Environmental Engineering Reference
In-Depth Information
The dominant technology used for almost two centuries to power both light and
heavy duty vehicles is based on internal combustion engines, that generate power
by converting chemical energy of the burning fuel into heat and then into
mechanical work, an energy transformation realized by either Otto or Diesel
thermodynamic cycle.
For a deep analysis of all issues regarding the internal combustion engines and
for a comprehensive examination of energy sources for transportation, which are
outside the scope of this topic, other monographs are recommended [
40
,
41
]. The
discussion of this section will be subsequently focused on interactions engine-fuel
and their effects on local and global environmental issues.
In the Otto cycle engine, the air-fuel mixture is burnt by using a timed
ignitiongeneratedbyanexternal electric source (SI, Spark Ignition engine). This
principle can be practically realized in two different basic ways, reciprocating
engine with four or two strokes, and rotary engines. The four-stroke recipro-
cating engine is the most frequently used in both light and heavy duty vehicles,
and utilizes four strokes for gas exchange processes, controlled by valves (during
one working stroke the crankshaft turns twice): air-fuel mixture intake, com-
pression and timed ignition shortly before the piston has reached the top dead
center, working stroke (combustion and expansion), exhaust of combusted gases.
In the more recent development of spark ignition engines (SIDI, Spark Ignition
Direct Injection), the air flow is regulated by a throttle valve and is compressed
in the cylinder, while the fuel, managed by electronic injection systems, is added
to the compressed air inside the cylinder. This system ensures more accurate
control of air-fuel mixture in terms of homogeneity and carburation, with con-
sequent benefits for fuel consumption, performance and response in transient
phases. After ignition induced by the spark plug, combustion must develop in a
controlled way under all operative conditions, without autoignition phenomena.
These occur if ignition quality of the fuel does not meet that required by the
engine, provoking very sharp increase of temperature and pressure (knocking
combustion) with consequent damages to the engine. A retarded ignition could
avoid knocking also with poor quality fuels, but would imply higher exhaust gas
temperature with efficiency losses. The ignition quality of the gasoline is
described by the octane number, which is determined by comparing in a
reference engine the ignition characteristics of a gasoline to those of isooctane
(2,2,4-trimethylpentane) and heptane. Isoctane is assigned octane number of 100,
due to its prerogative to burn smoothly with little knock, while heptane is given
an octane number of zero because of its bad knocking properties.
Two stroke Otto engines, in which a working cycle is achieved with each
rotation of the crankshaft, can be smaller and lighter with respect to four stroke
ones, and are used almost exclusively on small motorcycle, due to unsolved
problems regarding fuel consumption and emissions. The rotary engine (or Wankel
engine) can be considered a variant of the four-stroke reciprocating engine, from
which it differs for its rotary piston and for the use of ports instead of valves for
gas exchange phases. This type of engine is not diffused on modern vehicles due to
its relatively high fuel consumption and exhaust gas emissions.
