Historical
Perspective:
The
objective of internal combustion engines (IC Engines) is the production of
mechanical power from the chemical energy contained in the fuel. In internal
combustion engines, as
distinct from
external combustion
engines, this energy is released by burning or oxidizing the fuel
inside the
engine. The fuel-air mixture before combustion and the burned products after
combustion are the actual working fluids. The work transfers which provide the
desired power output occur directly between these working fluids and the
mechanical components of the engine. The internal combustion engines which are
the subject of this blog are spark-ignition engines (sometimes called Otto
engines, or gasoline or petrol engines, though other fuels can be used) and
compression-ignition or diesel engines. Because of their simplicity, ruggedness
and high power weight ratio, these two types of engine have found wide
application in transportation (land, sea, and air) and power generation.
It
is the fact that combustion takes place inside the work producing part of these
engines that makes their design and operating characteristics fundamentally
different from those of other types of engine. Practical heat engines have
served mankind for over two and a half centuries.
For the first 150 years, water, raised to
steam, was interposed between the combustion gases produced by burning the fuel
and the work-producing piston in- cylinder expander. It was not until the 1860s
that the internal combustion engine became a practical reality. The early
engines developed for commercial use burned coal-gas air mixtures at
atmospheric pressure there was no compression before combustion. J. J. E.
Lenoir (1822-1900) developed the first mar-' ketable engine of this type. Gas
and air were drawn into the cylinder during the first half of the piston
stroke. The charge was then ignited with a spark, the pressure increased, and
the burned gases then delivered power to the piston for the second half of the
stroke. The cycle was completed with an exhaust stroke. Some 5000 of these
engines were built between 1860 and 1865 in sizes up to six horsepower.
Efficiency was at best about 5 percent. A more successful development-an
atmospheric engine introduced in 1867 by Nicolaus A. Otto (1832-1891) and Eugen
Langen (1833-1895)-used the pressure rise resulting from combustion of the
fuel-air charge early in the outward stroke to accelerate a free piston and
rack assembly so its momentum would generate a vacuum in the cylinder.
Atmospheric pressure then pushed the piston inward, with the rack engaged
through a roller clutch to the output shaft. Production engines, of which about
5000 were built, obtained thermal efficiencies of up to 11 percent. A slide valve
controlled intake, ignition by a gas flame, and exhaust. To overcome this
engine's shortcomings of low thermal efficiency and excessive weight, Otto
proposed an engine cycle with four piston strokes: an intake stroke, then a
compression stroke before ignition, an expansion or power stroke where work was
delivered to the crankshaft, and finally an exhaust stroke. He also proposed
incorporating a stratified-charge induction system, though this was not
achieved in practice. His prototype four-stroke engine first ran in 1876. A
comparison between the Otto engine and its atmospheric-type predecessor
indicates the reason for its success: the enormous reduction in engine weight
and volume. This was the breakthrough that effectively founded the internal
combustion engine industry. By 1890, almost 50,000 of these engines had been
sold in Europe and the United States. In 1884, an unpublished French patent
issued in 1862 to Alphonse Beau de Rochas (1815-1893) was found which described
the principles of the four-stroke cycle. This chance discovery cast doubt on
the validity of Otto's own patent for this concept, and in Germany it was
declared invalid. Beau de Rochas also outlined the conditions under which
maximum efficiency in an internal combustion engine could be achieved.
These
were:
1.
The largest possible cylinder volume with the minimum boundary surface
2.
The greatest possible working speed
3.
The greatest possible expansion ratio
4.
The greatest possible pressure at the beginning of expansion
The
first two conditions hold heat losses from the charge to a minimum. The third
condition
recognizes that the greater the expansion of the post combustion
gases, the greater the work extracted. The fourth condition recognizes that
higher initial pressures make greater expansion possible, and give higher
pressures throughout the process, both resulting in greater work transfer.
Although Beau de Rochas' unpublished writings predate Otto's developments, he
never reduced these ideas to practice. Thus Otto, in the broader sense, was the
inventor of the modern internal combustion engine as we know it today. Further
developments followed fast once the full impact of what Otto had achieved
became apparent. By the 1880s several engineers (e.g., Dugald Clerk,
1854-1913,; and James Robson, 1833-1913, in England and Karl Benz, 1844- 1929,
in Germany) had successfully developed two-stroke internal combustion engines
where the exhaust and intake processes occur during the end of the power stroke
and the beginning of the compression stroke. James Atkinson (1846-1914) in
England made an engine with a longer expansion than compression stroke, which
had a high efficiency for the times but mechanical weaknesses. It was
recognized that efficiency was a direct function of expansion ratio, yet
compression ratios were limited to less than four if serious knock problems
were to be avoided with the available fuels. Substantial carburetor and
ignition system developments were required, and occurred, before high-speed
gasoline engines suitable for automobiles became available in the late 1880s.
