Fuel injectors are controlled by one of two possible injector control circuits called drivers: saturated (high impedance) and peak-and-hold (low impedance or current regulated).
An injector used in a saturated driver system requires a high resistance value across its coil (12-16 ohms). The high resistance values enable the injectors to operate at low current levels (.8-1 amp) which keep the circuitry cool, promoting longer component life. Unfortunately, because of the low current levels, injector response time is slow, sacrificing dynamic range. Ohm’s law (V=IR) can be used to show the relationship between injector resistance and current level, where V=Voltage, I=Current (amps) and R= Resistance (ohms). A high impedance injector (14 ohms) used in a 12 volt system would require an operating current of 0.86 amps.
I = 12 Volts/14 ohms = 0.86 amps
A peak-and-hold driver circuit utilizes fuel injectors with low resistance coils (2-2.5 ohms) which require more current (4-5 amps) to open (Ohm’s law: I = 12 Volts/2.5 ohms = 4.8 amps). The driver circuitry will overheat if the injector is constantly operated at 4 amps. Therefore, a switching mechanism is built into the circuit which will turn down the current to a lower, more acceptable level after the injector is opened. Once the injector is opened, it takes far less current to keep it open. Thus the term peak-and-hold is used to describe this type of circuitry. After the initial peak of 4 amps is reached, the driver turns down the current to 1 amp which holds the injector open for the duration of the pulse width. In most peak-and-hold Multi-Port applications, one driver operates two injectors in which case the peak current per injector is 2 amps and the hold current is 0.5 amps. The advantage of this system is the quick response time of the injector. The high initial current instantly creates the magnetic force require to raise the valve. This allows for a wide dynamic range, which is why peak-and-hold systems are used in small displacement, high horsepower engines such as 4 cylinder turbocharged engines.
Figure 1 shows the relationship of current and opening times for high and low impedance fuel injectors.
