Answer
Miller cycle is a modified engine cycle used to reduce compression work and peak combustion temperature while maintaining a high effective expansion ratio, thereby improving efficiency and reducing NOx emissions.
In marine engines, especially modern turbocharged engines, the Miller effect is normally achieved by changing the inlet-valve closing timing together with sufficiently high turbocharger pressure.
Two methods are possible:
- Early Inlet Valve Closing — EIVC
- Late Inlet Valve Closing — LIVC
With early closing, the inlet valve closes before BDC. The trapped charge expands slightly as the piston continues downward, so its temperature falls before compression starts.
With late closing, some air is pushed back into the inlet receiver during the early part of the compression stroke.
Thus, Effective compression ratio decreases while the geometric expansion ratio can remain high. In a conventional Diesel cycle, the compression ratio equals the expansion ratio.
- Internal Charge Air Cooling: When the inlet valve closes early (EIVC) during the intake stroke, the trapped air continues to expand inside the cylinder as the piston moves down to BDC. This expansion causes the charge air temperature to drop (10-15⁰C) before compression even begins.
- Lower Peak Temperatures (Tmax): Because compression starts from a lower temperature, the end-of-compression temperature and maximum combustion temperature (Tmax) are significantly lower.
- NOx Reduction: Thermal NOx formation follows the Zeldovich mechanism and increases exponentially with temperature above 1400⁰C. Lowering Tmax dramatically reduces NOx output.
- Prerequisite (High Boost Pressure): Because EIVC traps a smaller volume of air, the engine requires high-pressure turbocharging (e.g., pressure ratios of 4.5-5.0 or two-stage turbocharging) to supply sufficient oxygen density for complete combustion.
Why high turbocharger pressure is required
Because the effective compression stroke is reduced, less air would otherwise enter/trap in the cylinder.
Therefore, a high-efficiency turbocharger supplies air at a higher pressure so that sufficient air mass remains trapped.
Advantages
- Lower NOx formation.
- Improved thermal efficiency when properly optimized.
- Lower compression temperature.
- Can reduce specific fuel consumption in optimized designs.
- Allows higher engine output without proportionately increasing thermal loading.
Disadvantages
- Requires high turbocharger efficiency/boost pressure.
- Poor matching can reduce air supply and cause smoke.
- Starting and low-load operation can become more difficult.
- More sophisticated valve control may be required.