Stationary engine progress also continued. By the late 1890s, large
single-cylinder engines of 1.3-m bore fueled by low-energy blast furnace gas
produced 600 BHP at 90 revlmin. In Britain, legal restrictions on volatile
fuels turned their engine builders tower kerosene. Low compression ratio
"oil" engines with heated external fuel vaporizers and electric
ignition were developed with efficiencies comparable to those of gas engines
(14 to 18 percent). The Hornsby-Ackroyd engine became the most popular oil
engine in Britain, and was also built in large numbers in the United States
America (USA). In 1892, the German engineer Rudolf Diesel (1858-1913) outlined
in his patent a new form of internal combustion engine. His concept of
initiating combustion by injecting a liquid fuel into air heated solely by
compression permitted a doubling of efficiency over other internal combustion
engines. Much greater expansion ratios, without detonation or knock, were now
possible. However, even with the
efforts of Diesel and the resources of M.A.N.
in Ausburg combined, it took five years to develop a practical engine. Engine
developments, perhaps less fundamental but nonetheless important to the
steadily widening internal combustion engine markets, have continued ever inch
one more recent major development has been the rotary internal combustion
engine. Although a wide variety of experimental rotary engines have been
proposed over the years,' the first practical rotary internal combustion
engine, the Wankel, was not successfully tested until 1957. That engine, which
evolved through many years of research and development, was based on the designs
of the German inventor Felix WankeL6* ' Fuels have also had a major impact on
engine development. The earliest engines used for generating mechanical power
burned gas. Gasoline, and lighter fractions of crude oil, became available in
the late 1800s and various types of carburetors were developed to vaporize the
fuel and mix it with air. Before 1905 there were few problems with gasoline;
though compression ratios were low (4 or less) to avoid knock, the highly
volatile fuel made starting easy and gave good cold weather performance.
However, a serious crude oil shortage developed, and to meet the fivefold
increase in gasoline demand between 1907 and 1915, the yield from crude had to
be raised. Through the work of William Burton (1865-1954) and his associates of
Standard Oil of Indiana, a thermal cracking process was developed whereby
heavier oils were heated under pressure and decomposed into less complex more
volatile compounds. These thermally cracked gasolines satisfied demand, but
their higher boiling point range created cold weather starting problems.
Fortunately, electrically driven starters, introduced in 1912, came along just
in time. On the farm, kerosene was the logical fuel for internal combustion
engines since it was used for heat and light. Many early farm engines had
heated carburetors or vaporizers to enable them to operate with such a fuel.
The period following World War I saw a tremendous advance in our understanding
of how fuels affect combustion, and especially the problem of knock. The antiknock
effect of tetraethyl lead was discovered at General ~otors,' and it became
commercially available as a gasoline additive in the United States in 1923. In
the late 1930s, Eugene Houdry found that vaporized oils passed over an
activated catalyst at 450 to
480•‹C were converted to high quality
gasoline in much higher yields than was possible with thermal cracking. These
advances, and others, permitted
fuels with better and better antiknock
properties to be produced in large quantities; thus engine compression ratios
steadily increased, improving power and efficiency. During the past three
decades, new factors for change have become important and now significantly
affect engine design and operation. These factors are, first, the need to
control the automotive contribution to urban air pollution and, second, the
need to achieve significant improvements in automotive fuel consumption. The
automotive air-pollution problem became apparent in the 1940s in the ~oAsng
eles basin. In 1952, it was demonstrated by Prof. A. J. Haagen-Smit that the
smog problem there resulted from reactions between oxides of nitrogen and
hydrocarbon compounds in the presence of sunlight.' In due course it became
clear that the J automobile was a major contributor to hydrocarbon and oxides
of nitrogen emissions, as well as the prime cause of high carbon monoxide
levels in urban areas. Diesel engines are a significant source of small soot or
smoke particles, as well as hydrocarbons and oxides of nitrogen. As a result of
these developments, emission standards for automobiles were introduced first in
California, then nationwide in the United States, starting in the early 1960s.
Emission standards in Japan and Europe, and for other engine applications, have
followed. Substantial reductions in emissions from spark-ignition and diesel
engines have been achieved. Both the use of catalysts in spark-ignition engine
exhaust systems for emissions control and concern over the toxicity of lead
antiknock additives have resulted in the reappearance of unleaded gasoline as a
major part of the automotive fuels market. Also, the maximum lead content in
leaded gasoline has been substantially reduced. The emission-control
requirements and these fuel developments have produced significant changes in
the way internal combustion engines are designed and operated. Internal
combustion engines are also an important source of noise. There are several
sources of engine noise: the exhaust system, the intake system, the fan used
for cooling, and the engine block surface. The noise may be generated by
aerodynamic effects, may be due to forces that result from the combustion
process, or may result from mechanical excitation by rotating or reciprocating
engine components. Vehicle noise legislation to reduce emissions to the environment
was first introduced in the early 1970s. During the 1970s the price of crude
petroleum rose rapidly to several times its cost (in real terms) in 1970, and
concern built up regarding the longer-term availability of petroleum. Pressures
for substantial improvements in internal combustion engine efficiency (in all
its many applications) have become very substantial indeed. Yet
emission-control requirements have made improving engine fuel consumption more
difficult, and the removal and reduction of lead in gasoline has forced
spark-ignition engine compression ratios to be reduced. Much work is being done
on the use of alternative fuels to gasoline and diesel. Of the non-petroleum-based
fuels, natural gas, and methanol and ethanol (methyl and ethyl alcohols) are
receiving the greatest attention, while synthetic gasoline and diesel made from
shale oil or
coal, and hydrogen could be longer-term possibilities. It might be
thought that after over a century of development, the internal combustion
engine has reached its peak and little potential for further improvement remains.
Such is not the case. Conventional spark-ignition and diesel engines continue
to show substantial improvements in efficiency, power, and degree of emission
control. New materials now becoming available offer the possibilities of
reduced engine weight, cost, and heat losses, and of different and more efficient
internal combustion engine systems. Alternative types of internal combustion engines,
such as the stratified charge (which combines characteristics normally associated
with either the spark-ignition or diesel) with its wider fuel tolerance, may
become sufficiently attractive to reach large-scale production. The engine
development opportunities of the future are substantial. While they present a
formidable challenge to automotive engineers, they will be made pos- &le in
large part by the enormous expansion of our knowledge of engine processes which
the last twenty years has witnessed.
No comments:
Post a Comment