Miller Cycle

The Miller cycle is a thermodynamic cycle employed in certain internal combustion engines to improve efficiency and control combustion characteristics. Patented in 1957 by the American engineer Ralph Miller, the cycle modifies the conventional compression process by altering the timing of the intake valve. It can be implemented in both two-stroke and four-stroke engines and is compatible with diesel, gaseous, or dual-fuel operation. Initially developed for marine and stationary power-generation applications, the Miller cycle has since been used in railway locomotives and adapted for various automotive engines, including those by Mazda, Subaru, and Nissan.
A defining feature of the cycle is the use of supercharging or turbocharging to compensate for the power loss associated with reduced effective compression. This approach differentiates it from the naturally aspirated Atkinson cycle, which applies a similar delayed valve-closing principle but without forced induction.

Overview and operational principles

Conventional four-stroke engines follow a sequence of intake, compression, power, and exhaust strokes. In the Miller cycle, the intake valve remains open for a portion of the upward compression stroke, causing some of the mixture to be forced back into the intake manifold. This creates a modified compression process in which the piston compresses the charge only during the latter fraction—typically 70 to 80 per cent—of the upward stroke.
This delayed closing of the intake valve effectively shortens the compression stroke while leaving the expansion stroke unchanged. The resulting difference between effective compression ratio and expansion ratio is central to the efficiency benefits of the Miller cycle. The shortening of the effective compression duration is often described as introducing a symbolic “fifth stroke” within the compression phase, reflecting the split between the initial displacement of the mixture and the subsequent true compression.
Because part of the charge is expelled during early piston travel, forced induction is required to restore the mass of the air–fuel mixture and the engine’s power density. Positive-displacement superchargers, such as Roots or screw-type units, offer strong low-speed boost but impose parasitic losses. Turbochargers can provide higher overall efficiency, particularly at higher loads, although they may lead to Atkinson-like characteristics at low speeds due to turbo lag. Supplementary electric motors or hybrid systems can mitigate this shortfall.

Charge temperature and combustion behaviour

A characteristic advantage of the Miller cycle is the reduction of the pre-combustion charge temperature. Compressed air is typically cooled using an intercooler to a pressure and temperature conducive to filling the cylinders efficiently. With the intake valve closing later, part of the expansion occurs inside the cylinder, further reducing temperature before full compression begins.
Lower charge temperatures result in several benefits:

• increased charge density without raising peak pressure
• reduced tendency for knock in spark-ignition engines
• reduced formation of nitrogen oxides (NOₓ) in diesel engines
• capacity to advance ignition timing, improving overall thermal efficiency

The combination of supercharging and intercooling enables higher power output from a given displacement while staying within mechanical and thermal limits.

Compression ratio and efficiency

The Miller cycle increases efficiency by combining a lower effective compression ratio with a larger expansion ratio. In conventional spark-ignition engines, exhaust gases remain at high pressure—often around five atmospheres—at the end of the power stroke. The Miller cycle’s extended expansion extracts more useful work from the gases by allowing them to expand closer to atmospheric pressure before the exhaust valve opens.
By decoupling compression and expansion ratios, the cycle achieves improved thermal efficiency without sacrificing the mechanical simplicity of the basic engine architecture. This also aligns the engine with broader thermodynamic principles seen in other high-efficiency cycles.

Boosting systems: advantages and limitations

The use of forced-induction systems introduces trade-offs:

• Superchargers: Provide immediate boost but may consume 15–20 per cent of the engine’s output to operate.
• Turbochargers: Offer higher efficiency with lower parasitic loading but may suffer from lag, particularly at low engine speeds.
• Hybrid assistance: Electric motors can provide low-speed torque to bridge turbo lag, a concept used experimentally but not widely adopted for Miller-specific systems.

The choice of boosting method significantly affects engine response, fuel economy, and suitability for different applications.

Advantages and drawbacks

The principal advantage of the Miller cycle is the increased expansion ratio relative to the effective compression ratio, enabling greater energy extraction from combustion. Intercooling and charge cooling enhance this effect, reducing NOₓ emissions in diesel engines and suppressing knock in spark-ignition units. This makes the cycle valuable for large marine and stationary engines where emissions control and fuel efficiency are essential.
However, the system requires careful balancing of boosting system performance, thermal efficiency, mechanical friction, and engine displacement. In some configurations, parasitic losses, increased complexity, and the need for robust intake management systems can offset efficiency gains.

Patent principles

Ralph Miller’s original 1957 patent describes a supercharged, intercooled engine equipped with a compression control valve (CCV) in the cylinder head. The CCV, operated by a servo mechanism linked to intake manifold pressure, opens during part of the compression stroke to release excess air into the exhaust manifold.
This system effectively allows the engine to vary its compression ratio according to load:

• at high load, greater CCV lift reduces effective compression
• at low load, reduced CCV lift increases effective compression

This adaptability ensures reliable starting and combustion stability across operating conditions. The patent illustrations demonstrate the use of a turbocharger rather than a positive-displacement supercharger, marking a significant distinction from some modern interpretations of the Miller cycle.

Comparison with the Atkinson cycle

Modern Atkinson cycle engines also employ delayed intake valve closing but do so without supercharging. They rely on increased expansion relative to compression to enhance efficiency but typically exhibit lower power density. These engines are common in hybrid electric vehicles, where electric motors compensate for reduced peak power. In contrast, the Miller cycle uses forced induction to overcome this drawback, delivering both efficiency and higher power output when required.

Applications and contemporary relevance

The Miller cycle remains in active use across diverse sectors. Marine engines and stationary generators benefit from its favourable efficiency characteristics and reduced emissions. Railway locomotives such as the GE PowerHaul use Miller-cycle principles to meet operational demands. Automotive manufacturers have applied variations of the cycle to achieve performance and efficiency gains: Mazda’s KJ-ZEM V6, Subaru’s hybrid B5-TPH concept, and Nissan’s small variable-valve-timing engines are notable examples.