Haltech E6A - Manual (Page 106)

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Appendix C
Injector Impedance
Electrically, there are two different types of electronic fuel injectors. One type of injector,
characterised by a high coil impedance (> 12
Ω
) is known as saturation injectors. The other
sort of injector, typically with coils of less than 6 ohms impedance, is known as peak-and-hold
injectors. The names are derived from the current waveform that accompanies the injector
when it is switched on.
Saturation, or high impedance, injectors, require a simple switch to operate. When a 16 ohm
injector has 12V applied to it, the current that is expected to be drawn is 0.75 amps (by Ohm's
law). However, the current in a coil cannot change instantly. When power is applied to this
injector, the current builds in the coil over a period of around half a millisecond, until it reaches
its 0.75 Amp maximum, i.e. saturation. The coils remain in saturation until power is switched
off at the end of injection.
In order to reduce opening time, the coil impedance can be reduced. Very low impedance
would result in very large continuous currents, resulting in a great deal of heat and waste of
energy. However, a much smaller current is needed to hold the injector open than is needed to
initially switch it on. Thus, such an injector is described by two currents: a peak current for
opening, and a hold current for maintaining the fuel flow. These peak-and-hold, or low
impedance, injectors, are particularly good for high rpm engines needing large fuel-flow rates.
Peak-and-hold injectors come with a wide variety of current requirements. Most peak-and-hold
injectors need 2A to switch on with 0.5A hold current (2/0.5A peak-and-hold), or are 4/1A
peak-and-hold. Some very high performance injectors need as much as 6A to switch on,
especially under high fuel pressures.
C.1 The E6A Injector Drivers
The solid state devices within the E6A ECU that are used to switch the injectors are known as
the injector drivers. The E6A possesses four heavy-duty injector drivers that can be used on
either saturation or peak-and-hold injectors. Each driver limits its current draw to 4 amps, and,
upon reaching that limit, clamps the current to just 1 amp.
The drivers used in the E6A are each capable of firing two saturation injectors. Thus, with four
drivers, a total of eight injectors can be used. Each is also capable of firing one or two 2/0.5A
peak-and-hold injectors. If using 4/1A peak-and-hold injectors, you should not employ more
than one injector per driver. Most high current motorsport injectors can also be fired (one per
driver) providing fuel pressures are not excessive (>5 Bar). Running two high current, low
impedance injectors from one driver can result in difficulty in obtaining stable air:fuel ratios
when injection times are short (around idle and light load).