Fact-checked by Grok 2 weeks ago

Atkinson cycle

The Atkinson cycle is a thermodynamic cycle used in spark-ignition internal combustion engines, invented by British engineer James Atkinson in 1882 to improve fuel efficiency over the conventional Otto cycle.^[1] It achieves this by employing an expansion ratio greater than the compression ratio, extracting more work from the expanding combustion gases while reducing pumping losses, though at the cost of lower power density and torque output.^[2] The cycle follows a four-stroke process—intake, compression, power (expansion), and exhaust—but modifies the timing and stroke lengths to prioritize efficiency.^[1] Atkinson's original design, patented in the United States as US 367496 on August 2, 1887, utilized a multi-link connecting rod mechanism to vary piston stroke lengths, completing all four strokes within a single 360-degree crankshaft revolution rather than the Otto cycle's 720 degrees.^[3] This mechanical arrangement shortened the compression and intake strokes while extending the power stroke, allowing the engine to operate with higher thermal efficiency by better matching the expansion of hot gases to atmospheric pressure during exhaust.^[1] Early engines based on this cycle were produced in limited numbers but faced challenges with mechanical complexity and low power, limiting widespread adoption.^[4] In contemporary applications, the Atkinson cycle has been revived and simplified through electronic variable valve timing (VVT), particularly late intake valve closing (LIVC), which effectively shortens the compression stroke without altering the mechanical design.^[5] This approach delays the intake valve closure until after the piston begins its compression stroke, pushing some of the air-fuel mixture back into the intake manifold and reducing the effective compression ratio while maintaining a full expansion stroke.^[5] Modern Atkinson-cycle engines, often paired with electric motors in hybrid powertrains, offer thermal efficiencies up to 40% or more, significantly better than traditional Otto-cycle engines' 30-35%, making them ideal for fuel-efficient vehicles like the Toyota Prius.^[6]

History and Invention

James Atkinson's Original Concept

James Atkinson, a British engineer born on 31 October 1846 in Turton, Lancashire, and later based in Hampstead, London, developed the Atkinson cycle as an advancement in internal combustion engine design during the late 19th century.^[7] Apprenticed at Palmer Brothers in Jarrow-on-Tyne and later employed at the London and North Western Railway's works in Crewe, Atkinson sought to overcome the fuel inefficiencies prevalent in contemporary engines, including both steam engines and early gas engines like the Otto cycle, which suffered from incomplete expansion of combustion gases.^[8] His invention aimed to enhance thermodynamic efficiency by allowing the engine to perform more work from each combustion event without increasing fuel consumption.^[1] The core concept of Atkinson's cycle involved unequal piston strokes, where the expansion (power) stroke was significantly longer than the compression stroke, enabling the combustion gases to expand further and transfer more energy to the crankshaft before exhaust.^[9] This over-expansion principle was first detailed in his British patent No. 4,378, granted on September 14, 1882, which described a differential compression gas engine capable of achieving these variable strokes through a novel mechanical linkage.^[9] By extending the power stroke—typically to about 1.5 times the length of the intake, compression, and exhaust strokes—Atkinson's design extracted additional work from the cooling exhaust gases, theoretically improving thermal efficiency beyond that of the conventional Otto cycle.^[10] Despite its promising efficiency gains, implementing Atkinson's concept faced significant initial challenges, primarily due to the mechanical complexity required to realize the unequal strokes in a practical engine.^[11] The patented mechanism relied on intricate linkages and crankshaft arrangements, such as a two-bar linkage between the connecting rod and crank, which increased manufacturing costs and reduced reliability compared to simpler Otto designs.^[7] These hurdles limited early adoption, though the fundamental idea laid the groundwork for later refinements in engine technology.^[1] Atkinson died on 24 March 1914 in Thames Ditton, Surrey.^[7]

Early Patents and Prototypes

James Atkinson filed the initial patent for his cycle in the United Kingdom in 1882, describing a gas engine that utilized a longer expansion stroke than compression stroke to enhance thermal efficiency while circumventing the restrictive Otto cycle patents.^[4] This foundational design, often referred to as the basic Atkinson cycle, employed mechanical linkages to achieve the variable stroke lengths essential for the cycle's operation.^[11] In 1887, Atkinson secured U.S. Patent No. 367,496 for improvements to his gas engine, refining the mechanism to perform the full cycle with a single-acting piston in a single cylinder, enabling one working stroke per revolution while maintaining the efficiency advantages.^[3] These enhancements addressed practical implementation challenges, such as smoother operation and reduced mechanical complexity compared to earlier iterations.^[7] Early prototypes emerged shortly after the initial patent, with Atkinson's 1887 demonstration engine exemplifying the cycle's potential; it demonstrated superior thermal efficiency compared to contemporary Otto engines—but suffered from lower power density due to the extended expansion phase reducing mean effective pressure.^[4] These test models incorporated slide valves for intake and exhaust control, alongside complex linkages connecting the piston to the crankshaft, which allowed the expansion stroke to be approximately 1.5 times longer than the compression stroke.^[12] By around 1890, early production engines were built and tested by the British Gas Engine and Engineering Company, which Atkinson co-founded in 1883 as managing director to commercialize his inventions, with over 1,000 units produced by the time the company closed in 1893—marking a key 19th-century collaboration in advancing the cycle from concept to viable hardware.^[7]^[4] These efforts highlighted the cycle's promise for stationary applications, though mechanical intricacies limited broader adoption during the era.^[4]

