Gerald A. Szekely

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.

Combustion Characteristics of a Spray-Guided Direct-Injection Stratified-Charge Engine with a High-Squish Piston

This work describes an experimental investigation on the stratified combustion and engine-out emissions characteristics of a single-cylinder, spark-ignition, direct-injection, spray-guided engine employing an outward-opening injector, an optimized high-squish, bowled piston, and a variable swirl valve control. Experiments were performed using two different outward-opening injectors with 80° and 90° spray angles, each having a variable injector pintle-lift control allowing different rates of injection. The fuel consumption of the engine was found to improve with decreasing air-swirl motion, increasing spark-plug length, increasing spark energy, and decreasing effective rate of injection, but to be relatively insensitive to fuel-rail pressure in the range of 10-20 MPa. At optimal injection and ignition timings, no misfires were observed in 30,000 consecutive cycles. Multiple pulsing of the injector did not improve upon the single injection-pulse results either by changing the timing of the injection process or by changing the dwell time between injection events.

A Two-Stage Heat-Release Model for Diesel Engines

Modele de calcul du degagement de chaleur dans un moteur diesel considerant deux phases dans la combustion

Combining Instantaneous Temperature Measurements and CFD for Analysis of Fuel Impingement on the DISI Engine Piston Top

A two-pronged experimental and computational study was conducted to explore the formation, transport, and vaporization of a wall film located at the piston surface within a four-valve, pent-roof, direct-injection spark-ignition engine, with the fuel injector located between the two intake valves. Negative temperature swings were observed at three piston locations during early injection, thus confirming the ability of fast-response thermocouples to capture the effects of impingement and heat loss associated with fuel film evaporation. Computational fluid dynamics (CFD) simulation results indicated that the fuel film evaporation process is extremely fast under conditions present during intake. Hence, the heat loss measured on the surface can be directly tied to the heating of the fuel film and its complete evaporation, with the wetted area estimated based on CFD predictions. This finding is critical for estimating the local fuel film thickness from measured heat loss. The simulated fuel film thickness and transport corroborated well temporally and spatially with measurements at thermocouple locations directly in the path of the spray, thus validating the spray and impingement models. Under the strategies tested, up to 23% of fuel injected impinges upon the piston and creates a fuel film with thickness of up to 1.2 μm. In summary, the study demonstrates the usefulness of heat flux measurements to quantitatively characterize the fuel film on the piston top and allows for validation of the CFD code.

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.