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Miller cycle

The Miller cycle is a employed in internal combustion engines, characterized by an that exceeds the , which enhances by extracting more work from the expanding gases while reducing the work required for . This over-expansion is achieved through modified timing—either early closing (EIVC) or late closing (LIVC)—allowing the engine to operate with a lower effective than its geometric ratio, often paired with supercharging or turbocharging to offset any reduction in . Unlike the standard , where and expansion ratios are equal, the Miller cycle minimizes pumping losses and peak cylinder temperatures, making it suitable for both spark-ignition and -ignition engines. Developed by American engineer in the 1940s, the cycle builds on earlier concepts of explored in the and Atkinson's over-expansion ideas from the , but Miller's innovations focused on supercharged applications to achieve practical efficiency gains. He secured key U.S. patents in 1954 (US Patent 2,670,595) for a low-compression-ratio with supercharging and in 1956 (US Patent 2,773,490) for spark-ignition variants that mitigated through adjusted and richer fuel mixtures. Initially applied in large and stationary engines during the mid-20th century, the Miller cycle saw limited automotive adoption until the , when variable valve actuation technologies enabled its integration into production vehicles. In operation, the cycle reduces the intake air volume during the compression stroke via EIVC (pushing some mixture back into the intake manifold) or delays closure with LIVC to allow reverse flow, both lowering end-of-compression temperatures and enabling higher geometric compression ratios for better efficiency—up to 15% improvement over cycles in some configurations. For diesel engines, it primarily cuts NOx emissions at high loads by cooling the charge; in gasoline engines, it suppresses knock and reduces fuel consumption, though it requires boosting to maintain brake (BMEP). Modern implementations often combine it with two-stage turbocharging or electric superchargers to address challenges. Notable applications include Mazda's KJ-ZEM in the 1993 Millennia sedan, which used supercharging and EIVC for high efficiency, and Toyota's 1NZ-FXE Atkinson-cycle engine in the Prius since 1997, employing LIVC for naturally aspirated over-expansion. In diesels, integrated Miller cycle timing into its C11, C13, and C15 engines starting in 2004 to meet standards, while engines from manufacturers like and have adopted it for IMO Tier compliance. Today, the cycle influences many downsized, boosted engines from automakers like and , contributing to lower CO2 outputs amid tightening fuel economy regulations. As of 2025, ongoing developments include integration with fuels and advanced turbocharging for net-zero goals in and automotive sectors.

History and Development

Invention by Ralph Miller

, a Danish-born engineer who resided in the United States, specialized in development during the and , with a particular focus on enhancing the performance of supercharged engines. While working on these supercharged Otto-cycle engines, Miller observed significant inefficiencies stemming from elevated charge temperatures, which promoted knocking and , thereby limiting power output and operational reliability. Around 1947, Miller conceived the core principles of what would become the Miller cycle through initial theoretical analyses and sketches, aiming to decouple and ratios to enable higher for improved while avoiding the detonation risks associated with proportional high . This innovative approach sought to achieve greater and in supercharged applications by effectively reducing end-of- temperatures under varying loads, without necessitating excessive that would exacerbate knock. These foundational ideas culminated in Miller's key U.S. patent (US 2,670,595) for a high-pressure supercharging system granted in 1954 (filed October 19, 1949), describing early intake valve closing to reduce charge temperatures in supercharged engines.

