Lean Direct Injection Combustor Research Articles

Experimental and computational analysis of the combustion evolution in direct-injection spark-controlled jet ignition engines fuelled with gaseous fuels

Jet ignition and direct fuel injection are potential enablers of higher-efficiency, cleaner internal combustion engines (ICEs), where very lean mixtures of gaseous fuels could be burned with pollutants formation below Euro 6 levels, efficiencies approaching 50 per cent full load, and small efficiency penalties operating part load. The lean-burn direct-injection (DI) jet ignition ICE uses a fuel injection and mixture ignition system consisting of one main-chamber DI fuel injector and one small jet ignition pre-chamber per engine cylinder. The jet ignition pre-chamber is connected to the main chamber through calibrated orifices and accommodates a second DI fuel injector. In the spark plug version, the jet ignition pre-chamber includes a spark plug which ignites the slightly rich pre-chamber mixture which then, in turn, bulk ignites the ultra-lean stratified main-chamber mixture through the multiple jets of hot reacting gases entering the in-cylinder volume. The paper uses coupled computer-aided engineering and computational fluid dynamics (CFD) simulations to provide better details of the operation of the jet ignition pre-chamber (analysed so far with downstream experiments or stand-alone CFD simulations), thus resulting in a better understanding of the complex interactions between chemistry and turbulence that govern the pre-chamber flow and combustion.

NO x Reduction by Air-Side Versus Fuel-Side Dilution in Hydrogen Diffusion Flame Combustors

Lean-direct-injection (LDI) combustion is being considered at the National Energy Technology Laboratory as a means to attain low NOx emissions in a high-hydrogen gas turbine combustor. Integrated gasification combined cycle (IGCC) plant designs can create a high-hydrogen fuel using a water-gas shift reactor and subsequent CO2 separation. The IGCC’s air separation unit produces a volume of N2 roughly equivalent to the volume of H2 in the gasifier product stream, which can be used to help reduce peak flame temperatures and NOx in the diffusion flame combustor. Placement of this diluent in either the air or fuel streams is a matter of practical importance, and it has not been studied to date for LDI combustion. The current work discusses how diluent placement affects diffusion flame temperatures, residence times, and stability limits, and their resulting effects on NOx emissions. From a peak flame temperature perspective, greater NOx reduction should be attainable with fuel dilution rather than air or independent dilution in any diffusion flame combustor with excess combustion air, due to the complete utilization of the diluent as a heat sink at the flame front, although the importance of this mechanism is shown to diminish as flow conditions approach stoichiometric proportions. For simple LDI combustor designs, residence time scaling relationships yield a lower NOx production potential for fuel-side dilution due to its smaller flame size, whereas air dilution yields a larger air entrainment requirement and a subsequently larger flame, with longer residence times and higher thermal NOx generation. For more complex staged-air LDI combustor designs, the dilution of the primary combustion air at fuel-rich conditions can result in the full utilization of the diluent for reducing the peak flame temperature, while also controlling flame volume and residence time for NOx reduction purposes. However, differential diffusion of hydrogen out of a diluted hydrogen/nitrogen fuel jet can create regions of higher hydrogen content in the immediate vicinity of the fuel injection point than can be attained with the dilution of the air stream, leading to increased flame stability. By this mechanism, fuel-side dilution extends the operating envelope to areas with higher velocities in the experimental configurations tested, where faster mixing rates further reduce flame residence times and NOx emissions. Strategies for accurate computational modeling of LDI combustors’ stability characteristics are also discussed.