Thermodynamic Principles

Ideal Cycle Operation

The ideal Atkinson cycle models the theoretical operation of a heat engine using air as the working fluid with constant specific heats, consisting of four reversible processes that approximate the behavior of spark-ignition engines with extended expansion.^[13] These processes are isentropic compression, isochoric heat addition, isentropic expansion, and isobaric heat rejection, where "isentropic" refers to a reversible adiabatic process with no heat transfer and constant entropy.^[14] The cycle assumes a closed system, enabling analysis of energy conversion from heat to work via the first law of thermodynamics.^[15] In the first process (1-2), isentropic compression occurs as the piston moves from bottom dead center (state 1) to top dead center (state 2), reducing the volume from V_1 to V_2 without heat transfer or irreversibilities.^[13] The compression ratio is defined as r_c = V_1 / V_2 < 10 typically, leading to increased pressure P_2 = P_1 r_c^\gamma and temperature T_2 = T_1 r_c^{\gamma-1}, where \gamma is the specific heat ratio (approximately 1.4 for air).^[14] The second process (2-3) involves isochoric heat addition at constant volume V_3 = V_2, where fuel combustion adds heat Q_\text{in} = c_v (T_3 - T_2), raising pressure and temperature to state 3 while the piston remains at top dead center.^[13] The third process (3-4) is isentropic expansion, where the piston moves from top dead center toward bottom dead center, increasing volume from V_3 to V_4 > V_1, with the expansion ratio r_e = V_4 / V_3 > r_c.^[15] This longer expansion extracts more work from the hot gases, yielding T_4 = T_3 / r_e^{\gamma-1} and P_4 = P_3 / r_e^\gamma, with the model assuming P_4 = P_1 to simulate exhaust at atmospheric pressure.^[14] Finally, isobaric heat rejection (4-1) occurs at constant pressure P_4 = P_1, reducing the volume from V_4 to V_1 while releasing heat |Q_\text{out}| = c_p (T_4 - T_1) to the surroundings as the temperature drops back to T_1, completing the cycle.^[13] On a pressure-volume (PV) diagram, the cycle appears as a closed loop: a steep isentropic compression curve from (V_1, P_1) to (V_2, P_2), a vertical isochoric line upward to (V_2, P_3), a shallower isentropic expansion curve to (V_4, P_4) where V_4 > V_1 due to r_e > r_c, and a horizontal isobaric line leftward to (V_1, P_1).^[14] The lower compression ratio compared to the higher expansion ratio results in a larger enclosed area for net work and reduced heat rejection, enhancing thermal efficiency over cycles like the Otto where r_c = r_e.^[15] The thermal efficiency \eta of the ideal Atkinson cycle is given by

\eta = 1 - \frac{\gamma \left( \frac{r_e}{r_c} - 1 \right)}{r_c^{\gamma - 1} \left[ \left( \frac{r_e}{r_c} \right)^\gamma - 1 \right]}

where r_e is the expansion ratio (greater than the compression ratio r_c) and \gamma = c_p / c_v is the specific heat ratio.^[2] This formula arises from applying the first law of thermodynamics to the cycle: the net work output equals heat added minus heat rejected, so \eta = 1 - |Q_\text{out}| / Q_\text{in}.^[15] Substituting the expressions, Q_\text{in} = c_v (T_3 - T_2) and |Q_\text{out}| = c_p (T_4 - T_1), yields \eta = 1 - \gamma (T_4 - T_1)/(T_3 - T_2).^[13] To derive the efficiency formula, start with the isentropic relations: T_2 = T_1 r_c^{\gamma-1} and T_4 = T_3 r_e^{1-\gamma}. The condition P_4 = P_1 implies T_3 / T_2 = (r_e / r_c)^\gamma. Substituting these into the efficiency expression and simplifying gives the formula above.^[2] This results in higher efficiency than the Otto cycle's \eta = 1 - (1/r_c)^{\gamma-1} for the same r_c, due to minimized exhaust heat rejection from fuller expansion.^[13]