Patent Details and Initial Concepts

Miller secured several key patents related to the Miller cycle. US Patent No. 2,670,595, granted March 2, 1954, to , titled "High-Pressure Supercharging System," describes a supercharged using early closing (EIVC) to allow charge during the intake stroke, reducing end-of-compression temperatures and enabling higher pressures without . The patent focuses on applications with supercharging and intercooling to achieve low effective ratios while maintaining power. Another key patent, US 2,773,490, filed September 23, 1952, and granted December 11, 1956, titled "High Expansion, Spark Ignited, Gas Burning, Internal Combustion Engines," details spark-ignition variants with variable early closing based on load to achieve high ratios (up to 12:1) while keeping lower (e.g., 6:1), using richer mixtures to mitigate . The US 2,817,322, filed April 30, 1956, and granted December 24, 1957, titled "Supercharged Engine," further elaborates on these concepts for both . It claims late closing (LIVC), where the inlet remains open approximately 60 degrees after bottom dead center, allowing a portion of the charge to be expelled back toward the during the early stroke. This reduces the effective and entrapped charge volume compared to the geometric , thereby lowering peak cylinder pressures and temperatures. To counteract the resulting decrease in power from the reduced effective , the specifies supercharging via an exhaust-driven blower or , delivering boosted air at pressures exceeding 2 atmospheres at full load, often with intercooling to further control charge temperature. Additional claims in US 2,817,322 detail a variable-lift control valve in the that opens during the to reject excess air, with modulated by load—maximum at full load for substantial charge expulsion and minimal at idle. This mechanism complements the late , enabling precise control over the retained charge while maintaining scavenging in two- or four-cycle operations. The integration of the offsets power losses by increasing density, allowing the to achieve higher overall output than a naturally aspirated equivalent with similar geometric dimensions. The includes several diagrams illustrating these concepts, with Figure 8 providing and timing profiles for a two-cycle , showing the delayed inlet closure and the 's operation relative to position. Figures 1 through 6 depict layouts, including the connected to the and the blower delivering cooled, pressurized air to the intake port via an . These visuals highlight the spatial arrangement of actuators and the setup to facilitate the cycle's charge dynamics. Miller's concepts in these patents built upon his prior investigations into engine knock mitigation, where high compression temperatures were identified as a key factor in abnormal combustion.

Early Engine Prototypes

Following Ralph Miller's patents in the mid-1950s, early experimental engines incorporating the Miller cycle were constructed to validate the concept of over-expansion through modified intake valve timing in supercharged four-stroke internal combustion engines. These prototypes, often built by Miller and engineering collaborators, adapted existing engine designs—such as modifying standard configurations for boosted operation—to implement late intake valve closing (LIVC), as outlined in the core patent describing a supercharged system with variable valve mechanisms to control charge rejection and compression temperatures. Testing of these early prototypes in the late 1950s demonstrated notable improvements over conventional or cycles, primarily by enabling higher geometric compression ratios while mitigating knock and through reduced effective compression; however, the designs inherently suffered from lower power output due to the expulsion of part of the charge, necessitating supercharging to recover . Representative examples from the era, applied initially in large two-stroke low-speed diesel engines, confirmed gains in fuel economy and scavenging efficiency, though quantitative benchmarks varied by implementation and load conditions. Key challenges in prototype development included precise supercharger sizing to deliver sufficient boost pressure—typically exceeding 2 atmospheres absolute at full load—to offset the power penalty from charge rejection, as inadequate sizing led to suboptimal air-fuel ratios and reduced across load ranges. Valve train durability also posed issues, with the non-standard late-closing timing straining components and requiring robust, adjustable mechanisms that were technologically demanding for the period's materials and precision. Commercial interest remained limited in the and , as the automotive and stationary power sectors prioritized simpler, naturally aspirated or carbureted engines without the added complexity of and systems; this focus, combined with immature supporting technologies, resulted in the Miller cycle concepts being largely shelved until advancements in turbocharging and electronic controls revived them in the 1980s.

Thermodynamic Principles

Modification of the Otto Cycle

The standard , which forms the basis of most spark-ignition internal engines, operates through four distinct strokes: , where the moves downward with the open to draw in an air-fuel mixture; , where both valves are closed and the rises to compress the mixture; , where occurs near top dead center followed by as the descends; and exhaust, where the rises again to expel products with the exhaust open. In this cycle, the typically closes at or shortly before bottom dead center (BDC) of the stroke, ensuring the full displacement volume of the cylinder is filled with charge before the stroke begins. The Miller cycle modifies the by altering intake valve timing to reduce the effective , often employing such as supercharging or turbocharging to maintain , while using late intake valve closing (LIVC) after bottom dead center (BDC) in the original design. This late intake valve closing (LIVC) strategy, originally conceptualized by in his 1957 patent for supercharged engines, results in partial expulsion of the intake charge back into the manifold, effectively reducing the trapped volume available for compression without changing the geometric displacement. In the pressure-volume (P-V) diagram of the Miller cycle, this modification introduces a characteristic "pumping " during the transition from to : as the rises past BDC with the intake valve still open, cylinder drops below manifold pressure, pushing some charge back out and creating a small negative work that offsets part of the positive pumping work during . Consequently, the effective stroke is smaller than the full geometric , yielding a lower effective while preserving the full during the power stroke, which enhances by better extracting work from the expanding gases. The P-V diagram illustrates this as a shortened line (from the effective trapped point to top dead center) compared to the standard , with the expansion line extending over the full displacement for over-expansion benefits.

