Study on burning velocity of LPG fuel in a constant volume combustion chamber and an SI engine

Compared with petroleum fuel, liquefied petroleum gas (LPG) demonstrates advantages in low CO₂ emission. This is because of propane (C ₃ H₈), n -butane (n -C₄ H₁₀) and i -butane (i -C₄ H₁₀), which are the main components of LPG, making H/C ratio higher. In addition, LPG is suitable for high efficient operation of a spark ignition (SI) engine due to its higher research octane number (RON). Because of these advantages, that is, diversity of energy source and reduction of CO₂, in the past several years, LPG vehicles have widely used as the alternate gasoline vehicles all over the world. Consequently, it is absolutely essential for the performance increase in LPG vehicles to comprehend combustion characteristics of LPG. In this study, the differences of laminar burning velocity between C₃ H₈, n -C₄ H₁₀, i -C₄ H₁₀ and regular gasoline were evaluated experimentally with the use of a constant volume combustion chamber (CVCC). Similar comparisons were numerically conducted with one-dimensional premixed laminar flame simulation to examine combustion mechanisms of these fuels. The result showed that C₃ H₈ and n -C₄ H₁₀, which were the normal paraffin in LPG, had the characteristics that the laminar burning velocity was faster than those of gasoline and i -C₄ H₁₀. It was also showed that the factor was increase in O, H, OH and C₂ radicals in the flame zone. This radical formation mechanism was also analyzed in this study. Furthermore, the effect of above differences in burning velocity of SI combustion was investigated with an engine bench test and two-zone model analysis. In the results, it was showed that burning velocity of SI combustion was affected by the difference of laminar burning velocity between fuels. Especially, this effect was significant under the condition of low engine speed, because SI combustion in early phase of burning period is under laminar state. Therefore, burning velocity of SI combustion with LPG which is containing propane and n-butane is faster than that with gasoline.