Petter Dahlander

Effects of Injector Parameters on Mixture Formation for Multi-Hole Nozzles in A Spray-Guided Gasoline DI Engine

This paper focuses on ways of improving the spray formation from spray-guided multi-hole gasoline direct injection injectors. Work has been done both experimentally using laser diagnostic tools and numerically using Computational Fluid Dynamics. Laser Induced Exciplex Fluorescence (LIEF) measurements in a constant pressure spray chamber and optical engine measurements have shown that injectors with 6-hole nozzles and 50-degree umbrella angles are unsuitable for stratified combustion because they produce steep air-fuel ratio gradients and create a spray with overly-deep liquid fuel penetration as well as presence of liquid fuel around the spark plug. In order to study injector performance, numerical calculations using the AVL FIRE™ CFD code were performed. The numerical results indicate that by increasing the injector umbrella angle, the extent of piston wall wetting can be decreased. Also, changing the pattern of holes in the nozzle changes the spray pattern, enabling its optimization with respect to ignition and flame propagation. Furthermore, PDA and Direct Imaging experiments showed that increasing the l/d ratio by reducing the hole diameter resulted in a decrease in the mean droplet sizes (D32). The spray angle was found to increase with decreasing l/d ratios. It has also been shown that by choosing a suitable l/d ratio it is possible to control the local AFR and cross-flow velocity at the spark plug. Copyright

Spray Shape and Atomization Quality of an Outward-Opening Piezo Gasoline DI Injector

The spray formation and consequent atomization of an outward opening piezo-electric gasoline DI injector have been experimentally investigated in a constant pressure spray chamber. The sizes and velocities of the droplets and the resulting spray shape were evaluated, under different boundary conditions, using Planar Mie scattering and Planar Laser-induced Fluorescence (PLIF) in combination with Phase Doppler Anemometry (PDA) analyses and high-speed video photography. The use of piezo-electric actuation for gasoline DI injectors provides an additional means to control the atomization and spray shape that is not available with solenoid-driven injectors such as swirling and multi-hole type injectors. For instance, with piezo injectors up to four injections per cycle are possible, and the fuel flow rate can be controlled by adjusting needle lift. The captured high-speed video images show that a hollow-cone spray forms as the fuel exits the outward-opening nozzle. Shortly after the start of injection, the momentum exchange with the surrounding air creates a recirculation zone at the leading edge of the spray. The images also show that the position and size of this recirculation zone depends on the chamber back pressure and that it too can be controlled by the injection timing. In addition, the PDA analysis shows that the creation of the recirculation zone, with a fuel pressure of 20 MPa, results in good atomization, slow-moving droplets, and helps guide fuel to the spark plug in stratified charge operation. The effects of flash boiling on the injectors sprays were also investigated. The results indicate that flash boiling affects the spray shape less than when multi-hole injectors are used. Furthermore, it has been shown that the use of multiple injections per cycle can decrease the spray penetration and provide an extended window of ignitable mixture at the spark plug.

In-cylinder soot imaging and emissions of stratified combustion in a spark-ignited spray-guided direct-injection gasoline engine

The combustion in a spark-ignited spray-guided gasoline direct-injection engine operating in a stratified mode has been studied by in-cylinder imaging of the fuel, OH*, and soot distributions. Information on the fuel distribution was obtained by laser-induced fluorescence imaging of the aromatic molecules in the gasoline. The OH* and soot distributions were simultaneously visualized by detection of the natural emissions at 306 nm (OH*) and around 530 nm (soot) using two intensified charge-coupled device cameras. In addition to the in-cylinder observations, engine-out soot emissions, NOx, and HC were measured. The engine was operated at a speed of 2000 r/min and an indicated mean effective pressure of 2.5 bar, with a fully open throttle, resulting in a globally lean combustion with a fuel–air equivalence ratio of about 0.25. The gasoline was injected in single or double injections by an outward-opening piezo-actuated injector. The combustion was ignited efficiently at locally fuel-rich conditions. The soot formation and oxidation were investigated for the two injection strategies, each with three injection timings and two fixed ignition timings. The results showed that soot was efficiently formed and oxidized. From the in-cylinder measurements, it could be seen that the soot luminescence intensity quickly rose and then declined, while the combustion temperature was still increasing. Furthermore, the OH* intensity was still increasing as the soot luminescence was declining. The soot incandescence peak intensity occurred at a crank angle degree close to 50 per cent mass burned, and the OH* intensity peak arose later, shortly before the maximum soot temperature around top dead centre (TDC). When the injection timing was retarded, with constant ignition timing with respect to injection, it was found that the total soot luminosity increased. In addition, less OH* chemiluminescence was observed during the decrease of the soot incandescence, implying conditions less favourable for efficient soot oxidation in the later part of the combustion for retarded injections. This was confirmed by the engine-out soot emission measurements, which showed increased soot levels as the injection was retarded. It was also found that fuel impinged on the spark plug during the injections, resulting in a persistent jet flame close to the spark plug in the centre of the cylinder, which is believed to contribute to engine-out soot emissions.

