Sylvie Willis
The Atkinson cycle engine is a type of internal combustion engine invented by James Atkinson in 1882. Atkinson's design aimed to improve the efficiency of internal combustion engines by modifying the engine's operation to achieve a more efficient combustion process. The Atkinson cycle engine is often used in hybrid vehicles, where its improved fuel efficiency and reduced emissions are beneficial. Studies have shown that the Atkinson cycle can achieve fuel efficiency improvements of up to 20% compared to traditional Otto cycle engines. The Atkinson cycle engine operates on four stages: intake, compression, power, and exhaust. The key difference between the Atkinson cycle and the Otto cycle is the use of a longer expansion stroke, which allows for more efficient energy extraction from the combustion process. The Atkinson cycle engine is more fuel-efficient than a standard gasoline motor, and its development is likely to continue to play an increasingly important role as the demand for more efficient and environmentally friendly vehicles grows.
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What You'll Learn
The Atkinson cycle's longer expansion stroke
The Atkinson cycle engine, invented by James Atkinson in 1882, is a type of internal combustion engine designed to provide efficiency at the expense of power density. Atkinson's engine was not successful, but its thermodynamic cycle is widely used, mainly in gas-electric hybrids. The Atkinson cycle is ideal for hybrids because their electric motors compensate for the lost low-speed output.
The key advantage of the Atkinson cycle is its higher efficiency compared to the Otto cycle. This is because its expansion ratio is significantly larger than its compression ratio. 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. As a result, some of the fresh charge is driven back into the intake manifold by the rising piston, so the cylinder is never completely full. This results in a low-speed power reduction. However, the payoff comes after ignition when the piston begins descending on the expansion (or power) stroke. The shortened intake stroke combined with a full-length expansion stroke squeezes more work out of every increment of fuel.
The Atkinson cycle engine fine-tunes the combustion process to minimize the parasitic power loss that most normal engines experience. Because the combustion chamber isn't full of air, there is a low engine speed power reduction. This alleviates the load on the piston in the compression stroke by giving it more room to breathe, making the Atkinson cycle motor more efficient. As the combustion chamber has less air, the injector doesn't need to supplement as much fuel.
The Atkinson cycle can be used in a rotary engine, resulting in increased power and efficiency compared to the Otto cycle. This type of engine retains the one power phase per revolution, along with the different compression and expansion volumes of the original Atkinson cycle. This modification of the Atkinson cycle allows the use of alternative fuels such as diesel and hydrogen.
The Atkinson cycle has three different designs: the differential engine, the cycle engine, and the Utilite engine. The first Atkinson-cycle engine, the differential engine, used opposed pistons. The second and best-known design, the cycle engine, used an over-centre arm to create four piston strokes in one crankshaft revolution. This unusual reciprocating engine had the intake, compression, power, and exhaust strokes of the four-stroke cycle in a single turn of the crankshaft. The Utilite engine was even more efficient than Atkinson's previous designs. With this new design, Atkinson eliminated the linkages and made a more conventional, well-balanced engine capable of operating at speeds up to 600 rpm.
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Lower compression ratio
The Atkinson cycle engine is a type of internal combustion engine designed to provide efficiency at the expense of power density. Invented by James Atkinson in 1882, the Atkinson cycle is often used in hybrid vehicles, such as the Toyota Prius, due to its fuel efficiency. Atkinson cycle engines have a shorter compression stroke and a longer expansion stroke compared to traditional four-stroke engines. This design choice results in a lower power output but improves fuel efficiency by minimizing parasitic power loss.
The key advantage of the Atkinson cycle is its higher efficiency compared to the traditional Otto cycle engine. This efficiency gain is due to the Atkinson cycle's ability to achieve a higher expansion ratio than its compression ratio. By delaying the intake valve's closing during the compression stroke, the cylinder is never completely filled, leading to a reduction in low-speed power output. However, during the expansion stroke, the piston can extract more work from each increment of fuel, resulting in improved fuel efficiency.
While the Atkinson cycle engine provides good fuel efficiency, it does come with a trade-off in power-per-displacement when compared to traditional four-stroke engines. Atkinson cycle engines have a lower compression ratio, which reduces their energy efficiency. This lower compression ratio is a result of the delayed closing of the intake valve, allowing the piston to rise substantially above the bottom dead center before compression begins. The reduced compression ratio, combined with the shorter compression stroke, contributes to the lower power output of Atkinson cycle engines.
To compensate for the lower power, Atkinson cycle engines are often used in hybrid vehicles, where an electric motor can provide additional torque when needed. This combination of an Atkinson cycle engine with an electric motor forms a hybrid electric drivetrain, first introduced in the first-generation Toyota Prius. The electric motor supplements the engine's power during periods of higher demand, ensuring that the vehicle can deliver the required performance while still benefiting from the improved fuel efficiency of the Atkinson cycle.
In summary, the lower compression ratio in Atkinson cycle engines is a trade-off between power and efficiency. By sacrificing some power output, the Atkinson cycle achieves improved fuel efficiency through a higher expansion ratio and reduced parasitic power loss. This unique design makes the Atkinson cycle engine a valuable choice for hybrid vehicles, where the electric motor can compensate for the engine's lower power output, resulting in a more efficient and environmentally friendly propulsion system.
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Reduced power output
The Atkinson cycle engine is designed to be more efficient than a standard gasoline motor, but this comes at the expense of reduced power output.