Valve Timing and Charge Dynamics

In the Miller cycle, the intake valve timing is modified compared to the conventional , where the intake valve typically closes near bottom dead center (BDC) to maximize trapped charge volume. Specifically, the intake valve closing (IVC) is delayed by 20 to 60 degrees after BDC, allowing the piston to begin the compression stroke while the intake valve remains open. This late intake valve closing (LIVC) was a key feature in Ralph Miller's original supercharged engine design, where the inlet valve closes approximately 60 degrees after BDC to reduce the effective and control thermal loads. While late intake valve closing (LIVC) is the original method, early intake valve closing (EIVC) during the intake stroke is an alternative approach that limits charge intake to achieve a similar reduction in effective without reverse flow. The charge dynamics during the intake and early compression phases are characterized by an initial filling of the cylinder with air-fuel mixture as the piston descends to BDC, followed by a partial reversal as the piston ascends. With the valve still open post-BDC, the rising piston first compresses the charge slightly, but the momentum of the incoming and differences lead to blow-back or reverse back into the manifold, effectively reducing the trapped volume by expelling a portion of the charge. This results in lower as excess gas is discharged during the 20-60 degree window. In supercharged Miller cycle implementations, elevated manifold from the compensates for this lost charge mass, maintaining adequate cylinder filling while enabling the over-expansion benefits. The effective displacement V_{\text{eff}} in the Miller cycle can be approximated by the equation V_{\text{eff}} = V_d \left(1 - \frac{\theta_{\text{IVC}}}{360^\circ}\right), where V_d is the full geometric and \theta_{\text{IVC}} is the IVC angle in degrees after BDC. This simplification highlights how the delayed closure proportionally diminishes the active swept volume, directly influencing charge trapping and subsequent compression work.

Effective Ratios and Efficiency Gains

In the Miller cycle, the geometric compression ratio r_g = \frac{V_{\max}}{V_{\min}} is typically maintained at high values, such as 10:1 or greater, to support robust expansion while mitigating knock through modified valve timing. This ratio represents the full displacement volume V_{\max} at bottom dead center relative to the clearance volume V_{\min} at top dead center. However, late intake valve closure (IVC) reduces the trapped charge volume, resulting in a lower effective compression ratio r_e = \frac{V_{\text{IVC}}}{V_{\min}}, where V_{\text{IVC}} is the volume at IVC, which is less than V_{\max}. This discrepancy allows for reduced compression work without sacrificing the potential for high expansion. The in the Miller equals the geometric r_g, as the power stroke utilizes the full from V_{\max} to V_{\min}. This over- relative to the effective enables greater extraction of work from the gases, improving the overall efficiency by better matching the exhaust temperatures to ambient conditions. Late IVC serves as the key enabler for this reduced r_e, compression and processes. The of the standard is given by \eta_{\text{Otto}} = 1 - \frac{1}{r_g^{\gamma - 1}}, where \gamma is the specific heat ratio. In the Miller cycle, efficiency is influenced primarily by the lower r_e during , approximated as \eta \approx 1 - \frac{1}{r_e^{\gamma - 1}}, with adjustments for the full expansion at r_g and reduced pumping losses due to the over-expansion. This formulation demonstrates gains over the , as the lower r_e curbs peak temperatures and knock while the unchanged preserves work output; pumping losses are further minimized at part loads by avoiding throttling. Typical thermal efficiency improvements range from 5% to 10% higher than the , particularly at part loads where pumping benefits are pronounced.

Comparison to Related Cycles

Differences from the Otto Cycle

The Miller cycle modifies the conventional primarily through alterations in , enabling late intake valve closing (LIVC) or early intake valve closing (EIVC) to achieve an over-expanded configuration, whereas the employs fixed with intake valve closing at or near bottom dead center to fully trap the charge throughout the compression stroke. This late or early IVC in the Miller cycle reduces the effective at part loads, lowering peak cylinder temperatures and pressures to enhance knock resistance compared to the 's uniform compression that can lead to under high loads. In terms of , the maintains higher output at full load by trapping a complete air-fuel charge, resulting in greater and , while the Miller cycle sacrifices some for gains through partial charge expulsion or reduction during the process. The Miller cycle, particularly when supercharged, facilitates the use of lower-octane fuels by mitigating risks via reduced effective compression and controlled charge temperature, in contrast to the which typically requires higher-octane fuels to avoid knock in high-compression setups without such modifications. Historically, the dominated naturally aspirated spark-ignition engines through the pre-1980s era due to its simplicity and reliable power delivery in unboosted applications, while the emerged later as a specialized variant for boosted efficiency-focused designs.

