Abstract
Mixture Preparation and Combustion in an optically-accessible HCCI, Diesel EnginePlanar laser-induced fluorescence (LIF) imaging techniques have been applied in order to study the mixture preparation and combustion process in a single cylinder, optically-accessible homogeneous charge, compression ignition (HCCI) engine. In particular, the influence of piston bowl geometry on the in-cylinder mixture distribution and subsequent combustion process has been investigated. A new optically-accessible piston design enabled the application of LIF diagnostics directly within the combustion chamber bowl. Firstly, laser-induced exciplex fluorescence (LIEF) was exploited in order to characterise the in-cylinder fuel spray and vapour distribution. Subsequently a detailed study of the twostage CCI combustion process was conducted by a combination of direct chemiluminescence imaging, laser-induced fluorescence (LIF) of the intermediate species formaldehyde (CH2O) which is present during the cool flame and LIF of the OH radical which is subsequently present in the reaction and burned gas zones at higher temperature. Finally, spectrometry measurements were performed with the objective of determining the origin of the emitting species of the chemiluminescence signal. The experiments were performed on a single cylinder optical engine equipped with a direct-injection, common rail injection system and narrow angle injector. The experimental results presented reveal the significant role of the combustion chamber geometry on the mixture preparation and combustion characteristics for late HCCI injection strategies particularly in such cases where liquid impingement is unavoidable. Planar LIF 355 imaging revealed the presence of the intermediate species formaldehyde allowing the temporal and spatial detection of auto-ignition precursors prior to the signal observed by chemiluminescence in the early stages of the cool flame. Formaldehyde was then rapidly consumed at the start of main combustion which was marked not only by the increase in the main heat release but also by the first detection of OH, present within the reaction and burned gas zones. In the case of the flat piston geometry, soot precursors were also detected, as indicated by the strong polycyclic aromatic hydrocarbon (PAH) fluorescence signal observed later in the cycle (from 375 CAD).
Highlights
An improvement in thermal efficiency combined with significant reductions in NOx and soot emissions have been largely responsible for intensifying global research interest into Homogeneous Charge Compression Ignition (HCCI) combustion [1,2,3]
At 327 CAD, Figure 6 clearly shows that liquid fuel impinges on the piston face whilst the corresponding vapour phase images acquired at 327 and 330 CAD tend to indicate a certain degree of fuel stratification through the formation of a fuel rich region which appears to be confined to the centre of the piston following fuel impingement
The effect of piston geometry on the mixture preparation and combustion process has been studied in a direct injection HCCI Diesel engine
Summary
An improvement in thermal efficiency combined with significant reductions in NOx and soot emissions have been largely responsible for intensifying global research interest into Homogeneous Charge Compression Ignition (HCCI) combustion [1,2,3]. One of the major benefits of portinjected, pre-mixed HCCI combustion is that it is not limited by the rates of fuel atomisation, evaporation and mixing at the fuel/air interface which is in contrast to conventional direct injection Diesel engines since auto-ignition occurs whilst fuel is still being injected. In the latter case, combustion is inherently heterogeneous with fuel-rich zones existing in the non-premixed regions whilst the fuel-air charge essentially produces a number of turbulent diffusion flames. In contrast HCCI combustion is dominated to a large extent by local chemical-kinetic reaction rates while it has been proposed that turbulence effects probably have little direct effect local temperature variations might have a secondary influence since the chemical reaction rates are very sensitive to temperature [4]
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