Ignition Timing

The effect of ignition timing on NO and HC emissions is shown on Fig 3.2 When ignition occurs earlier in the cycle more heat is released before and around the top dead center. Thus, with advanced ignition timings higher peak cylinder pressures and temperatures result. As has been discussed lecture 5 with increase in combution temperatures NO formation increases. Hence , higher NO emissions are obtained as the ignition timing is advanced.
As the ignition timing is retarded more burning takes place during expansion stroke resulting in lower peak combustion pressures and a lower of mass of charge is pushed into crevice volume.  Also, at the retarded ignition timings exhaust gas temperature increases as the engine thermal efficiency is reduced.  In the hotter  exhaust gas with the retarded ignition timing higher oxidation rates of the HC and CO in the exhaust system are obtained.. Due to these reasons, lower HC emissions are obtained with retarded ignition timings. The disadvantage of the  retarded ignition timing is lower engine efficiency, lower power and a poorer fuel economy.
When the emission control legislation was introduced for the first time around 1970 in the USA and Europe, ignition timing versus speed and manifold vacuum curves were among the first engine parameters that were modified for control of NO_x emissions due to ease of their adjustment.