Efficiency Advantages Over Otto Cycle

The Otto cycle, the basis for conventional spark-ignition engines, operates with equal compression and expansion ratios (r_c = r_e), yielding a thermal efficiency of \eta_{Otto} = 1 - (1/r_c)^{\gamma-1}, where \gamma is the specific heat ratio of the working fluid, typically 1.4 for air-standard analysis.^[16] In the Atkinson cycle, the expansion ratio exceeds the compression ratio (r_e > r_c), enabling fuller extraction of work from the expanding combustion gases during the power stroke, which fundamentally increases thermal efficiency beyond that of the Otto cycle for the same compression ratio.^[17] This design choice allows the Atkinson cycle to approach the efficiency limits of cycles with higher expansion ratios while maintaining a practical compression ratio to avoid excessive mechanical stresses or knocking.^[18] The primary efficiency advantages stem from the over-expansion process, where the greater r_e reduces the exhaust gas temperature and pressure closer to ambient conditions, minimizing wasted energy and achieving better fuel efficiency under ideal conditions compared to the Otto cycle. Additionally, the Atkinson cycle exhibits lower pumping losses because the intake valve timing delays closure, effectively reducing the trapped charge during compression and easing the work required for gas exchange, which further boosts part-load efficiency—a critical factor in real-world operation.^[17] These gains are most pronounced in theoretical air-standard models, where the absence of irreversibilities like heat transfer or friction highlights the cycle's thermodynamic superiority. However, the higher expansion ratio in the Atkinson cycle leads to a trade-off in power density, as the shorter effective compression stroke results in less air-fuel mixture being trapped, yielding lower mean effective pressure and reduced power output per unit displacement relative to an equivalent Otto cycle engine.^[5] Modern implementations mitigate this by incorporating supercharging or turbocharging to increase intake manifold pressure, restoring power while preserving efficiency benefits.^[1] For illustration, consider an air-standard case with r_c = 8, r_e = 12, and \gamma = 1.4: the Otto cycle efficiency is approximately 56%, while the Atkinson cycle reaches about 60%, demonstrating a meaningful improvement from the extended expansion.^[18]

Historical Engine Designs

Differential Engine

The Differential Engine represented James Atkinson's first practical embodiment of his cycle concept, utilizing a novel mechanical arrangement to realize unequal stroke lengths. Patented in 1885 (corresponding to US Patent 336,505 in 1886), the design employed two pistons within a horizontal cylinder, with a differential linkage system that extended the expansion stroke to 1.5–2 times the length of the compression stroke, thereby extracting more work from the combustion gases. Power was delivered through a crankshaft driven by the linkage, enabling a four-stroke cycle to complete in one revolution while avoiding infringement on existing Otto patents.^[19] A prototype of the engine was constructed in 1886 by the British Gas Engine Company at their Albion Works in London and demonstrated at engineering exhibitions, including the London Inventors Exhibition where it earned a gold medal for its innovative efficiency. Operating at low power levels of around 1–2 hp, the engine showcased a 20–25% improvement in thermal efficiency compared to contemporary Otto-cycle designs of the era, primarily due to the over-expansion principle that better matched the cycle's thermodynamic potential. No surviving examples are known.^[20]^[21] Despite these advantages, the Differential Engine's complexity posed significant challenges, particularly high friction losses in the gear and linkage components, which accelerated wear and limited operational life to approximately 180 hours before maintenance was required. These mechanical drawbacks, including excessive joint stress and pivot wear, contributed to its limited commercial adoption despite successful demonstrations in applications like the Houses of Parliament's hydro-pneumatic systems. The expiration of the Otto patents in 1890 further reduced the need for such elaborate workarounds.^[21]

Cycle Engine

The Cycle Engine, patented by James Atkinson in 1887 under US Patent No. 367,496 (corresponding to a British patent), represented an evolution in his efforts to implement the Atkinson cycle through innovative mechanical design. This design utilized a complex mechanical linkage connected to the piston, which shortened the compression and intake strokes while extending the power stroke to 1.78 times longer, enabling a higher expansion ratio relative to the compression ratio and improving thermodynamic efficiency by extracting more work from the combustion gases. The engine was typically configured as a single-cylinder unit, simplifying construction while achieving the cycle's benefits in a compact form that completed all four strokes in one crankshaft revolution.^[4]^[3] In terms of performance, tests of the Cycle Engine demonstrated thermal efficiencies reaching up to 30%, surpassing contemporary Otto cycle engines by better utilizing the energy from combustion while reducing pumping losses. It delivered approximately 3 horsepower in practical applications, with notably smoother operation compared to Atkinson's earlier differential mechanism designs due to the elimination of complex gear systems and the reliance on precise linkage-driven events.^[4]^[7] Commercialization efforts from 1886 to 1893 were led by the British Gas Engine and Engineering Company, which produced and marketed over 1,000 units of the engine for industrial uses such as powering pumps and generators, with licensing to manufacturers like Manlove, Alliott & Co. However, despite its efficiency advantages, the design saw limited adoption, overshadowed by the widespread dominance of simpler Otto cycle engines that offered higher power density and easier manufacturing following the Otto patent expiration in 1890. The Cycle Engine's reliance on advanced mechanical control, while innovative, posed challenges in reliability and cost for the era's machining capabilities, contributing to its niche status. No surviving examples are known.^[7]

