Environmental Engineering Reference
In-Depth Information
In principle also Diesel engines can operate either with two or four strokes, but
in practice, while the four-stroke Diesel engines are present on the vast majority of
vehicles, the two stroke ones have some application only on large ships. The
modern four-stroke direct injection Diesel engine (CIDI, Compression Ignition
Direct Injection) differs from the SIDI Otto engine for the self ignition of the fuel
and more heterogeneous air-fuel mixture. The air intake is unthrottled and the
fuel/air ratio varies with the quantity of fuel injected. The energy required for
ignition comes from the high temperature of air compressed in the cylinder, which
determines the self-ignition of the fuel just before the end of the compression
stroke. This implies the adoption of very high compression ratios to reach the
temperatures necessary to ignition. The ignition delay (time between injection and
start of combustion) is necessary to be tuned in order to assure optimal engine
operation, with high efficiencies and low consumptions, exploiting the fuel self-
ignition properties. Too long ignition delay implies a steep pressure rise in the
combustion chamber, with consequent increase in noise and nitrogen oxides
emissions. The self-ignition properties of the Diesel fuel are quantified by the
cetane number, which is defined as volume percentage of cetane (hexadecane) in
a-methylnaphthalene. A cetane number of 100 is attributed to n-hexadecane, a
compound characterized by very high ignition quality, while a value of zero is
given to a-methylnaphthalene, with very low ignition properties. These engines
must operate in lean conditions (i.e. with air/fuel ratio higher than stoichiometric
requirements), in order to reach the complete combustion of the heterogeneous
mixture, with benefits in terms of efficiency and torque, but less power with
respect to SIDI engine of comparable size. The more recent versions of CIDI
engines use turbochargers to assure additional power (by feeding larger amounts of
fuel and air into the cylinders), and the so-called multi-jet common rail technol-
ogy, which permits reaching of very high injection pressures inside the combustion
chamber and utilizes sequential electronically controlled multi-injections of fuel,
with benefits on emissions, fuel consumption and noise. The CIDI engines reach
higher efficiencies when compared to SIDI engines, because of high compression
ratios and lean air-fuel mixtures.
Homogeneous charge compression ignition (HCCI) engines have received
much attention in recent years and are today becoming an industrial reality. They
combine the essential characteristics of Otto and Diesel engines; in particular, a
premixed homogeneous and lean air-fuel charge is compressed to the autoignition
point, permitting low NO
x
and particulate emissions, and efficiency values com-
parable to Diesel engines [
42
].
Nearly all motor vehicles today are propelled by either gasoline (Otto
engines) or gasoil (Diesel engines), which are carbon-containing liquids derived
from a non-renewable resource, such as crude oil. These fuels have been
always considered particularly suitable for vehicles because of their availability,
ease of use and high energy density. Because of different types of combus-
tion occurring in Otto and Diesel engines, completely different characteristics
are required for gasoline and gas oil, which are described in detail in the
reference [
41
].