Relation to the

The , invented by British engineer James Atkinson in 1882, achieves higher than the conventional through early closure of the intake using a mechanical linkage in the mechanism, which shortens the effective compression stroke while maintaining a longer expansion stroke in a naturally aspirated configuration. This design reduces the relative to the , minimizing pumping losses and improving fuel economy at the expense of . The Miller cycle, patented by American engineer in 1956 (US Patent 2,773,490), builds directly on this by incorporating late intake valve closing to similarly decouple the and ratios, but compensates for the resulting loss in through via a or . Often described as a "supercharged ," the Miller approach restores power output to levels comparable to standard Otto-cycle engines while preserving the efficiency gains from the extended stroke, avoiding the displacement penalties inherent in the original Atkinson's mechanical complexity. A key distinction lies in their approaches to power: the Atkinson cycle's naturally aspirated operation inherently limits torque and due to the reduced effective from early or late events, whereas the Miller cycle uses to achieve with conventional without increasing physical size. In modern , particularly with the advent of systems, some naturally aspirated designs employing late closing are classified as Atkinson cycles by regulatory bodies like the U.S. EPA, while boosted variants with similar strategies are more precisely termed Miller cycles to reflect the component.

Hybrid Cycle Variants

Variable valve timing (VVT) systems have enabled hybrid variants of the Miller cycle by allowing engines to dynamically switch between Miller-Atkinson operation for efficiency at low loads and modes for higher power output. In these designs, late intake valve closing (IVC) during low-speed or cruise conditions reduces the effective while maintaining a high , improving without sacrificing peak performance. For instance, advanced VVT mechanisms like continuously variable intake valve lift and timing can adjust valve events in real-time, decoupling compression from expansion to emulate Miller principles across operating ranges. In hybrid powertrains, the Miller cycle integrates seamlessly with electric motors to address limitations, using late IVC for fuel-efficient cruising while reverting to full events for supported by electric boost. This approach optimizes overall system efficiency, as the electric component compensates for the reduced inherent in Miller operation during transient demands. Toyota's Dynamic Force engine family exemplifies this, employing VVT-iW technology to alternate between Otto and Miller cycles, achieving up to 40% thermal efficiency in hybrid configurations through over-expansion akin to the Miller . Honda's i-VTEC system represents an variant (analogous to Miller principles without ) in certain naturally aspirated engines, where variable timing and lift create late IVC effects to enhance low-end and . Emerging integrations, such as electric supercharging in Miller cycle , further mitigate turbo lag and performance deficits by providing on-demand air supply, allowing sustained late IVC benefits across the drive cycle without trade-offs. These variants collectively advance engine design by balancing the Miller cycle's gains with responsive power delivery in electrified vehicles.

Advantages and Disadvantages

Thermal Efficiency Improvements

The Miller cycle enhances by significantly reducing pumping losses, especially at part-throttle operation, through late intake valve closing (LIVC). This strategy keeps the intake valve open beyond bottom dead center, permitting a portion of the intake charge to flow back into the manifold during the initial compression stroke, thereby lowering the effective and the required for operation. The resulting blow-back reduces the work needed to pump the charge, as quantified by the pumping work equation W_p = \oint P \, dV, where the integral over the pumping loop in the shows a minimized enclosed area compared to a conventional . Furthermore, the cycle lowers peak temperatures by compressing a smaller effective charge volume to top dead center, which decreases losses to the cylinder walls and improves second-law through reduced destruction during . This temperature reduction allows for higher geometric ratios without risking knock or excessive thermal stresses, further boosting the stroke's contribution to net work output. In boosted engine setups, these mechanisms translate to real-world brake specific fuel consumption (BSFC) improvements of up to 10%, with representative studies showing 4.7% better BSFC at high loads in high-compression-ratio direct-injection engines. Effective compression and expansion ratios play a key enabling role in realizing these gains by optimizing charge dynamics for higher overall .