Utilite Engine

The Utilite Engine, introduced in 1892, represented James Atkinson's final and most commercially oriented iteration of the Atkinson cycle, designed as a compact single-cylinder engine to facilitate easier manufacturing compared to his earlier, more complex models. This engine employed a conventional crankshaft but incorporated a complex linkage with a modified crank throw to create the necessary asymmetry between the compression and expansion strokes, enabling the longer power stroke characteristic of the cycle without relying on intricate multi-piston systems. The design simplified production by reducing mechanical components while maintaining the core thermodynamic advantages, making it suitable for broader industrial adoption, with an impulse every crankshaft revolution.^[7] In terms of performance, the Utilite Engine achieved thermal efficiencies of 25-30%, significantly higher than contemporary Otto cycle engines, and could operate at speeds up to 600 RPM. Small models typically produced around 5 horsepower, though larger variants reached up to 100 horsepower, and it was primarily deployed in stationary applications such as electric generators and pumping systems. The engine featured valve timing that optimized the extended expansion phase while maintaining four-stroke operation.^[7] Marketed and produced in limited numbers by companies including the British Gas Engine Company through the early 1900s, the design was protected under British Patent No. 2492, granted in 1892, which detailed the crank modifications and operational cycle. Despite its efficiency edge, the Utilite Engine saw limited production and ultimately declined as cheaper, more straightforward Otto cycle engines dominated the market, prioritizing cost and simplicity over thermal performance following the 1890 patent changes. Very few examples were built, and none are known to survive today.^[7]

Modern Implementations

Reciprocating Engines in Hybrids

In modern reciprocating implementations of the Atkinson cycle, engines typically feature high geometric compression ratios ranging from 12:1 to 14:1, combined with late intake valve closing (LIVC) to achieve effective expansion ratios that exceed compression ratios, thereby emulating the ideal Atkinson cycle's over-expansion for improved thermodynamic efficiency.^[22]^[23] This LIVC strategy reduces the effective compression stroke by delaying intake valve closure, minimizing pumping losses while maintaining a longer power stroke, which is particularly advantageous in hybrid applications where the electric motor compensates for any reduction in peak torque. To address the inherent power density limitations of the Atkinson cycle compared to the Otto cycle, many designs incorporate forced induction such as turbocharging or supercharging, alongside advanced variable valve timing systems, enabling seamless operation in electrified powertrains without sacrificing drivability.^[24] Prominent examples include Toyota's Dynamic Force engine family, such as the 2.5-liter A25A-FXS introduced in 2018, featuring a 14:1 compression ratio and LIVC via VVT-iW and VVT-i systems.^[25]^[26] Similarly, Honda's Earth Dreams technology incorporates Atkinson-cycle engines, exemplified by the 2.0-liter DOHC i-VTEC units in hybrid models, which use LIVC and high compression (around 13:1) to optimize efficiency in series-parallel hybrid configurations, as seen in the 2025 Civic Hybrid's updated 2.0 L four-cylinder.^[27]^[28] These engines leverage the Atkinson cycle's principles to prioritize fuel economy in hybrids, where the electric assist handles transient loads. The 2025 Toyota Camry, now hybrid-only, uses a similar 2.5 L Atkinson engine for enhanced efficiency.^[29] In hybrid powertrains, these reciprocating Atkinson engines deliver real-world thermal efficiencies of 40-42%, significantly outperforming conventional Otto-cycle engines that typically achieve 30-35%. For instance, Toyota's Prius models with the M20A-FXS engine reach a peak brake thermal efficiency of 41%, enabled by the cycle's reduced heat rejection and optimized combustion, as verified through benchmarking studies.^[30] This efficiency edge is amplified in hybrid synergies, where regenerative braking and electric-only modes further minimize fuel consumption. As of 2025, Atkinson-cycle reciprocating engines are increasingly integrated with 48V mild-hybrid systems in European models to meet stringent CO2 emissions regulations, such as the Euro 7 standards, by combining LIVC-based efficiency gains with low-voltage electrification for improved start-stop functionality and torque fill.^[31]^[32] This approach allows automakers like Toyota and Honda to extend Atkinson benefits to a broader range of vehicles, balancing compliance with performance in downsized powertrains.^[33]

