Abstract
Using the constant volume combustion vessel simulating the combustion system of 500 cc/cyl class DI gasoline engine, the stratification features in relation to fuel properties of fuel volatility and self-ignitability were studied based on indicated pressure analysis, high- speed combustion observation and fuel vapor concentration measurement inside spray immediately after break up. The nozzle used was a swirl nozzle with 90 spray cone angle. The fuels used were iso-Pentane, iso-Octane and n-Heptane.
The volumetric burning velocity (Sv)max, which has been proven to be useful as a measure of stratification degree in the previous study with gasoline fuel(1), was used for evaluating stratification features. A measurement method of fuel vapor concentration inside spray was developed applying He-Ne laser ray absorption method based on Beer-Lambert law to a vapor liquid two-phase mixture of spray. The decreased strength of the laser ray with an infrared-3.39Êm wave length transmitted through a spray and that with a visual-0.63Êm wave length were measured. The former is resulted from both of absorption by vapor phase and scattering by liquid phase, and the latter is resulted almost from scattering by liquid phase. The vapor concentration was determined by applying the light absorption coefficient of vapor to the deduction of above two values. Vapor phase equivalent concentration of liquid was also determined using the latter value and proposed as a liquid phase evaluating index.
The major burning features indicated by combustion analysis were explained in relation to the concentration behaviors. The high ambient temperature up to a certain point improves the stratification features with any one among the test fuels due to the enhanced vaporization. However, the condition of too high ambient temperature and too high swirl ratio combined with the fuel with high volatility and low self-ignitability brings about a significant adverse effects on (Sv)max. At the room temperature of 300 K, (Sv)max is poor even with iso-Pentane with high volatility.
The fuel vapor concentration decreases sharply against the distance from the spray axis, and also decreases rapidly as time elapses. The fuel vapor concentrations at the time from start of injection of 2`4 ms, when spark plug was discharged in the combustion tests, are near the lean limit of flammable concentration range, or rather in the markedly leaner side than the lean limit. The mixtures, however, were flammable in the combustion tests. This is to be caused by the existence of liquid phase, namely droplets crowd. That is, the rich concentration layers near the surface of droplets assist ignition even if total fuel vapor concentration is very lean.