Stratified cold start sprays of gasoline-ethanol blends

Gasoline and gasoline-ethanol sprays from an outward-opening piezo-injector were studied in a constant volume/pressure chamber using high-speed imaging and phase doppler anemometry (PDA) under stratified cold start conditions corresponding to a vehicle ambient temperature of 243 K (-30 °C/-22 °F); in-cylinder air pressure of 5 bar, air temperature of 350 K (-30 °C/-22 °F) and fuel temperature of 243 K. The effects of varying in-cylinder pressure and temperature, fuel injection pressure and fuel temperature on the formation of gasoline, E75 and pure ethanol sprays were investigated. The results indicate that fuel composition affects spray behaviour, but less than expected. Furthermore, varying the temperature of the fuel or the air surrounding the spray also had minor effects. As expected, the fuel injection pressure was found to have the strongest influence on spray formation under stratified conditions.

Multi-hole Injectors for DISI engines: Nozzle Hole Configuration Influence on Spray Formation

High-pressure multi-hole injectors are one candidate injector type for closed-spaced direct injection (DI) gasoline engines. In such a system, the spark plug must be located close to the spray and, during stratified operation, the spray is ignited very soon after the fuel droplets have been vaporized. Thus there are very high demands on the sprays used in such a system. An additional challenge is the positioning of the spark plug relative to the spray; both consistent ignitability and the absence of liquid fuel droplets must be achieved. Many injector parameters influence spray formation; for example, hole diameter, length to hole diameter ratio, nozzle hole configuration etc. This paper investigates the spray formation and spray induced air movement associated with rotational symmetrical and asymmetrical nozzle hole configurations. Four different nozzles with different hole configurations and umbrella angle were investigated both experimentally and numerically in a heated/ pressurized spray chamber. Their influence on spray formation, spray induced air motion, cross-flow velocity, fuel/air ratio, turbulence and cycle-to-cycle variations were studied. It was found that rotational symmetrical configurations produce non-coherent isolated clouds of fuel. If an asymmetrical configuration is used instead (holes positioned along a horseshoe-shaped arc) then, by choosing the injector configuration carefully, it is easier to obtain a coherent fuel cloud; this also facilitates better control over the conditions at the spark plug, for example the fuel/air ratio, cross-flow velocity and turbulence. Furthermore, asymmetrical nozzles benefit from smaller fuel gradients and enhanced mixing between spray plumes as a result of shorter spray plume distance and improved spray-induced air motion. All the nozzles tested produced partially premixed vapor clouds, with cycle-to-cycle variations. These variations may be an important issue for ignition stability in a closed-spaced combustion system.

Experimental Investigation of Soot in a Spray-Guided Single Cylinder GDI Engine Operating in a Stratified Mode

Forthcoming reductions in legal limits for emissions of particle matter (PM) from direct injection engines have increased the need for understanding particle distributions in the engines and the factors affecting them. Therefore, in the presented study the influence on PM-emissions of potentially important factors (fuel injection pressure, load, speed and 50% mass fraction burned phasing) on particle mass, number and size distributions were experimentally investigated. The experimental system was a spray-guided, direct injection, single-cylinder research engine operated in stratified charge mode (using gasoline with 10% ethanol as fuel), under five load and speed settings that are appropriate for stratified combustion. The particle distributions obtained from operating the engine in homogeneous combustion and stratified combustion modes were also compared. The particle distributions were measured using a Cambustion DMS500 fast particle analyzer in combination with a Dekati FPS4000 fine particle sampler and a thermodenuder in all tests except the comparison of distributions under stratified and homogeneous combustion conditions. The sampling system was designed to remove as much of the volatile unburned hydrocarbons as possible in order to sample mostly solid particles. Under all of the stratified operating conditions studied, the results indicate that the particle distribution has a characteristic shape with a tail and one large peak. The operating speed significantly affected the size of the largest particles and the quantity of the particles represented by the tail. An almost linear, positive relationship was found between the load and particle number. Increasing the fuel injection pressure reduced particle numbers whereas combustion phasing had no significant observed effects. More particles were generated in stratified combustion mode than in homogeneous mode.

