| The pumping loss of SI engine consumes a lot of energy which leads to the reduction of effective thermal efficiency at part load.The intake air mass of the unthrottled SI engine is controlled by adjusting the valve lift.The throttled is removed in the unthrottled SI engine,which improves the intake pressure and reduced pumping loss.A Fully Hydraulic Variable Valve System(FHVVS)is described in this paper which can realise continuous variation in valve lift,duration,and timing.The system was installed in a four-cylinder port fuel injection(PFI)spark ignition(SI)engine and achieved unthrottled load control through early intake valve close(EIVC).It is found experimentally that the pumping loss of the unthrottled SI engine is reduced at part load.However,its slow and unstable combustion reduced the indicated thermal efficiency.In order to improve combustion efficiency of the unthrottled SI engine at low load,a novel helical valve,which can generate intense swirl in a small valve lift,is proposed.The effect of the intake swirl produced by the helical valve scheme on the indicated thermal efficiency is studied and the combustion characteristics of the unthrottled SI engine are investigated at different loads.The main work of this paper cound be divided into four sections.(1)The unthrottled SI engine was modified and the test bench was built.The FHVVS was installed in BJ486EQ gasoline engine,which achieved unthrottled load control strategy.The engine test bench was built and the experimental equipment and data acquisition system were improved.In addition,the steady flow test bench was built and the experimental data was collected to analyze the characteristic of steady flow.(2)The combustion process and the reason of deterioration of combustion performance were studied in the unthrottled SI engine.It is found that the combustion rate is slow and afterburning becomes more evident,causing lower ηit.When the throttled is removed,the negative pressure in the intake port is disappeared and backflow residual exhaust is reduced which is 0.82mg.The decrease of backflow residual exhaust leads to the deterioration of fuel evaporation and fuel-air mixing in the intake port.The experiment of the effect of intake pressure on the combustion performance shows that the combustion performance deteriorates and indicated thermal efficiency decreases with the improvement of intake pressure,which indicates that the intake pressure has significant influence on fuel-air mixture.(3)In this paper the helical valve is proposed and the steady flow performance is studied.The basic structure and principle of the helical valve is introduced.It is found that the helical valve can generate intense intake swirl at small lift and not generate intense intake swirl at high lift.In addition,the tapered covering plate can increase the ability of generating intake swirl.The scheme of adopting one helical valve and one original valve can generate intense intake swirl and ensure the flow capacity,when Ω is 1.02 at 2mm valve lift.Compared with the masked valve,the helical grooves of helical valve are symmetrical and evenly distributed,and its stiffness is uniform.The helical valve does not cause asymmetrical wear,and does not need a complicated valve rotation preventing mechanism,which is useful for practical applications.(4)The effect of intake swirl on combustion performance in an unthrottled SI engine is studied.The intake swirl generated by the helical valve speeds up combustion rate,reduces COV IMEP and changes CA50 so that it approaches optimal range 5℃A～10℃A ATDC.The experiment with lean mixture and different θig shows that the intake swirl generated by the helical valves,makes the fuel-air mixing more homogeneous and flammable,thus improving combustion stability and reducing cycle variation.In addition,it avoids misfire cycle caused by lean mixture and different θig.The combustion duration is obviously shortened and indicated thermal efficiency is improved when using the helical valve scheme,especially at low load. |
PDF Full Text Request