Ethanol port injection and dual-fuel combustion in a common-rail diesel engine

摘要

Opposed to a conventional approach of using ethanol in a spark-ignition engine, this study demonstrates a potential of ethanol utilization in a diesel engine using dual-fuel combustion strategy where ethanol is injected into the intake manifold and diesel is directly injected into the combustion chamber. The main focus of this study is the effect of ethanol port fuel injector (PFI) sprays on dual-fuel combustion and emissions. Firstly, details of temporal and spatial development of ethanol PFI sprays were studied using Mie-scattering and high-speed shadowgraph imaging techniques. Momentum flux-based injection rate measurement was also performed. The influences of fuel flow-rate, injection duration, and ambient air cross-flow are of particular interest in an effort to understand ethanol PFI spray characteristics that are relevant to automobile engines. Ethanol sprays are also studied for various PFI positions to examine the potential effect of droplets-airflow interaction and wall wetting. With the clear understanding on ethanol PFI sprays, dual-fuel engine experiments were conducted for various ethanol energy ratios and PFI positions. It is found that the effect of PFI position on global phenomena such as in-cylinder pressure, apparent heat release rate and mean effective pressure is much less significant than the effect of ethanol energy fraction. However, the misfiring limit shows measurable difference such that the PFI position closer to the intake valves results in 10% higher ethanol energy fraction than that of the further upstream position. Reduced wall-wetting due to surface boiling occurring on the hot valve seat is suggested as a possible cause, which is consistent with 30% lower carbon monoxide and 64% lower unburnt hydrocarbon emissions. Detailed investigation for various ethanol energy fractions was also conducted. From the in-cylinder pressure measurements, it is found that the increased ethanol energy fraction increases the engine efficiency up to 10% until it is limited by misfiring. The results are compared to diesel-only operation with varying injection timings in order to explain whether the increased efficiency is due to the combustion phasing or improved combustion associated with fast burning of ethanol. Further analysis of the data reveals that the latter is the primary cause for the efficiency gain. By advancing the diesel injection timing, it is found that the maximum ethanol fraction can be extended to 70% without the misfiring problem but 20% increase in nitrogen oxide emissions is also observed, which raises a question on the advantages of utilizing ethanol in a diesel engine. However, negligible smoke emissions are measured at ethanol energy ratio of 20% or higher suggesting that optimization of these emissions is much easier compared with conventional diesel combustion.