Given the importance of the problem of cyclically unstable combustion with lean mixtures, a lean-burn engine requires fuel metering, which is all the more delicate because mixture cannot be constant over the entire operating range of the engine. The Institut François du Pétrole has developed a lean-mixture limit sensor based on the digital processing of data from ionization probes placed in combustion chambers. The novel strategy used takes a reduced sampling of 32 consecutive cycles to extrapolate information about combustion quality in the vicinity of the lean-mixture limit. This information has been used by Renault for regulating the Renix multipoint electronic injection system. A demonstration of the system was made on a European medium swept volume engine (displacement between 1,4 and 2 liters) for which the lean-mixture capability was improved by ignition system adjustments. Development of the control loop was optimized on a transient test-bench engine equipped with a piloting system simulating standardized cycles (ECE-15 and FTP) and a sampling system of the CVS type. These tests were completed on vehicle on a chassis dynamometer. They showed that nitrogen-oxide emission levels could be reached that were compatible with future European standards, provided the catalytic oxidation of unburned hydrocarbons was accepted.