Figs. 6 and 7 present BSFC and brake efficiency contours
over the throttle-less range of load at various speeds. With no
load, brake efficiency is assumed to be zero, while BSFC is
assumed to be equal to 500 g/kW h. For the reason that BSNOx
and knock index limit the maximum fuel-to-air equivalence
ratio with maximum brake torque spark advance, maximum
torque output is obtained with the best compromise between
a lean fuel-to-air equivalence ratio and a retarded spark
advance for each engine speed. Full load spark advances are
maximum brake torque values providing the computed knock
index and BSNOx fall below reference values. BMEP curves vs.
engine speed with various fuel-to-air ratios are superimposed.
Knock limits the spark advance for mixtures richer V¼ 0.43
and higher engine speed, when the MBT spark timing has to be
retarded to reduce the knock index to acceptable levels. These
areas are also those of major concern for formation of NOx
within the cylinder, and retarding the spark timing is also
effective in reducing the peak temperature and therefore
formation of NOx.