Compression Top Dead Center Research Articles

Partially Premixed Combustion in a Diesel Engine Induced by a Pilot Injection at the Low-pressure Top Dead Center

An experimental investigation performed on a single-cylinder heavy-duty diesel research engine operating at low-speed, medium load is presented in this paper. This study is focused on combining a homogeneous charge compression ignition and a conventional diesel combustion processes to produce a partially premixed combustion by means of an original injection strategy. This strategy is based on introducing an early pilot injection at the low-pressure top dead center, between the exhaust and the intake strokes, followed by a conventional injection close to the compression top dead center. The main advantage of this concept compared to other early injection strategies consists of injecting always inside the piston bowl, far of the cylinder liner walls, thereby avoiding the oil dilution problems. The effect of the distribution of the fuel injected between low-pressure and compression top dead centers, and also the effect of the start of injection timing for the conventional injection are investigated in terms of combustion process characteristics, engine out emissions, and fuel consumption. The results obtained show how NOx−soot emissions can be reduced with a slight increment in fuel consumption by increasing the fraction of fuel mass injected at low-pressure top dead center and delaying the conventional injection at the same time. Unburned hydrocarbon emissions increase together with the fuel mass injected at the low-pressure top dead center, whereas CO emissions initially increase before being reduced.

Reconstruction and prediction of the in-cylinder pressure attractor of an internal combustion engine using locally constant models

In this research, the auto-mutual information function and correlation dimension are used for determination of lag and embedding dimension needed for reconstruction of the attractor of in-cylinder pressure of an internal combustion engine. Subsequently, a locally constant model is used for a 20-step prediction. The time series are acquired at three different test conditions and consisting of succeeding pressures at compression Top Dead Center (TDC) position of one of four cylinders. We have concluded that at least at relatively low engine speeds (below 2000 rpm) this method produces acceptable results.

Multidimensional Predictions of In-Cylinder Turbulent Flows: Contribution to the Assessment of k-ε Turbulence Model Variants for Bowl-in-Piston Engines

Multidimensional computational fluid dynamics (CFD) codes with reliable turbulence models are useful investigation and design tools for internal combustion engines, in-cylinder flow phenomena being critical to the combustion process and related emission sources. Although a variety of turbulence models has long been proposed, the assessment of even the most widely used k-ε model is still lacking, especially for bowl-in-piston engines. This paper provides a survey of k-ε turbulence model variants and their numerical implementation for in-cylinder flow analysis. Mean motion and turbulence quantities were simulated in the axisymmetric combustion chamber of a motored model engine featuring one centrally located valve and each of a flat-piston and cylindrical bowl-in-piston arrangements. A noncommercial CFD code developed by the authors was applied for calculation, using a finite-volume conservative implicit method and applying various order-of-accuracy numerical schemes. Simulation results are presented at the engine speed of 200 rpm throughout the whole engine cycle. These were obtained using three k-ε turbulence model versions, standard, renormalization group (RNG) and two scale, each of which focuses on one main engine flow feature, i.e., compressibility, anisotropy, and high unsteadiness, respectively. Modified boundary conditions with respect to conventional logarithmic wall functions were applied. Effects of equation-differencing scheme and computational-grid spacing effects on flow predictions were tested. The numerical results were compared to those of laser Doppler velocimetry measurements and the influence of the k-ε model variants on the flow-field features was examined during the induction stroke and around compression top dead center. For the flat-piston case, a comparison between the homemade and commercial STAR-CD® code results was also made.

An experimental investigation of flow parameters inside a spherical swirl pre-chamber of a diesel engine under motoring condition

The main aim of the present investigation is to experimentally study the flow parameters inside a spherical combustion chamber of an indirect injection diesel engine. For this a spherical chamber has been fitted in the cylinder head. Using a constant temperature hot-wire anemometer, measurements have been made under non-firing condition at 400 and 500 rpm during compression stroke. Mean velocity and turbulence intensity values are evaluated from the hot-wire signal. Measurements are made at three axial planes, six angular locations and four radial positions. Large variations of mean velocity and turbulence intensity have been observed within the chamber. From the measurements, the mean velocity and turbulence intensity are found to be maximum at 30° before compression TDC and these values are found to decrease towards the centre of the pre-chamber. The mean velocity and turbulence intensity distribution with respect to various locations and crank angles are presented and discussed.

