Premixed Combustion in Spark Ignition Engines and the Influence of Operating Variables

Abstract

Advances in Internal Combustion Engines and Fuel Technologies 4 . . Overview of flame propagation mechanismDetailed observations of development and structure of the flame in SI engines can be made by using direct photographs or other methods such as Schlieren and shadowgraph photography techniques [ , ]. The initial stage of the combustion process is the development of a flame kernel, centred close to the spark-plug electrodes, that grows from the spark discharge with quasispherical, low-irregular surface its outer boundary corresponds to a thin sheet-like developing reaction front that separates burned and unburned gases. Engine combustion takes place in a turbulent environment produced by shear flows set up during the induction stroke and then modified during compression. Initially, the flame kernel is too small to incorporate most of the turbulence length scales available and, therefore, it is virtually not aware of the velocity fluctuations [ ]. Only the smallest scales of turbulence may influence the growing kernel, whereas bigger scales are presumed to only convect the flame-ball bodily the initial burning characteristics are similar to those found in a quiescent environment a laminar-like combustion development . As the kernel expands, it progressively experiences larger turbulent structures and the reaction front becomes increasingly wrinkled. During the main combustion stage, the thin reaction sheet becomes highly wrinkled and convoluted and the reaction zone, which separates burned and unburned gases, has been described as a thick turbulent flame brush. While the thickness of the initial sheet-like reaction front is of the order of . mm, the overall thickness of this turbulent flame brush can reach several millimetres this would depend on type of fuel, equivalence ratio and level of turbulence. The turbulent flow field, in particular velocity fluctuations, determines a conspicuous rate of entrainment in the reaction zone, which has been described [ , ] as being composed of many small pockets and isolated island of unburned gas within highly marked wrinkles that characterize a thin multi-connected reaction sheet. Theories have been advanced that describe the local boundary layer of this region as a quasi-spherical flame front, which diffuses outwards with laminar flame speed [ ].Gillespie and co-workers provide a useful review of those aspects of laminar and turbulent flame propagation, which are relevant to SI engines combustion [ ]. Similarly to laminar-like combustion taking place in a quiescent environment, two main definitions of time-based combustion rate can be proposed for turbulent combustion. The first one relates to the rate of formation of burned products velocity. The dependence of the combustion rate on turbulence is embodied in the velocity term, which is fundamentally modelled as a function of turbulence intensity, u ' , and laminar burning velocity, S L . The latter, loosely addressed to as laminar flame velocity in the context of simplified flame propagation models, has been demonstra...

Fuel economy analysis of part-load variable camshaft timing strategies in two modern small-capacity spark ignition engines

“…As clearly suggested by the COV data, a strong intake-to-exhaust pressure differential enables considerable internal burned gas recirculation and high levels of cylinder charge dilution; virtually, any valve overlap duration in excess of 30 CA deg results in degraded combustion and worsened fuel economy. As early IVO and late EVC are reported to exert similar influence upon charge dilution at low engine load[6], the experimental data suggest the effects of increased effective expansion ratio (more work extracted per cycle) would be more significant than those associated to increased effective compression ratio. As the engine speed is increased at low load for engine B(Figure 5), the overlap duration reduces in real time, limiting the amount of internal recirculation; here combustion regains stability and further retarding the EVC timing becomes beneficial.…”

“…At low engine speed, wide overlaps are generally detrimental, yielding higher residual gas fraction [8] which may significantly degrade the combustion stability. Improvements in Brake Specific Fuel Consumption (BSFC) of up to 10%, as a result of early Intake Valve Opening (IVO) strategies, have been reported in recent studies [9,10]; these explain the observed changes referring to the displacement of fresh mixture with residuals during valve overlap, ultimately reducing the need for throttling [6]. If the intake cam profile is fixed, the effects of IVO are inevitably linked to those of Intake Valve Closing (IVC) timing.…”

“…Today's engine technical developments are primarily focused on improved part-load driving performance and fuel consumption with stoichiometric operation [5,6]. One way of targeting these goals is through the implementation of VCT systems.…”