Numerical Simulation Of The Effect Of Alcohols Addition And Partial Fuel Stratification On HCCI Combustion

Homogeneous Charge Compression Ignition (HCCI), as a new type of combustion technology, with high efficiency,low temperature and ultra emmisions (NOx and PM), has received great attentions. The operating principle of HCCI combines the best features of SI and diesel engines. However, dominated by chemical reaction, it is difficult to control ignition timing, eventually causing engine knock on the high load. It is known that adjusting the ignition timing by changing the fuel properties of mixture gas has performance of easy operation and quick response, which has the potential to be an effective method of controling HCCI combustion. In this paper, two different methods were adopted for controling the process of HCCI combustion based on the different heat release characteristics of fuels when applied on the HCCI combustion.The first method is blending the high octane number fuel ethanol with diesel. The diesel fuel combustion typically has two different stages:the low temperature heat release and the main combustion. And, a multi-zone model is used to study the potential of controlling diesel HCCI ignition time by belending ethanol (in mole) and Exhaust Gas Recirculation (EGR). Diesel is represented by n-Heptane. The effect of ethanol addition on heat release rate, the maximum pressure rise rate and the emissions are numerically calculated. The difference between ethanol and EGR on diesel combustion is compared. The effect of ethanol addition on the active intermediate radicals OH, CH2O and H2O2is analyzed. Main conclusions of this Part:The ethanol addition delays the low temperature heat release (LTHR), and retards the onset of main combustion. The maximum pressure rise rate decreases. For equivalence ratio of0.3, blending57%(in mol) of the ethanol can retard LTHR and the main combustion by about2.4°nd6.3°, respectively. For the same equivalence ratio, further increasing the ethanol addition can reduce the indicated mean effective pressure (IMEP). The blending of the ethanol in diesel HCCI at high equivalence ratio offers the possibility to extend the high load limit. The ethanol addition effectively reduced the pressure rise rate, and the emissions of CO and NOx.The secondary way is to adopt gasoline with one stage of heat release as the subject investigated. Using the CHEMKIN software, the relation between ignite time and equivalence ratio (Φ) is analyzed under different intake pressure, intake temperature and external EGR. And the spray model of gasoline engine is builded by CFD software. Partial Fuel Stratification (PFS) is produced by premixing the majority of the fuel and then directly injecting the remanider (at most20%) in the latter part of the compression stroke. The degree of PFS can be changed by adjusting either amount of direct inject fuel or the timing. The effect of PFS on boosted gasoline HCCI combustion is studied numerically. Main conclusions of this Part:As for gasoline, it is not sensitive to Φ at naturally aspirated conditions, and PFS can not reduce the Ringing Intensity. While, the ignite timing of gasoline is found to become sensitive to Φ at boosted conditions, especially on the low intake temperature, and PFS can effectively reduce the Ringing Intensity. By delaying the inject timing and increasing the amount of inject fuel in sequence, controling the Ringing Intensity, PFS has the potential to extend the high load limit.