Samples with an O/M ratio of 2.00 had larger central-void diameters than those with O/M ratios of 1.96 and 1.98 (a). The calculation showed that the fuel-center temperature was lower for O/M 2.00 by about 200 ℃ than for O/M 1.96 and 1.98. The central-void diameter becomes larger in the sample with an O/M ratio of 2.00 because the vapor pressure is high (b).