THE OTTO CYCLE: FROM CHEMICAL TO MECHANICAL ENERGY

Session C7
Paper 3176
THE OTTO CYCLE: FROM CHEMICAL TO MECHANICAL ENERGY
Colin L. Detweiler (cld77@pitt.edu, Bursic 2 PM), Hal T. Hamilton Jr. (hth5@pitt.edu, Bursic 2 PM)
Abstract—The transfer of energy from the chemical
potential energy of covalent bonds to mechanical movement
has been utilized for centuries, whether that movement has
been to pump water out of a coal mine, turn the crankshaft
of an automobile, or rotate a generator's dynamo. This
paper will describe one method of performing that transfer:
the Otto cycle.
Heat engines, such as an automobile's engine or a
generator's turbine, use thermodynamic cycles to extract
mechanical energy from changes in the pressure and volume
of a “working fluid,” a liquid or gas that is enclosed in (or
passes through) the engine. In the case of the Otto cycle, the
pressure produced by the combustion of a mixture of air and
vaporized hydrocarbons is exploited to exert force over
distance—usually, on a piston through a cylinder—and
transform those hydrocarbons' chemical potential energy
(made into available heat and pressure by the combustion)
into the mechanical (kinetic) energy of a spinning crankshaft.
This paper will describe the four-stroke Otto cycle (and
its widespread variants, the Diesel and two-stroke cycles),
and detail how the Otto cycle may be improved with further
technological advancements, in order to increase efficiency
and mitigate deleterious effects on the environment.
Key Words—Brayton Cycle, Diesel Cycle, Gasoline, Heat
Engine, Otto Cycle, Rankine Cycle
THE IMPORTANCE OF THE OTTO CYCLE
Many processes useful to human civilization—pumping,
weaving, transportation—require mechanical energy. The
most obvious way to obtain and use mechanical energy is to
get it directly from moving objects; for example, a water
wheel steals mechanical energy from a river, and a windmill
exploits the motion of the air to turn a shaft. However, such
mechanical-to-mechanical transfers are limited to certain
locations (near a river, in a windy area) and can vary widely
in power output (spring thaw vs. winter freeze, hurricane vs.
doldrums), and humans generally don't want important
industrial processes to rely on the vagaries of the weather.
A more useful source of power is chemical energy,
released when certain substances bond with other
substances—for example, when wood burns in oxygen, the
reaction releases energy in the form of heat, and when
hydrogen burns in oxygen, a blast of high-pressure gases (an
explosion) is created. However, a major problem is how to
convert that heat and pressure into useful mechanical energy,
as well as preventing it from becoming dangerous.
Thermodynamic cycles are methods of doing exactly that,
using metal and moving parts. Before the invention of the
Otto cycle in the late nineteenth century, the predominant
method for the conversion of chemical into mechanical
energy was the reciprocating steam engine, which relied on
externally-combusted coal fuel and a heavy water boiler;
though it was useful, it was inefficient and difficult to move.
The Otto engine was the first useful internal-combustion
engine, allowing the use of gaseous and liquid fuels, such as
coal gas and gasoline, in a more compact and portable form.
Since its invention, it's been a major thermodynamic cycle,
and more liquid fuels (gasoline, Diesel fuel, etc.) have been
used than gaseous (natural gas) and solid (coal, wood, etc.)
fuels [1].
EXPLANATION OF THE CYCLE
Basic Process
An Otto engine is typically made up of several cylinders,
in each of which a separate Otto cycle takes place, out of
phase with the others so that power output is evenly spaced
over time—for example, in a V8 engine with eight cylinders,
one might go through the power stroke every eighth of a
second. In each cylinder, a piston moves up and down, and
converts its back-and-forth movement to the rotation of a
crankshaft (a long rod, which spins in a hollow crankcase
below the cylinder); the piston uses lubricating oil to form a
tight seal against the cylinder wall. Each cylinder has two
openings at the top: an intake valve, for the incoming fuelair
mixture; and an exhaust valve, for outgoing exhaust
gases. In addition, a spark plug at the top of the cylinder is
used to create sparks for ignition. The lubricating oil, stored
in the crankcase, is pumped into the rods connecting the
piston to the crankshaft, is sprayed into the engine from
small holes in those parts, and then drips back down into the
crankcase, where it's filtered and recycled [2].
