Environmental Engineering Reference
In-Depth Information
four-stroke-cycle engines is unlikely to change in the foreseeable future, especially because two-
stroke cycle engines have higher exhaust pollutant emissions. 6
Although SI and CI engines are similar in that they both burn fuel with air in the cylinder when
the piston is near TC, so as to create the pressure rise that eventually produces net mechanical
work during the four-stroke cycle of events, they differ in three important ways. In the SI engine
the fuel and air are mixed together outside the cylinder to form a uniform fuel-air mixture that is
ingested during the intake stroke; in the CI engine the fuel is sprayed into air in the cylinder, near
TC, forming a nonuniform fuel-air mixture. In the SI engine, the combustible mixture is burned
when ignited by a spark at the appropriate time in the cycle near TC, whereas the CI fuel-air
mixture ignites spontaneously, shortly after the fuel is sprayed into the engine combustion chamber
at the requisite time near TC. A third difference is the method of controlling the power output of
the engine, an important consideration because the vehicle engine must operate satisfactorily over
a wide range of power in a continuously and frequently variable manner. This is accomplished in
the SI engine by adjusting the air pressure in the intake manifold by means of a throttle valve that
lowers that pressure below the atmospheric value, the more so as the engine power is reduced. In
the CI engine, the power is lowered by reducing the amount of fuel injected into the cylinder at
each cycle. These differences have important implications for the formation of pollutants and the
fuel efficiency of each type of engine, SI or CI.
The power needed for a passenger vehicle engine is in the range of tens to hundreds of
kilowatts. For many practical reasons, the cylinder size in light-duty passenger vehicles and trucks
is approximately the same for all engines, with higher-powered engines using more cylinders
than small ones. Thus engines of increasing power may contain 2, 3, 4, 5, 6, 8, or 10 cylinders.
Geometrically, the piston motions in these cylinders are phased so that the cylinders fire at equally
timed intervals during the two revolutions of the crankshaft (720 ) that constitute the four-stroke
cycle. This also facilitates the mechanical balance of the moving parts, which would otherwise
cause excessive engine vibrations. It is general practice to construct 2-, 3-, 4-, 5-, and 6-cylinder
engines with all the cylinders aligned one behind the other, while 6-, 8-, and 10-cylinder engines
have two banks of cylinders in a V-shaped arrangement. This permits more efficient use of the
engine space in the vehicle, especially for front-wheel-drive vehicles.
Combustion in SI and CI Engines
To perform properly, any reciprocating internal combustion engine must burn a mixture of air and
fuel in a very short time, completing the combustion soon after the piston begins its outward power
stroke. The duration of combustion should not much exceed about 50 degrees of rotation of the
crankshaft, which would equal about 2 ms if the crankshaft were rotating at 3600 revolutions per
minute (rpm), a typical value for a vehicle traveling at high speed. In order for a fuel molecule to
burn, it must be mixed intimately with oxygen at the molecular level. Even then, the conversion
of a hydrocarbon molecule to carbon dioxide and water vapor molecules will not occur rapidly
enough unless the mixed air and fuel are sufficiently hot. This is assured by the rapid temperature
rise of the cylinder charge during the inward compression stroke.
6 For developments in the two-stroke-cycle engine field, see: Heywood, John B., and Eran Shaw, 1999. The
Two-Stroke Cycle Engine: Its Development, Operation, and Design. Philadelphia: Taylor and Francis.
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