Investigations On The Knock Combustion Mechanisms And Combustion Control Strategies Of Medium Duty Gasoline Engines Based On The Diesel Engine Platform

Medium-duty gasoline engines based on the diesel engine platform could achieve the similar low-speed torque characteristics as diesel engines,and reduce the cost of after-treatment due to the stoichiometric combustion,which show good application potential in the field of medium-duty commercial vehicles.However,the characteristics of highly downsizing degree,strong swirl flow field,and large bore could increase the knock tendency,limiting the fuel economy performance of the engine.To this end,this study conducted researches about the high efficiency combustion technology of medium duty gasoline engines based on the diesel engine platform using numerical simulation combined with bench test as the main research method.The mechanisms of knock suppression for medium duty gasoline engines based on the diesel engine platform were revealed,and a high efficiency and clean combustion control strategies for medium duty gasoline engines was proposed based on the cooperation of turbulence,fuel properties and exhaust dilution,which would provide a theoretical basis for the development of high-efficiency gasoline engine combustion theory for medium duty commercial vehicles.Firstly,the flame propagation speed impacts on the auto-ignition combustion mode and knock intensity during knocking combustion at low speed and high load condition were studied,and the potential of rapid combustion technology based on strong tumble to improve the knock-limited thermal efficiency was evaluated.The results show that knock intensity(KI)increases first and thereafter decreases with the increase of spark ignition(SI)flame speed under knocking condition.Four regimes of auto-ignition behaviors: sequential auto-ignition of a single hot spot,sequential autoignition of several hot spots and their interaction,sequential auto-ignition of initial hot spot thereafter a homogeneous autoignition,and flame-induced sequential autoignition are identified as the acceleration of SI flame front.The SI flame speed could affect the auto-ignition mode by controlling the homogeneity of temperature field in the unburned zone,and hence affect KI.During knock-limited combustion cases,there is a trade-off relationship between flame propagation speed and anti-knock performance when increasing tumble ratio(TR),which leads to a decrease followed by an increase in thermal efficiency.When TR comes to a critical value,KI is less sensitive to the variation of spark timing,and combustion characteristics are also insensitive to the variation of TR,resulting nearly the same knock limited spark timing and combustion duration.As a result,the thermal efficiency gradually levels off as improving TR further.Therefore,there is a limit to the improvement of TR in thermal efficiency.Secondly,to reveal the effect of charge motion on knocking combustion and thermal efficiency,five combustion system structures in a gasoline engine based on the flat cylinder head were designed,which could generate different types of large-scale flow field including strong swirl,inclined swirl and strong tumble inside the combustion chamber.The in-cylinder turbulence,knocking combustion and the normal combustion process within the knock boundary under low speed and high load conditions were studied using three-dimensional numerical simulation.The results show that by optimizing the intake port and combustion chamber structures cooperatively,the tumble level and turbulent kinetic energy in the near spark plug region could be improved by 3 times compared to the prototype and hence the initial flame propagation speed can be accelerated.However,the main flame speed is not correlated with turbulence intensity positively,which is affected by the net effect of turbulent kinetic energy and flame front shape.The asymmetrical shape of the flame front due to the tumble effect would prolong the main combustion duration as well as promoting the formation of “hot spots” at the locations where the flame speed is slower.In addition,the impact of flame propagation speed on KI is different under different large-scale flow field structures.For strong swirl combustion system,increasing TR can effectively suppress knock by shortening the residence time of end-gas.However,for inclined swirl and strong tumble combustion systems,the compression heating effect from the flame front is enhanced,and the end-gas reactivity is higher,resulting in a sharp increase in auto-ignition velocity and thus KI.An optimized inclined swirl motion based on the flat cylinder head exhibits 2.76%(absolute value)higher thermal efficiency due to the suppression of knock,reduction of heat losses as well as promotion of flame speed,which is more appropriate for medium duty gasoline engines based on the diesel engine platform.High octane sensitivity(OS)fuels have better knock resistance in highly downsized gasoline engine.but the underlying mechanism still needs to be further explored.Therefore,this study explored the effects of gasoline surrogate fuels with different OS on knock behavior as a function of spark timing(ST)and compression ratio(CR)under boosted high load condition,including a primary reference fuel(PRF),ethanol reference fuels(ERFs),and toluene reference fuels(TRFs).Then the effect of ethanol blending ratio on thermal efficiency was studied by bench test.It’s found that increasing OS to decrease end gas reactivity is conditional.The end-gas reactivity becomes less sensitive to OS with advancing ST.Fuel-specific variations beyond OS play an important role in knock tendency when increasing CR,where RON and OS value is insufficient to describe the fuel anti-knock performance.The difference in the ignition delay time behavior of ERF and TRF fuels under similar OS in the negative temperature coefficient(NTC)region is the main factor affecting the chemical effect of auto-ignition.With the increase of CR,the chemical anti-knock performance of high OS ERF fuel increases,which could combine with charge cooling effects to reduce the charge reactivity,showing better anti-knock potential.Furthermore,increasing the blending ratio of ethanol in gasoline can effectively inhibit knock and improve thermal efficiency.And with the increase of load,the improvement effect is more obvious.Finally,the effects of different combustion control strategies on the thermal efficiency and load boundary expansion of stoichiometric combustion for the medium duty gasoline engine based on the diesel engine platform was studied using bench test.The results show that the combustion system consisting of compound intake ports and the inverted wedge combustion chamber could improve the thermal efficiency most obviously,mainly due to that it could extend the tolerance of EGR and increase the combustion rate under high EGR atmosphere.And with the increase of load,the improvement effect is more obvious.At medium-high load condition,improve the ethanol blending ratio could advance combustion phasing by advancing ST and accelerate the medium-term combustion rate,and hence improve the thermal efficiency.As a result,based on the inclined swirl rapid combustion system,the load at the peak brake thermal efficiency(BTE)could be increased to 1.15 MPa when applying E20 fuel coupled with 23.9% EGR to control combustion phasing,the peak BTE is improved by 1.68%.