Research for Piston Secondary Motion by Numerical Simulation. 2nd Report. Consideration on Design Parameter Optimization.

The piston secondary motion calculation procedure had been developed with keeping both the simple modeling and the calculation accuracy of the piston noise. This report describes its application for piston design parameter optimization. The analysis work was focused both on the slap in a lower engine speed, which excites the thrust side cylinder wall at just after the compression top dead center, and the rattle in a higher engine speed, which excites the both side cylinder wall for several times at around the TDC. Taguchi method was used to optimize the piston design parameters for the slap and rattle. Three design parameters i. e., the cylinder bore offset, the center of gravity offset and the wrist pin offset of the piston are considered with including the interaction by each other. These results show the advantages of the numerical analysis in respect to the accuracy, cost and time compared with the experimental analysis.

A study on the formation of vertical vortex in the cylinder of an I.C. engine using CFD: Effect of intake valve closing timing

Air flows in the intake ports and cylinders of a four-valve gasoline engine were numerically analyzed by means of an original CFD code named “GTT”. In this code, the k-e turbulence model was used and the Chakravarthy-Osher 3rd order TVD scheme was applied to the convection terms in all the governing differential equations. The validity of the code was confirmed by comparing the calculated results of the mean velocity and turbulence intensity with the measured ones by means of LDV. Using the present code, the effect of the intake valve closing timing on the formation of vertical vortex in the cylinder of a four-valve engine was investigated. It was found that the turbulence energy in the combustion chamber near compression TDC can be increased by the retardation of intake valve closing timing in the case of intense vertical vortex, because the vertical vortex is intensified by the back flow from the cylinder to the intake ports.

Cycle-to-Cycle Variation of In-Cylinder Flow in a Motored Engine.

An experimental study investigating the cyclic variation of the internal air motion has been conducted in a motored engine having a cup-in-piston combustion chamber. The engine used for this study has a transparent quartz liner and a transparent acryl piston. The continuous wave beam of a 4 watt argon ion laser was formed into a thin light sheet to define the measurement plane. The scattered light from the particles which were supplied to the intake air stream, were recorded by a NAC E-10 high-speed camera and the bulk flow fields inside the combustion chamber were measured at planes parallel and perpendicular to the piston crown by means of an image processing technique. We reveal, for the first time, continuous cycle-resolved two-dimensional bulk flow fields inside the combustion chamber around the compression TDC. Also, comparison between cycle-resolved and ensemble-averaged flow patterns has shown that the latter may become artificial ones if there exists a strong effect of cyclic variation. Moreover, by examining the rms of the variation component and the ratios of the rms to ensemble-averaged velocity with and without swirl, it is found that the ratios of the rms to ensemble-averaged velocity can be used for evaluating the effect of the cyclic variation on the flow fields.

Study of the Cycle-to-Cycle Variation of In-Cylinder Flow in a Motor Engine.

An experimental study investigating cyclic variation of the internal air motion was made in a motor engine having a cup-in-piston combustion chamber. The engine used for this study had a transparent quartz liner and a transparent acryl piston. The continuous wave beam of a 4 watt argon ion laser was formed into a thin light sheet to define the measurement plane. The scattered light from the particles which were supplied to the intake air stream, were recorded by a NAC E-10 high-speed camera and the bulk flow fields inside the combustion chamber were measured at both planes parallel and vertical to the piston crown by means of an image processing technique. This study reveals for the first time the continuous cycle-resolved two-dimensional bulk flow fields inside the combustion chamber around the compression TDC. Also, comparison between cycle-resolved and ensemble-averaged flow patterns showed that ensemble-averaged flow patterns may become artificial ones if there exists a large effect of the cyclic variation. Moreover, by examining the rms of the cyclic variation component and the ratios of the rms to ensemble-averaged velocity with/without swirl conditions, it was found that the ratios of the rms to ensemble-averaged velocity can be used for evaluating the effect of the cyclic variation on the flow fields.

Effects of Piston Head Shape on Gas Flows in Engine Cylinders. Numerical Prediction Using Higher-Order Schemes.

