Investigation Of The In-cylinder Flow Field In A Diesel/Natural Gas Engine

The severe energy situation,the pollution of automobile emission and the increasingly stringent emission regulations have promoted the technological development of internal combustion engines.Diesel pilot ignited natural gas engine has been widely studied for its outstanding advantages in terms of use cost,emission cleanliness and high thermal efficiency.However,some problems limit its extensive applications,such as high THC emissions,low thermal efficiency at low and medium loads,engine knock at high load.In-cylinder gas flow and distribution directly determines the combustion effect.And the engine speed,intake and exhaust system,fuel supply strategy,the shape and size of the combustion chamber and so on influence the gas flow directly.The investigation is based on a pilot ignited natural gas engine which modified from diesel engine,using simulation platform,then the in-cylinder flow trajectory,mixture formation and combustion process are studied,which provides data for a deeply understanding and rationally organizing in-cylinder flow process.Firstly,in 1335 r/min,238N·m,in-cylinder flow was analyzed.The results show that the mixture is rotational flow in initial intake process,otherwise with tiny vortex,and rotation inertia can continue to the work stroke.The average turbulent kinetic energy has two main peaks while intake valve open and diesel injection,and the high turbulent kinetic energy in intake stroke cannot continue to the later stage of compression stroke,and dissipate quickly.The natural gas mainly mixes axially in intake stroke then radially in compression stroke,finally,a "weakly layered" is formed.The diesel is too narrow distribution and burning fast to fully play the role of pilot-ignition.And most of the diesel is distributed in the dilute area of natural gas,flame propagation is slow,in addition,gas flow is weak at the bottom of the combustion chamber,near the wall and at the head center of cylinder,most of mixture is not burned in these areas.Above is the reason for natural gas incomplete combustion,and it also provides the direction for organizing the flow field rationally.Based on the two conditions of 1335r/min,238N·m and 1656 r/min,286N·m,the single variable of injection timing,injection pressure,chamber shape and natural gas inject time is simulated.When advancing injection timing,radius of the tumble underneath the oil beam is bigger,diesel diffuses more rapidly to cylinder wall and chamber wall,turbulent kinetic energy caused by diesel injection increases,but the whole turbulent kinetic energy level is related to the turbulent kinetic energy while fuel injecting.Diesel is not uniformly distributed in the cylinder,at the same time,natural gas is denser from top to bottom of engine,the flame propagates more quickly.So,CA50 phase of natural gas and total fuel moves forward,and the residual natural gas decreases.Increasing injection pressure can improve the turbulent kinetic energy caused by diesel injection,but the increase scope is limited,and it can improve the diesel evaporation quality significantly,diesel earlier arrived in the surface of chamber,thus changing the flame propagation path,CA10 and CA50 phase of diesel and natural gas become earlier,the surplus of them are both reduced.Proper matching of injection pressure and injection timing can achieve good fuel distribution and combustion effect.Reasonable chamber design can significantly improve the turbulent kinetic energy of later stage of compression stroke and work stroke,chamber 2# enhances tumble motion,and chamber 3# enhances swirl motion,both of them make diesel steam extend to the bottom of the chamber,natural gas becomes thickened from inside to edge of chamber,the wide range turbulent motion reduces the diesel combustion rate,besides,increasing the natural gas combustion rate,residual natural gas of chamber 3# is reduced by 9% finally.It is obvious that natural gas inject time affects gas flow around intake valve opening maximum.CA10 and CA50 phase of natural gas moves forward,mainly due to natural gas changes thinner from top to bottom of cylinder while delaying injecting,the burning speed is accelerated when passing concentrated zone to dilute zone.In terms of the impact on the turbulent kinetic energy,flow of the intake stroke is mostly sensitive to the natural gas inject time,and flow of the compression stroke and the work stroke is mostly sensitive to the chamber shape.