The original form of the Otto cycle, and the one most
widely used in developed countries, is the four-stroke cycle,
so named because the piston goes through four strokes
(movements in different directions) over each repetition of
the cycle. First, in the intake stroke, the piston moves down,
decreasing the pressure in the cylinder and drawing air
through the opened intake valve, where fuel is sprayed and
mixed into the air. Second, in the compression stroke, the
intake valve closes and the piston moves up, compressing
the air at the top of the cylinder; when the cylinder reaches
the top and the air is at maximum compression, a spark plug
creates a spark to ignite the fuel-air mixture, which explodes
and puts pressure on the piston. Third, in the power stroke,
the increased pressure from the exploded fuel-air mixture
forces the piston down, exerting force over distance to add
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Colin L. Detweiler
Hal T. Hamilton Jr.
mechanical energy to the piston (and, thereby, the
crankshaft). Fourth, in the exhaust stroke, the piston rises,
forcing the burned exhaust gases out through the exhaust
valve, which closes behind them [2].
FIGURE 1
crankcase. Second, in the power/exhaust stroke, the
explosion of the mixture in the cylinder forces the piston
down, which uncovers the intake port (a simple hole, rather
than a valve, which connects the cylinder with the crankcase)
and the exhaust port (which connects the cylinder with the
outside air) and allows the mixture in the crankcase (which
is being compressed by the downward-moving piston) to
move upwards through the intake port and force the exhaust
gases out through the exhaust port [2].
FIGURE 2
A schematic drawing of the four-stroke cycle [3]. Blue
indicates fuel-air mixture, red indicates exhaust gases.
The fuel for an Otto engine is typically gasoline, a
mixture of light, liquid hydrocarbons—ideally octane (as the
“octane rating” on commercially-sold gasoline suggests), but
usually including such other impurities as heptane and
pentane, which can cause premature explosions in the
cylinder, which lower efficiency and reliability [4].
Major Variants
The two-stroke engine, developed a few decades after the
four-stroke, performs two strokes at once in different places.
First, in the intake/compression stroke, the piston moves up,
compressing a mixture of air, fuel, and lubricating oil in the
cylinder and simultaneously drawing a new mixture into the
A schematic drawing of the two-stroke cycle [5]. Pink
indicates fuel-air mixture, peach indicates exhaust gases.
The Diesel engine, invented by Rudolf Diesel, still uses
the four strokes of the standard Otto engine, but takes only
air in through the intake valve, and injects the fuel directly
into the cylinder at the end of the compression stroke. The
compression stroke produces a higher pressure than in the
four-stroke engine, igniting the fuel without the need for a
spark plug (or the concomitant electrical wiring, improving
reliability) [2]. Since the light hydrocarbons in gasoline
ignite prematurely in the high-pressure environment of the
Diesel engine, Diesel fuel is made up of longer, heavier
hydrocarbons, such as cetane [6], which is twice as large as
octane.
IMPORTANCE OF THE CYCLE
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Colin L. Detweiler
Hal T. Hamilton Jr.
Areas of Use
In 2011, fifty-seven billion gallons of Diesel fuel and 14
billion gallons of gasoline were sold in the United States
alone [7], and in 2009, four hundred billion gallons of Diesel
fuel and three hundred billion gallons of gasoline were used
worldwide [8]. Most automobiles use some form of the Otto
cycle, whether it’s the four-stroke, the two-stroke, or the
Diesel cycle; in addition, the Diesel engine is used in
industry and to power ships [11], and the two-stroke is often
used in such smaller applications as snowmobiles, ATVs,
and small boats [12]. It’s estimated that more than a quarter
of all greenhouse-gas emissions in the United States
originate from automobiles [13].
Advantages over the Competition
Other widely-used thermodynamic cycles include the
Rankine cycle and the Brayton cycle. The Rankine cycle,
once used in the reciprocating steam engines of locomotives,
is used in the steam turbines of electricity-generation plants:
water is boiled by the heat of externally burned fuel (usually
coal, oil, or natural gas), becomes high-pressure steam, and
forces its way through a turbine, which turns the crankshaft
that runs the electrical generator; the water then cools,
condenses, and is recycled [9]. Since the fuel is burned
outside of the engine, however, the separate combustion
chamber takes up more space than an internal-combustion
engine of equal power. In addition, since the fuel must be
burned at a minimum temperature to make the water boil at
all, the engine's efficiency is low at low power output [1].
In the Brayton cycle, used in gas-turbine engines in jet
airplanes and large ships, compressed air is taken in through
a large intake in the front of the engine, funneled through a
compressor to increase its pressure, and forced into a
combustion chamber where fuel is mixed with it and ignited.
The exhaust gases, now at even higher pressure and
temperature, drive a turbine at the rear of the engine (which
in turn drives a crankshaft) before exiting at lowered
pressure and temperature [9]. However, the high
temperatures and pressures that are necessary for efficient
combustion produce more power than can be used for such
small-scale purposes as automobiles [10].