Rotary and Other Variants

The LiquidPiston X-Engine represents a notable rotary implementation of Atkinson cycle principles, utilizing an eccentric rotor design inspired by but distinct from the Wankel engine to achieve over-expansion for improved efficiency. This patented hybrid thermodynamic cycle (HEHC) integrates elements of the Otto, Diesel, and Atkinson cycles, enabling a high expansion ratio while maintaining a lower compression ratio to reduce pumping losses. Developed in the 2010s, the X-Engine architecture features a high-speed rotor that creates variable chamber volumes, allowing for efficient combustion and exhaust in a compact form factor.^[34] The design targets brake thermal efficiencies up to 45% in compression-ignition variants, with the spark-ignition version incorporating Atkinson-like dwell near top-dead-center to optimize fuel economy. For instance, the X-Mini prototype delivers 3.5 horsepower at 10,000 RPM while weighing only 4 pounds, demonstrating 20-50% fuel consumption reductions compared to conventional engines of similar output.^[35]^[36] Free-piston linear generators (FPLGs) adapt Atkinson cycle principles by leveraging the piston's unconstrained motion to achieve variable compression and expansion ratios, suitable for range-extender applications in hybrid systems. In these designs, the absence of a crankshaft allows the expansion stroke to exceed the compression stroke, mimicking the over-expansion characteristic of the Atkinson cycle to enhance thermal efficiency without mechanical linkages. A 2022 study on a free-piston engine prototype using gasoline demonstrated that an Atkinson-based configuration achieved higher thermal efficiency and power output than equivalent diesel cycles, with peak efficiencies exceeding those of standard Otto operations due to reduced heat losses.^[37] These systems convert linear piston motion directly into electrical energy via integrated linear alternators, offering simplicity and scalability for compact power generation.^[38] Opposed-piston architectures have also incorporated Atkinson cycle modifications, particularly in two-stroke diesel engines, to balance efficiency gains with emissions control. By adjusting port timings and piston phasing, these designs enable an effective expansion ratio greater than the compression ratio, aligning with Atkinson over-expansion for improved fuel utilization. A 2019 analysis of a two-stroke opposed-piston diesel engine showed that implementing Atkinson-like parameters increased indicated thermal efficiency by up to 5% over baseline cycles, primarily through extended expansion durations that recovered more exhaust energy.^[39] Achates Power's ongoing opposed-piston developments, while primarily diesel-focused, explore such variants for heavy-duty applications, emphasizing reduced NOx without aftertreatment.^[40] Rotary and free-piston Atkinson variants face sealing challenges, particularly in high-pressure environments where apex or piston ring leaks can degrade efficiency, though innovations like LiquidPiston's coated surfaces mitigate this compared to traditional Wankels.^[41] Despite these hurdles, their compactness and high power density provide advantages for specialized uses, such as unmanned aerial vehicles (UAVs) and autonomous underwater vehicles (AUVs), where the X-Engine's lightweight design supports extended mission durations under fuel constraints.^[42]

Applications in Vehicles

Notable Vehicle Models

The Toyota Prius, introduced in 1997 as the world's first mass-produced hybrid vehicle, has utilized Atkinson-cycle engines across all generations, with the 2025 model featuring a 2.0-liter DOHC 16-valve Atkinson-cycle inline-four engine producing 150 horsepower and 139 lb-ft of torque, paired with two electric motors for a combined output of 194 horsepower in front-wheel-drive configuration.^[43] This design emphasizes efficiency, contributing to the Prius's role as a pioneer in hybrid technology. The Toyota Camry Hybrid, starting with the 2018 model year and updated for 2025, employs a 2.5-liter Atkinson-cycle inline-four engine delivering 184 horsepower and 163 lb-ft of torque from the gas engine alone, integrated with electric motors for a total system output of 225 horsepower (FWD) or 232 horsepower (AWD).^[44] Other popular Toyota models, such as the 2025 RAV4 Hybrid and Corolla Hybrid, also incorporate Atkinson-cycle engines for enhanced efficiency in SUV and compact car segments.^[45]^[46] Ford's Escape Hybrid, available since the 2020 model year and continuing into 2025, incorporates a 2.5-liter iVCT Atkinson-cycle inline-four engine combined with electric motors to achieve 192 horsepower and 155 lb-ft of torque overall, enabling up to 42 mpg city and 36 mpg highway in EPA ratings.^[47] The Honda Insight, relaunched in 2018 and discontinued after 2022, used a 1.5-liter DOHC i-VTEC Atkinson-cycle four-cylinder engine rated at 107 horsepower and 99 lb-ft of torque, supplemented by an electric motor for a combined 151 horsepower, focusing on refined hybrid performance in a compact sedan.^[28] The 2025 Honda Civic Hybrid continues this legacy with a 2.0-liter Atkinson-cycle engine producing 200 horsepower combined.^[48] Hyundai's Ioniq Hybrid, produced from 2017 to 2022, featured a 1.6-liter GDI Atkinson-cycle inline-four engine producing 104 horsepower and 109 lb-ft of torque, paired with an electric motor for 139 horsepower total, achieving up to 59 mpg combined.^[49] By 2025, Toyota had sold over 15 million hybrid vehicles worldwide, many incorporating Atkinson-cycle engines, underscoring the cycle's widespread adoption in electrified powertrains.^[50]

