Optical Study On Jet Ignition With Pre-Chamber In Natural Gas Engine

The characteristics of natural gas make it a suitable alternative motor fuel as it's good performance in abundant reserves,readily complete combustion,less emissions of harmful gases and high antiknock value.However,it's laminar flame speed is relatively slower comparing with other motor fuels,causing long sustained combustion period and severe post-combustion in natural gas engine.This problem is more worse for large bore engine which even occurs “piston crown ablation”.Meanwhile,natural gas has high ignition energy and misfire easily happens for conventional spark plug ignition as the mixture is lean.The pre-chamber jets contain a large amount of active free radicals and high energy which helps to fulfill multi-point ignition,effectively accelerating flame propagation speed.The pre-chamber gets widespread use in natural gas engine especially for large bore engine.In order to study the role of active free radicals in the jet ignition,an experiment based on the constant volume vessel and optical engine was performed.An idea that facilitating the gather of active free radicals was put forward and ignition zones can be controlled through this method.The system of the constant volume vessel was successfully set up.Two different structure types,namely double-deck and intersecting jet pre-chamber,were designed to fulfill the aim of the gather of active free radicals.For double-deck pre-chamber,the ignition zones lie in the vicinity of holes of outer deck and it increases the lean-burn limit(excess air ratio)by up to 0.3 comparing with single-deck pre-chamber.For intersecting jet pre-chamber,the ignition zones are above(considering the direction of jet movement as upside)regional intersection but also adjacent to it.For non-intersecting jet pre-chamber,the ignition zones are in the much higher location and more disperse and its lean-burn limit is reduced by 0.1 comparing with the intersecting jet pre-chamber.When the regional intersection of jets is close to the hole of pre-chamber,the pre-chamber shows increased ability in igniting lean mixture.Increasing the distance of regional intersection of jets can reduce the main combustion period badly.Before ignition taking place,more space is covered with jet,meaning the shorter main combustion period.The results of experiment about double-deck andintersecting jet pre-chamber demonstrates that the gather of active free radicals can control the ignition zones and expand the lean-burn limit.In the experiment of optical engine,part of a cylinder head of diesel engine was transformed to be used in spark plug pre-chamber ignition.The picture shows that ignition zones lies in the location of intersecting jet and the condition of ignition zone is distinct from the conventional pre-chamber design.When the diameter of hole is 0.8mm,intersecting jet pre-chamber demonstrates faster heat release in the initial stage of combustion comparing with non-intersecting jet pre-chamber,but it has longer time concerning the stage of 50%-90% mass fraction burned.As the diameter of hole is1.5mm,there is no distinct differences between them.When the pre-chamber jets are both intersecting,the delay time is longer when the diameter of holes are small and the main combustion period has reverse phenomenon.