Investigation on the Influence of Organic Polymer Additives on n-Heptane Autoignition

Abstract

Increasing compression ratio of spark ignition (SI) engines can improve fuel efficiency; however, engines with higher compression ratios are more likely to experience knock or pre-ignition, a potentially damaging phenomenon. One method to suppress knock is to use fuel additives; however, traditional organometallic additives like tetraethyl lead, while effective create toxic lead oxide nanoparticles. This study examined the hypothesis that organic polymer nanoparticles could be used as environmentally benign anti-knock additives by scavenging gas-phase radicals responsible for autoignition and then be consumed in the eventual combustion event. The organic polymer polycyclohexylethylene (PCHE) in n-heptane, a single component gasoline fuel surrogate, was experimentally investigated. To imitate fuel atomization and vaporization in a SI engine, a pneumatic atomizer was used to generate PCHE nanoparticles entrained in gaseous n-heptane and air mixtures. Liquid phase concentrations of 0.01, 0.05 and 0.10 percent by weight PCHE in n-heptane were compared to n-heptane with no additive. Particle number as a function of diameter produced by the atomization process was quantified for each solution. In addition, the thermal stability and autoignition characteristics of the additive-doped fuels were examined using a laminar flow reactor. Results of the study showed that the pneumatic atomization process was able to successfully produce solid PCHE nanoparticles. Number of particles increased with additive concentration indicating that the PCHE was miscible in liquid n-heptane. Thermal degradation of the PCHE nanoparticles began when the reactor temperature reached approximately 775 ˚C. Although droplet evaporation produced PCHE nanoparticles in significant concentrations, the reactive experiments showed no difference in autoignition temperature between the additive-containing fuels and n-heptane alone. Although the central hypothesis of the study was not proven for PCHE particles, the same experimental design can be tested with other organic polymers.