Lean burn combustion became an important concept for improving fuel
economy and reducing exhaust emissions. However. lean burning is associated
with increased cycle-to-cycle combustion variations due t o the ignition
instabilities and reduced propagation rates. Engine stability under lean
mixture conditions could be improved by increasing burning rates through
enhanced flow characteristics and by securing ignitability with better ignition
systems.
The effects of flow and ignition characteristics on the combustion performances
are investigated in a 4-valve SI engine. Flow characteristics of tumble-swirl were
varied with swirl control valve attached at the inlet ports. while ignition energy
and its distribution were controlled in a high-frequency ignition system by
changing spark duration and spark frequency. The improvement of lean burn
performance by the optimum flow and ignition characteristics was discussed.