Investigation On Intake And Combustion Performance Of Diesel Engine Based On Early Intake Valve Closure

Conventional diesel engine is characterized with diffusion combustion which is mainly affected by fuel-air mixing rate. Local over-rich fuel regions and local high temperature regions are widespread, resulting in high level emissions of NOx and soot. LTC (Low Temperature Combustion) has become research focus, of which some new combustion modes such as HCCI (Homogeneous Charge Compression Ignition) and PCCI (Premixed Charge Compression Ignition) provide simultaneously reduction in NOx and soot, and meanwhile maintain high indicated thermal efficiency. However, difficulties in commercial application of LTC are control of combustion and limited operating range. FVVS (Fully Variable Valve System) can adjust effective compression ratio continuously, thus control combustion phase by regulating compression temperature and pressure. In addition, FVVS is beneficial in increasing engine out power, attracting more and more attention.A fully hydraulic variable valve actuation using EIVC (Early Intake Valve Closing) mode was developed and assembled on a SD2100TA diesel engine to adjust intake mass flow and ECR (Effective Compression Ratio) in large range. The maximum valve lift, opening duration and IVC (Intake Valve Closing) timing were varied. The main work of this paper could be divided into four sections.(1) The effects of EIVC on intake and combustion performance of naturally aspirated diesel engine were investigated. With the advance of IVC, the excess air coefficient and effective compression ratio are decreased, the cylinder pressure and temperature at the end of compression stroke are lowered, the ignition delay is increased, the start of heat release is delayed, the proportion of premixed combustion is increased, the proportion of diffusion combustion is decreased, and the exhaust temperature is increased because of reduction in heat capacity. A significant reduction in peak pressure is attained with small variation in indicated thermal efficiency. The peak pressure is reduced by 26% at 1650r/min and IMEP of 0.64 MPa while IVC is advanced from 30° CAABDC to-50° CAABDC.(2) The effects of EIVC on cyclic variations of combustion were investigated. With the advance of IVC, ECR is decreased, the start of combustion is delayed, the peak pressure is decreased and corresponding crank angle is delayed; While the ECR is decrease to 13, it is observed that the maximum combustion pressure is less than the motored peak pressure and the extreme point is not even formed during expansion stroke in some cycles, which leads to significant cyclic variations. The combination of boost and EIVC to achieve lower combustion temperature and peak pressure shows higher cyclic variations; at the same IVC, cyclic variations are increased with excess air coefficient because of low blends concentration. The further research indicates:the cyclic variations caused by EIVC show strong correlation with the combustion development period (A(p), the greater the △φ, the later the center of heat release (CA50), which leads to large cyclic variations; Advancing the fuel supply angle based on △φ can reduced the cyclic variations and improve indicated thermal efficiency (ηi).(3) Theoretical thermodynamic evolution and experiments were adopted to analyze the potential of extending load range for EIVC matched with boost and intercooling. EIVC makes indicator diagram develop downward and upward, leading to fatter indicator diagram. On the condition of not exceeding limited pressure value, extension of load range is studied under constant intake air mass and excess air coefficient respectively.(4) Gasoline was blended with diesel at volume fractions of 30% and 50%, and the combustion characteristics of blended fuels were investigated under various IVCs. The experimental results show that:both EIVC and increasing gasoline blended ratio prolong ignition delay, retarding CA10 and CA50. With the advance of IVC, peak rate of heat release increases firstly and then decreases, CA10 to CA50 combustion duration is shortened, exhaust gas temperature is enhanced, and cyclic variation is increased. With the increase of gasoline blended ratio, peak rate of heat release is increased, peak pressure is firstly decreased and then increased, exhaust gas temperature is reduced, and cyclic variation is nearly unchanged while IVC is close to that of the baseline. While IVC is advanced to -49℃A ABDC, combustion is deteriorated and there is a turning point for peak rate of heat release. Cool flame is observed at gasoline blended ratio of 50% and IVC of -58℃AABDC.