Zhenkan Wang

High-Speed Particle Image Velocimetry Measurement of Partially Premixed Combustion (PPC) in a Light Duty Engine for Different Injection Strategies

It has been proven that partially premixed combustion (PPC) has the capability of high combustion efficiency with low soot and NOx emissions, which meet the requirements of increasingly restricted emission regulations. In order to obtain more homogenous combustion and longer ignition delay in PPC, different fuel injection strategies were employed which could affect the fuel air mixing and control the combustion. In the present work, a light duty optical diesel engine was used to conduct high speed particle image velocimetry (PIV) for single, double and triple injections with different timings. A quartz piston and a cylinder liner were installed in the Bowditch configuration to enable optical access. The geometry of the quartz piston crown is based on the standard diesel combustion chamber design for this commercial passenger car engine, including a re-entrant bowl shape. The severe image distortions caused by the optical piston shape are minimized through recordings of reference targets and an image dewarping algorithm. To the authors knowledge this is the first time the flow field inside such realistic re-entrant piston bowl has been mapped through high speed PIV. PRF 70 was used as fuel in these measurements. The in-cylinder flow field was evaluated and investigated with high temporal and spatial resolution to provide additional understanding of the fuel air mixing process. Formation of the vortices and turbulence enhance the air fuel interaction. The vector field of 40 consecutive cycles, mean velocity, and turbulence kinetic energy were calculated and evaluated. All the results based on PIV experiment also provide a quantitative dataset being useful for model validation of the computational fluid dynamics (CFD) simulation of this PPC engine.

Effects of Injection Strategies on Fluid Flow and Turbulence in Partially Premixed Combustion (PPC) in a Light Duty Engine

Partially premixed combustion (PPC) is used to meet the increasing demands of emission legislation and to improve fuel efficiency. With gasoline fuels, PPC has the advantage of a longer premixed duration of the fuel/air mixture, which prevents soot formation. In addition, the overall combustion stability can be increased with a longer ignition delay, providing proper fuel injection strategies. In this work, the effects of multiple injections on the generation of in-cylinder turbulence at a single swirl ratio are investigated. High-speed particle image velocimetry (PIV) is conducted in an optical direct-injection (DI) engine to obtain the turbulence structure during fired conditions. Primary reference fuel (PRF) 70 (30% n-heptane and 70% iso-octane) is used as the PPC fuel. In order to maintain the in-cylinder flow as similarly as possible to the flow that would exist in a production engine, the quartz piston retains a realistic bowl geometry. The distortion caused by the complex shape of the optical piston is corrected by an advanced image-dewarping algorithm. The in-cylinder charge motion is evaluated and investigated over a range of crank angles in the compression and expansion strokes in order to understand the turbulence level, especially the late-cycle turbulence. The results show the spatial and temporal development of the flow-field structures in the piston bowl. The PIV data, obtained in the vertical plan, are used to calculate the ensemble average velocity turbulent kinetic energy (TKE), cycle-resolved turbulence, and mean velocity of the instantaneous fluid motion.

Evaluation of optical arrangements for ballistic imaging in sprays

This work investigates the imaging performance, in terms of contrast and resolution, of two different time-gated ballistic imaging setups commonly used in spray research. It is shown that the two setups generate similar spatial resolution in the presence of scattering media. The simpler (2f) setup, however, is less sensitive to component misalignments and time-gate induced aberrations than the commonly used (4f) system. Measurements comparing both arrangements indicated slightly higher contrast for the 2f system under the densest conditions for small scatterers. Subsequent computational modeling confirmed the observed tolerance of the 2f system to misalignment and gate effects. The best performing setup was also compared experimentally to its non-time-gated shadow-imaging equivalent, to establish when the time-gate enhances imaging performance. It is shown that the time-gated setup generates higher contrast under almost all of the scattering conditions tested, while the non-time-gated setup generates higher spatial resolution only in the lower scatterer size range at the lowest scatterer concentrations.

Transition from HCCI to PPC: Investigation of Fuel Distribution by Planar Laser Induced Fluorescence (PLIF)

In a previous study, in order to investigate the effect of charge stratification on combustion behavior such as combustion efficiency and combustion phasing which also largely affects the emissions, an experiment was conducted in a heavy-duty compression ignition (CI) metal engine. The engine behavior and emission characteristics were studied in the transition from HCCI mode to PPC mode by varying the start of injection (SOI) timing. To gain more detailed information of the mixing process, in-cylinder laser diagnostic measurements, namely fuel-tracer planar laser induced fluorescence (PLIF) imaging, were conducted in an optical version of the heavy-duty CI engine mentioned above. To the authors’ best knowledge, this is the first time to perform fuel-tracer PLIF measurements in an optical engine with a close to production bowl in piston combustion chamber, under transition conditions from HCCI to PPC mode. Results show that four mixing schemes can be distinguished as the SOI timings are varied during the transition. They are linked to the results presented in the reference paper, where emissions were varied in different zones. For SOI at -100 crank angle degree (CAD), fuel distribution is homogeneous as expected. With other SOI timings, a significant part of the fuel mixture was trapped in the squish region and crevice area before start of combustion (SOC) as shown by PLIF results. The observations in the reference metal engine paper were confirmed by this measurement. Results are also in good agreement with computational fluid dynamics (CFD) simulations performed for this engine. (Less)

