This review article describes and discusses the literature on reducing agen
t control systems for NOx emission reduction in the exhaust gas of full lea
n-burn engines. The literature can be classified as feedback, feedforward,
feedforward-feedback, feediorward/feedforward-feedback and periodical dosin
g control systems, which can be further classified as classical, override,
co-ordinated, constrained and split systems or a combination of those.
As long as reproducible, fast and cheap NOx sensors are not commercially av
ailable, the control system should be an inferential feedforward or feedfor
ward-feedback system supplied with constraints for environmental and econom
ical reasons. For future applications an oxidation catalyst downstream of t
he deNO(x) catalyst should be applied to convert any inevitable reductant s
lip, due to modelling errors, to harmless compounds.