Performance and Efficiency Impacts

The Atkinson cycle significantly enhances fuel economy in hybrid vehicles, contributing 20-30% to the overall efficiency gains compared to conventional Otto cycle engines, primarily through reduced pumping losses and higher expansion ratios.^[51] For instance, the 2025 Toyota Prius, utilizing an Atkinson cycle engine, achieves an EPA-estimated 57 mpg combined, enabling real-world operation with substantially lower fuel consumption under varied driving conditions.^[43] This efficiency translates to reduced carbon dioxide (CO2) emissions, with hybrid systems incorporating the Atkinson cycle demonstrating approximately 25% lower CO2 output relative to equivalent Otto-based non-hybrids, as fuel savings directly correlate with greenhouse gas reductions.^[52] While the Atkinson cycle prioritizes efficiency over power density, it introduces performance trade-offs such as lower engine torque, which can result in slower acceleration without electric assistance; however, hybrid electric motors effectively compensate, delivering 0-60 mph times of 7-10 seconds in models like the Prius, versus 6-8 seconds for comparable conventional vehicles in the compact segment.^[53]^[54] The leaner operation and lower peak combustion temperatures inherent to the cycle further reduce nitrogen oxide (NOx) emissions by up to 40% compared to Otto cycles, supporting cleaner exhaust profiles.^[55] In 2025, EPA data confirms that Atkinson-equipped hybrids readily comply with updated multi-pollutant standards, including those aligned with Euro 7 requirements for criteria pollutants and greenhouse gases, ensuring minimal environmental impact while maintaining high efficiency.^[56] Broader adoption of the Atkinson cycle in hybrids has propelled their global market share to 21% of new vehicle sales in 2025, positioning them as key range extenders in the transition to full electrification by bridging efficiency gaps in mixed-use scenarios.^[57]

