Optical investigation of ethanol and n-heptane dual-fuel partially premixed combustion in a reciprocating engine.

摘要

The details of the dual-fuel combustion of n-heptane and ethanol were investigated in an optically accessible engine representative of light-duty applications. Experimental work involved the application of optical diagnostics such as high-speed imaging of combustion chemiluminescence and soot radiation, spectral measurements and laser induced fluorescence. At the same time, the experimental conditions have been replicated using numerical simulation of chemically reactive in-cylinder flow.;The numerical simulation results, which showed good agreement with the experimental data across several validation approaches, provided an improved understanding of the mechanism responsible for the control of the rate of combustion for dual-fuel operation. The comparison of ethanol/heptane dual-fuel operation with an n-heptane baseline investigated two possible combustion rate control mechanisms previously identified in literature. The first mechanism relies on the direct impact of auto-ignition characteristics stratification on the rate of combustion and could be termed kinetically controlled. The second mechanism is based on the formation of fast propagating flame fronts and is termed flame propagation control.;While previous studies have identified the kinetic stratification as being the dominant control mechanism, under the conditions used in the current investigation, the opposite conclusion has been reached. This seems to suggest that the mechanism responsible for control of the rate of combustion shifts as a function of the value of local state variables such as temperature, pressure and equivalence ratio and further investigation is warranted.