DIY ECU Handles Predator 212cc Engine Fuel Injection Conversion

Fuel injection may be almost universal in the automotive world today, but small engines are another story. Though carburetors may be temperamental and more difficult to understand, they are simple mechanical devices that cost very little to manufacture and that don’t require computers — perfect for lawnmowers, generators, and other applications where efficiency and power output are low priorities. But Carlos Takeshita, a mechanical engineering student at UC San Diego, wanted his 212cc Predator engine to have fuel injection, so he built this DIY ECU to control the conversion.

This is an inexpensive 6.5HP engine from Harbor Freight originally used for a mini bike project. You could buy this engine brand new today for just $119.99 and it would, of course, be carbureted. It is intended for lawnmowers and other simple machines, so low cost was likely the top design goal.

But Takeshita wanted a way to experiment with electronics and firmware, and this engine was the perfect platform for that. It also gave him the opportunity to practice some of the mechanical engineering he was learning in school.

The parts required for the conversion consisted of a fuel cell, a fuel pump, an intake manifold, the injector itself, a battery, a custom crankshaft position sensor system, and the DIY ECU. That last part was made using a Teensy 4.0 microcontroller development board.

Timing is critical, so the crankshaft position sensor was very important. It is a missing-tooth wheel made of ferrous steel and the Teensy can monitor its rotation through a Hall effect sensor. The missing tooth helps the Teensy verify TDC (Top Dead Center) with each rotation. This lets the Teensy control injector timing and could also give it the ability to control spark timing in the future, if Takeshita decides to add that.

The Teensy knows the crankshaft’s position and speed, and also monitors a MAP (Manifold Air Pressure) sensor mounted on the custom intake manifold. That data is all it needs and the custom firmware is responsible for controlling the injector through a TI LM1949.

But tuning is tricky — especially without a way to reliably measure power output — and Takeshita hasn’t perfected that yet. All it takes is tweaking the variables in firmware, so he can experiment as much as he likes. Still, engine tuning is almost an artform of its own. Regardless, the engineering was a success and Takeshita achieved his goal of giving this small engine fuel injection.