Optical Study On Flame Propagation Characteristics Of Laser Ignition Using Ethonal

As more strict regulations on exhaust emissions and limitations on engine fuel consumption are launched,development of more efficient and clean engines are demanded for the sake of Energy Conservation and Emission Reduction policy.Lean burn is effective to increase fuel economy whilst reducing emissions.However,current ignition system performs poorly under lean mixtures by increasing misfire rate as well as cycle coefficient of variations.To ignite lean mixtures,higher the secondary ignition voltage of ignition coil is required,which will surely shorten lifespan and reliability of the ignition system.Under this circumstances,laser ignition is a novel and promising technology and holds many benefits over the spark ignition.For example,the absence of electrodes promotes the development of initial flame kernel without any quenching effect.And laser ignition extends the ignitability of lean mixtures and reduces cycle variations.To further understand the mechanism of laser ignition process and explore the possibilities of applying laser ignition on thermal engines,this thesis carried out a experimental study on laser ignition in comparison to spark ignition in a constant volume combustion bomb as well as a single-cylinder optical engine.In the current thesis,a comparison of flame morphology between laser ignition and spark ignition was studied using the Schlieren method in a constant volume combustion bomb.Effect of laser energy on flame propagation characteristics was discussed.Flame with three lobes was an identical sign of laser ignition.The flame propagation velocities of different directions differs due to a complex interaction of plasma,shock wave and air motion with flame.The instant flame velocity in x-direct of laser ignition could be 10 times the flame velocity of spark ignition under the same initial conditions.Flame images recorded with ultra-high temporal and spatial resolutions showed the existence and development of shock wave.Effect of laser energy and equivalence ratio on shock wave propagation was further studied.Taylor’s Blast Wave Theory performed poorly predicting the radius of shock wave because its basic assumption of large pressure ratio between shock wave front pressure and ambient pressure was not met in a typical laser ignition process under engine-like conditions.Due to this reason,an empirical formula to calculate the development of shock wave was proposed in this work.What’s more,the “overdrive” effect of plasma and air motion on initial flame propagation was studied in details.Based on the experimental studies in this work,the result showed that the effect of plasma on flame kernel lasted for about 1 ms,and this was more dominant with lean mixtures.The effect of air motion on flame kernel lasted longer,for more than 1.5 ms.A comparison between laser ignition and spark ignition was carried out in a singlecylinder optical engine.The optical flame images showed that flame developed faster with laser ignition compared to flame with spark ignition under all conditions(λ = 0.9 ~ 1.3),but this was more dominant with leaner mixtures.It was proven that cycle variation was greatly reduced by using laser ignition under lean mixtures.Similar results were discovered by analyzing in-cylinder pressure data.Besides,the effect of ignition location were discussed as laser ignition was performed in different locations under spark plug electrodes,for laser ignition hold the advantage of highly flexible ignition location.The result showed that the advantage of flexible igniting locations could be adopted to improve engine combustion process.