Study On In Cylinder Process Of Dual Fuel Engine Based On Chemical Reaction Dynamics

The internal combustion engine plays an important role in transportation because of its high thermal efficiency and good energy utilization.However,due to the shortage of energy and environmental pollution,the natural gas engine has become a research hotspot,especially the dual fuel engine that diesel ignites natural gas.However,the current natural gas engines supply natural gas into the cylinder in the form of gas,which brings about the following problems In order to solve these problems,the design concept of"liquid injection in LNG cylinder"is proposed,that is,natural gas is directly injected into the cylinder as liquid.In this paper,the R12V280ZJ diesel engine is used as a prototype,and it is converted into a LNG cylinder liquid-injected diesel-ignited natural gas dual-fuel engine.Based on the ANSYS-FORTE software,in order to fully express the common influence of turbulent flow and chemical reaction in the turbulent combustion process of dual-fuel engine,the turbulent flow and chemical reaction mechanism model are coupled in the numerical calculation of the combustion process,and the interaction between the two is considered.Turbulent flow adopts RNG k-?model.The chemical reaction mechanism model is composed of reduced n-heptane mechanism,reduced methane mechanism and Extended Zeldovich mechanism.The interaction between turbulent flow and chemical reaction is established by Kong model.This article is divided into two parts:The first part is to construct the combustion chemical reaction mechanism of diesel/natural gas,First,select the detailed mechanism of diesel engine LLNL3.1 and the detailed mechanism of natural gas GRI3.0 as the reduction objects,based on the direct relationship graph method and its derivative method for mechanism reduction,and in the reduction process,A new strategy is proposed that the threshold is gradually increased for multi-step reduction,and multiple reduction methods are cross-used,The LLNL3.1 is reduced to the reduced mechanism including 162 components and 692 elementary reactions,the component is reduced by about 75.2%,GRI3.0 is reduced to the reduced mechanism including 26components and 122 elementary reactions,the component is reduced by about 50.9%.And compare the simplified mechanism and the detailed mechanism separately to verify the ignition delay,combustion performance in cylinder and emission performance,The results show that the maximum delay error of n-heptane reduced mechanism is within 30%,the prediction error of cylinder pressure,cylinder temperature,CO and CO₂is within 5%,the maximum delay error of methane reduced mechanism is within 10%,and the prediction error of cylinder pressure,cylinder temperature and CO₂is within 4%.For the prediction error of CO,when the equivalence ratio is 0.5,the maximum error is reaching 9%,because only the ignition delay period is selected as the target parameter in the reduction process,which leads to the elimination of substances with high CO correlation when the components are deleted,but overall the forecast is performing well.In the second part,based on ANSYS-FORTE software,the numerical model of the dual fuel engine?3-speed idle condition?is established,and the Extended Zeldovich mechanism is selected as the NO emission model according to the needs.Since there is no relevant experimental data of dual fuel engine,it can only be verified for the original diesel engine,the reduced mechanism of n-heptane and Extended Zeldovich mechanism are introduced into the engine numerical model,the accuracy of the initial conditions and boundary conditions of the numerical model is verified according to the measured cylinder pressure curve and nitrogen oxide curve of the model at the third idle speed,and the effectiveness of CFD software coupled with chemical reaction mechanism,Based on this,the reduced mechanism of n-heptane,the reduced mechanism of methane?NOx removal?and the Extended Zeldovich mechanism are introduced into the established engine numerical model.Firstly,the calculation and analysis of the combustion process of pilot diesel are carried out,The calculation results show that when the amount of pilot diesel injected into the cylinder is 0.5%of the original diesel oil,it can not be compressed,but when the amount of pilot diesel injected into the cylinder is 0.6%?0.9%of the original diesel oil,a small part of the diesel oil is compressed,and when the amount of diesel oil is 1%of the original diesel oil,all the pilot diesel oil can be compressed,forming a stable fire source,With the increase of the initial temperature,the amount of diesel fuel needed for the pilot diesel fuel to be compression ignited will be reduced,and the amount of diesel fuel needed for the pilot diesel fuel to be fully compression ignited and to form a stable fire source will also be reduced,which provides a theoretical basis for the determination of the minimum amount of pilot diesel fuel in the later experimental refitting.When the diesel engine is refitted into a dual fuel engine,LNG substitution through low-calorie value conversion algorithm,Although the pilot diesel fuel quantity is 1%of the original diesel fuel quantity,it can be all compressed and form a fire source,but it can not completely ignite the natural gas which then enters the cylinder,until the pilot diesel fuel quantity is 2.8%of the original diesel fuel quantity,it can completely ignite the natural gas and form a continuous combustion,and the engine can be started The engine can work normally,with the increase of initial temperature,the amount of diesel fuel needed to ignite LNG will decrease.On this basis,the limit replacement rate of LNG for the dual fuel engine is 97.2%,and the numerical calculation of dual fuel engine with different LNG substitution rate?I=97.2%,97%,95%,92.5%,90%?was carried out,and compare the power performance and emission performance with the original diesel engine.The calculation results show that:When the LNG replacement rate is 90%or less,the in-cylinder pressure and the in-cylinder temperature under dual fuel conditions are lower than that of pure diesel,but the production of nitrogen oxides is also less than that of pure diesel.I=90%,the peak in-cylinder pressure drops by about 4.98%,the peak in-cylinder temperature drops by about 2.90%,and NO production drops by about 11.80%.And as the replacement rate of LNG increases,the starting point of natural gas combustion will gradually extend,the pressure in the cylinder and the peak temperature in the cylinder will also decrease,and the generation of nitrogen oxides will also decrease,because LNG enters into the cylinder,which will absorb part of the heat by gasification.With the increase of LNG substitution rate,more and more LNG will be injected into the cylinder,and more and more heat will be absorbed by gasification,which will lead to the decrease of the peak temperature in the cylinder,the delay of the starting point of natural gas combustion and the decrease of NO generation.