The effects of piston head shape on gas flows in the cylinders of four-valve gasoline engines have been numerically analyzed using the authors' GTT method with higher-order schemes for the convection terms of Navier-Stokes equations without any turbulence models. It has been found that among third-order schemes the Chakravarthy-Osher third-order TVD scheme is stabler than the UTOPIA and Kawamura-Kuwahara schemes. Therefore, the TVD scheme allows a larger time increment and enables us to conduct a practical calculation on an engineering workstation. The flow anaysis using the TVD scheme has revealed the following : The convexity or concavity of the piston head greatly affects the forming and collapsing processes of tumbling vortices in the cylinder. Both the decay of the spactially averaged kinetic energy of gas in the cylinder between intake BDC and compression TDC and the averaged kinetic energy of gas around an ignition plug near ignition timing may be used as measures for correlating the gas flow with the combustion phenomenon.

Turbulence measurements in a direct injection diesel engine by LDV.

The airflow was measured by a LDV in a motored four-stroke direct injection diesel engine with a troidal cavity combustion chamber, under three different swirl intensity condition. The turbulence scale was calculated by a random sample data analysis utilizing multicycle velocity data. It has been clearly demonstrated that even though the swirl intensity was varied, the integral time scale increased once at the beginning of the compression stroke and decreased toward compression top dead center. On the other hand, the micro time scale hardly changed in suction and compression strokes. Furthermore, it has been revealed that the axial micro time scale was almost uniform in the cylinder under the high swirl condition, but the integral time scale remained large near the piston cavity center.

Measurement of turbulence of air swirl in a 4-stroke cycle engine cylinder.

The turbulence of air swirl in the cylinder of a motored engine with a shrouded intake valve was measured with a hot-wire anemometer. In order to determine the turbulence characteristics such as turbulence intensity, integral time scale, etc., the flow velocity data were analyzed by the "stationary time average method" during a time interval of compression TDC±Δt/2. By changing the time interval Δt, different values of turbulence characteristics were obtained. It was made clear that the use of a finite time interval Δt meant the use of a high pass filter with a cut-off frequency of about 1/2(2Δt). As the angular speed of swirl, Ω, increased, the turbulence intensity, (u')v, increased and the integral time scale, (Lt)v, and the micro time scale, (λt)v, decreased. Here, (u')v, (Lt)v and (λt)v are spatial average values in a cylinder. Different correlations between (u')v and Ω were obtained in different intake valves.

The back scattered LDV measurements of swirl flow in an internal combution engine.

The purpose of this paper is to clarify the air motion of swirl flow in a reciprocating engine. The swirl velocities under motoring conditions were measured by a back scattered LDV using a bottom-view research engine. Experiments were conducted with three different combustion chambers having the same compression ratios. The results are as follows: (1) The swirl velocity in the cylinder increases in proportion to the engine speed. (2) The swirl velocity at the compression stroke increases due to the fact that the swirl flow is pushed into the cavity on the piston top. (3) At the compression TDC, the swirl ratio of a deep dish chamber is higher than the ratios of a pancake and a shallow dish chamber.

Squish and Swirl-Squish Interaction in Motored Model Engines

Measurements of the three components of velocity and their corresponding fluctuations have been obtained by laser-Doppler anemometry mainly near TDC of compression in a model IC engine motored at 200 rpm with compression ratio of 6.7. The flow configurations comprised an axisymmetric cylinder head with and without upstream induced swirl and each of a flat piston and two centrally located, cylindrical and re-entrant, bowl-in-piston arrangements. In the absence of swirl and squish, the intake-generated mean motion and turbulence decayed considerably by the end of compression. The two piston-bowl configurations, however, resulted in a compression-induced squish motion with consequent formation of a toroidal vortex occupying the whole bowl space. Interacton of swirl, carried from intake and persisting through compression, with squish generated near TDC profoundly altered the axial flow structure. In the case of the cylindrical bowl, the sense of the vortex was reversed by swirl and, in the reentrant bowl, increased the number of vortices to two. The swirling motion inside the cylindrical bowl was close to solid body rotation while the re-entrant bowl gave rise to more complex flow patterns. Squish, in the presence or absence of swirl, did not augment the turbulent energy inside the cylindrical bowl contrary to the reentrant configuration where turbulence generation was observed.