Arun S. Solomon

Piston Fuel Films as a Source of Smoke and Hydrocarbon Emissions from a Wall-Controlled Spark-Ignited Direct-Injection Engine

Thin films of liquid fuel can form on the piston surface in spark-ignited direct-injection (SIDI) engines. These fuel films can result in pool fires that lead to deposit formation and increased hydrocarbon (HC) and smoke emissions. Previous investigations of the effects of piston fuel films on engine-out HC and smoke emissions have been hampered by their inability to measure the fuel-film mass in operating direct-injection engines. In this paper, a recently developed high-speed refractive-index-matching imaging technique is used for quantitative time- and space-resolved measurements of fuel-film mass on a quartz piston window of an optically-accessible direct-injection engine operating over a range of fully-warmed-up stratified-charge conditions with both a high-pressure hollow-cone swirl-type injector and with a high-pressure multihole injector. Measured fuel-film mass is a small percentage of the total fuel injected with the high-pressure swirl injector (maximum of ∼1% with gasoline fuel and ∼0.1% with isooctane fuel). Most of the piston fuel-film mass evaporates during the cycle and burns as a pool fire. These pool fires are observed by endoscopic and through-the-piston imaging, and the occurrence and location of the pool fires are consistent with the measured piston fuel films. The fuel-film data are also correlated with engine-out HC and smoke emissions measurements from a conventional all-metal single-cylinder engine of the same design. Smoke emissions from the engine with a high-pressure swirl injector increase linearly with the measured fuel-film mass. Fuel films are found to be the dominant source of smoke emissions with the swirl injector in this engine, with ∼10% of the wall-film mass converted to emitted smoke mass. Smoke emissions from the engine with a high-pressure multihole injector are very small or zero, consistent with the much smaller measured fuel-film mass (∼0.05% of the injected gasoline fuel volume). In contrast, engine-out HC emissions do not correlate with fuel-film mass. For optimum injection timings, the measured fuel-film mass is so small that even in the unlikely event that all of the fuel film mass was converted to engine-out HC emissions, fuel films could account for less than 15% of the total HC emissions for the swirl injector and less than 2% for the multihole injector. For off-optimum injection timings, the HC emissions are significantly larger, but wall films can account for at most 35% of the unburned HC emissions. This is contrary to some previous studies that claimed fuel films were the largest contributor to HC emissions (∼80%) in stratified SIDI engines. The data from this engine support overmixing as the dominant source of HC emissions for optimum engine operating conditions. However, fuel films may be a significant source of HC emissions for cold start or low-speed engine-operating conditions.

Plasma-Jet Ignition of Fuel Sprays in a Rapid Compression Machine

Plasma-jet ignition of fuel sprays for direct-injection engine application was evaluated in two phases. In the first phase, data on the penetration characteristics of the plasma puff were obtained as a function of the input electrical energy to the igniter and the igniter cavity dimensions over a range of ambient density conditions. In the next phase, the three-dimensional arrangement of the combustion bowl, injector and spark plug in an engine was simulated by a simplified two-dimensional arrangement in a rapid compression machine. High-speed schlieren photography and pressure-time data ware used to compare fuel-spray ignition with plasma jets and results obtained by conventional high-energy ignition (HEI) with a spark plug

Particulate Matter Sampling and Volatile Organic Compound Removal for Characterization of Spark Ignited Direct Injection Engine Emissions

More stringent emissions regulations are continually being proposed to mitigate adverse human health and environmental impacts of internal combustion engines. With that in mind, it has been proposed that vehicular particulate matter (PM) emissions should be regulated based on particle number in addition to particle mass. One aspect of this project is to study different sample handling methods for number based aerosol measurements, specifically, two different methods for removing volatile organic compounds (VOCs). One method is a thermodenuder (TD) and the other is an evaporative chamber/diluter (EvCh). These sample handling methods have been implemented in an engine test cell with a spark ignited direct injection (SIDI) engine. The engine was designed for stoichiometric, homogeneous combustion. SIDI is of particular interest for improved fuel efficiency compared to other SI engines, however, the efficiency benefit comes with greater PM emissions and may therefore be subject to the proposed number based PM regulation. Another aspect of this project is to characterize PM from this engine in terms of particle number and composition.

Optimization of the Stratified-Charge Regime of the Reverse-Tumble Wall-Controlled Gasoline Direct-Injection Engine

An optimum combustion chamber was designed for a reverse-tumble wall-controlled gasoline direct-injection engine by systematically optimizing each design element of the combustion system. The optimization was based on fuel-economy, hydrocarbon, combustion-stability and smoke measurements at a 2000 rev/min test-point representation of road-load operating condition. The combustion-chamber design parameters that were optimized in this study included: piston-bowl depth, piston-bowl opening width, piston-bowl-volume ratio, exhaust-side squish height, bowl-lip draft angle, distance between spark-plug electrode and piston-bowl lip, sparkplug-electrode length, and injector spray-cone angle. No attempt was made to optimize the gross engine parameters such as bore and stroke or the intake system, since this study focused on optimizing a reverse-tumble wall-controlled gasoline direct-injection variant of an existing port-fueled injection engine. The results of this study showed that numerous tradeoffs in engine performance parameters have to be made in selecting the geometric features of the piston bowl, spray angle and spark-plug protrusion in arriving at the optimum design. The results of this study also showed that by preserving certain physical characteristics of the piston, operating characteristics of the engine with that piston could be passed on to other piston designs. A piston bowl that combines the design features of a positive draft-angle lip in the back of the bowl and a negative draft-angle lip on the sides of the bowls generates a hybrid piston bowl that possesses features from both original designs. In the final design, the positive draft-angle lip in the back of the piston bowl as well as its increased bowl depth allowed it to achieve very low smoke. While the negative draft-angle lips on the side of the piston bowl allowed the final piston-bowl design to achieve very low hydrocarbons emissions.