Don't confuse the noun with the adjective.
In your sentence, 'ones' is used in place of 'engines' in order to avoid repetition. Here are more examples:
I usually order the double-beef hamburger, but today I'm not very hungry so I'm going to order the single one.
My friend likes to wear a fancy dress to parties but I prefer to wear a simpler one.
'Ones' is also used as a possessive pronoun referring to someone in general.
One (subject pronoun) should obey ones (possessive pronoun) parents.