IT IS A METHOD OF REDUCING THE NOX EMISSION FROM THE ENGINE. FOR NOX REDUCTION BY MILLER PRINCIPLE THE SCAVENGE AIR PORTS ARE LOWER AND THEIR OPENINGS ARE MADE NARROW. MOREOVER THE STROKE LENGTH IS INCREASED BEFORE THE AIR PORTS ARE OPENED.
ONCE THE PORT OPENS, THE AIR RUSHES INSIDE THE CYLINDER AND EXPANDS IN THE CYLINDER THROUGH NARROWED AIR PORTS. DURING THIS EXPANSION, THE TEMPERATURE OF AIR REDUCED FURTHER BY 8-10°C AND HENCE LOWERS THE CYLINDER TEMPERATURE LEVEL AND HENCE NOX FORMATION.
IN FOUR STROKE ENGINE, AIR INLET VALVES ARE CLOSED BEFORE PISTON REACHES B.D.C DURING INTAKE STROKE. AS THE PISTON GOES DOWN THE AIR EXPANDS AND TEMPERATURE OF AIR DROPS AND HENCE NOX FORMATION.