System modelling and analysis for design and development of a lean exhaust after-treatment system

A comprehensive system-level modelling methodology for vehicle exhaust and after-treatment system development is presented. Within the constraints required for a development vehicle equipped with a lean-burn direct-injection gasoline engine, system-level analysis was extensively applied to guide the design of the after-treatment system. The size and location of the lean nitrogen oxide (NO x) after-treatment device was determined to achieve the best trade-off between engine back pressure, catalyst light-off, and acceptable thermal ageing under high-speed and high-load conditions. In addition, various after-treatment architecture and layout concepts were efficiently evaluated and screened, requirements for after-treatment components and exhaust thermal management devices were defined, and promising technologies were recommended for further development and testing. Beyond immediate implementation in the development vehicle, several exhaust thermal management strategies (electrically heated converter, lambda split control, engine stop—start, and multiple warm-up) were explored. These technologies have been assessed individually and in combination to formulate effective thermal management strategies under fuel penalty constraints. While the specific applications described in this paper are for a lean-burn gasoline direct-injection development vehicle, the methodology is equally applicable to advanced powertrains with one or more after-treatment devices (diesel engines, alternative fuels, hybrid vehicles, etc.)

Computational analysis of the lean-burn direct-injection jet ignition hydrogen engine

This paper presents a new in-cylinder mixture preparation and ignition system for various gaseous fuels including hydrogen. The system consists of a centrally located direct-injection (DI) injector and a jet ignition (JI) device for combustion of the main chamber (MC) mixture. The fuel is injected in the MC with a new-generation, fast-actuating, high-pressure, high-flowrate DI injector capable of injection shaping and multiple events. This injector produces a bulk lean stratified mixture. The JI system uses a second DI injector to inject a small amount of fuel in a small pre-chamber (PC). A spark plug then ignites a slightly rich mixture. The MC mixture is then bulk ignited through multiple jets of hot reacting gases. Bulk ignition and combustion of the lean jet-controlled stratified MC mixture resulting from coupling DI with JI makes it possible to burn MC mixtures with fuel-to-air equivalence ratios reducing almost to zero for a throttleless control of load diesel-like and high efficiencies over almost the full range of loads. Computations are performed with hydrogen as the PC and MC fuel.

Flame Spectra of a Turbulent Liquid-Fueled Swirl-Stabilized Lean-Direct Injection Combustor

Flame spectra within the UV/visible-light range are measured for a turbulent, liquid-fueled, swirl-stabilized, lean-direct injection combustor. The flame spectra are quite similar to those of lean premixed combustion, which can be attributed to the small droplet size and fast fuel/air mixing. Broadband background emissions around 431 nm, which mainly consist of CO * 2 chemiluminescence, are found to be self-similar with wavelength for both stable and unstable combustion. Chemiluminescence from OH * , CH * , and CO * 2 is found to be a nonlinear function of the airflow rate, the equivalence ratio, preheat temperature, and pressure. Procedures for the determination of the instantaneous heat release rate and equivalence ratios along the flame front are developed. For both combustion instability and forced flame responses, the prediction errors are within 2.0 % in the mean heat release rate, the mean air consumption rate, and the mean equivalence ratio. The assumption that chemiluminescence is proportional to the instantaneous heat releaser rate is generally invalid. Probably the proportionality is valid only in the weakly turbulent or wrinkled flamelet region in the absence of equivalence ratio variations and strong acoustic oscillations.

The lean burn direct injection jet ignition gas engine

This paper presents a new in-cylinder mixture preparation and ignition system for various fuels including hydrogen, methane and propane. The system comprises a centrally located direct injection (DI) injector and a jet ignition (JI) device for combustion of the main chamber (MC) mixture. The fuel is injected in the MC with a new generation, fast actuating, high pressure, high flow rate DI injector capable of injection shaping and multiple events. This injector produces a bulk, lean stratified mixture. The JI system uses a second DI injector to inject a small amount of fuel in a small pre-chamber (PC). In the spark ignition (SI) version, a spark plug then ignites a slightly rich mixture. In the auto ignition version, a DI injector injects a small amount of higher pressure fuel in the small PC having a hot glow plug (GP) surface, and the fuel auto ignites in the hot air or when in contact with the hot surface. Either way the MC mixture is then bulk ignited through multiple jets of hot reacting gases. Bulk ignition of the lean, jet controlled, stratified MC mixture resulting from coupling DI with JI makes it possible to burn MC mixtures with fuel to air equivalence ratios reducing almost to zero for a throttle-less control of load diesel-like and high efficiencies over almost the full range of loads.

