Fundamental Research On Fuels’ Spray And Combustion Characteristics By Optical Technology

Modern IC engines are now facing the challenges of fossil fuel reserves and their environmental effects. By applying advanced technic such as gasoline direct injection (GDI), stratified charge lean burn, Homogeneous Charge Compression Ignition (HCCI), Reactivity Controlled Compression Ignition (RCCI) and developing new alternate fuels can ease the situation. The atomization and combustion characteristics play an important role in the direct injection engines (both gasoline and diesel engines). The atomization and combustion characteristics of different fuels were detailed discussed in this thesis by using different optical diagnostic techniques.The macroscopic and microscopic spray characteristics of 2-methruan (MF) isooctane and ther blend fuel (MF50) were investigated by using high speed imaging and PDPA. Under low injection pressure, the effect of inertial force on penetration length was higher than that of viscous force. Ambient temperature had higher influence on MF than isooctane when compared with their penetration length. Back pressure showed more inhibiting effect on isooctane than MF. For all the fuels, droplet sizes generally showed good symmetry along the axis, the droplet size in the center was generally larger than the droplet size on the edge. For isooctane, its SMD increased with the increase of back pressure while SMD of MF increased first and then decreased.The laminar and turbulent flame characteristics of isooctane, MF and MF50 were studied by using schlieren and optical engine. At all conditions, the laminar burring velocity of MF was highest followed by isooctane and MF50, and their laminar burring velocity increased with the increase of initial temperature. When equivalence ration was less than 1.0, MF always had the smallest Markstein lengths, while when equivalence ration was larger than 1.0, Markstein lengths of all fuels showed no clear relation. The flame area of MF was also faster than isooctane and both of them have a relation with 2/3 degree of their mass fraction burned(MFB). The turbulent flame propagation speed of MF was higher than that of isooctane, and the difference was larger than their laminar flame speed. Fuel itself will mostly affect its turbulent flame propagation speed.The near nozzle spay patterns at the start of the fuel spray of ethanol, winter fuel, diesel, and RME were investigated with an ultra-high speed imaging technique. The commonly seen spray structure during the start of injection included a mushroom and a trail zone. The initial mushroom was generated by the residual fuel in SAC volume. Mushroom length and fuel viscosity were closely related, with higher viscosity leading to a longer mushroom length. The mushroom traveled faster than the trail region at first, leading to the increase of mushroom length. Development of micro cone angle during this transient stage experienced a sudden expansion of cone angle followed by a relatively steady stage. The average spray cone angle fluctuation with time may be considered as a level of instability or as a measure of the cavitation effects during the opening stage. The cone angle greatly depends on the properties of fuels and injection pressure. The pressure drop had less effect on penetration than that of stable stage, and the fuels’ density had more influence on qusi-stage penetration under lower pressure drop.