Claus Borgnakke

Conditionally-Sampled Velocity and Turbulence Measurements in a Spark Ignition Engine

Abstract Laser Doppler velocimeter measurements have been made in a homogeneously-charged spark ignition engine. With ignition at the side wall of the disc-shaped combustion chamber, the fluid motion in the direction of flame propagation was measured at the center of the chamber. A simultaneous ionization probe measurement was used to identify the time of flame arrival at the velocimeter probe volume. Phase-averaged measurements recorded from many engine cycles were conditionally sampled according to flame arrival time. The results presented show an increase in the unburned gas turbulence from compression, and strongly suggest that cyclic variation in burn duration is caused by cyclic variation in the bulk turbulence intensity ahead of the flame.

Measurements and Predictions of the Precombustion Fluid Motion and Combustion Rates in a Spark Ignition Engine

Laser Doppler velocimeter results are presented for the mean velocity and turbulence intensity measured in a motored research engine. The compression of complex bulk motions created during induction produces turbulence as the piston approaches top dead center. The turbulence field is shown to be isotropic but nonhomogeneous. A zero-dimensional computer simulation based on an averaged k-epsilon model is shown to adequately predict the decay of turbulence at a point in the flow after the production phase is completed. Cylinder pressure measurements were recorded for homogeneous stoichiometric combustion for a range of engine speeds and ignition locations. A two-zone (burned and unburned gases) thermodynamic model accurately predicts the measured pressure histories when the turbulence results determined from the motored tests are used to establish initial conditions for the combustion model.

Characterization of Flow Produced by a High-Swirl Inlet Port

The flow produced by an experimental high-swirl intake port was studied by several techniques. These included measurements of flow rate and swirl as a function of valve lift on a steady state bench rig, hot-wire measurements of flow issuing from the valve, and flow visualization in water and air. By applying these techniques together to a single port, a body of data was generated which is presented as an addition to what is known about intake port flows and swirl generation. Data include flow and swirl coefficients, information on the effects of valve offset and port orientation angle, swirl generation by velocity non-uniformity around the valve, swirl decay in the rig due to air friction on the walls, and forward/backward flow coefficients. The definition of the appropriate dimensionless parameters for port flow characterization is also discussed.

Fluid Motion during Flame Propagation in a Spark Ignition Engine

Laser Doppler velocimeter results are presented for the mean velocity and turbulence intensity measured during combustion in a research engine. Simultaneously with each LDV measurement, the cylinder pressure and gas state (unburned or burned) were measures so that conditional sampling techniques could be used in the data-averaging procedure. Measurements of the mean velocity component in the direction of flame propagation agree well with a computer simulation of the induced velocities generated by the volume expansion of the burned gases. Mean velocities measured parallel to the flame surface are shown to be complex because a small amount of swirl was present. Conditional sampling on the time of flame arrival at the LDV probe volume revealed a thirty percent cyclic-variation bias error in the turbulence component normal to the flame. The turbulence field ahead of the flame appears to be enhanced by compression, with the component normal to the flame increased twice as much as the parallel component. 28 references, 16 references.

Combustion Effects on the Preflame Flow Field in a Research Engine

Measurements are presented for the turbulence intensities and mean velocities obtained in a research engine in which a grid was used to create a flow field characterized by negligible mean motions and homogeneous and isotropic turbulence at the time of ignition. Pressure measurements for homogeneous stoichiometric combustion indicate a very low level of cyclic variation. The combustion-induced mean flow field is shown to be characteristic of a one-dimensional compression of the unburned gases, which produces a small increase in the bulk turbulent kinetic energy ahead of the flame. Most of the effect of combustion appears to occur locally, as the turbulence in the preflame gases close to the flame front is strongly amplified in the direction of flame propagation. Parallel to the flame surface there is little effect until the flame has propagated nearly all the way across the chamber.

Multiparameter conditionally sampled laser velocimetry measurements during flame propagation in a spark ignition engine

Laser Doppler velocimeter results are presented for the mean velocity and turbulence intensity measured during combustion in a research engine. The cylinder pressure and time of flame arrival at the LDV measurement volume were also recorded for subsequent application of conditional sampling techniques in the data-averaging procedure. For velocity measurements made at different locations along the path of flame propagation, it is shown how the pressure history for each individual engine cycle can be used to relate the fluid mechanics from point to point. Additional conditional sampling, using the flame arrival time, is applied to further identify a set of similar combustion events with reduced bias from cyclic variations in the combustion rate. The fluid mechanics in the engine are very complex with strong mean motions and anisotropic and nonhomogeneous turbulence. Combustion appears to amplify the turbulence in the preflame gas although the measured changes may be due, in part, to flame-induced convection of nonhomogenous turbulence.