ETHICAL PROBLEMS
Problems with the Cycle
In two-stroke engines, since the crankcase is used in the
intake/power stroke, it can't be used to store lubricating oil,
so the lubricating oil is vaporized and mixed into the intake
air with the fuel. However, this causes the lubricating oil to
be burned with the fuel in the intake/power stroke; this
increases harmful emissions, including carbon monoxide and
soot (unburned fuel) [14]. Carbon monoxide directly impairs
humans’ absorption of oxygen [15], while both carbon
monoxide and soot contribute to smog formation [16]. This
is a major problem in large cities in undeveloped countries,
such as in China and India, where many vehicles use twostroke
engines, which tend to be cheaper and simpler to
maintain than four-strokes. In addition, many motorcycles in
such places have sidecars attached, whose added weight
forces their engines above their intended power output,
further reducing efficiency and increasing emissions. The
World Health Organization ranks outdoor air pollution
thirteenth in worldwide death contributors. In Calcutta and
Delhi in India, two-thirds of residents suffer from chronic
colds and coughs because of the effects of air pollution,
which lead to asthma and lung cancer [17].
Engines based on the Otto cycle also have effects on the
environment in general. In the United States, due to the
increasing use of automobiles, both air pollution and acid
rain have increased [18]. As a side effect of the high
temperatures of combustion in Otto engines, the nitrogen
that composes the majority of the air combines with oxygen
to form various oxides of nitrogen, primarily nitric oxide
[14]. After rising into the atmosphere, nitric oxide reacts
with water vapor to form corrosive nitric acid, which
dissolves into rainwater. Due to these mixtures' acidic pH,
the rain that accumulates from these clouds affects the
environment in many different ways, from eroding buildings
and damaging cars to deleteriously affecting the
concentrations of elements in soil and waterways [18].
Problems with the Fuel
Through the refining process of gasoline, toxic artificial
compounds are added such as methyl tert-butyl ether are
added [19]. Methyl tertiary-butyl ether, or MTBE for short,
is used to raise the oxygen content of gasoline to make
engines run more smoothly and burn fuel more completely,
increasing efficiency and reducing emissions [19]. However,
studies done by the U.S. Environmental Protection Agency
show that animals who inhaled high concentrations of
MTBE “developed cancers or experienced other noncancerous
health effects” [19]. In addition, MTBE is very
soluble in water and has the potential to contaminate
drinking water, though its carcinogenicity when drunk has
not been thoroughly researched [19].
Another prominent additive is tetraethyl lead. Invented in
the early twentieth century to increase the smoothness and
efficiency of Otto engines, it was banned in most developed
countries after it was determined that the exhaust gases of
engines that used it contained high levels of lead, and tended
to afflict large amounts of people in factories and urban
areas with lead poisoning [21]; however, as of 2011, two
hundred million people lived in undeveloped countries
where tetraethyl lead was still used [22] as a cheap
alternative to expensive catalytic converters [23].
In addition to artificial additives, the natural components
of gasoline can also be detrimental to human health. Toxic
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Colin L. Detweiler
Hal T. Hamilton Jr.
components of unadulterated gasoline include carcinogenic
“aromatic” hydrocarbons, most notably benzene [20]. In
addition, petroleum naturally contains sulfur, which is
combusted into sulfur dioxide. Sulfur dioxide is as damaging
as nitric oxide (described above) in the creation of acid rain,
and is also directly toxic to humans [24].
In a more long-term fashion, though, one of the more
dangerous emissions of burned hydrocarbons is carbon
dioxide. While not directly or indirectly toxic, it contributes
to global climate change by preventing infrared radiation
from escaping from Earth’s surface to interplanetary space
[25].
Petroleum and Sustainability
Much of the petroleum that remains to be extracted from
the Earth is located in chronically undeveloped and unstable
places, such as Iran, Iraq, Libya, Nigeria, and Saudi Arabia
[26]. Since gasoline and Diesel fuel are refined from
petroleum, and these countries control a large proportion of
the petroleum supply, disruptions in that supply caused by
natural disasters, diplomatic impasses, or military action (by
states or by non-state entities) can cause extreme
fluctuations in the prices of those fuels. An eminent example
is, of course, the oil crisis of the 1970s, in which several oilproducing
Arab countries, angered by the United States'
support of their enemy Israel in the Yom Kippur War,
lowered petroleum production and increased prices,
quadrupling the price of oil and severely impacting
economies worldwide [27]. This means that using
petroleum to fuel Otto cycle engines is not very sustainable
as this small number of countries could drastically curtail
petroleum production and export at any time, leading to
worldwide petroleum shortages. These shortages would
become worse because the other areas where petroleum is
produced would not be able to sustain the world’s demands.