Simulation of spray–turbulence–flame interactions in a lean direct injection combustor

Large-eddy simulation (LES) of a liquid-fueled lean-direct injection (LDI) combustor is carried out by resolving the entire inlet flow path through the swirl vanes and the combustor. A localized dynamic subgrid closure is combined with a subgrid mixing and combustion model so that no adjustable parameters are required. The inflow spray is specified by a Kelvin–Helmholtz (or aerodynamic) breakup model and compared with LES without breakup, where the incoming spray is approximated using measured data just downstream of the injector. Overall, both time-averaged gas and droplet velocity predictions compare well with the measured data. The major impact of breakup is on fuel evaporation in the vicinity of the injector. Further downstream, a broad spectrum of drop sizes are recovered by the breakup simulation and produces spray quality, as in the no-breakup case. It is shown that the vortex breakdown bubble (VBB) is smaller with more intense reverse flow when there is heat release. The swirling shear layer plays a major role in spray dispersion and the VBB provides an efficient flame-holding mechanism to stabilize the flame. Unsteady features such as the efficient dispersion of the spray by the precessing vortex core (PVC) are well captured. Flame structure analysis using the Takeno flame index shows the presence of a diffusion flame in the central portion, whereas premixed burning mode is observed farther away. Instantaneous thermochemical states of fuel–air mixing and oxidation indicate significant departure from the gaseous diffusion limits, consistent with earlier observations. Additionally, particle–particle and particle–gas correlations are analyzed and discussed.

Simulation of spray combustion in a lean-direct injection combustor

Large-eddy simulation (LES) of a liquid-fueled lean-direct injection (LDI) combustor is carried out by resolving the entire inlet flow path through the swirl vanes and the combustor. A localized dynamic subgrid closure is combined with a subgrid mixing and combustion model so that no adjustable parameters are required for both non-reacting and reacting LES. Time-averaged velocity predictions compare well with the measured data. The unsteady flow features that play a major role in spray dispersion, fuel–air mixing and flame stabilization are identified from the simulation data. It is shown that the vortex breakdown bubble (VBB) is smaller with more intense reverse flow when there is heat release. The swirling shear layer plays a major role in spray dispersion and the VBB provides an efficient flameholding mechanism to stabilize the flame.

Numerical Study of Lean-Direct Injection Combustor With Discrete-Jet Swirlers Using Reynolds Stress Model

The flowfield in a lean-direct injection (LDI) combustor with discrete-jet swirlers is described and analyzed using a computational fluid dynamics (CFD) code with a Reynolds stress turbulence model (RSTM). The results from the RSTM are compared to time-averaged laser-Doppler velocimetry (LDV) data, as well as results from the National Combustion Code (NCC) that has a cubic nonlinear κ-ε turbulence model, and from the KIVA code using the standard κ-ε model. The comparisons of results indicate that the RSTM accurately describes the flow details and resolves recirculation zones and high velocity gradients while the κ-ε models are unable to capture most flow structures. This confirms that, within the Reynolds averaging approach, the higher-order RSTM is preferred for simulating complex flowfields where separations, strong anisotropy, and high swirl are present.

Time-Averaged Gradient Control of Thermoacoustic Instabilities

Active control of thermoacoustic instabilities has been an important topic of research for many years. In particular, adaptive-control approaches are continually investigated because of their potential ability to adjust to changing and unknown operating conditions. Most adaptive controllers are model based and require accurate knowledge of the plant to which they are applied; this is currently an extremely challenging solution for the limit-cycling combustion plant. A more versatile controller approach, based on a gradient descent algorithm, is presented here. The time-averaged gradient controller can automatically adapt to changing plants and can assume a variety of different controller architectures and actuator styles, without the need for any explicit knowledge of the plant dynamics. We present the controller in a variety of forms and provide simulation and experimental results illustrating its effectiveness in stabilizing an unstable Rijke tube combustor and a 50-kW kerosene-fueled lean direct-injection combustor.