>>1356151Basically, if you starve a fire of oxygen or the draft is too strong, the fuel doesn't fully burn in the firebox and can produce a lot of soot that gets pulled out of the chimney.
Having air coming over the top of the fire and reducing the airflow coming up through the grate helps prevent small bits of coal getting pulled off the grate before they have been fully burnt, reducing fuel loss and inefficiency.
P&LE Berkshire:
https://www.railarchive.net/nyccollection/ple9405.htmFor Porta's GPCS:
https://www.martynbane.co.uk/modernsteam/ldp/gpcs/gpcs-rc.htmFor Porta's locomotives in action:
https://www.youtube.com/watch?v=y-8_OdndN-M 