Variations In Spark Ignition Engines Research Articles

DNS study of the global heat release rate during early flame kernel development under engine conditions

Despite the high technical relevance of early flame kernel development for the reduction of cycle-to-cycle variations in spark ignition engines, there is still a need for a better fundamental understanding of the governing in-cylinder phenomena in order to enable resilient early flame growth. To isolate the effects of small- and large-scale turbulent flow motion on the young flame kernel, a three-dimensional DNS database has been designed to be representative for engine part load conditions. The analysis is focussed on flame displacement speed and flame area in order to investigate effects of flame structure and flame geometry on the global burning rate evolution. It is shown that despite a Karlovitz number of up to 13, which is at the upper range of conventional engine operation, thickening of the averaged flame structure by small-scale turbulent mixing is not observed. After ignition effects have decayed, the flame normal displacement speed recovers the behavior of a laminar unstretched premixed flame under the considered unity-Lewis-number conditions. Run-to-run variations in the global heat release rate are shown to be primarily caused by flame kernel area dynamics. The analysis of the flame area balance equation shows that turbulence causes stochastic flame kernel area growth by affecting the curvature evolution, rather than by inducing variations in total flame area production by strain. Further, it is shown that in local segments of a fully-developed planar flame with similar surface area as the investigated flame kernels, temporal variations in flame area rate-of-change occur. Contrasting to early flame kernels, these effects can be exclusively attributed to curvature variations in negatively curved flame regions.

Analysis of premixed flame kernel/turbulence interactions under engine conditions based on direct numerical simulation data

Although the evolution of premixed flames in turbulence has been frequently studied, it is not well understood how small flames interact with large-scale turbulent flow motion. Since this question is of practical importance for the occurrence of cycle-to-cycle variations in spark ignition engines, the objective of the present work is to fundamentally differentiate early flame kernel development from well-established turbulent flame configurations. For this purpose, a DNS database consisting of three flames propagating in homogeneous isotropic turbulence (Falkenstein et al., Combust. Flame, 2019) is considered. The flames feature different ratios of the initially laminar flame diameter to the integral length scale. To quantify flame kernel development, the time evolution of flame topology and flame front geometry are analysed in detail. It is shown that some realizations of the early flame kernel are substantially influenced by high compressive strain caused by large-scale turbulent flow motion with characteristic length scales greater than the flame kernel size. As a result, the initial spherical kernel topology may become highly distorted, which is reflected in the stochastic occurrence of excessive curvature variance. Two mechanisms of curvature production resulting from early flame kernel/turbulence interactions are identified by analysis of the mean curvature balance equation. Further, it is shown that the curvature distribution of small flame kernels becomes strongly skewed towards positive curvatures, which is contrary to developed turbulent flames. Hence, the transition of ignition kernels to self-sustaining turbulent flames is very different in nature compared with the development of a statistically planar flame brush.

A Fast CFD-Based Methodology for Determining the Cyclic Variability and Its Effects on Performance and Emissions of Spark-Ignition Engines

A methodology for determining the cyclic variability in spark-ignition (SI) engines has been developed recently, with the use of an in-house computational fluid dynamics (CFD) code. The simulation of a large number of engine cycles is required for the coefficient of variation (COV) of the indicated mean effective pressure (IMEP) to converge, usually more than 50 cycles. This is valid for any CFD methodology applied for this kind of simulation activity. In order to reduce the total computational time, but without reducing the accuracy of the calculations, the methodology is expanded here by simulating just five representative cycles and calculating their main parameters of concern, such as the IMEP, peak pressure, and NO and CO emissions. A regression analysis then follows for producing fitted correlations for each parameter as a function of the key variable that affects cyclic variability as has been identified by the authors so far, namely, the relative location of the local turbulent eddy with the spark plug. The application of these fitted correlations for a large number of engine cycles then leads to a fast estimation of the key parameters. This methodology is applied here for a methane-fueled SI engine, while future activities will examine cyclic variations in SI engines when fueled with different fuels and their mixtures, such as methane/hydrogen blends, and their associated pollutant emissions.

Computationally efficient prediction of cycle-to-cycle variations in spark-ignition engines

Cycle-to-cycle variations are important to consider in the development of spark-ignition engines to further increase fuel conversion efficiency. Direct numerical simulation and large eddy simulation can predict the stochastics of flows and therefore cycle-to-cycle variations. However, the computational costs are too high for engineering purposes if detailed chemistry is applied. Detailed chemistry can predict the fuels’ tendency to auto-ignite for different octane ratings as well as locally changing thermodynamic and chemical conditions which is a prerequisite for the analysis of knocking combustion. In this work, the joint use of unsteady Reynolds-averaged Navier–Stokes simulations for the analysis of the average engine cycle and the spark-ignition stochastic reactor model for the analysis of cycle-to-cycle variations is proposed. Thanks to the stochastic approach for the modeling of mixing and heat transfer, the spark-ignition stochastic reactor model can mimic the randomness of turbulent flows that is missing in the Reynolds-averaged Navier–Stokes modeling framework. The capability to predict cycle-to-cycle variations by the spark-ignition stochastic reactor model is extended by imposing two probability density functions. The probability density function for the scalar mixing time constant introduces a variation in the turbulent mixing time that is extracted from the unsteady Reynolds-averaged Navier–Stokes simulations and leads to variations in the overall mixing process. The probability density function for the inflammation time accounts for the delay or advancement of the early flame development. The combination of unsteady Reynolds-averaged Navier–Stokes and spark-ignition stochastic reactor model enables one to predict cycle-to-cycle variations using detailed chemistry in a fraction of computational time needed for a single large eddy simulation cycle.

