Comparison of Primary and Secondary Emissions from an Internal Combustion Engine - Novel Combustion Concepts for Sustainable Energy Development - page 414

loads, a relatively higher cylinder temperature leads to lower amount of unburnt
organic species. This led to lower BSOF in the emitted particles. With increasing
engine load the total number of particles emitted has lower BSOF which is critical
for growth of soot nuclei. Sulfur content of the fuel also decides the number of
sulfate nuclei formed in the cylinder. As biodiesel is essentially sulfur free, it further
explains lower number concentration of primary particles from B20. Biodiesel
contains approximately 10 % (w/w) oxygen this may also led to the sharp reduction
in particle number-size distribution for the primary particles.
In comparison to the primary emissions, we see a striking decrease in particle
number concentration in secondary aerosols (Fig. 5 ). This can be attributed to
gravitational settling, wall losses, sampling losses and various other chemical
reactions occurring inside the photo-chemical chamber (Gupta et al. 2011 ; Ruiz
et al. 2007 ). Lower particle number concentration in secondary emissions was
noticed for no load conditions for both the fuels. It is interesting to note that tiny
particles (510 nm) emitted by B20 were prevailing as secondary aerosols. Particles
emitted (in primary emissions) at higher engine loads were dominated by relatively
larger particles, which somehow showed lower tendency to undergo agglomeration.
At 25 % engine load for both the fuels, a sudden increase in number concen-
tration was observed. It is possible that the conditions inside the photochemical
chamber (37.5

°

C and 50 % RH) were the most favorable for SOA formation for
this case. In line with the observations of Robinson et al. 2007 , a sign of fresh nuclei
formation in the photochemical chamber was observed. It seems that the particles
were in a heterogeneous transient phase within the chamber. This further explains
high variability in the secondary aerosol size distribution.
For both fuels as the engine load increased, the peak of particle number con-
centration shifted toward larger diameters for primary as well as secondary aerosols.
The particle number concentration for both primary and secondary diesel exhaust
was an order of magnitude higher than B20 (Agarwal et al. 2011 ). At a constant rated
engine speed of 1,800 rpm, the total PM mass in primary emissions from diesel
increases with engine load (Fig. 6 ). Particle number concentrations at lower engine
loads were higher, but particle diameters were smaller. Hence, they cannot con-
tribute signi

cantly to the PM mass. For B20, at lower engine loads, PM mass in the
primary emissions increased with engine load, with a maxima at 50 % engine load.
The PM mass in primary and secondary emissions from diesel was higher than B20
by an order of magnitude. Numerous studies in the literature have indicated similar
trends (Agarwal et al. 2011 ; Cowley et al. 1993 ; Den Ouden et al. 1994 ; Kalligeros
et al. 2003 ; Lange 1991 ). Oxygen content of B20 favors ef cient fuel combustion

resulting in the lower PM mass (Akasaka et al. 1997 ; Owen et al. 1995 ).
Total surface area of the particles is directly proportional to the particle s number

and is inversely proportional to the particle size. Reduction in primary emission
particle number concentration and increase in the particle size were observed with
increasing engine load for B20. Higher number concentration at lower engine loads
(Fig. 5 ) for primary diesel emissions leads to lower PM mass and higher particulate
surface area (Fig. 6 ). For B20, the trend of particulate surface area curve is very well
correlated to the PM mass curve, with a peak at 50 % engine load. At 100 % engine

'