One way to decrease the world’s dependence on
petroleum (which affects more of the world's economy than
merely Otto engines, such as plastic production) is to modify
the fuel intake of these machines. This is explained further in
the next section, but overall, this would make a large impact
on the sustainability of the Otto cycle. The world simply is
not the same as it was when Nikolaus Otto invented the Otto
cycle in the 1800s. The Earth’s natural resources are being
depleted at exponential rates as third-world countries
continue to develop and industrialize and the world’s
population increases. In order to make the Otto cycle
sustainable, alternative fuel sources such as biodiesel must
be considered.
Another, possibly less effective, method to decrease this
vital dependence is by increasing the Otto cycle’s efficiency.
There exists a variety of methods to do this, including
adding air compressors to increase energy output from
combustion, modifying the way fuel is introduced to the
engine, and adding additional components to recover
additional energy form the machine that the Otto cycle
powers. These methods will be further explained in the next
section.
HOW IT CAN BE IMPROVED
Minor Adjustments
Various methods of improving the Otto cycle’s
efficiency and emissions characteristics have been proposed
and adopted. One of the most prevalent is the catalytic
converter, which attaches to an engine’s exhaust system and
converts nitric oxide, carbon monoxide, and soot into
harmless water and nitrogen, and relatively harmless carbon
dioxide [28]; however, it uses expensive metals (including
platinum) to promote the necessary chemical reactions, and
its use is therefore largely limited to developed countries
[28].
The supercharger, a fan placed before the intake valve,
effectively steals a page from the Brayton cycle’s book of
tricks by using part of the energy produced in the Otto
engine’s power stroke to compress the air taken in in the
intake stroke and force more of the fuel-air mixture into the
cylinder, increasing power and efficiency; the
turbosupercharger, or “turbocharger,” does the same, but
takes the energy from the pressure of the escaping exhaust
gases, which would otherwise be wasted [9].
In direct injection, fuel is injected into an engine’s
cylinder in several places, rather than solely at the top of the
cylinder. This allows combustion to occur smoothly and
evenly throughout the cylinder, rather than wherever the airfuel
mixture happens to swirl after entering through the
intake valve, and also increases efficiency [29].
The addition of electric motors and batteries to an engine
can improve efficiency and emissions by using the motors as
generators to recoup wasted heat energy from the brakes,
and by using electric motors rather than the Otto engine
when possible [30]; however, if the original electricity was
generated with coal or some other similarly-polluting fuel,
there may be a net increase in emissions, rather than a
decrease [31].
Major Adjustments
Fuel need not be refined from petroleum; several
methods of obtaining fuel from other sources have been
devised, though the engine must be extensively modified in
order to have the correct temperature and compression ratio
for the new fuel. Biodiesel, a mixture of oxygen-containing
hydrocarbons derived from vegetable oil and animal fat,
greatly reduces most unwanted emissions and aids in
lubricating the engine, but tends to congeal at low
temperatures [32]. Ethanol (the same alcohol that’s found in
whiskey), derived from fermenting the sugar or cellulose in
certain plants (most prominently, corn in the United States
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Colin L. Detweiler
Hal T. Hamilton Jr.
and sugarcane in Brazil), is also a potential alternative fuel
for standard Otto engines [33]; however, there are problems
with finding enough arable land to grow enough plants to
satisfy energy needs without raising food prices [34].
A proposed “six-stroke engine” would use a secondary
Rankine cycle to recover the otherwise wasted heat energy
of the still-warm exhaust gases of an Otto or Diesel engine,
by retaining some hot exhaust gases in the cylinder and
injecting water into the cylinder, which expands to produce a
second power stroke; however, difficulties include
preventing the steam from condensing on the soot in the
exhaust, which would damage the engine [35].
THE FUTURE OF THE OTTO CYCLE
The Otto cycle and its variants are well-suited for such
small and medium-sized portable applications as
automobiles and portable generators, and will continue to be
used widely in the future. It's to be hoped that, by
implementing some of the major and minor adjustments to
the Otto cycle mentioned, the cycle can continue to grow in
its number of applications and improve efficiency, which
will ultimately improve the Otto cycle’s sustainability.
While they do have some drawbacks in the areas of pollution
and efficiency, those can be mitigated with such further
technological advancements as the tacking-on of two
additional strokes, or the use of fuels other than petroleumbased
ones. By continuing to make these improvements,
there are great hopes that the Otto cycle, although more than
a century old, can have a competitive edge over the
multitude of energy-transformation methods.
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Hal T. Hamilton Jr.
[20] “Benzene.” United States Environmental Protection
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ACKNOWLEDGEMENTS
The efforts of Nikolaus Otto, Rudolf Diesel, Joseph Day,
and the other mechanical and chemical engineers who
invented and refined the Otto engine and its variants are
greatly appreciated.
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