Power Density and Response Limitations

The Miller cycle achieves higher through over-expansion, but this comes at the cost of reduced due to diminished trapped air mass in the . By employing early or late closing, the cycle intentionally limits the amount of air inducted during the stroke, resulting in a reduction of approximately 10-20% compared to a conventional engine. This charge loss directly translates to lower output, particularly at low engine speeds where the effect of or shortened duration is most pronounced, potentially decreasing peak by similar margins without boosting compensation. To counteract this inherent power deficit, the Miller cycle typically incorporates via a or , yet these systems introduce additional limitations. Mechanically driven impose parasitic losses, consuming up to 20% of the engine's gross output power to drive the , which offsets some of the cycle's gains under steady-state operation. In exhaust-driven configurations, turbo lag exacerbates response issues, with noticeable delays before full boost is achieved during transient acceleration, stemming from the time required to spool the using exhaust energy—particularly challenging in Miller cycle engines where reduced charge mass can slow exhaust flow buildup. Mitigation strategies for these power density and response drawbacks often involve hybrid boosting approaches, such as electric-assisted superchargers or compound turbo-supercharger setups, which provide immediate air supply to maintain torque across RPM ranges. Electric superchargers, powered by the vehicle's or regenerative systems, can dramatically shorten response times to fractions of a second (e.g., 0.3-0.7 seconds) by delivering boost without relying on mechanical or exhaust drive, effectively preserving low-RPM drivability in Miller cycle implementations. However, these enhancements increase system complexity, weight, and manufacturing costs, potentially limiting their adoption in cost-sensitive applications.

Emissions and Fuel Economy Impacts

The Miller cycle reduces () emissions compared to the conventional by employing early or late intake closing, which results in a cooler charge at the end of compression and lower peak temperatures. This effect is particularly pronounced at high loads in engines and across various loads in spark-ignition applications, with reported NOx reductions ranging from 8% to 46% depending on the strategy and operating conditions. Fuel economy in Miller cycle engines benefits significantly from enhanced part-load efficiency, driven by minimized pumping losses and an effective that exceeds the , leading to better utilization of the . Improvements of 5% to 20% over engines have been documented, with greater gains—up to 15%—observed in urban driving scenarios characterized by frequent low-speed operation. Recent implementations, such as the 2025 XC90's mild-hybrid Miller cycle engine, have demonstrated approximately 4% fuel economy improvements. Carbon dioxide (CO2) emissions decrease in alignment with these fuel economy enhancements, as the cycle's thermodynamic advantages directly lower fuel consumption and thus tailpipe CO2 output. However, in diesel implementations, particulate matter (PM) emissions may increase by up to 30% without optimization, due to altered charge motion and combustion characteristics, though such penalties can be mitigated through turbocharging and exhaust aftertreatment tuning. Standardized drive cycle testing underscores the Miller cycle's advantages, particularly in hybrid powertrains where its efficiency shines during stop-and-go conditions; for instance, evaluations under the FTP-75 urban cycle have shown substantial fuel economy edges, while WLTP assessments confirm similar benefits in real-world mixed driving for downsized boosted engines.

Modern Applications and Implementations

Supercharged and Turbocharged Engines

In early implementations of the Miller cycle, superchargers were preferred to provide immediate boost and compensate for the reduced effective resulting from late intake valve closing (IVC), which allows some charge to blow back into the manifold. Roots-type positive superchargers, such as those with multi-lobe rotors, were commonly selected for their ability to deliver consistent boost at low speeds, enhancing low-end in configurations where the cycle's over-expansion reduces . Centrifugal superchargers, driven directly by the , offered an alternative in these designs by providing progressive boost that scales with RPM, though they were less effective at idle and low loads compared to positive types. This thermodynamic necessity for offsets the blow-back effect, maintaining cylinder filling comparable to standard engines. Turbocharging adaptations for Miller cycle engines address the inherent turbo lag exacerbated by late IVC, which reduces exhaust energy available for drive at low speeds. Variable geometry turbochargers (VGTs) are particularly suited, as their adjustable vanes optimize exhaust flow to the , improving and minimizing lag by matching boost to the delayed across a broader RPM range. Twin-scroll turbochargers, which separate exhaust pulses from pairs, further reduce lag in these setups by enhancing and pulse energy recovery, allowing quicker spool-up despite the cycle's reduced charge trapping. Intercooling remains essential in both supercharged and turbocharged configurations to cool the boosted charge, preventing knock and enabling higher ratios while preserving the cycle's gains. Typical boost levels in Miller cycle engines range from 1.5 to 2.0 absolute to restore equivalent to counterparts, with the exact value depending on engine load and strategy. Positive displacement , often integrated in systems with , excel in providing this at low RPMs, prioritizing low-end for responsive in Miller cycle applications where natural aspiration would yield insufficient output. For instance, boosting arrangements—such as a upstream of a —can achieve these pressures while reducing overall parasitic losses, with intercoolers placed between stages to manage effectively.

