Closed-loop engine control: Analog or digital circuits to aid emissions control may be the forerunner of onboard engine “tuning”

To meet current U.S. emissions standards for automobiles, hydrocarbon (HC) and carbon monoxide (CO) emissions are usually controlled by a catalytic converter in the automobile exhaust. Federal standards for nitrogen oxides (NO
x
) emissions are met mainly by exhaust-gas recirculation (EGR) techniques. However, in 1981, when NO
x
emissions standards are tightened from the current limit of 2.0 grams per mile to 1.0 gram per mile, these techniques alone may no longer be sufficient.