The Atkinson cycle engine is a type of internal combustion engine invented by James Atkinson in 1882. Atkinson's engine was designed to improve on the Otto four-stroke combustion engine, which had been produced in 1876. Atkinson's engine had variable stroke lengths provided by a multilink connecting rod between the piston and the flywheel. This allowed for a shorter compression stroke and a longer expansion stroke, which is the key to the engine's efficiency.
The Atkinson cycle engine is more fuel-efficient because its expansion ratio is significantly larger than its compression ratio. This means that the pressure in the combustion chamber at the end of the power stroke is equal to atmospheric pressure, maximising the energy obtained from the combustion process. However, this also means that the engine does not take in as much air as a similarly designed and sized Otto-cycle engine, resulting in a lower power output.
The Atkinson cycle engine delays the intake valve's closing until the piston has completed 20 to 30 percent of its upward travel on the compression stroke. As a result, some of the fresh charge is driven back into the intake manifold by the rising piston, so the cylinder is never completely filled. This leads to a low-speed power reduction, as the combustion chamber is not "chock full of air". However, the payoff comes after ignition when the piston begins descending on the expansion (or power) stroke, squeezing more work out of every increment of fuel.
The Atkinson cycle engine is commonly used in hybrid vehicles, where the electric motor compensates for the lost low-speed output. This combination of the Atkinson cycle engine and an electric motor provides the most efficient means of producing the desired power.
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Improved fuel economy
The Atkinson cycle engine is a type of internal combustion engine invented by James Atkinson in 1882. The Atkinson cycle is designed to provide efficiency at the expense of power density. Atkinson produced three different designs that had a short compression stroke and a longer expansion stroke. This longer expansion stroke allows for more efficient energy extraction from the combustion process. The compression ratio is lower than in a traditional Otto cycle engine, which reduces the energy lost during compression. The expansion ratio is higher than the compression ratio, allowing for more efficient energy extraction. This is why the Atkinson cycle wins on efficiency.
The Atkinson cycle is ideal for hybrids because their electric motors make up for the lost low-speed output. 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. As a result, some of the fresh charges are driven back into the intake manifold by the rising piston so the cylinder is never completely full, hence the low-speed power reduction. The payoff comes after ignition when the piston begins descending on the expansion (also called power) stroke. Consistent with Atkinson’s original thinking, the shortened intake stroke combined with a full-length expansion stroke squeezes more work out of every increment of fuel.
The Atkinson cycle engine plays a key role in the development of hybrid electronic vehicles (HEVs) because of its greater fuel economy than the Otto cycle engine. The Atkinson cycle has the significant advantage of cycle efficiency, and the ratio of expansion ratio to effective compression ratio and pressure rise ratio have a great influence on efficiency. The increase of mechanical efficiency due to the reduction in pumping mean effective pressure (PMEP) is the main reason for the fuel economy improvement at low and medium load, while the increase of indicated thermal efficiency is the main reason at a high load. The Atkinson cycle can also achieve more obvious constant volume combustion, and the combustion quality has been greatly improved.
The Atkinson cycle can be used in a rotary engine. In this configuration, an increase in both power and efficiency can be achieved when compared to the Otto cycle. This type of engine retains the one power phase per revolution, together with the different compression and expansion volumes of the original Atkinson cycle. This modification of the Atkinson cycle allows the use of alternative fuels such as diesel and hydrogen.
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Reduced emissions
The Atkinson cycle engine is designed to be more fuel-efficient than a standard gasoline motor, such as the traditional Otto cycle engine. This is achieved by reducing the power density of the engine, which in turn reduces the amount of fuel needed. 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. As a result, some of the fresh charge is pushed back into the intake manifold by the rising piston, so the cylinder is never completely filled. This means that there is less air in the combustion chamber, and therefore less fuel needs to be added.
The Atkinson cycle engine is particularly useful in hybrid vehicles, where the electric motor can compensate for the loss of low-speed output caused by the reduced power density. This combination of the Atkinson cycle engine and an electric motor provides the most efficient means of producing the desired power.
The Atkinson cycle engine has been shown to reduce emissions. For example, in the 2018 Camry Hybrid, Toyota's A25A-FXS engine, a naturally aspirated Atkinson-cycle engine with a compression ratio of 14:1, achieved an 18.6% reduction in NOx emissions compared to the engine in the 2015 model without Atkinson and EGR features. Furthermore, Toyota's 2ZR-FXE engine, used in the third-generation Prius and Lexus CT200h, combines the Atkinson cycle with cooled EGR and port fuel injection (PFI), achieving a reduction of 3%–8% in NOx emissions.
In addition to these models, Mazda's SKYACTIV engine, which utilizes the Atkinson cycle, has also exhibited great fuel-saving potential. The Atkinson cycle has been shown to be the most efficient solution under a combined urban-highway driving cycle, achieving a 6% reduction in energy consumption and a 9% reduction in well-to-wheel emissions.
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Frequently asked questions
The Atkinson cycle engine is more fuel-efficient than a standard gasoline motor. Studies have shown that the Atkinson cycle can achieve fuel efficiency improvements of up to 20% compared to traditional Otto cycle engines.
The Atkinson cycle engine has a longer expansion stroke and a lower compression ratio than a traditional Otto cycle engine. This results in improved fuel efficiency and reduced emissions. The Atkinson cycle engine also has fewer pumping losses during the compression cycle, which improves fuel economy.
The Atkinson cycle engine is commonly used in hybrid vehicles, where its improved fuel efficiency and reduced emissions are beneficial. Atkinson cycle engines have been used in the Toyota Prius, Kia Seltos, and Hyundai Kona.
