In cylinder visualization of stratified combustion of E85 and main sources of soot formation

The combustion process and soot formation in spark ignited spray guided stratified combustion of E85 was investigated in a single cylinder optical engine with direct injection of fuel using an outward opening piezo actuated injector. The effect of engine rotation frequency, fuel quantity, injection sequence and ignition timing was studied. Combustion, soot formation and soot oxidation was analyzed using cylinder pressure measurements, images recorded using high speed video cameras, the flame emission spectrum and OH^∗ chemiluminescence and soot incandescence imaging. A maximum injection duration was found to exist for direct ignition of the fuel spray. Engine rotation frequency had little effect on the initial and maximum rate of combustion. The maximum rate of combustion decreased with increasing cycle fuel mass when a single injection was used. The rate of combustion and indicated mean effective pressure increased and the combustion variability decreased when the single injection was split into multiple injections in close succession to deliver the same total fuel mass and the last fuel spray was ignited. Ignition of the first fuel spray resulted in a more pronounced change. The absence of soot incandescence during the initial flame propagation suggested flame propagation in a partially mixed fuel and air mixture with stoichiometric to fuel lean regions. A single fuel injection resulted in piston pool fires due to fuel spray impingement on the piston and was the primary source of soot formation. The pool fires persisted until after conditions favorable to oxidation of the soot had ended. Soot formation in the gas phase occurred while favorable soot oxidation conditions existed and was efficiently oxidized. The magnitude of the piston pool fires was reduced using multiple injections. The reduction is attributed to a reduction of the fuel spray penetration length and a smaller effective injection orifice area, resulting in a shorter total duration of fuel spray impingement on the piston crown. Soot formation occurred primarily in the gas phase when the first of two fuel sprays was ignited and persisted due to the second fuel spray entering an existing flame leading to fuel rich combustion

Characterization of stratified fuel distribution and charge mixing in a DISI engine using Rayleigh scattering

The stratified fuel distribution and early flame development in a firing spray-guided direct-injection spark-ignition (DISI) engine were characterized applying optical diagnostics. The objectives were to compare effects of single and double injections on the stratified air-fuel mixing and early flame development. Vaporized in-cylinder fuel distributions resulting from both single and double injections before, during and after ignition were selectively visualized applying Rayleigh scattering. The optical investigation of the in-cylinder fuel distributions and early flame propagation corroborated the better mixing, showing that double injections were associated with more evenly distributed fuel, fewer local areas with high fuel concentrations, faster initial flame spread and more even flame propagation (more circular flame spreading). The results support the hypothesis that delivering fuel in closely coupled double injections results in better mixing than corresponding single injections. The improved mixing is believed to stem from the longer time available for mixing of the air and fuel in double injection events

Optical Diagnostics Applied to Internal Combustion Engines: Catalysis, Sprays and Combustion

Gasoline direct injection with spray-guided charge formation is one of the most promising concepts to reduce fuel consumption and CO2 emissions of spark ignited engines. The advantages with the system are owing to un-throttled operation and stratification of the injected fuel resulting in a globally lean combustion. In these engines, the mixture formation, which is controlled by the fuel injection system, play a crucial role for the success of the combustion event. The stratification of the fuel creates locally fuel rich areas promoting efficient soot formation, and one of the drawbacks of stratified combustion is increased engine-out soot levels.In this work optical diagnostics were applied to fuel sprays in order to investigate the overall spray formation, the internal structure and to some extent the rate of evaporation. The formation of hollow cone sprays from an outward opening injector, at conditions corresponding to start-up in stratified mode, was studied in more detail. Sprays of gasoline-ethanol blends and ethanol were compared to sprays of gasoline. The obtained sprays were highly reproducible and had large recirculation zones that were somewhat reduced for low fuel temperatures. Optical diagnostics were also applied to investigate stratified combustion inside a firing engine. The aim was to investigate the sources of soot formation at stratified combustion. The charge formation, and the OH chemiluminescence and soot luminescence of the flame were visualized from below, through a quartz window in the piston. The in-cylinder observations showed that the fuel formed a compact cloud in the centre of the cylinder with fuel rich islands. Soot was efficiently formed and also efficiently oxidized except late in the cycle. The in-cylinder observations were compared to engine out emissions.Engine-out emissions of HC, CO and NOX may be removed by oxidation or reduction promoted by a catalyst. The amount of desorbed species outside a catalyst gives information about the reaction kinetics. In this work the concentration of desorbed OH molecules, formed during the water formation reaction, was measured outside a platinum catalyst using cavity ringdown spectroscopy

Optical Diagnostics Applied to Internal Combustion Engines: Catalysis, Sprays and Combustion

