Mech covered it pretty good, but one more thing to touch up on.
NOX pollution is created by high combustion temperatures, the creation of NOX absorbs energy from the combustion. NOX is one of the primary pollutants that cause smog, and is not good for your health or for the plants arround you. NOX = BAD.
High compression makes higher combustion temperatures, which is why during the early years of smog control compression ratios were droped to the lowest spec I have seen was 7:1
Like any devleoping technology the early systems were junk, now they are highly effective and efficient.