The Miller cycle involves early or late intake valve closure to reduce the compression temperature by allowing the air to expand or decrease the net compression ratio. This results in reduced NOx emissions due to lower combustion temperatures. The reduced charge air is compensated by using a high-pressure supercharger or turbocharger to maintain the desired air supply for combustion.

The Miller cycle reduces air temperature during compression in the following ways:

1. Early Intake Valve Closing: Closing the intake valve before the piston reaches Bottom Dead Center (BDC) allows the air to expand as the piston continues downward. This expansion cools the air, lowering the compression temperature.
2. Late Intake Valve Closing: By keeping the intake valve open during the early phase of the compression stroke, part of the charge air is expelled, reducing the net compression ratio and, consequently, the temperature during compression.

This reduction in temperature minimises thermal stress and contributes to lower exhaust temperatures, reducing NOx emissions.

The early or late closing of the intake valve reduces the amount of charge air, resulting in a potential loss of pressure and power. To address this:

1. High-Pressure Turbocharging: The intake pressure is increased using a high-pressure ratio turbocharger, ensuring an adequate quantity of charge air during the shortened intake stroke. This helps maintain cylinder pressure and minimizes the increase in specific fuel consumption.
2. Supercharger: Positive displacement superchargers are used to recover pressure in the combustion chamber. However, they require some of the engine’s power output to drive, which slightly offsets the efficiency gains.

Specific Fuel Consumption:

• The Miller cycle slightly increases specific fuel consumption due to the loss of charge air.
• However, the use of high-efficiency turbochargers and superchargers brings the fuel consumption closer to that of conventional cycles.

NOx Emission:

• The reduced compression and exhaust temperatures in the Miller cycle significantly lower NOx emissions, achieving reductions of up to 30% without substantial penalties in fuel consumption.

• The use of supercharger has made low load operation very effective without much compromise in specific fuel oil consumption and power developed.
• Energy efficiency has improved, exhaust gas temperature has reduced and NOx emissions have reduced.