Automotive and Heavy-Duty Uses

In automotive applications, pioneered the commercial use of the Miller cycle in production vehicles through its KJ-ZEM 2.3-liter supercharged , introduced in the Millenia luxury sedan from 1995 to 2002. This employed late intake valve closing to achieve an effective lower than the , enabling higher while maintaining power output comparable to larger displacement engines; it delivered 210 horsepower and fuel economy approaching that of smaller 1.8-liter naturally aspirated units under similar conditions, marking a significant step in downsized, efficient design. In heavy-duty sectors, the Miller cycle has been explored in truck engines to meet stringent Euro VI emissions standards since the 2010s, with strategies implementing late intake valve closing for reduction. Experimental implementations in heavy-duty have demonstrated potential reductions of up to 57% through reduced peak temperatures with parity to conventional cycles when optimized with high , aiding compliance without excessive reliance on aftertreatment systems. For marine and stationary power generation, companies such as and have incorporated late intake valve closing in large diesel engines to optimize efficiency under variable loads, with simulations showing potential savings of up to 5-8% in dual-fuel and low-speed configurations. These adaptations lower pumping losses and enable higher expansion ratios, particularly beneficial in where sustained operation at partial loads predominates. The adoption of Miller cycle principles is expanding in hybrid powertrains, particularly in , where hybrid vehicles accounted for over 40% of new car sales in the first half of 2025, with models like Subaru's e-BOXER systems utilizing Atkinson-Miller variants for enhanced efficiency in sedans and crossovers, including the 2025 Hybrid. This growth reflects a broader trend toward over-expansion cycles in electrified drivelines to complement electric motor and achieve superior fuel economy without sacrificing drivability.

Recent Technological Advances

Recent advancements in variable valve lift systems have enabled more precise dynamic adjustment of intake valve closing (IVC) timing in Miller cycle engines, allowing for real-time optimization of the effective compression ratio. A novel fully variable valve timing and lift mechanism, known as CD-HFVVS, has been developed for diesel engines operating on the Miller cycle, facilitating continuous adjustment of IVC to achieve early or late closure strategies that enhance thermal efficiency while mitigating knock. This approach decouples the intake duration and lift, improving part-load performance in downsized engines by reducing pumping losses without compromising volumetric efficiency. Electrified boosting technologies in 48V mild hybrid systems have addressed the inherent power density limitations of the Miller cycle by integrating electric superchargers or turbos, which provide instantaneous boost to compensate for delayed IVC and eliminate turbo lag. collaborated with Garrett to develop an electric powered by the 48V system, enhancing in high-performance engines that could benefit Miller cycle designs. strategies for these powertrains optimize air path management, achieving up to 5% fuel savings in dynamic cycles through coordinated electric supercharging and belt starter generator assist. Artificial intelligence and techniques are increasingly applied to optimize in for Miller cycle engines, adjusting IVC based on load conditions to maximize efficiency. AI-driven modeling enables dynamic tuning of and other parameters cycle-by-cycle, reducing emissions and improving economy by integrating data for predictive . In engines employing the Miller cycle, -based predictive knock models co-optimize Miller degree and , yielding efficiency gains of approximately 3-5% under varying loads. Looking toward future trends, the integration of the Miller cycle with fuels is poised to enable zero-carbon internal combustion engines by 2030, leveraging 's clean properties to eliminate CO2 emissions. FAW has developed a high-efficiency Miller cycle achieving 42% brake and near-zero emissions through ultra-lean burn and high-pressure direct injection, with commercialization targeted post-2030 to support carbon neutrality goals. Simulations of engines using Miller cycle with demonstrate up to 30% improvements in brake power at low loads while maintaining net-zero potential, though control via EGR remains essential.