The ignition delay in a diesel engine is defined as the time interval
between the start of injection and the start of combustion. This delay period
consists of (a) physical delay, wherein atomisation, vaporization and mixing of air
fuel occur and (b) of chemical delay attributed to pre-combustion reactions.
Physical and chemical delays occur simultaneously. To reduce NOx, the method
adapted in modern engines is to reduce the ignition delay. For predicting heat
release in modern engines, therefore, the estimation of ignition delay is no more
important. However, the ceiling on NOx is dipping to such a low level that
accurate prediction of ignition delay has become important even if it is small.
Ignition delay of diesel sprays is a strong function of ambient temperature and
pressure. However, the physical delay has not been modelled satisfactorily in the
literature. In this chapter, phenomenological calculations of the cooling of spray
surface have shown that the physical parameters and fuel type influence the
temperature of the mixture of air and the vapour produced by the first parcel of
the injected fuel throughout its life up to ignition. A unique thin-ring like zone on
the spray surface is postulated where the preflame reactions have reached a critical
level beyond which uncontrolled reactions take place. The time, at which the spray
just touches the ring, the ignition is predicted. However, due to turbulence, ignition
will take place at only a few points in the neighbourhood of the ring. Detailed
consideration of droplet formation, evaporation fuel and preflame reaction has
enabled prediction of delay period and location of the ignition accurately within
the experimental errors and errors in the input to the calculations.
Ignition delay of a fuel, in the context of diesel engines, is the period from the
time the first parcel of fuel enters the chamber to the point when the first flame is
observed in the spray. This is different from the combustion performance of the
fuel, which influences the efficiency of the engine. The ignition delay of a diesel
spray is important from the viewpoint of preparing the fuel before injecting into
the engine as well as selecting optimum injection timing. The delay is primarily
dependent on the ambient temperature. This has been shown by Wolfer (1950).
His correlation showed exponential dependence on temperature as in Arrhenius’
experimentally shown the strong influence of the injection parameters like the
hole size, injection pressure and types of fuel and hence quality of fuel on the
measured delay. The physical parameters mentioned above are normally lumped
as physical delay (Thelliez and Ji 1987, Woschni and Anisits 1974). This is either
a constant or variable according to the characteristics of the injector and has a
form of Wolfer’s correlation. Ahmed (1980) and Tsao, Myers and Uyehara (1962)
related physical delay to engine operating conditions. Thelliez and Ji (1987)
equation for rate of reaction. Pischinger, Scheid and Reuter (1987) have
60 Modelling Diesel Combustion
attempted to incorporate a constant physical delay. It was explained as the time
taken for the fuel to reach the length at which ignition occurs. This characteristic
length was assumed to depend only on the chamber pressure and temperature at
the start of injection contrary to the experimental results (Pischinger et al. 1987),
which show its dependence on injection parameters as well. This period is about
0.5 ms for the injection systems that are encountered. Andrea and Paschernegg
(1969) emphasized the importance of basic research to investigate the physical
and chemical complexities involved in igniting and burning hydrocarbon fuels.
Since conclusive information was not available, a combination of empirical
observations with modified working theories was used to fill the gap in
understanding this persistent problem of diesel combustion. An integral of the
difference between the temperature of the gases in the engine and the ignition
temperature of the fuel over time, from the time of start of injection till the end of
ignition delay was found to be nearly a constant irrespective of fuel or engine
speed (Andrea and Paschernegg 1969). The present work describes an attempt to
incorporate the effects of injection parameters, dimensions of the nozzle orifice,
fuel type and ambient conditions into a comprehensive ignition delay model
(Chandorkar et al. 1988).
between the start of injection and the start of combustion. This delay period
consists of (a) physical delay, wherein atomisation, vaporization and mixing of air
fuel occur and (b) of chemical delay attributed to pre-combustion reactions.
Physical and chemical delays occur simultaneously. To reduce NOx, the method
adapted in modern engines is to reduce the ignition delay. For predicting heat
release in modern engines, therefore, the estimation of ignition delay is no more
important. However, the ceiling on NOx is dipping to such a low level that
accurate prediction of ignition delay has become important even if it is small.
Ignition delay of diesel sprays is a strong function of ambient temperature and
pressure. However, the physical delay has not been modelled satisfactorily in the
literature. In this chapter, phenomenological calculations of the cooling of spray
surface have shown that the physical parameters and fuel type influence the
temperature of the mixture of air and the vapour produced by the first parcel of
the injected fuel throughout its life up to ignition. A unique thin-ring like zone on
the spray surface is postulated where the preflame reactions have reached a critical
level beyond which uncontrolled reactions take place. The time, at which the spray
just touches the ring, the ignition is predicted. However, due to turbulence, ignition
will take place at only a few points in the neighbourhood of the ring. Detailed
consideration of droplet formation, evaporation fuel and preflame reaction has
enabled prediction of delay period and location of the ignition accurately within
the experimental errors and errors in the input to the calculations.
Ignition delay of a fuel, in the context of diesel engines, is the period from the
time the first parcel of fuel enters the chamber to the point when the first flame is
observed in the spray. This is different from the combustion performance of the
fuel, which influences the efficiency of the engine. The ignition delay of a diesel
spray is important from the viewpoint of preparing the fuel before injecting into
the engine as well as selecting optimum injection timing. The delay is primarily
dependent on the ambient temperature. This has been shown by Wolfer (1950).
His correlation showed exponential dependence on temperature as in Arrhenius’
experimentally shown the strong influence of the injection parameters like the
hole size, injection pressure and types of fuel and hence quality of fuel on the
measured delay. The physical parameters mentioned above are normally lumped
as physical delay (Thelliez and Ji 1987, Woschni and Anisits 1974). This is either
a constant or variable according to the characteristics of the injector and has a
form of Wolfer’s correlation. Ahmed (1980) and Tsao, Myers and Uyehara (1962)
related physical delay to engine operating conditions. Thelliez and Ji (1987)
equation for rate of reaction. Pischinger, Scheid and Reuter (1987) have
60 Modelling Diesel Combustion
attempted to incorporate a constant physical delay. It was explained as the time
taken for the fuel to reach the length at which ignition occurs. This characteristic
length was assumed to depend only on the chamber pressure and temperature at
the start of injection contrary to the experimental results (Pischinger et al. 1987),
which show its dependence on injection parameters as well. This period is about
0.5 ms for the injection systems that are encountered. Andrea and Paschernegg
(1969) emphasized the importance of basic research to investigate the physical
and chemical complexities involved in igniting and burning hydrocarbon fuels.
Since conclusive information was not available, a combination of empirical
observations with modified working theories was used to fill the gap in
understanding this persistent problem of diesel combustion. An integral of the
difference between the temperature of the gases in the engine and the ignition
temperature of the fuel over time, from the time of start of injection till the end of
ignition delay was found to be nearly a constant irrespective of fuel or engine
speed (Andrea and Paschernegg 1969). The present work describes an attempt to
incorporate the effects of injection parameters, dimensions of the nozzle orifice,
fuel type and ambient conditions into a comprehensive ignition delay model
(Chandorkar et al. 1988).
No comments:
Post a Comment