respectively. Maximum brake torque (MBT) is always used to optimize the spark timing to get maximum power and efficiency. Therefore, for each operating condition in this study, the engine was run at the fuel’s MBT spark timing. Equivalence ratio was varied from 0.83 to 1.25, covering lean, stoichiometric and rich conditions. Measurements
of engine torque, equivalence ratio and NOx emission were averaged over a 60-s period, while UHC and CO emissions were
recorded directly from the emissions analyzer.