Fuel Injector Drivers: Peak and Hold and Saturated
The Problem:
Back in the 1970s while doing EFI research at Mellon Institute I was tasked with finding out why fuel injector flow was not consistent at low pulse widths. Flow when the injector was held open was very consistent and easy to measure, but flow was not as predictable at short pulse widths. Here’s a simple plot of fuel flow vs pulse widths.
Note that at short pulse widths the injector just sat there with no flow. This is called the injector dead time and was caused by the inertia of the pintle valve itself, and is dramatically affected by the operating voltage.
This would seem to be all you needed to know, but alas that is not the case. A fuel injector is just an electric solenoid that opens a valve. A solenoid is a coil with a lot of inductance. When power was removed a reverse polarity, inductive spike is created by the collapsing magnetic field. This spike could be a relatively high voltage which induced noise into wires of close proximity. “No biggee,” I said. “I’ll just clamp that with a diode.”
That was a disaster! The reason is basic electronic fundamentals. When the magnetic flux field around a coil collapses the reverse polarity spike happens, but if you slow down the release of that stored energy everything slows down until the energy has been dissipated. A diode clamped the voltage but it also slowed down the injector closing time. We thought all we had to handle was the opening time, but now also had to handle the closing time too.
This simple type of injector driver is called a saturated driver because the transistor is totally on, or saturated, when the injector is activated. If the injector’s inductance is low as would be the case if the injectors impedance is high (DC ohms > 8 ohms), the spike still happens, but the current and energy level are low. Low impedance injectors (DC ohms < 2 ohms) were a problem because they required much higher currents and therefor the stored energy was high. But you needed a low impedance injector for higher fuel flows at higher pressures.
The “eureka moment” was when we realized that although a full high current hit was required to minimize the opening dead time, we could back way off on the current once we had the injector open. Since this “hold current” is much lower than the opening current, the stored energy is also much lower so the injector closes at a fast, consistent rate which also causes minimal noise spikes. This is called a Peak and Hold driver.
