Diesel fuels have been tested in both a naturally aspirated and an externally supercharged single cylinder, air cooled KHD DI-diesel engine, to determine the influence of poor fuel quality on combustion and emissions. A thermodynamic analysis of the cylinder pressure was conducted and the emissions were measured both gaseous as well as the particle emission (by means of a dilution tunnel). Additionally, extensive cold start tests were conducted.Under steady state conditions the cetane number seems to be a good parameter which describes the ignition behavior of different fuels. At low load, a change in combustion and a high increase in CO, HC and particle emissions were found with decreasing cetane number.During cold starting and warming up, a clear deterioration of the emission and combustion characteristics was also observed with decreasing cetane number when basic fuels were used. Fuels containing ignition improvers exhibited poor results, similar to the basic fuel without ignition improvers.