Page 80 - Automotive Engineering

Emissions control CHAPTER 3.1

3.1.26 Black smoke have either a beneficial or detrimental effect on com-
bustion efficiency. Similarly, the resultant change in
droplet size and fuel penetration relative to the air
The effect of sulphur content on the formation of par-
ticulates has been covered in Section 3.1.23. Other fac- swirl could have either a beneficial or detrimental
tors include volatility and cetane number. As regards effect.
visibility, however, the carbon content is much more The reason why the cetane number does not have
significant. Suggestions that volatility per se influences a significant effect on the output of black smoke is
black smoke are without foundation. Smoke is reduced simple. It is that smoke density is largely determined
with increasing volatility for two reasons: the first is the during the burning of the last few drops of fuel to be
correspondingly falling viscosity, and the second the as- injected into the combustion chamber.
sociated rising API gravity of the fuel. A consequence of
the first is increased leakage of fuel through the clear-
ances around both the pumping elements and the in- 3.1.27 White smoke
jector needles and, of the second, the weight of the fuel
injected falls. Therefore, for any given fuel pump de- White smoke is a mixture of partially vaporised droplets
livery setting, the power output decreases with in- of water and fuel, the former being products of com-
creasing volatility. In fact, the real influence of volatility bustion and the latter arising because the temperature of
depends on an extremely complex combination of cir- the droplets fails to rise to that needed for ignition. It
cumstances, and varies with factors such as speed, load can be measured by passing the exhaust through a box,
and type of engine. one side of which is transparent and the other painted
The reason is that each engine is designed to operate matt black. A beam of light is directed through the
at maximum efficiency over a given range of speeds and transparent wall on to the matt black surface. If there is
loads with a given grade of fuel. Therefore, at any given no white smoke, no light is reflected back to a sensor
speed and load, a change of fuel might increase the alongside the light source; the degree of reflection
combustion efficiency, yet at another speed and load therefore is a function of the density of the white
the same change might reduce it. This is because smoke. For testing fuels, the criterion is the time taken,
a certain weight of fuel is required to produce a given after starting from a specified low temperature, for the
engine power output so, if the API gravity is increased, smoke level to reduce to an acceptable level. After

a commensurately larger volume of fuel must be starting at 0 C, satisfactory smoke levels are generally
supplied, and this entails injection for a longer period obtainable with a Diesel Index of 57 and a cetane
which, for any given engine operating condition could number of 53.5.

73