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EGR (Exhaust gas recirculation) can reduce the pumping loss and improve the thermal efficiency of spark ignition engines. The techniques for combustion enhancement under high EGR rate condition has been required for further improvement of the thermal efficiency. In order to develop the technique of combustion enhancement by turbulence, the influences of turbulence scale on combustion properties, such as probability of flame propagation, EGR limit of flame propagation, flame quenching and combustion duration were investigated under the condition of same turbulence intensity. Experiments were carried out for stoichiometric spherically propagating turbulent i-C8H18/Air/N2 flames using a constant volume vessel. It was clarified that all of these combustion properties were affected by the turbulence scale. The development of spherically propagating turbulent flame during flame propagation was affected by the turbulence scale.

Syngas, is an alternative fuel consisting mainly of hydrogen and carbon monoxide in various proportions. An understanding of the effects of the varying constituents on the combustion characteristics is important for improvement of the thermal efficiency of syngas-fueled engines. The effects of hydrogen concentration and mixture pressure on the turbulent burning velocity of outwardly propagating stoichiometric flames of hydrogen-carbon monoxide-air were studied in a constant volume fan-stirred combustion chamber at a constant mixture temperature of 350 K. The mole fraction of hydrogen in the binary fuel was varied from 0 to 1.0, at mixture pressures of 0.10, 0.25 and 0.50 MPa. The turbulence intensity was kept constant at 3.27 m/s. For fixed mixture pressures, it was found that the turbulent burning velocity increased with an increase in hydrogen fraction primarily due to increase in the unstretched laminar burning velocity.

Outwardly propagating stoichiometric flames of H2/CH4/air were studied in a constant volume fan-stirred combustion chamber in order to investigate the effects of hydrogen concentration on the turbulent burning velocities. The experiments were conducted at mixture temperature of 350 K and mixture pressure of 0.10 MPa. The mole fraction of hydrogen in the binary fuel was varied from 0 to 1.0 for turbulence intensities equal to 1.23, 1.64 and 2.46 m/s. Laminar flames of the mixtures were first investigated to obtain the unstretched laminar burning velocities and the associated Markstein numbers. The unstretched laminar burning velocity increased non-linearly with increase in hydrogen fraction. The Markstein number and the effective Lewis number of the mixtures varied non-monotonically with hydrogen mole fraction. The Markstein number was used to investigate the influence of thermo-diffusive effects on the turbulent burning velocity.

The influence of pressure on spherically propagating premixed turbulent flames was examined with experiments carried out using a constant volume fan-stirred combustion vessel. The effects of mixture strength and turbulence intensity on the propagation and quench of turbulent flames were studied for mixture pressures of 0.10 to 0.50 MPa. Two mechanisms of quench were observed and are discussed for lean and rich flames. Possible correlations of turbulent burning velocity were developed in terms of Lewis and turbulence Reynolds numbers.

Combustion characteristics of the stratified mixture were investingated by the experiments on the combustion of the transient fuel jet and the numerical simulations of counterflow premixed flames. In the experiments, some characteristic features such as “secondary flame” and “bulk quenching” were observed. The secondary flame came out in the burned region after the primary flame had propagated within the fuel jet. The bulk quenching was found to occur in the periphery of the jet due to the low fuel concentration. Then the “flame inertia” was found in the investigation of the flame propagation into the lean region. The experiment was accomplished by the injection of propane into the lean premixed propane-air mixture charge, whose equivalence ratio was less than the lower flammability limit of the premixed mixture. The flame generated in the fuel jet propagated into the lean premixed mixture charge as if it had an “inertia”.

Combustion characteristics of the transient methane jet were investigated using a constant volume bomb. The amount of unburned fuel increased as the ignition timing was delayed. Bulk quenching was found to occur in the trailing part of the jet due to the low fuel concentration. Then the characteristics of the flame propagation into the lean region was investigated. This is accomplished by the injection of methane into the lean methane-air mixture charge, whose equivalence ratio was less than the lower flammability limit of the premixed methane-air mixture. The effects of methane concentration of the charge on the flame propagation was examined. The flame generated in the fuel jet propagated into the lean mixture charge. Though the flame propagated in the lean mixture charge for a longer duration with the increase of its methane concentration, it was quenched in the charge before it reached the chamber wall.