the eddy current and collision are considered, the larger current value can be seen compared to when they are not considered. For more information about the cause, see the Application Note “66 – Operating Time Analysis of an Injector “.
When the plunger moves in the direction to open the valve, there will be a delay in response of approximately 1.5(ms) due to the fuel pressure, spring and the weight of the plunger in the case that eddy current and collision are not accounted for. In the case when eddy current and collision are accounted for, the delay in response is approximately 2 (ms) and it can be confirmed that the cause of delay in response is the decrease of the attractive force due to eddy current. The plunger will move to the direction of closing the valve from 7 (ms). Also, the eddy current caused magnetic flux and result in a delay of the plunger displacement. In addition, the plunger will collide and reflect, and it can be confirmed that more time is required for the open/close of the valve.