| Homogeneous charge compression ignition(HCCI)low-temperature combustion technology is a current method to realize high efficiency and low emission in engine.Inside,the combination of fuel design and injection strategy is the key.Catalytic hydrogenation biodiesel(CHB)has been seen as the optimal alternative fuels on the incylinder combustion due to the absence of oxygen,high cetane number and pretty stability of the mixture with fossil diesel.In the paper,several optical techniques were used to investigate the in-flame soot formation of CHB and in-flame soot formation of diesel in different double-injection strategies on a constant volume combustion vessel.The main results are shown as following:(1)Diffused back-illumination extinction imaging and OH* chemiluminescence were simultaneously applied to investigate the combustion and soot formation characteristics of HCB/diesel mixture(B50).Meanwhile,DBIEI was used to simultaneously capture spray liquid length and soot formation.Results show that flame lift-off length(LOL)of B50 is shorter than spray liquid length(LL),and the heat release from combustion did not shorten the LL.Besides,a consistent trend shows that LOL and LL of B50 are shorter than that of diesel under all parameters.Due to the low fuel density,low boiling point and high cetane number of B50,the difference between LOL and LL(D-value)of B50 is longer than that of diesel.Meanwhile,soot onset time of B50 is shorter than that of diesel and soot onset length of B50 is shorter than that of diesel.In the downstream of optical window,soot concentration along spray radial of B50 is more averaged than that of diesel,by the way,soot width of B50 is shorter than that of diesel.Soot formation of both fuels increases with ambient temperature increasing,oxygen concentration increasing and injection pressure decreasing.Soot net formation rate depends on soot formation rate and soot oxidation rate.Hence,soot net formation rate of B50 decreases with oxygen concentration increasing.(2)In double-injection,non-reacting spray liquid penetration and spray vapor penetration of diesel were measured by LED-Mie scatter and schlieren separately.Results show that spray liquid penetration of the second injection is faster than that of the first injection before it reaches the maximum spray liquid length.At quasi-steady period,spray liquid length shows no difference between successive injections.Spray evaporation rate and spray-air mixture are different under different ambient conditions.Exchange of momentum flux exists in spray vapor of successive injections.Therefore,the first injection penetration rate is less than the second injection.(3)Soot formation process in double-injection was measured by direct-radiation method(DRM)and diffused back-illumination extinction imaging(DBIEI).Due to the high-temperature products of the first injection,ambient temperature and mixture fraction near nozzle was changed before the start of the second injection.Hence,soot onset time of the second injection is shorter than that of the first injection.Meanwhile,soot onset length locates at a closer position to nozzle.Besides,soot width in the tip of flame of the second injection is shorter,and high soot concentration regions in the flame would move to the upstream of spray liquid.Therefore,the second injection has more soot formation than the first injection,especially in low ambient temperature and low back pressure conditions.With longer dwell time,the interval effect between injections declines sharply.Soot formation process of HCB is investigated in the paper,and the reasons why it can reduce soot emission has been found,which can provide fundamental data for its application in diesel engines.With the employment of several optical techniques,spray and combustion characteristics in double-injection were investigated to provide a deeper understanding the interval effect on spray,combustion and soot formation,the results are also useful for injection strategies optimization and engine efficiency improvement. |
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