Research on Engine Performance and Chemical Mechanism Model of Butanol-gasoline Blends in High Speed Gasoline Engine
Author:Liu Kai Min
Driven by draining energy sources and increasingly serious environment problems,bio-butanol is considered as a superior substitute fuel for conventional petroleum fuels.Compared with low-carbon alcohols,butanol can be produced by renewable fermentation of various organic materials,and it has much higher energy density than methanol and ethanol,the physicochemical property of butanol has a closer resembalance to current conventional fuels such as gasoline and diesel.This means that it could require minimal or no modifications both to engine design as well as to fuel delivery infrastructure.Therefore,it has important strategic significance to carry out the research on bio-butanol application in internal combustion(IC)engines.Based on a single cylinder,high speed,four strokes,spark ignition(SI)engine,the engine performance and combustion characteristics of butanol-gasoline blends with adopting bench test and numerical simulation,the n-butanol blended ratio is 0%,30%and 35%by volume to gasoline.And then,the brake specific energy consumption(BSEC)saving potential is studied with using genetic algorithm(GA).With adopting the reaction path analysis,sensitivity analysis and semi-decoupling methodology,a reduced chemical kinetic model of toluene reference fuel(TRF)/diisobutylene(DIB)-butanol is developed.This model consists of the fuels iso-octane,n-heptane,toluene,DIB and butanol,and has good prediction accuracy for ignition delay time,laminar flame velocity and in-cylinder combustion characteristics to butanol-gasoline blend fuels.The main research contents and achievements are as follows:(1)The experiment was set up,and the engine performance and operating parameters were measured.According to the theory of in-cylinder thermodynamic process and the numerical solution method of one-dimensional(1-D)unsteady flow.the corresponding GT-Power simulation models of engine fueled with pure gasoline and butanol-gasoline blends were built,and the simulation models were calibrated by experimental data.(2)According to the experimental data and numerical results,the common regularities about the effects of key operating variables such as butanol blend ratio,ignition timing,engine load,exhaust gas recirculation(EGR)rate and compression ratio combined with knock index(KI)on combustion characteristics of butanol-gasoline blends in high speed engine are revealed,and the influences of ignition timing and valve timing on engine performance are also studied.This study can largely extend the operating range and engine type of engine fueled with gasoline-butanol blends,and provides more data support for the promotion and application of butanol.The highest engine speed is 8500 rpm,which rarely appears in other available literatures.The present work can also give the readers a more intuitive explanation that how different operating variable interact with each other,and some listed combustion parameters can present the reasons why other listed combustion parameters show some kind of change trend.(3)The method of GT-Power coupling with MATLAB/Simulink is proposed,and the operating variables under full load are optimized with using GA,which make the BSEC of engine fueled with 35%n-butanol by volume to gasoline improved effectively.Besides,this method can be also applied to calibrate the universal characteristics of engines,and provide guidance in the actual engine development.(4)Applying CHEMKIN PRO zero-dimensional(0-D)engine model,the iso-octane detailed chemical reaction kinetics mechanism is studied with the methods of reaction path analysis and sensitivity analysis.On this basis,a iso-octane reduced mechanism including 37 species and 48 reactions is developed,the mechanism can be applied to simulate the combustion process in HCCI engine.(5)Several typical primary reference fuel(PRF)reduced chemical mechanism are compared,it is found that most of the current reduced mechanism for gasoline surrogate fuel are validated against ignition delay time,but rarely considering the evolution of key species and laminar flame velocity.Thus,the reaction which has greater impact on laminar flame velocity can be easily eliminated with this chemical mechanism developing method.The high coupling between small molecules and radicals will be also destroyed.Furthermore,the prediction of the evolution of key species and laminar flame velocity are not accurate.(6)With adopting the reaction path analysis,sensitivity analysis and semi-decoupling methodology,the macromolecules reaction for iso-octane,n-heptane,toluene and DIB are simplied,and then merged into a new skeletal kinetic model of TRF/DIB,which consists of 68 species and 183 reactions.(7)The TRF/DIB model is applied to simulate actual gasoline surrogate fuel with different liquid volume fraction,and the simulation results are compared with experimental data from shock tube(ST),jet stirred reactor(JSR),lamibar flame speed(LFS)and three-dimensional(3-D)computational fluid dynamics(CFD)combustion.Results show that the predictions of ignition delay time,the evolution of key species,laminar flame velocity and in-cylinder combustion characteristics have good agreement with experiments in collected literatures.(8)A new n-butanol reduced chemical mechanism which consists of 70 species and 150 reactions is developed by using reaction path analysis and sensitivity analysis,and its predictions for ignition delay time and laminar flame velocity have good agreement with experiments and the calculated results of detailed mechanism under different working conditions(initial temperature,pressure and equivalent ratio).(9)The TRF/DIB skeletal model and n-butanol reduced mechanism are merged into a final surrogate mechanism,named TRF/DIB-butanol mechanism hereby,which consists of 113 species and 280 reactions.The TRF/DIB-butanol mechanism is applied to simulate butanol-gasoline blends coupling with 3-D CFD calculation,the study shows that the simulated combustion heat release characteristics with TRF/DIB-butanol mechanism have good agreement with experiments in our high speed gasoline engine fueled with blend fuels.(10)Based on the empirical correction model of laminar flame speed,the power exponent of pressure and temperature is extended to four,which includes equivalent ratio.Then,the fitting constants for the laminar flame speed prediction data of TRF/DIB,n-butanol and blend fuel are resolved within a wide equivalent range.The corrected values of laminar flame speed are close to the prediction data,and the fitting constants are also similar to recommended values proposed by other researchers.