Cetane numbers rate the ignition properties of diesel fuel as a measure of the fuel’s ignition on compression as measured by ignition delay. Cetane affects smoke produc­tion on start-up, drivability before warm-up and diesel knock at idle. Cetane number is measured in a single-cylinder engine compared with reference blends of n-cetane and heptane. Cetane numbers for biodiesel are influenced by the fatty acid ester pro­file making up the biodiesel (Table 7.15). In general, as the number of carbon atoms in the fatty acid esters increases, so does the cetane number (Fig. 7.15). However, as the number of double bonds increases the cetane number decreases (Fig. 7.16), so for a high cetane number long-chain saturated fatty acids are needed (Graboski and McCormick, 1998; Knothe et al., 2003). The formation of pollutants is dependant on ignition delay and cetane number and a high cetane number gives less NOx.