A study of hydrous ethanol combustion in an optical central direct injection spark ignition engine

Abstract

The aim of this experimental work was to improve understanding of the influence of hydrous ethanol on combustion in an engine demonstrating a tendency for biased flame migration towards the hotter exhaust walls as often reported for typical modern pent roof design IC engines. The work aimed to uncover the degree of residual water content that can be reasonably tolerated in terms of combustion characteristics in future ethanol SI engines (with the energy required to reduce water levels then potentially reduced).The experiments were performed in a single cylinder optical research engine equipped with a modern central direct injection combustion chamber and Bowditch type optical piston. Results were obtained under part-load engine operating conditions (selected to represent typical highway cruising conditions) with hydrous ethanol at 5%, 12% and 20% volume water. Baseline results were obtained using pure isooctane. High-speed cross-correlated particle image velocimetry was undertaken at 1500rpm under motoring conditions with the intake plenum pressure set to 0.5 bar absolute. The horizontal imaging plane was fixed 10mm below the combustion chamber "fire face". Comparisons were made to CFD computations of the in-cylinder flow. Complimentary flame images were obtained via the "natural light" (chemiluminescence) technique over multiple engine cycles. The flame images revealed the tendency of an iso-octane fueled flame to migrate towards the exhaust side of the combustion chamber, with no complimentary bulk air motion apparent in this area in the horizontal imaging plane. The faster-burning ethanol offset this tendency of the flame to migrate towards the hotter exhaust walls. The fastest combustion rate occurred with pure ethanol, with higher water content (>5%) generally slowing down the flame speed rate to 10.64 m/s from 10.92 of ethanol and offsetting the flame speed/migration benefit (in good agreement with recent laminar burning velocity correlations for hydrous ethanol). When adding 20% water to ethanol the combustion rate was significantly slower (8.2 m/s) with a considerable increase in flame shape distortion as quantified by flame image shape factor values. The results demonstrate how the added water increases flame distortion and leads to higher flame centre displacement. Such flame centre displacement could potentially be offset in the future with a spark plug location biased further towards the intake side of the chamber (albeit sometimes practically constrained by the priorities given to intake valve sizing and local cooling jacket design). The results indicate that ethanol fuels offset such bias flame growth and allow residual water to be tolerated for an equivalent degree of biased flame migration. The implication is reduced fuel production energy and cost required to produce usable ethanol fuels.

The effects of residual gas trapping on part load performance and emissions of a spark ignition direct injection engine fuelled with wet ethanol

“…The optical results showed that the increase in water content decreased the flame distortion and corrugation. Other combustion visualization study showed that the longer combustion duration resulted from lower apparent flame speed of high water-in-ethanol fuels provided more time for flame front distortion caused by in-cylinder flow characteristics [15]. Iso-octane combustion duration was larger than anhydrous ethanol but faster than any wet ethanol mixture.…”

Spray development of iso-octane, ethanol, hydrous ethanol and water from a multi-hole injector under ultra cold fuel temperature conditions

“…However, the high latent heat of ethanol and the presence of water in ethanol that enhances latent heat effects further, reduces air intake charge temperature, and offers higher volumetric efficiency and lower NO X emissions, increases brake thermal efficiency and can decrease unburnt hydrocarbons and CO emissions under certain conditions [13][14][15][16][17]. Some studies have also been performed in optical spark-ignition engines where the decrease in flame speed has been quantified [18,19]. In general 10-15% water content has been considered quite tolerable and with good life-cycle benefits, noting that optimizing engine design to leverage gains in octane number and charge cooling corresponding to water addition on fuel economy could offset the reduction in fuel heating value [20].…”

Exhaust Emission Measurements from a Spark-Ignition Engine Using Fuels with Different Ethanol Content for Aircraft Applications

“… 18 This study also did not utilize alternative fuels for the characterization. Other studies performed tests for gaining a deeper understanding of the combustion processes with hydrous ethanol in a single piston test, 19 emissions and combustion efficiency with n-butanol/kerosene in an SI aircraft engine to full load, 20 and studies on general SI engines, with gasoline ethanol and acetylene fuel mixtures only at partial loads for engine combustion and emission analysis 21 among many others. Still, the findings of higher than 10% ethanol–gasoline mixtures over the full range of aircraft engine operation are not completely understood.…”