Lmt Bart Somers

Experimental Study of Fuel Composition Impact on PCCI Combustion in a Heavy-Duty Diesel Engine

Premixed Charge Compression Ignition (PCCI) is a combustion concept that holds the promise of combining emission levels of a spark-ignition engine with the efficiency of a compression-ignition engine. In a short term scenario, PCCI would be used in the lower load operating range only, combined with conventional diesel combustion at higher loads. This scenario relies on using near standard components and conventional fuels; therefore a set of fuels is selected that only reflects short term changes in diesel fuel composition. Experiments have been conducted in one dedicated test cylinder of a modified 6-cylinder 12.6 liter heavy duty DAF engine. This test cylinder is equipped with a stand-alone fuel injection system, EGR circuit and air compressor. For the low load operating range the compression ratio has been lowered to 12:1 by means of a thicker head gasket. It is shown that emission levels and performance strongly correlate with the combustion delay (CD=CA50-SOI), independent of how this combustion delay is achieved. In a longer term scenario, both engine hardware and fuels can be adapted to overcome intrinsic PCCI challenges. At higher loads and at 15:1 compression ratio, necessary for good full load efficiency, a less reactive fuel is required to delay auto-ignition and phase combustion correctly. A number of low reactivity fuel blends have been used to investigate the desired Cetane Number for PCCI operation at different loads. For these blends too, all emission levels as well as the efficiency are shown to greatly correlate with the combustion delay. With an improved efficiency because of the higher compression ratio, the blend with an estimated CN of 25 was found to be the most flexible in being able to choose the optimum CD for the conditions and load used.

Modeling of the Small Scale Structure of Flat Burner-Stabilized Flames

ABSTRACT The small scale structure of premixed methane/air flames, stabilized on perforated or porous flat-flame burners has been studied numerically. The two-dimensional combustion equations have been solved, using the vorticity-stream function formulation for the flow field and two different chemistry models: the skeletal mechanism for lean methane combustion and a one-step scheme. The curvature of the flames, predicted by both models is comparable. The range of pore sizes, porosities and mixture velocities, which can be used to create practically undistored flat flames has been determined. It is found that more-dimensional transport effects are negligible in flames, stabilized on the flat-flame burners recently introduced by (Maaren van, 1994) for measuring burning velocities with the Heatflux method. This guarantees that the measurement data are not obscured by small scale distortions and can be compared with ID modeling results.

Response of burner-stabilized flat flames to acoustic perturbations

The response of burner-stabilized flat flames to acoustic velocity perturbations is studied numerically and analytically. The numerical setup involves the set of one-dimensional transport equations for the low-Mach number reacting flow using a simple and a more complex reaction mechanism. The physical background of the phenomena observed numerically is explained by a simple analytical model. The model uncouples the unsteady transport equations into two parts: the first part describes the flame motion through the G-equation and the second flamelet part describes the inner flame structure and mass burning rate of the flame. The G-equation can be solved exactly in the case of a quasi-steady flame structure. The mass burning rate is assumed to be directly related to the flame temperature. Relations for the fluctuating heat release and heat loss to the burner are derived, from which the coupling between the velocity fluctuations at both sides of the flame is found. Comparison of the numerical and analytical results with earlier work of McIntosh and with primary experimental results on a lean methane/air flame shows the validity of the models. The origin of the differences encountered is discussed. The resulting transfer function for the velocity perturbation can be applied to the acoustic stability analysis of combustion systems. The most interesting application is the acoustic behaviour of central heating boilers.

