Environmental Engineering Reference
In-Depth Information
Understanding the physical and chemical processes is important to thoroughly describing the
ignition delay.
Hydrocarbon combustion occurs only in the gas phase. Thus, for a liquid fuel, the first step
toward ignition involves transitioning from a liquid to a gas phase. The time required for this transi-
tion is the “physical delay” in ignition and includes the amount of time required for a droplet of fuel
to heat, vaporize, and mix with hot air in the cylinder.
The physical delay is influenced by the density and temperature of air in the cylinder; velocity
and turbulence of the air; atomization; penetration; shape of the spray; and the properties of the
fuel, including density, viscosity, surface tension, specific heat, enthalpy of vaporization, and vapor
pressure.
Combustion is a sequence of chemical reactions in which the gas-phase fuel reacts with oxygen.
These reactions proceed stepwise through a mechanism involving free radicals. For ignition to
occur, the fuel must be heated to a temperature sufficient for some of the weaker bonds to break
and form radicals. The finite rate of these radical-forming oxidation reactions is responsible for the
chemical delay in compression ignition. Once a sufficient concentration of free radicals is reached,
rapid oxidation occurs (ignition). The heat-release pattern can be considerably influenced by igni-
tion delay, which in turn affects fuel economy and pollutant emissions. The start of injection is
usually taken as the time when the injector lifts from its seat (determined by a needle lift indicator).
The correlation coefficients among ignition delay, fuel CN, density, percentage of unsaturation,
and start of injection were found and shown in Table 25.9.
From the table, it can be observed that the ignition delay is negatively correlated with CN and
positively correlated with fuel density, percentage of unsaturation, and dynamic injection timing.
Apart from CN, the reason behind relating ignition delay with density, percentage of unsaturation,
and dynamic injection timing can be explained as follows:
• CN has a greater inluence on chemical delay whereas fuel density, viscosity, and surface
tension can have a significant influence on physical delay (Burman and Deluca 1962).
• The density and CN were found to be highly correlated with one another (McCormick
et al. 2005).
• The ignitability of an ester fuel depends not only upon the CN but also upon the fatty acid
ester composition (Kinoshita et al. 2006).
• If the injection timing is advanced, the fuel is injected to the combustion chamber at a
lower temperature and pressure. If the prevailing temperature and pressure are lower, it is
obvious that there would be a longer ignition delay.
From Table 25.9, it can be observed that the ignition delay increases with an increase in per-
centage of unsaturation. The effect of unsaturation percentage on ignition delay is illustrated in
Figure 25.10.
taBle 25.9
correlation coefficient among Ignition delay, cetane
number, density, Percentage of unsaturation, and
dynamic Injection timing
x
variable
y
variable
correlation coefficient
Cetane number
Ignition delay
-0.771
Density
0.933
Percentage of unsaturation
0.823
Dynamic injection timing
0.873
