k1 is an SPDT relay - that's connected to the power supply of the high voltage module/ignitor
C1 is just a decoupling capacitor - it's 47uF & 36V IIRC
I needed a decoupling cap as the wires from the ULN driver to the relay coil acted like antenae and picked up EMF. If you think of it the high voltage module/igniter can easily fry arduino 
and it creates some really nasty EMF. occasionally it even did something to USB ports on my desktop (you can hear it the second video - I get the same sound it makes when I insert new USB devices). Even though arduino was not connected through USB cable and the entire circuit was placed like 3 meters away