Lift-Off Lengths in an Optical Heavy-Duty Engine Operated at High Load with Low and High Octane Number Fuels

The influence of the ignition quality of diesel-and gasoline-like fuels on the lift-off length of the jet were investigated in an optical heavy duty engine. The engine was operated at a load of 22 bar IMEPg and 1200 rpm. A production type injector with standard holes were used. The lift-off length was recorded with high speed video Different injection pressures and inlet temperatures were used to affect conditions that consequently affect the lift-off length. No matter which fuel used nor injection pressure or inlet temperature, all lift-off lengths showed equal or close to equal lift-off length when stabilized. The higher octane fuel had a longer ignition delay and therefore the fuel penetrate the combustion chamber before auto ignition. This gave a longer lift-off length at the initial stage of combustion before reaching the same stabilized lift-off length. These results indicate that the hot combustion gases are a dominant factor to the lift-off length. Also, that possible soot reductions using high octanes fuels are feasible because of a longer ignition delay that allow more premixing, and an initially longer lift-off length due to longer penetration into the combustion chamber. (Less)

Effects of Post-Injections Strategies on UHC and CO at Gasoline PPC Conditions in a Heavy-Duty Optical Engine

Gasoline partially premixed combustion (PPC) has shown potential in terms of high efficiency with low emissions of oxides of nitrogen (NOx) and soot. Despite these benefits, emissions of unburned hydrocarbons (UHC) and carbon monoxide (CO) are the main shortcomings of the concept. These are caused, among other things, by overlean zones near the injector tip and injector dribble. Previous diesel low temperature combustion (LTC) research has demonstrated post injections to be an effective strategy to mitigate these emissions. The main objective of this work is to investigate the impact of post injections on CO and UHC emissions in a quiescent (non-swirling) combustion system. A blend of primary reference fuels, PRF87, having properties similar to US pump gasoline was used at PPC conditions in a heavy duty optical engine. The start of the main injection was maintained constant. Dwell and mass repartition between the main and post injections were varied to evaluate their effect. All points were run at 7 bar IMEPg. High-speed imaging of the natural combustion luminescence was performed together with measurements of performance and engine out emissions. Results show reduction in both CO and UHC with close coupled injections. A large close coupled post injection show the largest reduction in UHC. Analysis show that a post injection prior to combustion reduces the dribble and increases the recirculation in the downstream region of the fuel jet, hence reaching more of the UHC in the area near the injector. General observations show that the partition of fuel between the injections have the largest impact on the CO while the dwell time affects UHC emissions. Injector dribble seems to be a significant contributor to the UHC emissions. (Less)

Ultra-high-speed PLIF imaging for simultaneous visualization of multiple species in turbulent flames

In order to obtain more detailed characteristics and information in highly turbulent flames, for a better understanding of the transient behavior of eddies in such flames, a measurement technique with sufficient temporal resolution is requested. However, the probing of species distributions relevant in combustion (e.g. OH, CH2O) with ultra-high-speed laser diagnostics still remains a challenge. Nd:YAG clusters commercially available can generate only 4-8 pulses, although with high laser energy. Systems based on a diode-pumped solid-state Nd:YAG laser combined with a dye laser produce only about 100 μJ pulse energy at ultra-high repetition rates (≥50 kHz). Even more comprehensive information on the flame structure can be gained if simultaneous recording of multi-species is performed. In the present work, the development of the first ultra-high-speed diagnostic technique capable of simultaneous probing of hydroxyl radicals and formaldehyde distributions at a repetition rate of 50 kHz is outlined. This has been achieved by employing a burst laser pumped optical parametric oscillator system for the simultaneous detection of CH2O excited at 355 nm and OH-radicals excited at 283 nm, where the interference of scattering laser light can be avoided. The applicability of the proposed technique was demonstrated in a highly turbulent jet flame. Moreover, the presented improvement in terms of the number of consecutive images recorded with ultra-high-speed planar laser induced fluorescence imaging is significant. Due to the high temporal resolution, the movement of CH2O pocket enclosed by OH at the flame tip can be clearly captured. The transport velocity of the CH2O pocket was calculated and found to be in good agreement with previous LDV results.

Optical Arrangements for Time-Gated Ballistic Imaging

We report on a comparison of two optical setups used in time-gated ballistic imaging simulating monodisperse scattering environments with polystyrene spheres in different sizes and concentrations suspended in water.