Gasoline direct injection with spray-guided charge formation is one of the most promising concepts to reduce fuel consumption and CO2 emissions of spark ignited engines. The advantages with the system are owing to un-throttled operation and stratification of the injected fuel resulting in a globally lean combustion. In these engines, the mixture formation, which is controlled by the fuel injection system, play a crucial role for the success of the combustion event. The stratification of the fuel creates locally fuel rich areas promoting efficient soot formation, and one of the drawbacks of stratified combustion is increased engine-out soot levels.In this work optical diagnostics were applied to fuel sprays in order to investigate the overall spray formation, the internal structure and to some extent the rate of evaporation. The formation of hollow cone sprays from an outward opening injector, at conditions corresponding to start-up in stratified mode, was studied in more detail. Sprays of gasoline-ethanol blends and ethanol were compared to sprays of gasoline. The obtained sprays were highly reproducible and had large recirculation zones that were somewhat reduced for low fuel temperatures. Optical diagnostics were also applied to investigate stratified combustion inside a firing engine. The aim was to investigate the sources of soot formation at stratified combustion. The charge formation, and the OH chemiluminescence and soot luminescence of the flame were visualized from below, through a quartz window in the piston. The in-cylinder observations showed that the fuel formed a compact cloud in the centre of the cylinder with fuel rich islands. Soot was efficiently formed and also efficiently oxidized except late in the cycle. The in-cylinder observations were compared to engine out emissions.Engine-out emissions of HC, CO and NOX may be removed by oxidation or reduction promoted by a catalyst. The amount of desorbed species outside a catalyst gives information about the reaction kinetics. In this work the concentration of desorbed OH molecules, formed during the water formation reaction, was measured outside a platinum catalyst using cavity ringdown spectroscopy

High-Speed Photography of Stratified Combustion in an Optical GDI Engine for Different Triple Injection Strategies

To contribute to knowledge required to meet new emission requirements, relationships between multiple injection parameters, degrees of fuel stratification, combustion events, work output and flame luminosity (indicative of particulate abundance) were experimentally investigated using a single-cylinder optical GDI engine. A tested hypothesis was that advancing portions of the mass injected would enhance the fuel-air mixing and thus reduce flame luminescence. An outward-opening piezo actuated fuel injector capable of multiple injections was used to inject the fuel using different triple injection strategies, with various combinations of late and earlier injections leading to various degrees of fuel stratification. Sprays and combustion events were captured using two high-speed cameras and cylinder pressure measurements. The data were analyzed to assess effects of fuel stratification on yellow flame luminescence(assumed to be dominated by soot luminescence), flame propagation, jet flames, pool fires and heat release. The combustion phasing, amount of fuel injected and engine speed were kept constant and the engine was unthrottled for all tested cases. Image sectorization was used to analyze events captured in different parts of the cylinder. The results show that the injection strategy influences fuel spray behavior and the combustion in terms of both flame luminescence patterns and work output. Injecting some of the fuel earlier results in increased spray liquid penetration, streakier sprays (due to the lower backpressure), and less intense yellow flame luminescence, but also reductions in work output. The greater portions of fuel injected close to the ignition results in increased soot luminescence

Investigation of Charge Mixing and Stratified Fuel Distribution in a DISI Engine Using Rayleigh Scattering and Numerical Simulations

The stratified fuel distribution and early flame development in a firing spray-guided direct-injection spark-ignition (DISI) engine are characterized applying optical diagnostics.The goal is to compare effects of single and double injections on\ua0the stratified air–fuel mixing and early flame development. Vaporized in-cylinder fuel\ua0distributions resulting from both single and double injections before, during and after\ua0ignition are selectively visualized applying Rayleigh scattering. Reynolds-averaged\ua0Navier–Stokes (RANS) simulations are performed to facilitate interpretation of the\ua0obtained experimental data. Two hypotheses are tested. First, injecting the fuel as\ua0a closely coupled double injections can improve mixing. Second, the better mixing\ua0putatively associated with double injections is mainly due to either a longer mixing\ua0time or higher mixing rate (driven by turbulence generated by the injections). The\ua0optical investigation of the in-cylinder fuel distributions and early flame propagation\ua0corroborated the better mixing, showing that double injections are associated with\ua0more evenly distributed fuel, fewer local areas with high fuel concentrations, faster\ua0initial flame spread and more even flame propagation (more circular flame spreading).\ua0The results from both the experiments and the simulations support the hypothesis\ua0that delivering fuel in closely coupled double injections results in better mixing than\ua0corresponding single injections. According to the simulations, the improved mixing\ua0stems from the longer time available for mixing of the air and fuel in double injection\ua0events, which has stronger effects than the higher computed peak bulk mixing rate\ua0for single injections

Reduction of Soot Formation in an Optical Single-Cylinder Gasoline Direct-Injected Engine Operated in Stratified Mode Using 350 Bar Fuel Injection Pressure, Dual-Coil and High-Frequency Ignition Systems

The current trend toward more fuel efficient vehicles with lower emission levels has prompted development of new combustion techniques for use in gasoline engines. Stratified combustion has been shown to be a promising approach for increasing the fuel efficiency. However, this technique is hampered by drawbacks such as increased particulate and standard emissions. This study attempts to address the issues of increased emission levels by investigating the influence of high frequency ionizing ignition systems, 350 bar fuel injection pressure and various tumble levels on particulate emissions and combustion characteristics in an optical SGDI engine operated in stratified mode on isooctane. Tests were performed at one engine load of 2.63 bar BMEP and speed of 1200 rpm. Combustion was recorded with two high speed color cameras from bottom and side views using optical filters for OH and soot luminescence. The results indicated that increasing the fuel injection pressure led to faster burn as well as a reduction in soot luminescence. The ionizing ignition system generated faster initial combustion. Increasing the tumble level reduced the soot luminescence at all injection pressures, but the influence was largest at the lowest fuel injection pressure. The combination of an ionizing ignition system and high fuel pressure was most beneficial for lowering soot luminescence