$25 Additional Injector Drive
Warning
This note contains many modifications which may void your vehicle's
warranty, or even be illegal in some areas. It is your responsibility to check
the validity, safety and legality of anything here.
If you want an all-singing, all-dancing controller this is not for you. If you want an effective,
low cost solution to getting a little extra fuel into the engine when it needs it, then read on.
Theory
Additional injectors under the control of the factory computer are one solution to dealing with
increased boost, and possibly a fuel-cut defender. The problem is, how do you drive the additional
injectors?
The simplest answer would be to wire an extra injector in parallel with the original. This will work
if your computer has the current ability to handle it, but many don't and a blown computer is very
expensive. A better way is to use the drive signal to control, but not drive the additional
injector. Sounds simple huh? Actually it is...
The Circuit
The circuit is hardly original. It's a textbook Darlington switch, which requires just 700 microAmps
from the computer to turn on. Two transistors, a diode and a resistor are all that you really need, other
than a suitable fuel injector of course. Here's the parts list:
| Label | Item |
| Q1 | BC558 Transistor |
| Q2 | 2N3055 Power Transistor |
| D1 | 10A 500PIV Diode |
| R1 | 12K 0.5W Resistor |
| R2 | 10K 0.5W Resistor |
| SW1 | Switch |
The schematic for the driver is shown in the next picture.
If you prefer, you can download the Protel Schematic file by clicking
here
Construction
It would be best to make a printed circuit board, but it really is not essential. Perf-board will do
nicely if you are careful.
The 2N3055 transistor will be carrying several amps, so it needs a heatsink.
If you mount the circuit in an aluminium box you can use the box as the heatsink, but be careful to insulate
the 2N3055 with a proper mounting kit. It's case (the collector) is at 12V and if it shorts to earth things will
go very badly indeed.
Given the current that a fuel injector can draw, a heavy-duty relay is required. Headlight
relays are ideal for this task. This way, only light-duty cable needs to be run to the switch, which is best
mounted in the cabin.
The mounting and plumbing of the injector is entirely up to you, but don't wire it in just yet
Picking an Injector
This is tricky. Since an additional injector is triggered from a single original injector, the
engine will receive 1/(cylinders) extra fuel than the computer expected if a same size injector was used. This is
probably going to be way too much fuel, so a smaller (ie lower flow rate) injector is going to be required, or use a
pressure restrictor in the line to the additional injector. Experimentation is going to be required, and remember that
if the computer operates in closed loop mode (almost all factory ones do) it will attempt to trim back the overly
rich mixture, which will help somewhat.
Installation and testing
Power for the driver is supplied from a switched 12V supply. Make sure that power goes away when the ignition is OFF.
The open end of R1 is connected to the switched side of the target injector. With the engine running, one side of the
injector will be at a constant 12V, while the other will oscilate, because it is being switched to ground every time the
injector "fires". This is side you need to connect to.
To test the unit, don't connect the new injector just yet. Rather, use a 12V lamp in it's place, and hook everything else
up. With the switch off, the lamp should be OFF.
Start the engine. The lamp should still be OFF.
Now, turn the switch on. At idle, the lamp should be on dimly or even flickering. As revs increase, the lamp brightness will
also increase, indicating that duty cycle of the original injectors is being followed.
If all went well, switch off, shut down and you can now connect the new injector.
Going Further
So what if you need even more fuel? You can run multiple copies of the circuit, connected to other injectors.
A more clever approach would be to monitor the injector rate and/or duty cycle to determine the best time to
switch in extra injection. This is something I'm working on at present, so watch this space!
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