Fuel is combusted in the stroke of a four- stroke engine

intake
compression
power
exhuast

it should be power stroke if talking in truck terms

an ion is a charged atom or molecule. it is charged because the number of electrons do not equal the number of protons in the atom or molecule. an atom can acquire a positive charge or a negative charge depending on whether the number of electrons in an atom is greater or less then the number of protons in the atom.

whichever one it is, it's definitely not one that you included on the list you provided.