|
Lean burn is effective because it uses a higher mixture ratio instead of a more restricted throttle position. Lean burn allows higher BSFC due to leaner mixture, when loads are only a fraction of best BSFC points in an otherwise identical engine without lean burn.
Combustion characteristics when lean burn is incorporated only exist when effective in cylinder compression is much lower than possible with high loads applied to the engine.
One way to look at it is to just consider lean burn as displacement reduction. While you are not actually reducing the calculated displacement of the engine, partial throttle positions that maintain a manifold vacuum of less than 50% of atmospheric pressure reduce the "effective" displacement of the engine as well as the actual compression pressure at the instant before combustion occurs.
That same lower effective compression is the essential component of lean burn, but for lean burn to be most effective better atomization of the fuel-air mixture is another component of the strategy.
regards
Mech
|