The processes of particle formation during
diesel combustion have been described in
detail [26]. Carbonaceous spherules (soot) are
initially created in the cylinder. A phase of particle
growth then follows, involving the adsorption of
gas-phase components, as well as coagulation and
agglomeration. Almost all the particles found in
the exhaust prior to dilution are present as these
carbonaceous agglomerates (hence, accumulation
mode), with a small amount of metallic ash. The
processes leading to the presence of nucleation
mode particles in the exhaust during measurement
are quite complex. Most nucleation mode particles are thought to originate from the condensation of volatile material (hydrocarbons, hydrated
sulfuric acid and salts) in the exhaust gas during
dilution, rather than during combustion itself, and
their formation is a function of measurement
parameters such as temperature, dilution ratio,
residence time, and humidity [28–30], as well as
fuel sulfur content [31, 32]. Particles in the coarse
mode are formed by re-entrainment of material
previously deposited on engine cylinder and
exhaust system surfaces. Some of the pollutants
emitted by fuel combustion (sulfur oxides and
nitrogen oxides) are also precursors of secondary
particles in the atmosphere.
The main factors that govern exhaust emissions are the vehicle type, the fuel used, the vehicle technology, and the driving conditions. The
term “vehicle technology” can be used to refer to
either a specific type of engine or exhaust aftertreatment, or more generally compliance with a
particular emission standard. Some examples of
these effects are shown for road vehicles in Fig. 7.
Other factors include vehicle weight, road gradient, vehicle load, and the use of auxiliary equipment such as air-conditioning. Another important
consideration is the so-called dynamics of driving
(see section “Modelling Transport Emissions”).
The thermal state of the engine and exhaust
emission-control system also has a strong influence on emissions from road vehicles. Most emissions occur under what are known as thermally
stable or “hot” engine and exhaust conditions. The
temperature of engine coolant during normal
operation is typically between around 70
C and
90
C, whereas the temperature of the exhaust
system reaches several hundred degrees centigrade. During the warm-up period following an
engine start, emissions and fuel consumption are
elevated as a result of incomplete combustion, the
low conversion rate of pollutants in the catalyst,
and increased viscous friction due to the low
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.001
0.01
0.1
1
10
Diameter (mm)
dlogDp
Number
weighted
Mass
weighted
Nanoparticles
(Dp < 50 nm)
Ultrafine particles
(Dp < 100 nm)
Nucleation
mode
Accumulation
mode
Coarse
mode
Fine particles
(Dp < 2.5 µm)
Air Quality, Surface Transportation Impacts on, Fig. 6 Typical particle size distributions in vehicle exhaust. The
y-axis is a normalized log scale. (Adapted from [28])
54
Air Quality, Surface Transportation Impacts on
diesel combustion have been described in
detail [26]. Carbonaceous spherules (soot) are
initially created in the cylinder. A phase of particle
growth then follows, involving the adsorption of
gas-phase components, as well as coagulation and
agglomeration. Almost all the particles found in
the exhaust prior to dilution are present as these
carbonaceous agglomerates (hence, accumulation
mode), with a small amount of metallic ash. The
processes leading to the presence of nucleation
mode particles in the exhaust during measurement
are quite complex. Most nucleation mode particles are thought to originate from the condensation of volatile material (hydrocarbons, hydrated
sulfuric acid and salts) in the exhaust gas during
dilution, rather than during combustion itself, and
their formation is a function of measurement
parameters such as temperature, dilution ratio,
residence time, and humidity [28–30], as well as
fuel sulfur content [31, 32]. Particles in the coarse
mode are formed by re-entrainment of material
previously deposited on engine cylinder and
exhaust system surfaces. Some of the pollutants
emitted by fuel combustion (sulfur oxides and
nitrogen oxides) are also precursors of secondary
particles in the atmosphere.
The main factors that govern exhaust emissions are the vehicle type, the fuel used, the vehicle technology, and the driving conditions. The
term “vehicle technology” can be used to refer to
either a specific type of engine or exhaust aftertreatment, or more generally compliance with a
particular emission standard. Some examples of
these effects are shown for road vehicles in Fig. 7.
Other factors include vehicle weight, road gradient, vehicle load, and the use of auxiliary equipment such as air-conditioning. Another important
consideration is the so-called dynamics of driving
(see section “Modelling Transport Emissions”).
The thermal state of the engine and exhaust
emission-control system also has a strong influence on emissions from road vehicles. Most emissions occur under what are known as thermally
stable or “hot” engine and exhaust conditions. The
temperature of engine coolant during normal
operation is typically between around 70
C and
90
C, whereas the temperature of the exhaust
system reaches several hundred degrees centigrade. During the warm-up period following an
engine start, emissions and fuel consumption are
elevated as a result of incomplete combustion, the
low conversion rate of pollutants in the catalyst,
and increased viscous friction due to the low
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.001
0.01
0.1
1
10
Diameter (mm)
dlogDp
Number
weighted
Mass
weighted
Nanoparticles
(Dp < 50 nm)
Ultrafine particles
(Dp < 100 nm)
Nucleation
mode
Accumulation
mode
Coarse
mode
Fine particles
(Dp < 2.5 µm)
Air Quality, Surface Transportation Impacts on, Fig. 6 Typical particle size distributions in vehicle exhaust. The
y-axis is a normalized log scale. (Adapted from [28])
54
Air Quality, Surface Transportation Impacts on