References

[1]
Atkinson, Miller, or Otto: Internal Combustion Cycles Explained
Mar 23, 2023 · The Otto cycle uses compression and ignition. Atkinson uses variable stroke lengths. Miller uses forced induction and longer intake strokes.
[2]
Atkinson Cycle - Atkinson Engine | Definition | nuclear-power.com
Atkinson Cycle - Atkinson Engine. The Atkinson cycle is a thermodynamic cycle designed to provide higher efficiency at the expense of power density.
[3]
US367496A - atkinson - Google Patents
A compression gas-engine to be performed by one piston in a single-acting 7 cylinder, and gives a working-stroke for each revolution.
[4]
The Atkinson Engine. - Douglas Self
James Atkinson called this the "Cycle" engine to distinguish it from some ... Left: The Atkinson Cycle engine patent: 1887. The connecting rod E drives ...
[5]
What Is the Atkinson Combustion Cycle, and What Are Its Benefits?
Aug 4, 2016 · The Atkinson cycle delays the intake valve's closing until the piston has completed 20 to 30 percent of its upward travel on the compression stroke.
[6]
Why does Toyota use Atkinson cycle engines?
In 1882, a British engineer named James Atkinson developed and patented a modified four-stroke cycle that used a variable length piston stroke and delayed ...
[7]
James Atkinson - Graces Guide
Mar 5, 2025 · 1886 In the Atkinson Cycle engine, by the use of a two-bar linkage between the connecting rod and the crank, four unequal strokes were obtained ...
[8]
James Atkinson | Science Museum Group Collection
James Atkinson, 1846 - 1914, apprenticed to Palmer Brothers, of Jarrow-on-Tyne, employed by the London and North Western Railway in the Railway Works at Crewe.Missing: 1882 | Show results with:1882
[9]
US Patent: 306,712 - Gas Engine - DATAMP
US patent 306712 Gas Engine was issued to James Atkinson of London, Middlesex County, England. It was granted on 10/21/1884. Patented in England, 22 Sep ...
[10]
British Gas Engine & Engineering Co., Ltd. - Assigned Patents ...
If you find a patent number or patent date by ... Patented in England, 22 Sep 1881, #4,086 & 14 Sep 1882, #4,378. ... Cyclic Gas Engine, James Atkinson, , England ...Missing: cycle | Show results with:cycle
[11]
Atkinson engine - Model Engineering Website
The Atkinson-cycle engine is a type of internal combustion engine invented by James Atkinson in 1882. He produced three different designs.
[12]
Atkinson - Animated Engines
The Atkinson engine is essentially an Otto four stroke engine with a different means of linking the piston to the crankshaft.
[13]
[PDF] Effect of Volume Ratio of Heat Rejection Process on Performance of ...
Process 1 →2 is an isentropic compression. The heat addition occurs in the constant volume process 2 → 3. The process 3 → 4 is an isen- tropic expansion process ...<|control11|><|separator|>
[14]
theoretical analysis of determining the thermal efficiency in engine ...
Jul 11, 2019 · 4. The thermal efficiency in an engine with the Atkinson cycle ; V = const. can be expressed by differing degrees of compression: εs and de ...
[15]
[PDF] Efficiency of Atkinson Engine at Maximum Power Density using ...
Thermodynamic analysis of an ideal air-standard Atkinson cycle with temperature dependant specific heat is presented in this.
[16]
None
### Efficiency Formulas and Numerical Examples
[17]
The Atkinson cycle and improving the ICE's efficiency - Claytex
Oct 22, 2019 · The Atkinson cycle, a modified Otto cycle, has a greater expansion stroke, using more fuel energy and reducing temperature/speed, improving ...Missing: explanation | Show results with:explanation
[18]
(PDF) Comparative analysis of the Atkinson and the Otto cycles with ...
In this article, the performance of air standard Atkinson and Otto cycles with irreversibility factors, i.e. efficiencies of compression and expansion ...
[19]
The Atkinson Cycle Engine - The Old Machinery Magazine
Aug 13, 2024 · One of the leading UK pioneers of the internal combustion engine was James Atkinson who, as early as 1879, had been granted his first non-steam ...
[20]
US336505A - atkinson - Google Patents
This invention relates to and consists in a novel arrangement of parts forming a gas-engine of the compression type. As hitherto constructed compression ...
[21]
Original Atkinson patent 1885 differential gas engine 1 hp
Original Atkinson patent 1885 differential gas engine 1 hp ; maker: British Gas Engine Company Limited ; Related people. James Atkinson (patentee) ; Object Number: ...Missing: cycle | Show results with:cycle
[22]
Benchmarking a 2018 Toyota Camry 2.5-liter Atkinson Cycle Engine ...
Atkinson Cycle is implemented using late intake valve closing (LIVC). Effective compression ratio is varied by varying intake camshaft phasing. The A25A-FKS ...
[23]
Hybrid electric vehicle specific engines: State-of-the-art review
In the 2018 Camry Hybrid, Toyota's A25A-FXS engine, which is a naturally aspirated Atkinson-cycle engine with a compression ratio of 14:1, is used and equipped ...
[24]
(PDF) Benchmarking a 2018 Toyota Camry 2.5-Liter Atkinson Cycle ...
Aug 10, 2025 · A 2018 Toyota Camry A25A-FKS 4-cylinder, 2.5-liter, naturally aspirated, Atkinson Cycle engine with cooled exhaust gas recirculation (cEGR) was benchmarked.
[25]
[PDF] Benchmarking a 2018 Toyota Camry 2.5-liter Atkinson Cycle Engine ...
Compression Ratio in the A25A-FKS engine, on Tier 2 fuel, 1 mm reference lift, (initial interval). Figure 19. Effective. Expansion Ratio in the. A25A-FKS engine ...Missing: FXS LIVC
[26]
Toyota Develops New Series of Gas Engines That Will Boost Fuel ...
top-level among mass-produced engines.
[27]
https://www.hondainfocenter.com/2025/Civic-Sedan/Feature-Guide/Engine-Chassis-Features/Atkinson-Cycle-2-Liter-4-Cylinder-Engine/
[28]
Honda Two-Motor Hybrid-Electric Powertrain
Sep 18, 2024 · With the addition of direct multistage fuel injection, the new Atkinson-cycle DOHC i-VTEC® engine increases power to 145 hp (SAE net at 6,100 ...Missing: Dreams | Show results with:Dreams
