automotivebreath
Automotive
- Jul 5, 2006
- 66
Lately I have been doing research regarding advancements in the understanding auto-ignition and detonation. I have located great information and have learned plenty about formation of hot spots, pre-flame reactions, radicals and transition from deflagration to detonation.
I have one question that I have not been able to answer, I have found several references linking hot spots that lead to auto-ignition to improper air/fuel mixing and residual gas, see some quotes below. What are the effects of poor air/fuel/residual gas mixing on hot spots, specifically concentrations of exhaust gas and/or wet fuel rich areas in the chamber? Does anyone have a source for information on the subject?
"Around 12.5 CAD ATDC some hot spots were observed in the end gas as it is evident in the picture. In less than 1 CAD, the number and luminosity of the hot spots increased significantly. The hot spots corresponded to the very small centers of auto igniting exothermic reactions. The hot spots were characterized by steep gradients of temperature and residual gas concentration and weak pressure oscillations."
source:
"If autoignitive combustion occurs at the same time throughout a homogeneous charge the maximum pressure is limited to that for constant volume combustion. In this mode of combustion the rate of pressure rise increases as stoichiometric conditions are approached. In contrast, if the mixture is not homogeneous, it is possible with these same overall stoichiometric changes for autoignition to be associated with the development of a detonation. In this mode, localised maximum pressures are significantly in excess of constant volume combustion values. A developing detonation comprises supersonic combustion with very high peak pressures, which, although short lived, may cause damage to the engine, as is observed with severe conventional spark ignition engine knock. The ensuing reaction front can assume a number of forms. These include a near-instantaneous thermal explosion, an autoignitive deflagration propagating supersonically through a favourable spatial distribution of ignition delay times, a coalescence of pressure and reaction fronts in a developing detonation, an autoignitive deflagration propagating subsonically, or a conventional flame propagating by molecular diffusive and conductive mechanisms.
These developing detonations are associated with gradients in ignition delay time, caused by gradients of temperature, mixture strength, or active radicals. They can occur near the cylinder walls or in the body of the charge as a result of imperfect charge mixing. They will be seen as "hot spots", where the autoignition starts in a region of high temperature (which may have been caused by chemical reaction) and spreads out into the surrounding, slightly cooler, gas. There are situations when the autoignition front and the pressure wave from the combustion mutually re-enforce and travel at the same speed, giving rise eventually to a detonation."
source:
I have one question that I have not been able to answer, I have found several references linking hot spots that lead to auto-ignition to improper air/fuel mixing and residual gas, see some quotes below. What are the effects of poor air/fuel/residual gas mixing on hot spots, specifically concentrations of exhaust gas and/or wet fuel rich areas in the chamber? Does anyone have a source for information on the subject?
"Around 12.5 CAD ATDC some hot spots were observed in the end gas as it is evident in the picture. In less than 1 CAD, the number and luminosity of the hot spots increased significantly. The hot spots corresponded to the very small centers of auto igniting exothermic reactions. The hot spots were characterized by steep gradients of temperature and residual gas concentration and weak pressure oscillations."
source:
"If autoignitive combustion occurs at the same time throughout a homogeneous charge the maximum pressure is limited to that for constant volume combustion. In this mode of combustion the rate of pressure rise increases as stoichiometric conditions are approached. In contrast, if the mixture is not homogeneous, it is possible with these same overall stoichiometric changes for autoignition to be associated with the development of a detonation. In this mode, localised maximum pressures are significantly in excess of constant volume combustion values. A developing detonation comprises supersonic combustion with very high peak pressures, which, although short lived, may cause damage to the engine, as is observed with severe conventional spark ignition engine knock. The ensuing reaction front can assume a number of forms. These include a near-instantaneous thermal explosion, an autoignitive deflagration propagating supersonically through a favourable spatial distribution of ignition delay times, a coalescence of pressure and reaction fronts in a developing detonation, an autoignitive deflagration propagating subsonically, or a conventional flame propagating by molecular diffusive and conductive mechanisms.
These developing detonations are associated with gradients in ignition delay time, caused by gradients of temperature, mixture strength, or active radicals. They can occur near the cylinder walls or in the body of the charge as a result of imperfect charge mixing. They will be seen as "hot spots", where the autoignition starts in a region of high temperature (which may have been caused by chemical reaction) and spreads out into the surrounding, slightly cooler, gas. There are situations when the autoignition front and the pressure wave from the combustion mutually re-enforce and travel at the same speed, giving rise eventually to a detonation."
source:
