Atkinson cycle
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Thermodynamics
Branches
Classical · Statistical · Chemical
Equilibrium / Non-equilibrium
Thermofluids
Laws
Zeroth · First · Second · Third
Systems
State:
Equation of state
Ideal gas · Real gas
Phase of matter · Equilibrium
Control volume · Instruments
Processes:
Isobaric · Isochoric · Isothermal
Adiabatic · Isentropic · Isenthalpic
Quasistatic · Polytropic
Free expansion
Reversibility · Irreversibility
Endoreversibility
Cycles:
Heat engines · Heat pumps
Thermal efficiency
System properties
Property diagrams
Intensive and extensive properties
State functions:
Temperature / Entropy (intro.) †
Pressure / Volume †
Chemical potential / Particle no. †
(† Conjugate variables)
Vapor quality
Reduced properties
Process functions:
Work · Heat
Material properties
Specific heat
c=
capacity
Compressibility
T
N
β= 1
−
V
Thermal expansion α =
1
V
Property database
Equations
Carnot's theorem
Clausius theorem
Fundamental relation
Ideal gas law
Maxwell relations
Table of thermodynamic
equations
Potentials
Free energy · Free entropy
Internal energy
U(S,V)
H(S,p)
Enthalpy
= U +pV
Helmholtz free
A(T,V)
energy
= U −TS
G(T,p)
Gibbs free energy
= H −TS
History and culture
Philosophy:
Entropy and time · Entropy and
life
Brownian ratchet
Maxwell's demon
Heat death paradox
Loschmidt's paradox
Synergetics
History:
General · Heat · Entropy · Gas
laws
Perpetual motion
Theories:
Caloric theory · Vis viva
Theory of heat
Mechanical equivalent of heat
Motive power
Publications:
"An Experimental Enquiry
Concerning ... Heat"
"On the Equilibrium of
Heterogeneous Substances"
"Reflections on the
Motive Power of Fire"
Timelines of:
Thermodynamics · Heat engines
Art:
Maxwell's thermodynamic surface
Education:
Entropy as energy dispersal
Scientists
Daniel Bernoulli
Sadi Carnot
Benoît Paul Émile Clapeyron
Rudolf Clausius
Hermann von Helmholtz
Constantin Carathéodory
James Prescott Joule
Julius Robert von Mayer
William Rankine
John Smeaton
Georg Ernst Stahl
Benjamin Thompson
William Thomson, 1st Baron
Kelvin
John James Waterston
v·d·e
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, and is used in some modern hybrid electric applications.
Contents








1 Design
2 Ideal thermodynamic cycle
3 Modern Atkinson cycle engines
4 Rotary Atkinson cycle engine
5 Vehicles using Atkinson cycle engines
6 See also
7 References
8 External links
[edit] Design
The original Atkinson cycle piston engine allowed the intake, compression, power,
and exhaust strokes of the four-stroke cycle to occur in a single turn of
the crankshaft and was designed to avoid infringing certain patents covering Otto
cycle engines.[1] Because of the unique crankshaft design of the Atkinson, its
expansion ratio can differ from its compression ratio and, with a power stroke longer
than its compression stroke, the engine can achieve greater thermal efficiency than a
traditional piston engine. While Atkinson's original design is no more than an
historical curiosity, many modern engines use unconventional valve timing to produce
the effect of a shorter compression stroke/longer power stroke, thus realizing the fuel
economy improvements the Atkinson cycle can provide.[2]
[edit] Ideal thermodynamic cycle
Figure 1: Atkinson Gas Cycle
The ideal Atkinson cycle consists of following operations:






1-2 Isentropic or reversible adiabatic compression
2-3 Isochoric heating (Qp)
3-4 Isobaric heating (Qp')
4-5 Isentropic expansion
5-6 Isochoric cooling (Qo)
6-1 Isobaric cooling (Qo')
[edit] Modern Atkinson cycle engines
A small engine with Atkinson-style linkages between the piston and flywheel. Modern
Atkinson cycle engines do away with this complex energy path.
Recently Atkinson cycle has been used to describe a modified Otto cycle engine in
which the intake valve is held open longer than normal to allow a reverse flow of
intake air into the intake manifold. The effective compression ratio is reduced (for a
time the air is escaping the cylinder freely rather than being compressed) but the
expansion ratio is unchanged. This means the compression ratio is smaller than the
expansion ratio. Heat gained from burning fuel increases the pressure, thereby forcing
the piston to move, expanding the air volume beyond the volume when compression
began. The goal of the modern Atkinson cycle is to allow the pressure in the
combustion chamber at the end of the power stroke to be equal to atmospheric
pressure; when this occurs, all the available energy has been obtained from the
combustion process. For any given portion of air, the greater expansion ratio allows
more energy to be converted from heat to useful mechanical energy meaning the
engine is more efficient.
The disadvantage of the four-stroke Atkinson cycle engine versus the more common
Otto cycle engine is reduced power density. Because a smaller portion of the
compression stroke is devoted to compressing the intake air, an Atkinson cycle engine
does not take in as much air as would a similarly designed and sized Otto cycle
engine.
Four-stroke engines of this type with this same type of intake valve motion but with
a supercharger to make up for the loss of power density are known as Miller
cycle engines.
[edit] Rotary Atkinson cycle engine
Rotary Atkinson cycle engine
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. Exhaust gases are
expelled from the engine by compressed-air scavenging. This modification of the
Atkinson cycle allows for the use of alternative fuels like diesel and hydrogen.
Disadvantages of this design include the requirement that rotor tips seal very tightly
on the outer housing wall and the mechanical losses suffered through friction between
rapidly oscillating parts of irregular shape. See External Links for more information.
[edit] Vehicles using Atkinson cycle engines
2004 Toyota Prius hybrid
2009 Ford Fusion Hybrid, as sold in the United States. Unrelated to the European car
of the same name.
While a modified Otto cycle engine using the Atkinson cycle provides good fuel
economy, it is at the expense of a lower power-per-displacement as compared to a
traditional four-stroke engine.[3] If demand for more power is intermittent, the power
of the engine can be supplemented by an electric motor during times when more
power is needed. This forms the basis of an Atkinson cycle-based hybrid
electric drivetrain. These electric motors can be used independently of, or in
combination with, the Atkinson cycle engine, to provide the most efficient means of
producing the desired power.
Several production vehicles use Atkinson cycle engines:









Chevrolet Tahoe Hybrid hybrid electric (four-wheel drive) with a compression
ratio of 10.8:1
Ford Escape/Mercury Mariner/Mazda Tribute hybrid electric (front- and fourwheel drive) with a compression ratio of 12.4:1
Ford Fusion Hybrid/Mercury Milan Hybrid/Lincoln MKZ Hybrid hybrid
electric (front-wheel drive) with a compression ratio of 12.3:1
Hyundai Sonata Hybrid (front-wheel drive)
Infiniti M35h Hybrid (rear-wheel drive)
Kia Optima Hybrid (front-wheel drive)
Lexus CT200H (front-wheel drive)
Lexus HS250h (front-wheel drive)
Lexus RX 450h hybrid electric (front-wheel drive)




Mercedes ML450 Hybrid (four-wheel drive) hybrid electric
Mercedes S400 Blue Hybrid (rear-wheel drive) hybrid electric
Toyota Prius hybrid electric (front-wheel drive) with a (purely geometric)
compression ratio of 13.0:1
Toyota Camry Hybrid hybrid electric (front-wheel drive) with a compression
ratio of 12.5:1
Note that the compression ratios shown above reflect the expansion ratio, which is the
ratio of the combustion chamber volumes when the piston is at bottom dead
centre and top dead centre. The effective compression ratio of the air-fuel mixture in
an Atkinson cycle engine, with respect to atmospheric pressure, is lower due to the
aforementioned delay in closing the intake valve
[edit] See also

History of the internal combustion engine
[edit] References
1. ^ U.S. Patent 367,496
2. ^ http://www.canadiandriver.com/2010/07/14/auto-tech-atkinson-cycleengines-and-hybrids.htm
3. ^ Heywood, John B. Internal Combustion Engine Fundamentals, pp. 184-186.