Investigation of Influences of Secondary Butyl-alcohol Blends on Performance and Cycle-to-cycle Variations in a Spark Ignition Engines

Depletion of fossilised fuels coupled with stringent energy security has directed automotive researches to continuously investigate the use of alternative fuels in internal combustion engines. Within this subject, the research presented in this work is focused on using butanol as gasoline fuels substitution in a spark ignition engines. The present study reveals the combustion analysis specifically on cycle-to-cycle variations in spark ignition engines and performance characteristics of secondary butyl alcohol (sec-butanol) – gasoline blends at 5%, 10% and 15% by volume of sec-butanol in gasoline fuels. The engine performance characteristics was evaluated at 1000 to 4000 RPM with 50% of wide open throttle positions, while the in-cylinder pressure data were collected over 200 consecutive engine cycles for engine test at 3500 RPM. The indicated mean effective pressure time series is analyzed using the coefficient of variations and the wavelet analysis method. Comparative analysis of the performance characteristics showed that 2-butanol–gasoline blended fuel produced less brake power and brake mean effective pressure by 12.3%, 9.2% and 4.3%, for GBu5, GBu10 and GBu15, respectively, with regard to G100. From the wavelet power spectrum, it is observed that the long-term periodicities appear in gasoline fuels, while the short period oscillations become intermittently visible in blended fuels. Reduction of coefficient of variations occur as the additive of sec-butanol ratios increased, which agrees with the wavelet analysis results. Furthermore, the spectral power reduced with an increase in the additive ratio, indicating that the additive has a noticeable influence on reducing the cycle-to-cycle variations.

Controlling spark timing for consecutive cycles to reduce the cyclic variations of SI engines

Minimization of the cyclic variations is one of the most important design goal for spark-ignited engines. Primary motivation of this study is to reduce the cyclic variations in spark ignition engines by controlling the spark timing for consecutive cycles. A stochastic model was performed between spark timing and in–cylinder maximum pressure by using the system identification techniques. The incylinder maximum pressure of the next cycle was predicted with this model. Minimum variance and generalized minimum variance controllers were designed to regulate the in–cylinder maximum pressure by changing the spark timing for consecutive cycles of the test engine. The produced control algorithms were built in LabView environment and installed to the Field Programmable Gate Arrays (FPGA) chassis. According to the test results, the in–cylinder maximum pressure of the next pressure cycle can be predicted fairly well, and the spark timing can be regulated to keep the in–cylinder maximum pressure in a desired band to reduce the cyclic variations. At fixed spark timing experiments, the COVPmax and COVimep were 3.764 and 0.677%, whereas they decreased to 3.208 and 0.533% when GMV controller was applied, respectively.

Experimental Study on the Cycle-to-Cycle Combustion Variations in a Spark Ignition Engine

A cyclic variability has long been recognized as limiting the range of operating conditions of spark ignition engines, in particular, under idling conditions. The coefficient of variation (COV) in indicated mean effective pressure (IMEP) defines the cyclic variability in indicated work per cycle, and it has been found that vehicle drivability problems usually result. For analysis of the cyclic variations in spark ignition engines at idling, the results show that cyclic variability by the COV, COV of IMEP, the lowest normalized value (LNV), and burn angles can help to design the spark ignition engine.

Planar Laser-Induced Fluorescence of H20 to Study the Influence of Residual Gases on Cycle-to-Cycle Variations in SI Engines

A tunable KrF excimer laser operating at 248 nm is used for visualization of water vapor (H2O) at engine-like conditions by two-photon laser-induced fluorescence (LIF). The effects of two-photon absorption in water, laser sheet focusing and two-dimensional fluorescence imaging conditions are discussed, and an estimation of the reliability of the image data is given. The water vapor content measured close to the spark plug prior to ignition in a high-pressure combustion cell is correlated to the ensuing combustion rate, which is determined from the evolving pressure during combustion. The combustion rate is shown to be dependent on the water vapor concentration close to the spark plug prior to ignition, where a high water concentration yields a slow combustion. Using two-photon LIF of water vapor, the influence of residual gases on cycle-to-cycle variations is also investigated in a running spark-ignition (SI) engine. With a low inlet manifold pressure, a correlation coefficient of 0·6 is found between the combustion rate and the residual gas concentration close to the spark plug at the time of ignition. Further, a fast combustion in one cycle is found to yield more residual gases in the subsequent cycle.

The relationship between cyclic variations in spark-ignition engines and the small structure of turbulence

The hypothesis that cyclic variations in combustion in spark-ignition engines originate in the small-scale structure of turbulence has been further examined in the light of experimental data from a single-cylinder research engine. The data cover a wide range of engine speed and equivalence ratio. The effects of spark electrode geometry, spark gap, chamber geometry, and throttling have also been examined. The general conclusion is that the standard deviation in burning time, deduced for the smallest size flame kernels, is estimated within experimental uncertainty by a parameter λ 4u l , in which λ is the Taylor microscale and u l is the laminar burning velocity of the unburned mixture. The experimental results are thus consistent with the interaction of an effectively point-source ignition with the turbulence structure model of Tennekes, and with the idea that rapid burning takes place in the “vortex tube” regions of high dissipation.

A Turbulent Combustion Model of Cycle-to-Cycle Combustion Variations in Spark-Ignition Engines

A Turbulent Combustion Model of Cycle-to-Cycle Combustion Variations in Spark-Ignition Engines