Study On Ignition Characteristic And Mechanism Of Lean Mixture Based On The Flexible Discharge

The carbon neutral technologies has become the focus in China and even the whole world,one of the effective methods is to use the natural gas instead of diesel and gasoline for the power system.With the developing of natural gas filling equipments and the improvement of the natural gas engine,the natural gas engine will meet more opportunities in the fields of ship,road transportation and power generation.For the natural gas engine,diluted combustion can improve the thermal efficiency and reduce NO_x emissions.However,the flame propagation speed decreases and the cycle-to-cycle variation increases with the increase of dilution ratio,leading to partial combustion and even misfire.The in-cylinder flow field should be improved to accelerate the flame propagation,bringing the challenge for the ignition.Therefore,the ignition technology is important for natural gas engines.For some state-of-art ignition technologies like laser ignition,corona ignition,microwave assistant ignition etc.,they are not suitable for commercializing as the reliability and cost.Therefore,it is of great importance to study the ignition characteristic and mechanism for the universal spark ignition.An flexibly discharge technology is built based on the multi-coil.Based on this discharge technology,the ignition characteristic and mechanism are studied for the lean natural gas/air mixture.For the ignition mechanism,the studies include the breakdown processes,the short circuit and restrike phenomena,and the plasma temperatures.For the ignition characteristics of the flexibly discharge technology,the effects of discharge technology on the ignition processes are studied under the constant volume combustion vessel,the rapid compression and expansion machine,and the natural gas engine.For visual tests in the constant volume combustion vessel,the rapid compression and expansion machine,the comparisons include:(1)different discharge energies,(2)power and duration enhancements with similar discharge energies,(3)the continuous discharge and multi-strike.The discharge characteristics of multiple coils connected in series and parallel are firstly studied.The results show that the discharge duration can be increased with the series mode,and the discharge power can be enhanced with the parallel mode.Meanwhile,some advanced discharge strategies such as multi-strike and continuous discharge are developed based on the structure of multi-coil in parallel.Based on these strategies,the ignition mechanisms,i.e.,the breakdown processes,plasma temperatures,short circuit and restrike phenomena are studied.(1)For the breakdown processes,according to the results,the parallel mode could slightly increase the breakdown voltage,but not obvious.Meanwhile,the parallel mode could accelerate the pre-breakdown process and increase the breakdown current.The series mode has little effects on the breakdown voltage and current.The breakdown voltage and current show the nonlinear correlation with the ambient pressure or density and gap size of the electrodes.The breakdown voltage and current are affected by the ambient gas compositions.Nevertheless,the fuel concentration species have little effects on the breakdown voltage when the fuel concentration is applicable for engine operations.Based on these experimental results,a novel expression of the breakdown voltage is developed as a function of ambient pressure,ambient temperature and spark plug gap distance for spark-ignition engine.(2)For the short circuit and restrike phenomena,the results show that the short-circuit and restrike phenomena could be suppressed by increasing discharge power.Increasing spark plug gap size is benefit for suppressing the short circuit and restrike phenomena.Meanwhile,the bigger gap size increases the spark channel length.A new spark plug prototype with the triangular electrode head is proposed,which can increase the length of plasma channel.This is because that the distance of plasma roots increases under the gas flow.(3)Finally,the vibrational and rotational temperatures of the spark plasma are measured based on the spectroscopic diagnostic technology.The vibrational and rotational temperatures measured under atmospheric condition are in the range of 3700-4300 K and 1400-2600 K,respectively.These results show that the plasma is in the state of non-thermal equilibrium.However,the enhanced pressure can accelerate the plasma into the thermal equilibrium state.The plasma temperatures increase with the increase of discharge power,which means that the plasma temperatures can be controlled by discharge energies and strategies.The gas compositions can greatly affect the vibrational and rotational temperatures.The rotational temperature increases with the increase of spark plug gap size,and the highest area of the rotational temperature occurs near the center of the gap.Meanwhile,the rotational temperature near the central electrode is higher than that near the ground electrode.For the ignition characteristics of the flexibly discharge technologies,the studies are carried out at constant volume combustion vessel,rapid compression and expansion machine,and natural gas engine.(1)For the quiescent condition in the constant volume combustion vessel,the results show that the discharge duration,power and strategy have little on the ignition processes when the lean limit is not approached.Once the kernel can be successfully established,the enhanced discharge energy can only slightly accelerate the flame development.However,with the condition of lean limit,the multi-strike strategy has the highest ignition success rate compared to the single strike and continuous discharge.(2)For the flow condition in the rapid compression and expansion machine,enlengthening the discharge duration is more effective than enhancing the discharge power when the gas flow reaches a certain value.Meanwhile,the continuous discharge strategy is more effective than the multi-strike on the ignition processes.The continuous strategy is more benefit for extending the lean limit than the multi-strike.(3)Finally,the confirmatory experiments for the discharge strategies are carried out with a multi-cylinder natural gas engine test bench.The air-fuel-ratio were changed with the fixed best spark timing.For the electrical load of 70%,compared to the single strike of single coil,the lean limit can be extended from 1.51 to 1.61 for the advanced discharge strategies.For the electrical load of 90%,the lean limit can only be extended from 1.66 to 1.69.In addition,multi-strike is benefit for improving the stability of the combustion phase.