Study On The Mechanism Of Laser Induced Spark Ignition Of Liquid Fuel

The development of internal combustion engine has been impacting with environmental and energy issues. Lean combustion technique in gasoline engines can improve fuel economy and emission performance. High-energy ignition is the key to implementation technology, which will aggravate the electrode degradation and erosion. Laser ignition has many potential benefits and has become an issue in research now. The major benefits are greater control over the timing and locations of ignition. Moreover, it is accomplished without electrodes, which allows the lifetime of a laser-ignition system to be significantly longer. What is more, laser induced spark ignition would also allow ignition in multiple locations inside the chamber to shorten the combustion time of lean mixtures. Laser ignition and spark ignition test systems have been built. In this paper, premixed gasoline-air mixture by laser induced spark ignition has been investigated and the results are compared with central electrode spark ignition ones.1. Laser induced spark ignition test system has been built including ignition laser, combustion chamber, temperature and pressure measuring system, heating system and intake and exhaust system. The investigation of spark ignition could be conducted by using the spark plug installed on the metal disc instead of the quartz glass.2. Laser ignition energy has been studied. The research indicates that within the flammable range, the probability increases when the ignition energy increases and100%of ignition could be obtained within equivalence ratios of0.8-1.6. The initial free electrons are produced from impurities in gas mixture because the intensity in the focus (1012W/cm2) is too low to ionize gas molecules via the multiphoton ionization process, which requires higher irradiance (≥1014W/cm2). The distribution of MIE (minimum ignition energy) with the equivalence ratios is U-shape. The MIE for the equivalence ratio of1is13.5mJ and9.5mJ for532nm and1064nm, respectively. Toward both the lean side and the rich side of the equivalence ratio, the MIE is increasing.3. The process of laser induced spark was observed by high-speed schlieren photography. The plasma volume is an elongated ellipsoid, with the major axis orientated along the laser beam, which is due to the exponential decay of the rate of energy absorption by the plasma along the beam. Interactions with waves, the plasma generated deform the ellipsoid of hot gas so that its principal axis is displaced orthogonally. Toroidal ring at left will decay more rapidly than that at the right and the gas velocity toward the laser, generated by this toroidal ring gives rise to a third lobe. Although the third lobe arises before chemical induction period, it plays an important role in enhancing the subsequent spreading of the flame.4. Pressure history in the combustion chamber after laser induced spark ignition was studied. Both of peak pressure and pressure rise rate firstly increases and then decreases with the increase of the equivalence ratio. The maximum value of peak pressure and pressure rise rate, both of532and1064nm, were obtained at equivalence ratio of1.6.5. Comparative study of laser ignition energy and spark ignition energy has been conducted. the distribution of minimum spark ignition energy with the equivalence ratios is also U-shape. The least values of minimum spark ignition energy was obtained at the equivalence ratio of1.6and it was3.76mJ. The minimum ignition energies obtained with a laser-spark ignition are greater than those measured by electrical sparks. The maximum probability for equivalence ratio of0.6was65%for532nm and70%for1064nm by laser ignition, nevertheless, we failed to obtain a successful ignition with spark ignition, no matter how much energy was deposited in the spark. It can be concluded that laser ignition extends the lean flammability limit.