Haltech F9A User Manual download pdf (Page 70)

Appendix B

Injector Impedance

Electrically, there are two different types of electronic fuel injectors. One type of injector,

characterised by a high coil impedance (> 12Ω) 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.

B.1 The F9 Injector Drivers

The solid state devices within the F9 ECU that are used to switch the injectors are known as

the injector drivers. The F9 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 F9 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).

In general, an F9 driver will reliably fire two injectors with more than 2.5 ohms coil

resistance. Coil resistance can be measured with an ordinary low-ohm resistance meter. If you

do not have enough driver power then you will require an Extra Driver Box. If unsure, contact

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Haltech F9A User Manual Page 70