The Miller cycle is a modification of the four-stroke cycle in which the inlet valve is closed earlier (or later) than normal, so that the effective compression stroke is shorter than the expansion stroke. In the "early inlet valve closing" (EIVC) version, the inlet valve is closed well before BDC, so the air charge is expanded and cooled during the remainder of the downward stroke; the effective compression ratio is lower than the expansion ratio. In the "late inlet valve closing" (LIVC) version, the inlet valve is held open past BDC so some air is pushed back into the inlet manifold, again reducing the effective compression ratio. The result is that the charge air temperature at the end of compression is lower than in a normal cycle. Because NOx formation is strongly dependent on the peak combustion (flame) temperature, the lower compression temperature reduces the peak combustion temperature and hence reduces thermal NOx formation. The Miller cycle therefore lowers NOx without the fuel penalty of retarding injection, and is a primary internal engine measure for Tier II/III compliance. The engine must be turbocharged to compensate for the reduced air mass (higher boost) to maintain power.

1. Inlet valve timing: the camshaft/cam profile (or the electronic valve control) must be modified to close the inlet valve early (or late) - a new cam profile or a variable valve timing system.
2. Higher turbocharging/boost: because the effective compression ratio is reduced, the engine needs a higher charge-air pressure (higher turbocharger pressure ratio) to maintain the same trapped air mass and power; this may require a larger or two-stage turbocharger, and a charge-air cooler to keep the air temperature low.
3. Charge air cooling: an efficient charge-air cooler is needed to keep the compressed air temperature low (the Miller effect relies on low charge temperature).
4. Combustion chamber/injection: the injection timing and possibly the compression ratio may be adjusted to maintain good combustion and Pmax; the piston/cylinder head may be modified to suit the lower effective compression.
5. Valve gear/actuation: the valve train must be able to close the inlet valve at the required early/late angle reliably (stronger springs or hydraulic/electronic actuation).
6. Control system: the engine management must be updated to set the correct valve timing and injection for the Miller operation, and to protect against the higher boost and lower compression.

These modifications allow the medium-speed engine to run on the Miller cycle, reducing NOx while maintaining power and efficiency.