Particulate formation studies have been conducted using both spark-ignition premixed pre-vaporized (PMPV) operation and spark-ignition direct-injection (SIDI) operation. The effects of fuel chemistry, mixture enrichment, pressure, temperature, and residence time have been characterized for premixed flames using PMPV operation. The influence of fuel physical properties, in-cylinder liquid films, pressure, and residence time were studied using SIDI operation. The PMPV studies demonstrated a non-fuel baseline (NFB) particulate size distribution (PSD) that was insensitive to fuel composition and enrichment below a critical enrichment level. The critical enrichment level needed to generate significant soot above the NFB as a function of fuel type and operating condition was then determined.
A toluene reference fuel (TRF30: 50% isooctane, 20% n-heptane, and 30% toluene by volume) was demonstrated to match the in-cylinder chemical sooting tendencies of a tier II certification grade gasoline due to matching of the gasoline’s aromatic fraction and carbon to hydrogen ratio (C/H). Sensitivity of the sooting tendency to aromatic fraction was validated by testing fuels with varying toluene volume fraction. A PMPV combustion phasing sweep established that extreme changes in the main combustion temperature and pressure result in minor changes in particulate formation. Equivalence ratio sweeps at several loads and two speeds revealed that both increased pressure and in-cylinder residence time reduce the critical enrichment threshold under premixed conditions.
Injection timing sweeps during SIDI operation using EEE and TRF30 proved that the high boiling point components in EEE result in higher particulate emission when compared with TRF30 which vaporizes more easily. Under SIDI operation, lower speed reduced total particle number but produced similar particulate mass due to a tradeoff between increased mixing time and increased residence time. Increased load under stoichiometric SIDI operation was shown to increase particulate with a dependence similar to that seen for PMPV operation at the richest mixtures. Comparisons between the PMPV and SIDI cases provide a method for interpreting the relative importance of chemical and physical parameters in an SI engine at different operating conditions by providing a separation of the effects of fuel chemical sooting tendency from fuel physical property impacts on in-cylinder mixture formation.