[29]
[PDF] Hybrid vehicle technology developments and opportunities in the ...
Feb 9, 2025 · Honda and. Toyota's most efficient current engines achieve 40% and 41% peak BTE, respectively, when applied in hybrid vehicles (American Honda ...<|control11|><|separator|>
[30]
Toyota Unveils More New Gasoline ICEs with 40% Thermal Efficiency
Apr 4, 2018 · New Toyota 2.0L gasoline four achieves 40% BTE in conventional version, 41% in Atkinson cycle version for hybrid models.
[31]
[PDF] 2025 Passenger car and light commercial vehicle powertrain ...
The. Simulation tools include in-house built models for the longitudinal simulations of powertrains and emissions. Summary/Results: The technology potentials ...
[32]
Hybrid powertrains with dedicated internal combustion engines are ...
In order to still reach the 1.5°C or at least 2°C target, a rapid and significant reduction in the CO2 contribution of all sectors must be achieved.
[33]
Honda's new hybrid engines merge efficiency with performance
Jan 6, 2025 · Honda has announced it is developing two new four-cylinder hybrid engines operating on the Atkinson cycle. These engines will have a new front-drive unit and ...
[34]
How Our HEHC Cycle Powers Our Rotary Engines - LiquidPiston
The X Mini utilizes a spark-ignition (SI-HEHC) version of the cycle with a lower compression ratio standard for gasoline engines. A dwell near top-dead ...
[35]
Small engine packs a punch | Massachusetts Institute of Technology
Dec 5, 2014 · LiquidPiston's 70-cubic-centimeter engine, the X Mini, produces about 3.5 horsepower at 10,000 RPM; at 4 pounds, it's also about 30 percent ...
[36]
Benefits of Our Rotary Engine - LiquidPiston
HIGH-EFFICIENCY · 20% decrease in fuel consumption possible for SI gasoline engines · 50% decrease in fuel consumption possible for CI diesel engines ...
[37]
[PDF] An investigation of the performance of a free-piston engine using ...
Feb 9, 2022 · The thermal efficiency and power output of the Atkinson cycle are higher than those of the diesel cycle system. ... from gasoline free-piston ...
[38]
Thermodynamic Simulation and Prototype Testing of a Four-Stroke ...
As a promising future technology for a hybrid electric vehicle (HEV), the free-piston ... The Atkinson cycle is characterized by a low compression ratio ...
[39]
Study of Atkinson cycle in two-stroke diesel engine with opposed ...
Jun 20, 2019 · PDF | The paper presents possibilities of change working parameters of two-stroke diesel engine with opposed pistons.
[40]
Here's Why We Don't Have Opposed-Piston Engines—at Least Not Yet
Apr 28, 2021 · Achates Power was founded in 2004 and began developing the opposed-piston engine. Achates Power is not a manufacturing company; that would ...
[41]
How Our Rotary Engine Works | LiquidPiston
While it is a rotary engine, LiquidPiston's X Engine is NOT a Wankel engine. It has a fundamentally different thermodynamic cycle, architecture and operation.Missing: Atkinson | Show results with:Atkinson
[42]
LiquidPiston's "inside-out" rotary X-Engine wins Army research ...
Dec 15, 2020 · ... Atkinson cycle engine, while solving the lubrication and sealing ... 3 hp.
[43]
2025 Toyota Prius Review, Pricing, and Specs - Car and Driver
Rating 5.0 DOHC 16-valve 2.0-liter Atkinson-cycle inline-4, 150 hp, 139 lb-ft + 2 AC motors (combined output: 194 hp; 0.5-kWh lithium-ion battery pack (C/D est))More Features and Specs · 2024 Toyota Prius Prime · 2019 Toyota Prius AWD-e
[44]
Tested: 2018 Toyota Camry XLE Hybrid - Car and Driver
Jul 18, 2017 · ENGINE TYPE DOHC 16-valve Atkinson-capable 2.5-liter inline-4, 176 hp, 163 lb-ft; 1 permanent-magnet synchronous AC motor, 118 hp, 149 lb-ft ...
[45]
First Drive: 2020 Ford Escape Hybrid - MotorTrend
Sep 19, 2019 · The 2020 Ford Escape Hybrid makes use of an Atkinson-cycle 2.5L I-4 paired to a 1.1-kWh lithium-ion battery pack and two electric motors that function as a ...
[46]
2022 Hyundai Ioniq Review, Pricing, and Specs - Car and Driver
Rating 4.0 · Review by Eric StaffordDOHC 16-valve Atkinson-cycle 1.6-liter inline-4, 104 hp, 109 lb-ft ... View 2023 Ioniq 6 Details. Starting at $42,715 · 10/10. Chevron Down Icon.
[47]
Toyota Hits 15 Million Hybrids Sold, Expands 2025 Lineup as EV ...
Sep 18, 2025 · Toyota has sold 15M+ hybrids and now offers 15+ models in 2025, with hybrids making up 40% of sales as it prepares 30 EVs by 2030.
[48]
Research on performance optimization and fuel-saving mechanism ...
Apr 1, 2020 · A performance optimization strategy for the Atkinson cycle engine is proposed through analysis of compression ratio range according to cycle efficiency.
[49]
What's Up With Hybrids Using The Atkinson Cycle Engine? - CarBuzz
Feb 2, 2025 · What Is An Atkinson Cycle Engine, And Why Do Hybrids Use Them?
[50]
Hybrid Car 0-60 | Find 0 to 60 & Quarter Mile Times for HEV, MHEV ...
This section contains numerous 0-60 times for hybrid cars. These hybrid vehicles are powered by both a conventional internal combustion engine and an electric ...
[51]
Atkinson Cycle Engine: How They Work and Why They're Efficient
Nov 28, 2024 · Reduced Emissions: The Atkinson cycle can reduce certain emissions, such as nitrogen oxides (NOx), due to the lower peak combustion temperatures ...
[52]
Multi-Pollutant Emissions Standards for Model Years 2027 and Later ...
Apr 18, 2024 · The program establishes new, more stringent vehicle emissions standards for criteria pollutant and greenhouse gas (GHG) emissions from motor ...
[53]
Charted: Global EV Adoption (2019 vs. 2025) - Visual Capitalist
Aug 20, 2025 · In 2025, hybrids make up 21% of sales globally, up from just 6% in 2019. For many consumers, hybrids offer a practical middle ground—familiar ...Hybrids As A Transition... · Data & Discussion · Tesla Holds Nearly Half The...<|control11|><|separator|>