Fundamental Study Of Auto-ignition Characters Of Fuel And Its Effect On Combustion Mode In Confined Space

The worldwide shortage of fossil fuel and the intensifying global warming have put forward new standard of carbon emission to our country.To satisfy decreasing carbon emission specification,the emission reduction of automobile engines,which contributes a large percentage of total emission,is a key issue to be solved.Engine downsizing could effectively increase thermal efficiency,thus reducing carbon emission.However,as downsized engines are operated with increasing compression ratio and inlet charging,downsizing technique is facing problems of knock and super-knock,which is a bottle neck when increasing thermal efficiency to the next level.The research into mechanism of knock would provide useful theory foundation and technique methods for suppression of knock.The research contents of this paper proceed with two phenomenon in company with appearance of knock,which are auto-ignition and transition of flame.To explore combustion process through the view of auto-ignition,pressure wave and their interaction with flame,we carry out a series of numerical simulation and experiment study to explain the inherent relation among auto-gnition,pressure wave and flame,which would contribute to explanation of mechanism of knock and super-konck.First,by employing ANSYS Fluent,interactions between pressure wave and flame and end gas auto-ignition induced by pressure wave under engine-relevant condition in two-dimensional space is simulated.It is found that pressure wave accelerate and decelerate flame by changing speed of unburnd gas.Propagation and reflection of pressure wave could promote end gas auto-ignition,and different temperature distribution of end gas would lead to different pressure oscillation.Moreover,sensitivity analysis is carried out to research chemical kinetics process during auto-ignition,it is found that inducement and reflection of pressure wave changes competition and promotion relation between elementary reactions.Second,one-dimensional simulation is carried out to simulating auto-ignition and transition of flame front of hydrogen/air mixture in a closed domain.Subsonic deflagration,detonation and supersonic deflagration are observed under different initial conditions.In order to analyze relation between different combustion modes and auto-ignition of unburned gas,concept of auto-ignition progress is employed.By monitoring auto-ignition progress,it is found that pressure wave can obviously raise temperature and pressure of end gas and flame front,which promotes auto-ignition.Auto-ignition ahead of flame front would trigger transition of combustion mode,the final form of which depends on absolute value and gradient distribution of auto-ignition progress.Third,influence of temperature gradient inducing auto-igniting flame and pressure wave generated by it on combustion mode is then studied.During flame propagation along temperature gradient,transition from supersonic deflagration to detonation is first observed.By analyzing relation between auto-igniting flame speed and C-J speed,it is found that relation between auto-igniting flame speed,C-J speed and ignition delay of unburned mixture would determine transition condition between supersonic deflagration and detonation.By monitoring structure of detonation during propagation,the reason of degeneration from detonation to subsonic deflagration is studied.Results show that insufficient reactivity of mixture behind precursor shock is one of the reasons of degeneration.Transition locations under different equivalence ration and initial pressure show some kind of regularity,indicating transition from detonation to subsonic deflagration is the consequence of multiple factors.Last,in order to study and verify relation between auto-ignition,pressure wave and flame,a RCM device,which can implement experiments on auto-ignition and knock combustion,is designed and developed.By analyzing experiment results of compression ignition of hydrogen/air mixture,it is found that RCM can achieve short compression time,good compression repeatability and enough high compressed state at TDC,thus is capable of for following experiments.Based on RCM,effect of different initial pressure and equivalence ratio on hydrogen combustion is examined.It is observed that though higher flame speed increases pressure oscillation during combustion,it prohibits strong pressure oscillation because of its low energy density.To evaluate the effect of energy density on pressure oscillation,experiments using propane are carried out to study transition of combustion modes.It is found that though flame speed of propane is slower,its higher energy density would generate pressure wave with greater amplitude.To study the effect of flame acceleration on pressure oscillation,crevice pistons are employed to accelerate propane flame,it is found that flame acceleration increases energy density and auto-ignition trend of end gas,which leads to stronger pressure oscillation or even appearance of detonation after auto-ignition,indicating that high energy density and high flame speed are both factors that lead to strong knock.Optical tests of propane combustion under different initial conditions are carried out.As initial pressure increases,combustion modes such as normal combustion,mild knock,weak knock and super knock are observed.By comparing images and pressure traces,it is found that the increase of energy density increases auto-ignition trend and amplitude of formed pressure wave.Supersonic propagation of auto-igniting flame leads to formation of DDT and C-J detonation,which forms super-knock with high pressure peak.