An Otto cycle is an idealized thermodynamic cycle which describes the functioning of a typical spark ignition reciprocating piston engine,^{} the thermodynamic cycle most commonly found in automobile engines.^{}
PressureVolume diagram TemperatureEntropy diagram
 TOP and BOTTOM of the loop: a pair of quasiparallel adiabatic processes
 LEFT and RIGHT sides of the loop: a pair of parallel isochoric processes
The adiabatic processes are impermeable to heat: heat flows into the
loop through the left pressurizing process and some of it flows back out
through the right depressurizing process, and the heat which remains
does the work.
The processes are described by:^{[2]}
 Process 12 is an isentropic compression of the air as the piston moves from bottom dead centre (BDC) to top dead centre (TDC).
 Process 23 is a constantvolume heat transfer to the air from an external source while the piston is at top dead centre. This process is intended to represent the ignition of the fuelair mixture and the subsequent rapid burning.
 Process 34 is an isentropic expansion (power stroke).
 Process 41 completes the cycle by a constantvolume process in which heat is rejected from the air while the piston is a bottom dead centre.
The Otto cycle consists of adiabatic
compression, heat addition at constant volume, adiabatic expansion, and
rejection of heat at constant volume. In the case of a fourstroke Otto
cycle, technically there are two additional processes: one for the
exhaust of waste heat and combustion products (by isobaric
compression), and one for the intake of cool oxygenrich air (by
isobaric expansion); however, these are often omitted in a simplified
analysis. Even though these two processes are critical to the
functioning of a real engine, wherein the details of heat transfer and
combustion chemistry are relevant, for the simplified analysis of the
thermodynamic cycle, it is simpler and more convenient to assume that
all of the wasteheat is removed during a single volume change.
A PV animation of the Otto cycle is very useful in the analysis of the entire process.^{[3]}
History
The fourstroke engine was first patented by Alphonse Beau de Rochas in 1861. Before, in about 1854–57, two Italians (Eugenio Barsanti and Felice Matteucci) invented an engine that was rumored to be very similar, but the patent was lost.
"The request bears the no. 700 of Volume VII of the Patent Office of
the Reign of Piedmont. We do not have the text of the patent request,
only a photo of the table which contains a drawing of the engine. We do
not even know if it was a new patent or an extension of the patent
granted three days earlier, on December 30, 1857, at Turin."
f. Eugenio Barsanti and Felice Matteucci, June 4, 1853 ^{[4]}
The first person to build a working four stroke engine, a stationary engine using a coal gasair mixture for fuel (a gas engine), was German engineer Nicolaus Otto.^{[5]} This is why the fourstroke principle today is commonly known as the Otto cycle and fourstroke engines using spark plugs often are called Otto engines.
Processes
Process 12 (B on diagrams)
Piston moves from crank end (bottom dead centre) to cover end (top
dead centre) and an ideal gas with initial state 1 is compressed
isentropically to state point 2, through compression ratio .
Mechanically this is the adiabatic compression of the air/fuel mixture
in the cylinder, also known as the compression stroke. Generally the
compression ratio is around 910:1 (V1:V2) for a typical engine.^{[6]}
Process 23 (C on diagrams)
The piston is momentarily at rest at TDC and heat is added to the
working fluid at constant volume from an external heat source which is
brought into contact with the cylinder head. The pressure rises and the
ratio
is called the "explosion ratio". At this instant the air/fuel mixture
is compressed at the top of the compression stroke with the volume
essentially held constant, also known as ignition phase.
Process 34 (D on diagrams)
The increased high pressure exerts a greater amount of force on the
piston and pushes it towards the BDC. Expansion of working fluid takes
place isentropically and work is done by the system. The volume ratio
is called "isentropic expansion ratio". Mechanically this is the
adiabatic expansion of the hot gaseous mixture in the cylinder head,
also known as expansion (power) stroke.
Process 41 (A on diagrams)
The piston is momentarily at rest at BDC and heat is rejected to the
external sink by bringing it in contact with the cylinder head. The
process is so controlled that ultimately the working fluid comes to its
initial state 1 and the cycle is completed.
