Cl engines typically run on a leaner fuel-air mixture even at full load operation, as compared to SI engines. The fuel-air mixture cannot be enriched beyond a limiting value in diesel engine due to the tendency of increased smoke formation. It should also be noted that the air-fuel mixture in a CI engine is not homogenous like that in an SI engine. There will be rich and lean pockets as the liquid fuel is sprayed into the cylinder and there is no much time to mix homogenously before the combustion. In contrast, the fuel and air is mixed prior to the intake in an SI engine. But if you compare the global air-fuel ratio (i.e. total air intake/total fuel intake of the engine), SI engine air-fuel mixtures are richer than their CI engine counterparts.