Gasoline–diesel dual fuel : effect of injection timing and fuel balance

Recently, some studies have shown high efficiencies using controlled auto-ignition by blending gasoline and diesel to a desired reactivity. This concept has been shown to give high efficiency and, because of the largely premixed charge, low emission levels. The origin of this high efficiency, however, has only partly been explained. Part of it was attributed to a lower temperature combustion, originating in lower heat losses. Another part of the gain was attributed to a faster, more Otto-like (i.e. constant volume) combustion. Since the concept was mainly demonstrated on one single test setup so far, an experimental study has been performed to reproduce these results and gain more insight into their origin. Therefore one cylinder of a heavy duty test engine has been equipped with an intake port gasoline injection system, primarily to investigate the effects of the balance between the two fuels, and the timing of the diesel injection. Besides studying trends in the dual-fuel regime, this also allows to find best points to compare with conventional diesel combustion. Results show that compared to more conventional combustion regimes, this dual-fuel concept can escape from the common NOx-smoke trade-off, reducing both to near-zero values. Although hydrocarbon emissions are somewhat increased, indicated efficiencies are significantly improved. The absolute efficiencies are not as high as reported in other work, but the increase does confirm the potential of the concept. The increase in indicated efficiency is shown to originate from a higher thermal efficiency, because short burn durations at high gasoline fractions enable for CA50 to be phased closer to TDC, without combustion occurring too much before TDC. Pressure rise rates are as low as with conventional diesel combustion, when using the same Exhaust Gas Recirculation (EGR) percentage. Although the dual fuel concept has a much higher rate of heat release, this is phased better after TDC. A dedicated set of experiments has also shown that the late-cycle diesel injection is dominant in combustion phasing and that control has to be found in this diesel injections.

A numerical study of a premixed flame on a slit burner

A numerical study of a premixed methane/air flame on a 4 mm slit burner is presented. A local grid refinement technique is used to deal with large gradients and curvature of all variables encountered in the flame, keeping the number of grid points within reasonable bounds. The method used here leads to a large reduction in the number of mesh points, compared to global or line-by-line refinement techniques. The procedure to obtain the initial guess for the detailed model simulation, needed for the Newton-like solution method, is discussed. The method uses the result of a one-step global model simulation of the same geometry and an appropriate one-dimensional detailed model simulation. The method works fine for the computations presented here. The results of the detailed model are compared to those of the one-step global reaction model computation. Furthermore, using a direct photograph of the luminescence, the flame-tip height and the general flame shape of the detailed simulation are verified with the corresponding experimental flame. Although only a qualitative comparison can be made, both the height and the flame shape compare well.

Commercial Naphtha Blends for Partially Premixed Combustion

Partially Premixed Combustion has shown the potential of low emissions of nitrogen oxides (NOx) and soot with a simultaneous improvement in fuel efficiency. Several research groups have shown that a load range from idle to full load is possible, when using low-octane-number refinery streams, in the gasoline boiling range. As such refinery streams are not expected to be commercially available on the short term, the use of naphtha blends that are commercially available could provide a practical solution. The three blends used in this investigation have been tested in a single-cylinder engine for their emission and efficiency performance. Besides a presentation of the sensitivity to injection strategies, dilution levels and fuel pressure, emission performance is compared to legislated emission levels. Conventional diesel combustion benchmarks are used for reference to show possible improvements in indicated efficiency. Analysis of the heat release patterns revealed an interesting and strong correlation between the premixed fraction and the amount of soot produced. To be specific, each of the fuels showed a decrease in this fraction as either fuel pressure was lowered or load was increased, showing a transition from more premixed to mainly mixing-controlled combustion, with the corresponding soot emissions. For one blend, over the whole load range EURO VI PM levels were approached or achieved, combined with a peak gross indicated efficiency of 50% clearly indicating the potential of this concept.

Experimental validation of extended NO and Soot model for advanced HD Diesel Engine Combustion

A computationally efficient engine model is developed based on an extended NO emission model and state-of-the-art soot model. The model predicts exhaust NO and soot emission for both conventional and advanced, high-EGR (up to 50%), heavy-duty DI diesel combustion. Modeling activities have aimed at limiting the computational effort while maintaining a sound physical/chemical basis. The main inputs to the model are the fuel injection rate profile, in-cylinder pressure data and trapped in-cylinder conditions together with basic fuel spray information. Obtaining accurate values for these inputs is part of the model validation process which is thoroughly described. Modeling results are compared with single-cylinder as well as multi-cylinder heavy-duty diesel engine data. NO and soot level predictions show good agreement with measurement data for conventional and high-EGR combustion with conventional timing.