Exhaust and intake strokes
Exhaust strokeejection of the gaseous mixture via an exhaust valve
through the cylinder head. Induction strokeintake of the next air
charge into the cylinder. The volume of the exhaust gasses is the same
as the air charge.^{[6]}
Cycle Analysis
Processes 12 and 34 do work on the system but no heat transfer
occurs during adiabatic expansion and compression. Processes 23 and 41
are isochoric; therefore, heat transfer occurs but no work is done. No
work is done during a isochoric (constant volume) because work requires
movement; when the piston volume does not change no shaft work is
produced by the system. Four different equations can be derived by
neglecting kinetic and potential energy and considering the first law of
thermodynamics (energy conservation). Assuming these conditions the first law is rewritten as:^{[2]}
Applying this to the Otto cycle the four process equations can be derived:
Since the first law is expressed as heat added to the system and work expelled from the system then () and ()
will always produce positive values. However, since work always
involves movement, processes 23 and 41 will be omitted because they
occur at a constant volume. The net work can be expressed as:
The net work can also be found by evaluating the heat added minus the heat leaving or expelled.
Thermal efficiency
is the quotient of the net work to the heat addition into system. Upon
rearrangement the thermal efficiency can be obtained (Net Work/Heat
added):
Equation 1:
Alternatively, thermal efficiency can be derived by strictly heat added and heat rejected.
In the Otto cycle, there is no heat transfer during the process 12
and 34 as they are reversible adiabatic processes. Heat is supplied
only during the constant volume processes 23 and heat is rejected only
during the constant volume processes 41.^{[7]}
Equation 1 can now be related to the specific heat equation for constant volume. The specific heats are particularly useful for thermodynamic calculations involving the ideal gas model.
Rearranging yields:
Inserting the specific heat equation into the thermal efficiency equation (Equation 1) yields.
Upon rearrangement:
Next, noting from the diagrams , thus both of these can be omitted. The equation then reduces to:
Equation 2:
Since the Otto cycle is an isentropic process the isentropic equations of ideal gases and the constant pressure/volume relations can be used to yield Equations 3 & 4.
Equation 3:
Equation 4:



 The derivation of the previous equations are found by solving these four equations respectively (where is the gas constant):


Further simplifying Equation 4, where is the compression ratio :
Equation 5:
Also, note that
where is the specific heat ratio
From inverting Equation 4 and inserting it into Equation 2 the final thermal efficiency can be expressed as:^{[7]}
Equation 6:
From analyzing equation 6 it is evident that the Otto cycle depends directly upon the compression ratio . Since the for air is 1.4, an increase in will produce an increase in . However, the
for the combustion products of the fuel/air mixture is taken at
approximately 1.3. The foregoing discussion implies that it is more
efficient to have a high compression ratio. The standard ratio is
approximately 10:1 for typical automobiles. Usually this does not
increase much because of the possibility of autoignition, or "knock", which places an upper limit on the compression ratio.^{[2]}
During the compression process 12 the temperature rises, therefore an
increase in the compression ratio causes an increase in temperature.
Autoignition occurs when the temperature of the fuel/air mixture becomes
too high before it is ignited by the flame front. The compression
stroke is intended to compress the products before the flame ignites the
mixture. Therefore if the compression ratio was increased, the mixture
could be compressed before ignition leading to "engine knocking". This
can damage engine components and will decrease the original horsepower
of the engine.
References
 ^ Wu, Chih. Thermodynamic Cycles: Computeraided Design and Optimization. New York: M. Dekker, 2004. Print.
 ^ ^{a} ^{b} ^{c} Moran, Michael J., and Howard N. Shapiro. Fundamentals of Engineering Thermodynamics. 6th ed. Hoboken, N.J. : Chichester: Wiley ; John Wiley, 2008. Print.
 ^ "Animated Diagram". Leipzig. 2006. Retrieved 20100922.
 ^ "Documenti Storici". Barsantiematteucci.it. Retrieved 20100922.
 ^ Gunston, Bill (1999). Development of Piston Aero Engines (2 ed.). Sparkford, UK: Patrick Stephens Ltd. p. 21. ISBN 0750944781.
 ^ ^{a} ^{b} "Heat Cycles  Electropeaedia". Woodbank Communications Ltd. Retrieved 20110411.
 ^ ^{a} ^{b} Gupta, H. N. Fundamentals of Internal Combustion. New Delhi: PrenticeHall, 2006. Print.
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