Flame propagation visualization in a spark-ignition engine using laser-induced fluorescence of cool-flame species

The flame propagation in a spark-ignition engine has been studied using laser-induced fluorescence (LIF) of species formed during the first ignition stage of hydrocarbon combustion. The detected two-dimensional LIF images showed the distribution of unburned regions. For the excitation, two Nd:YAG lasers operating at 355 nm were used for two consecutive measurements within the same engine cycle with adjustable time separation between the pulses. Two ICCD cameras that were synchronized to each of the laser pulses recorded pairs of fluorescence images, i.e. the movement of the flame front could be tracked. It is well known that formaldehyde is excited using a wavelength of 355 nm and a spectral signature of this species was also identified in engine LIF spectra. Programme routines were developed and used for evaluation of the flame propagation velocity from the fluorescence images. This paper presents the potential and the characteristics of the experimental technique as well as the evaluation procedure. The measurements of cool-flame intermediates have also been compared with measurements of fuel-tracer as an indicator of unburned fuel–air mixture. A good agreement between position and shape of the signal areas was obtained at crank angles where both fluorescence signal from cool-flame species excited at 355 nm and added 3-pentanone excited at 266 nm could be detected.

Analysis of Transient Compressible Gas Jets Using High Speed Schlieren Imaging

Transient compressible gas jets, as encountered in direct injection gas fuel engines, have been examined using schlieren visualization. Helium has been injected into air in a pressure chamber to create the jets examined. The structure of the jets is studied from the mean and coefficient of variation of the penetration length, jet width and jet angle. The quantities are calculated by digital image processing of schlieren images captured with a high-speed camera. Injection pressure and chamber pressure have been varied to determine whether they have an effect on the response variables. Design of experiments methods have been used to develop the scheme employed in performing the experiments. The mean normalized penetration length of the jets is found to scale with injection to chamber pressure ratio and is in agreement with a momentum conserving relation given in the literature. The dispersion of the penetration length has been found to be in agreement with a normal distribution. The Turner model for a jet has been found to be in agreement with the observed jets. The jet has been observed to transition to self-similarity within 20 nozzle diameters. The calculated penetration constant Γ has been observed to vary with the pressure ratio and is smaller than previously reported values. The jet angle approaches a constant value for all cases and has also been found to be in approximate agreement with a normal distribution.

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.

Experimental Investigation of Methane Direct Injection with Stratified Charge Combustion in Optical SI Single Cylinder Engine

This paper assesses methane low pressure direct injection with stratified charge in a SI engine to highlight its potential and downsides. Experiments were carried out in a spark ignited single cylinder optical engine with stratified, homogeneous lean and stoichiometric operational mode, with focus on stratified mode. A dual coil ignition system was used in stratified mode in order to achieve sufficient combustion stability. The fuel injection pressure for the methane was 18 bar. Results show that stratified combustion with methane spark ignited direct injection is possible at 18 bar fuel pressure and that the indicated specific fuel consumption in stratified mode was 28% lower compared to the stoichiometric mode. Combustion and emission spectrums during the combustion process were captured with two high-speed video cameras. Combustion images, cylinder pressure data and heat release analysis showed that there are fairly high cycle-to-cycle variations in the combustion. Both blue pre-mixed flame and soot luminescence occurred in the combustion. The occurrence of soot luminescence was also supported by the emission spectrum. Soot formation sources were found to be localized randomly in the bulk flame but not on the piston nor in the vicinity of the spark plug. These findings illustrate the difficulty of achieving proper mixing between air and methane resulting in fairly high cycle-to-cycle variations in the combustion and fuel rich areas which create a source of soot.