groundFungus:
For the timing part you can use the millis() function. Here is a tutorial that may help.An Arduino output pin cannot drive an injector so an external circuit is necessary (like your relay). A MOSFET is a good choice. Here is an injector driver circuit. Choose a MOSFET that has a logic level input and can handle the current required by the injector. The diode is necessary to protect the MOSFET from the high reverse voltage when the injector is turned off.
Thank you for the tips! On the MOSFET, does it matter what type? I keep seeing logic MOSFET referred to, but all I find are the IRF type on breakout boards.
Would something like this be appropriate? Adafruit 12-Channel 16-bit PWM LED Driver - SPI Interface [TLC59711] : ID 1455 : $7.50 : Adafruit Industries, Unique & fun DIY electronics and kits
Or would using a IRL540 be preferable? This is my first project, so I'm unsure if these MOSFET's can be used "loose" or if they want to be fastened to a board.