Investigation on the Self-Stabilization Feature of HCCI Combustion 2014-01-2663
The combustion timing, work output and in-cylinder peak pressure for HCCI engines often converge to a stable equilibrium point, which implies that the HCCI combustion may have a self-stabilization feature. It is thought that this behavior is due to the competing residual-induced heating and dilution of the reactant gas. As one of the most important features of HCCI combustion, the self-stabilization behavior can give great guidance to people for designing controller for HCCI engine control.
The self-stabilization features of HCCI combustion had been observed by many researchers and mentioned in some publications. However, there is no report to experimentally analyze this phenomenon individually. Due to the fuel injection normally ending during the NVO process and the spark plug is turned off for HCCI engines, there is no direct control approach between the Intake Valve Close (IVC) and the start of combustion. The experimental proof or numerical validation of this feature can deliver great confidence to engine designer for controlling the HCCI combustion more reliably and efficiently.
In this paper, the self-stabilization feature of HCCI combustion is discussed by experimental studies. The detailed analysis and discussion are given. To study the HCCI self-stabilization behavior experimentally, the experimental data for COVs of cylinder B2 in the HCCI engine is used. Transient responses with different operating condition changes are utilized as indicators of stability. The change of COV indicates how the engine is stabilized from one relatively stable point to another. From the experimental study, it is found that the higher engine speed leads to faster stabilization, and higher COVs. Moreover, the transient process at higher engine speeds is smoother than at lower engine speeds with the same IMEP change amplitude. Furthermore, during the transient process, larger IMEP change amplitudes leads to higher overshoot, i.e. a more severe transient process. It is also observed that for higher IMEP the COVs are higher. From the experimental comparison between SI and HCCI combustion, it is found that the HCCI combustion has a smoother performance than that of SI engines when the engine operating condition is changed. Compared with SI combustion, the peak pressure and MFB50 location fluctuation of HCCI combustion is much smaller. It is mainly because the HCCI combustion is a continuous cycle-based self-tuning process; the combustion performance of the current engine cycle is affected by the preceding engine cycle and will affect the following engine cycle. Therefore, it is able to consider the in-cylinder conditions as cycle-based continuous control signals.