Page 162 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN006K-933 June 29, 2001 12:14

272 Fuel Chemistry

that burns in the gas phase. Evaporation is caused by heat B. Diesel Engines
transfer to the surface of the droplet. The time required for
Diesel engines are also IC engines. However, in Diesel
complete evaporation is given by
engines, there is no carburetor. Only air is compressed to
8λ much higher pressures and the fuel is injected into the
β = ln (1 + B T ), compressed air. As the fuel and air are mixed, the fuel
ρ l c p
evaporates and ignites (hence called compression igni-
where, B T is transfer coefficient, lambda is thermal tion). The pressures used in the engines are almost twice
Conductivity, ρ l = liquid density, and c p is the heat those of the gasoline engines. Rate of injection and mixing
capacity. of fuel and air determine the rate of combustion. Diesel
engines are classified based on fuel injection, direct in-
jection (DI), and indirect injection (IDI). Fuel quality is
measured by cetane number (CN).
XII. MAJOR COMBUSTION METHODS
C. Gas Turbines
Internal combustion engines are devices that produce
work using the hot combustion gases directly rather than Another class of internal combustion engine is the gas tur-
steam. Three major types of IC engines are commonly bine. Air is compressed to high pressures (10–30 atm) in
used today using the top three products from a refinery. a centrifugal compressor. Fuel is sprayed into the primary
They are (1) spark engine (gasoline engine), (2) compres- combustion zone where the fuel burns and increases the
sion engine (diesel engine), and (3) gas turbine (aircraft temerpature of the gases. The gas volume increases with
engines). combustion and the gases expand though a turbine. The
power generated exceeds that required for the compres-
sor. This drives the shaft to run an electric generator. In
A. Spark or Gasoline Engines the aircraft applications, the gases are released at high ve-
locity to provide the thrust. These systems are light weight
The most common engine is a 4-stroke engine. During the compared to land-based systems. Land-based systems use
intake stroke, the fuel and air mixture is drawn into the either distillate oil or natural gas. Gas turbine-based power
cylinder with the exhaust valve closed. Then the air and generation is used commonly to meet the peak power re-
fuel mixture is compressed in a compression stroke. At quirements rather than for base load operation.
the top of the stroke, the spark plug ignites the mixture.
During the expansion or power stroke, the high-pressure
D. Environmental Challenges
combustion gases expand moving the piston down and
for Liquid Fuel Utilization
delivering the power. The gases expand completely, the
exhaust valve opens, and the gases are expelled out dur- Carbon monoxide is present in any combustion gas from
ing the exhaust stroke. The fuel and air are atomized and anycarboncontainingfuel.Themainfactorthatleadstoits
premixed in a carburetor. The higher the compression ratio formation is incomplete combustion and in the IC engines
the higher the efficiency of the engine. However, higher continuous change from fuel-lean to fuel-rich conditions
compression ratios also require higher octane number fu- results in large emissions of CO. More than 70% of the
els. The octane number of a fuel is indicative of its an- CO emitted in the United States is from the transportation
tiknock properties. At equivalence ratios below 0.7 and sector. CO emissions are also a function of vehicle speed.
above 1.4, the mixtures are generally not combustible. At lower speeds the emissions are higher. Cold starts also
The equivalence ratio changes as the power requirement to contribute to higher emissions of CO. Oxygenates in
changes. For example, as the vehicle accelerates, high the fuel aid complete combustion and result in a decrease
torque and power are required for which a fuel-rich mix- of CO emissions. Catalytic converters placed at the end
ture is used, whereas when the vehicle is cruising at high of the exhaust pipe oxidize the CO catalytically at lower
speeds the vehicle needs fuel lean mixtures. Therefore, in temperatures.
IC engines it is difficult to maintain the air to fuel ratio Most of the hydrocarbon emissions are emitted through
constant. Combustion in IC engines takes place in both the exhaust. However, methane, ethane, acetylene, propy-
oxygen-deficient and oxygen-rich environments, and the lene, and aldehydes were found in the exhaust but were
air and fuel mixtures are preheated by compression. Ev- not present in the fuel. It can be deduced that these
ery time a fresh batch of fuel comes in flame is produced were formed during combustion. A significant amount of
and quenched resulting in unsteady combustion. This re- hydrocarbon emissions also come from the combustion
sults in continuous changes in the pollutant generation and chamber wall crevices and solid deposits. These hydro-
emission. carbon emissions reduce the NO x emissions. However,