Enhanced Spray and Evaporation Model with Multi-Fuel Mixtures for Direct Injection Internal Combustion Engines

A spray and evaporation model has been developed and embedded in a quasi-dimensional multi-zone direct injection internal combustion engine simulation framework. The model accounts for the behavior of the spray zone and fuel evaporation including sub-models for spray breakup, improved zone velocity estimations with transient fuel injection, spray penetration and tracking of evaporated fuel components. Each sub-model deals with multi-component fuel surrogates to simulate the real fuel effects. To enhance sensitivity of the model to the fuel components, additional thermophysical properties to the traditional spray model are incorporated. The ideal gas and the ideal solution are assumed in the model to retain computational efficiency. Finally, the model was validated against experiments and compared with other numerical methods. The ability of the new model to treat a multi-component fuel provides an improvement in the simulations of modern direct injection internal combustion engine when used with alternative fuels.

A thermodynamic model for homogeneous charge compression ignition combustion with recompression valve events and direct injection: Part I – Adiabatic core ignition model

This two-part article presents a model for boosted and moderately stratified homogeneous charge compression ignition combustion for use in thermodynamic engine cycle simulations. The model consists of two components: one an ignition model for the prediction of auto-ignition onset and the other an empirical combustion rate model. This article focuses on the development and validation of the homogeneous charge compression ignition model for use under a broad range of operating conditions. Using computational fluid dynamics simulations of the negative valve overlap valve events typical of homogeneous charge compression ignition operation, it is shown that there is no noticeable reaction progress from low-temperature heat release, and that ignition is within the high-temperature regime (T > 1000 K), starting within the highest temperature cells of the computational fluid dynamics domain. Additional parametric sweeps from the computational fluid dynamics simulations, including sweeps of speed, load, intake manifold pressures and temperature, dilution level and valve and direct injection timings, showed that the assumption of a homogeneous charge (equivalence ratio and residuals) is appropriate for ignition modelling under the conditions studied, considering the strong sensitivity of ignition timing to temperature and its weak compositional dependence. Use of the adiabatic core temperature predicted from the adiabatic core model resulted in temperatures within ±1% of the peak temperatures of the computational fluid dynamics domain near the time of ignition. Thus, the adiabatic core temperature can be used within an auto-ignition integral as a simple and effective method for estimating the onset of homogeneous charge compression ignition auto-ignition. The ignition model is then validated with an experimental 92.6 anti-knock index gasoline-fuelled homogeneous charge compression ignition dataset consisting of 290 data points covering a wide range of operating conditions. The tuned ignition model predictions of θ 50 have a root mean square error of 1.7° crank angle and R2 = 0.63 compared to the experiments.

Flame Propagation and Heat-Transfer Effects in Spark Ignition Engines

The essential characteristics of the internal combustion engine are ultimately determined by the processes that take place in the combustion chamber. It is these processes that generate the power output, the heat losses, and the formation of pollutants and therefore determine the trade-off between efficiency and emissions. The need thus arises to identify and qualify the significant physical processes that take place in the combustion chamber. Only after this accomplishment is it possible to establish a control of the major parameters that influence the combustion process so desired design goals can be achieved. In the past an acceptable level of understanding of the phenomenon involved has been reached through an analysis of experimental evidence and the pursuit of theoretical investigations. The processes inside the combustion chamber of an internal combustion engine involve a broad range of different subjects. The characterization includes chemistry, thermodynamics, fluid mechanics, and heat transfer, to mention a few of the most important fields as outlined in Fig. 1. Furthermore, the combustion process takes place in such a highly complex environment that an accurate and detailed description on a fundamental level cannot be done with present-day knowledge. Through numerous experiments a large amount of information has been acquired allowing the key processes to be identified and investigated in more detail. Guided by these experiments and the theoretical analysis, semiempirical methods have been used to develop the internal combustion engine, and such two-sided investigations are necessary for the successful development of the internal combustion engine. Though today’s engine is highly sophisticated, continued research efforts produce an increased understanding of the different processes leading to an improvement of the overall engine performance.

Thermodynamic model for homogeneous charge compression ignition combustion with recompression valve events and direct injection: Part II—Combustion model and evaluation against transient experiments:

This two-part article presents a combustion model for boosted and moderately stratified homogeneous charge compression ignition combustion for use in thermodynamic engine cycle simulations. The model consists of two parts: one an ignition model for the prediction of auto-ignition onset and the other an empirical combustion rate model. This article focuses on the development of the combustion model which is algebraic in form and is based on the key physical variables affecting the combustion process. The model is fit with experimental data collected from 290 discrete automotive homogeneous charge compression ignition operating conditions with moderate stratification resulting from both the direct injection and negative valve overlap valve events. Both the ignition model from part 1 and the combustion model from this article are implemented in GT-Power and validated against experimental homogeneous charge compression ignition data under steady-state and transient conditions. The ignition and combustion model are then exercised to identify the dominant variables affecting the homogeneous charge compression ignition and combustion processes. Sensitivity analysis reveals that ignition timing is primarily a function of the charge temperature, and that combustion duration is largely a function of ignition timing.