The Flamelet-Generated Manifold Method applied to steady planar partially premixed counterflow flames

ABSTRACT The recently introduced Flamelet Generated Manifold (FGM) method has proved to be an accurate and efficient reduction method for the modelling of premixed flames. The FGM method uses a chemical library based on one-dimensional unstrained premixed flames to model the chemistry of a multi-dimensional flame. Recently, the method has also been applied successfully to a so-called triple flame configuration, which is partially premixed. In this configuration, the gradient of the mixture fraction was relatively small compared to the flame thickness. In this paper the applicability of FGM in partially premixed combustion systems is investigated further. The method is tested in a planar counterflow configuration, which enables the control of the mixture fraction gradient. The gradient of the mixture fraction is changed by modifying the applied strain in one case and the inlet mixture fraction in the other case. The results show that, even though the mixing and flow time scales are of the same order as the flame time scales from the database. FGM is still relatively accurate. This can be explained by the fact that in a large part of the flame, the chemistry is not far from equilibrium, which means that the chemistry is still much faster than the flow and mixing processes. It has already been shown that this holds for strain, but this paper shows that it is also true for the dissipation rate. For very high strain and dissipation rates (up to 2000 s−1), errors up to 10% are obtained.

Study of Turbulent Flow Structures of a Practical Steady Engine Head Flow Using Large-Eddy Simulations

The prediction performance of two computational fluid dynamics codes is compared to each other and to experimental data of a complex swirling and tumbling flow in a practical complex configuration. This configuration consists of a flow in a production-type heavy-duty diesel engine head with 130-mm cylinder bore. One unsteady Reynolds-averaged Navier-Stokes (URANS)-based simulation and two large-eddy simulations (LES) with different inflow conditions have been performed with the KIVA-3V code. Two LES with different resolutions have been performed with the FASTEST-3D code. The parallelization of the this code allows for a more resolved mesh compared to the KIVA-3V code. This kind of simulations gives a complete image of the phenomena that occur in such configurations, and therefore represents a valuable contribution to experimental data. The complex flow structures gives rise to an inhomogeneous turbulence distribution. Such inhomogeneous behavior of the turbulence is well captured by the LES, but naturally damped by the URANS simulation. In the LES, it is confirmed that the inflow conditions play a decisive role for all main flow features. When no particular treatment of the flow through the runners can be made, the best results are achieved by computing a large part of the upstream region, once performed with the FASTEST-3D code. If the inflow conditions are tuned, all main complex flow structures are also recovered by KIVA-3V. The application of upwinding schemes in both codes is in this respect not crucial.

Butanol-diesel blends for partially premixed combustion

Partially Premixed Combustion has shown the potential of high efficiency, emissions of nitrogen oxides (NOx) and soot below future emissions regulations, and acceptable acoustic noise. Low-octane-number gasoline fuels were shown to be most suitable for this concept, with the reactivity determining the possible load range. Other researchers have used several refinery streams, which might be produced by a refinery if they were required to do so without additional investment. Some of refinery streams are, however, not expected to be commercially available on the short term. For the present investigation, n-butanol (BuOH) has been selected as a blend component in diesel, and is used from 50 – 100%. The blends then have a reactivity range similar to the refinery streams, so single-cylinder engine tests for their emission and efficiency performance can also be used to determine their applicable load range. The current paper presents a summary of the performance of such BuOH-diesel blends with respect to emissions and efficiency in the Partially Premixed Combustion regime. Besides a presentation of the sensitivity to injection strategies, dilution levels and fuel pressure, emission performance is compared to upcoming legislated emission levels. The effect of the blend ratio on load ranges is shown and conventional diesel combustion benchmarks are used to show improvements in indicated efficiency. Butanol-diesel blends are shown to be a viable approach to partially premixed combustion, with its high soot reduction potential and stable operation. EURO VI emission levels can therefore be achieved, with moderate or slightly increased fuel pressure. Combustion efficiency is shown to be very reasonable over the whole load range, similar to that of conventional diesel combustion. Combined with an improved thermal efficiency a moderate butanol-diesel blend is shown to have an average gross indicated efficiency of 50% over the whole load range.