To provide a diagnostic device for engine capable of reducing manufacturing costs and correctly diagnosing adsorbing performance of a HC adsorbing material without deteriorating exhaust when HC of a hydrocarbon HC adsorbing material is separated.
This diagnostic device for an engine is provided with the HC adsorbing material disposed in an exhaust pipe, an upstream air-fuel ratio sensor 12 for detecting a wide air-fuel ratio of exhaust gas, a downstream air-fuel ratio sensor 3 for detecting a rich or lean condition of exhaust gas, and a controller 16. The controller is provided with means 31 for correcting an air fuel ratio of an upstream side of the HC adsorbing material, means 30 for correcting an air-fuel ratio of a downstream side of the HC adsorbing material, and means 28 for estimating an HC adsorbing amount of the HC adsorbing material. The means for estimating the HC adsorbing amount estimates it on the basis of a difference between a stoichiometric air-fuel ratio C and an output value of the upstream air-fuel ratio sensor for a period when a temperature of the HC adsorbing material attains a HC separating temperature A or more, and then it attains a prescribed value B and less after the output value of the upstream air-fuel ratio sensor is corrected and controlled in a lean condition.
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OSUGA MINORU
ISHII TOSHIO
TAKAKU YUTAKA
KATOGI KOZO
