ISO 5165:2017 establishes the rating of diesel fuel oil in terms of an arbitrary scale of cetane numbers (CNs) using a standard single cylinder, four-stroke cycle, variable compression ratio, indirect injected diesel engine. The CN provides a measure of the ignition characteristics of diesel fuel oil in compression ignition engines. The CN is determined at constant speed in a pre-combustion chamber-type compression ignition test engine. However, the relationship of test engine performance to full scale, variable speed and variable load engines is not completely understood.ISO 5165:2017 is applicable for the entire scale range from 0 CN to 100 CN but typical testing is in the range of 30 CN to 65 CN. An interlaboratory study executed by CEN in 2013 (10 samples in the range 52,4 CN to 73,8 CN)[1] confirmed that paraffinic diesel from synthesis or hydrotreatment, containing up to 7 % (V/V) fatty acid methyl ester (FAME) can be tested by this test method and that the precision is comparable to conventional fuels.This test can be used for unconventional fuels such as synthetics, vegetable oils, etc. However, the relationship to the performance of such materials in full scale engines is not completely understood.Samples with fluid properties that interfere with the gravity flow of fuel to the fuel pump or delivery through the injector nozzle are not suitable for rating by this method.NOTE This document specifies operating conditions in SI units but engine measurements are specified in inch-pound units because these are the historical units used in the manufacture of the equipment, and thus some references in this document include these units in parenthesis.