Comparison of Primary and Secondary Emissions from an Internal Combustion Engine - Novel Combustion Concepts for Sustainable Energy Development - page 408

2 Primary Particulate Composition and Its Formation

2.1 Particulate Composition

Engine exhaust particles are composed of volatile (soluble) and non-volatile
(insoluble) fractions. Soluble part consist organic carbon (OC), sulphate and nitrate
compounds whereas insoluble part comprises of carbonaceous (soot) fraction and
ash content (Eastwood 2008 ). The level of particle toxicity depends on its chemical
composition and size. Figure 1 (Kittelson 1998 ) shows a typical chemical com-
position of diesel particulates. The soluble fraction is the most toxic component of
the particulates. This fraction can also be present in the gas phase and it can
eventually get condensed over the surface of solid particle. Such primary particles
(15 40 nm in diameter) formed can further agglomerate and form bigger particles

(Eastwood 2008 ). The size of these particles is expressed as electrical mobility
diameter. Electrical mobility diameter refers to the particle velocity at which it
migrates in an applied electrical

-

field.

2.2 Particulate Formation

Many past studies have explored the soot formation process in diesel exhaust.
Figure 2 explains the critical steps of the particle formation. Diesel is injected in the
combustion chamber at high pressure and temperature. Spray penetration and fuel
atomization depends on the actual combustion chamber pressure and temperature.
Higher degree of fuel penetration and atomization results in improved air and fuel
mixing. Injected fuel at high pressure forms tiny droplets that get dispersed in the
chamber with the incoming air

fl flow. Excess air ratio ( γ ) is not uniform within the

' γ '

combustion chamber. value is very low near the droplet surface and attains

Fig. 1 Typical particle
composition for a heavy-duty
diesel engine tested in a
heavy-duty transient cycle
(adapted from Kittleson 1998 )