Homebrew EFI -- the ECU

Existing ECUs

The simplest way to build the ECU would be to use an existing design such as the Bowling & Grippo MegaSquirt or the MegaSquirtAVR. On the other hand, part of the fun of this project is to hack together something out of nothing, so I'm considering designing my own ECU.

Designing New Hardware

Injector Drivers

Ignition

Idiot Lights

Other Outputs

Designing New Software

Older Thoughts:

General:

• The environment on a motorbike is even tougher than that in a car engine bay, with cold rain, hot oil and warm beer frequently sloshing around. Also, the electronics must be protected from impact and vibration, especially on an off-roader. Locking the circuit inside a sealed, die-cast aluminium box seems to be the way to go here. Cable entry would need to be via waterproof cable glands. The Alumnimium box would also provide an effective heat-sink. (as long as it wasn't mounted too close to the exhaust pipe). Foam inserts (rather than solid screw mounts) could protect the board from vibration, although having a bunch of TO-220 components bolted to the case might stop this working. Maybe bending their legs into an S shape would be sufficient.
• Size is at a premium too. On a car-oriented design, you've got a lot of spare volume under the hood. For a bike, you're much more restricted: 6" x 4" x 2" would be about the limit on the XLV. The MegaSquirt is 6.25" x 4.25" x 1.75" so that's at least the right ballpark. Jaycar sell nice sealed boxes 115 x 90 x 55 (4" x 3.5" x 2.1") which might be a goer if it can all be made to fit.
• The CPU should do as much as possible, thus minimizing interface components. It's easier to fit more code in the box than more parts. However, a one-chip solution is far preferable to a two-chip solution, because tracks eat up board space. This indicates a CPU with onboard RAM, ROM, ADCs and PWM. Fortunately, modern microcontrollers come with an amazing amount of that kind of thing.

Outputs:

• Use MOSFETs and SCRs where appropriate to minimize heat dissipation. TO220 and TO126 devices fit more easily than TO3!
• It should also control the ignition (CDI) timing and (solid state) fuel pump relay, and maybe even an electronic cooling fan for bikes which have one. If a flyback is needed to produce the CDI voltages, the CPU can control that as well. A bypass valve may be needed to raise cold idle. K-bikes apparently send the starter motor signal via the ECU to stop it spinning when the engine is turning, which is a thought.
• Also has to switch on O2 sensor heaters. Should be in the order of 2A each
• Communication to the user should be via a single 'idiot' light LED on the dash. Green for go, Orange for trouble, Red for failure. Flashing patterns can indicate what the problem is. More idiot lights on the board can help with side-of-the-road diagnosis too. And now Jaycar sell RGB LEDs! More lights, more lights!

Inputs:

• Should use existing crank sensors where possible. Generator phase may provide extra information here if needed ... check frequency per RPM (bike generators are often mounted on the end of the crank). On the other hand, the current CDI system uses these sensors, and apparently CDI phase is more critical than EFI phase anyway.
• Lambda / O2 / EGO sensor(s) -- apparently vary between 0 and 1 volt, impedance in the order of 1 Mohm. So it'll need some kind of shielding (or at least noise rejection) and Hi-Z buffering. It'd want to be a heated one, so that's 4-wire.
• Manifold vacuum, TPS, CHT -- use commercially available sensors and interfaces to avoid disappointment.
• Killswitch should immediately stop ignition, fuel pump, everything stone dead. It should not just signal the controller, it should operate an override relay or something in case of disaster.
• Starter button feeds into the ECU so that it can refuse to run the starter w/ the engine going. Maybe the button could be used to communicate with the ECU when the engine _is_ running? Eg: to change maps.
• Power/Economy switch? Wheel speed? Accelerometer? Speedshifter? iButton for an ignition lock? The possibilities are endless :-)