Stijn Broekaert

Assessment of Empirical Heat Transfer Models for a CFR Engine Operated in HCCI Mode

Homogeneous charge compression ignition (HCCI) engines are a promising alternative to traditional spark- and compression-ignition engines, due to their high thermal efficiency and near-zero emissions of NOx and soot. Simulation software is an essential tool in the development and optimization of these engines. The heat transfer submodel used in simulation software has a large influence on the accuracy of the simulation results, due to its significant effect on the combustion. In this work several empirical heat transfer models are assessed on their ability to accurately predict the heat flux in a CFR engine during HCCI operation. Models are investigated that are developed for traditional spark- and compression-ignition engines such as those from Annand [1], Woschni [2] and Hohenberg [3] and also models developed for HCCI engines such as those from Chang et al. [4] and Hensel et al. [5]. The heat flux is measured in a CFR engine operated in both motored and HCCI mode and compared to the predicted heat flux by the aforementioned models. It is shown that these models are unable to accurately predict the heat flux during HCCI operation if the model coefficients are not properly calibrated. The models from Annand, Hohenberg and Woschni overestimate the heat flux, whereas the models from Chang et al. and Hensel et al. underestimate it during the entire engine cycle if the original model coefficients are used. If the model coefficients are properly calibrated, the models from Annand, Hohenberg and Hensel et al. are able to predict the heat flux during HCCI operation for one engine operating point. However, if the same model coefficients are used for another operating point, the models are unable to satisfactorily predict the heat flux.

Calibration of a TFG Sensor for Heat Flux Measurements in a S.I. Engine

In the development of internal combustion engines, measurements of the heat transfer to the cylinder walls play an important role. These measurements are necessary to provide data for building a model of the heat transfer, which can be used to further develop simulation tools for engine optimization. This research will focus on the Thin Film Gauge (TFG) heat flux sensor. This sensor consists of a platinum RTD (Resistance Temperature Detector) on an insulating Macor® (ceramic) substrate. The sensor has a high frequency response (up to 100 kHz) and is small and robust. These properties make the TFG sensor adequate for measurements in the combustion chamber of an internal combustion engine. To use this sensor, its thermal properties - namely the temperature sensitivity coefficient and the thermal product - must be correctly calibrated. First, different calibration setups with a different temperature range are used to calibrate the temperature sensitivity coefficient of the TFG sensor. These results will be analyzed and discussed. Second, the DED (Double Electrical Discharge) calibration setup for the thermal product is extensively discussed.

Demonstrating the Use of Thin Film Gauges for Heat Flux Measurements in ICEs: Measurements on an Inlet Valve in Motored Operation

To optimize internal combustion engines (ICEs), a good understanding of engine operation is essential. The heat transfer from the working gases to the combustion chamber walls plays an important role, not only for the performance, but also for the emissions of the engine. Besides, thermal management of ICEs is becoming more and more important as an additional tool for optimizing efficiency and emission aftertreatment. In contrast little is known about the convective heat transfer inside the combustion chamber due to the complexity of the working processes. Heat transfer measurements inside the combustion chamber pose a challenge in instrumentation due to the harsh environment. Additionally, the heat loss in a spark ignition (SI) engine shows a high temporal and spatial variation. This poses certain requirements on the heat flux sensor. In this paper we examine the heat transfer in a production SI ICE through the use of Thin Film Gauge (TFG) heat flux sensors. An inlet valve has been equipped with 7 TFG sensors in a row. In literature only measurements on the piston, cylinder liner or cylinder head could be found. First, the construction of the heat flux sensor will be discussed. Second, the heat flux measurement technique and the implementation of the TFG sensors are discussed. The choice for Thin Film sensors is highlighted. Only compression operation (motored) measurements are currently considered and compared to literature. The effect of a variation in manifold air pressure on the heat flux is analysed.

A Heat Transfer Model for Low Temperature Combustion Engines

Low Temperature Combustion is a technology that enables achieving both a higher efficiency and simultaneously lower emissions of NOx and particulate matter. It is a noun for combustion regimes that operate with a lean air-fuel mixture and where the combustion occurs at a low temperature, such as Homogeneous Charge Compression Ignition and Partially Premixed Combustion. In this work a new model is proposed to predict the instantaneous heat flux in engines with Low Temperature Combustion. In-cylinder heat flux measurements were used to construct this model. The new model addresses two shortcomings of the existing heat transfer models already present during motored operation: the phasing of the instantaneous heat flux and the overprediction of the heat flux during the expansion phase. This was achieved by implementing the in-cylinder turbulence in the heat transfer model. The heat transfer during the combustion was taken into account by using the turbulence generated in the burned zone. This allowed the model to accurately predict the instantaneous heat flux and the effect of varying the engine settings and the fuel on the heat transfer. Contrary to the existing heat transfer models, the new model does not require a recalibration of its model coefficients. Moreover, the model implements the mixture’s gas properties to make it fuel independent.

Experimental Investigation and Modelling of the In-Cylinder Heat Transfer during Ringing Combustion in an HCCI Engine

Homogeneous Charge Compression Ignition (HCCI) engines can achieve both a high thermal efficiency and near-zero emissions of NOx and soot. However, their maximum attainable load is limited by the occurrence of a ringing combustion. At high loads, the fast combustion rate gives rise to pressure oscillations in the combustion chamber accompanied by a ringing or knocking sound. In this work, it is investigated how these pressure oscillations affect the in-cylinder heat transfer and what the best approach is to model the heat transfer during ringing combustion. The heat transfer is measured with a thermopile heat flux sensor inside a CFR engine converted to HCCI operation. A variation of the mass fuel rate at different compression ratios is performed to measure the heat transfer during three different operating conditions: no, light and severe ringing. The occurrence of ringing increases both the peak heat flux and the total heat loss. This effect should be accounted for in the heat transfer models by increasing the convection coefficient. It is shown that the heat transfer correlations of Annand and Woschni are not able to accurately model the heat transfer during ringing combustion. Two modifications to Annand’s model are proposed that take into account the additional heat transfer during ringing combustion.

Studying the Effect of the Flame Passage on the Convective Heat Transfer in a S.I. Engine

Engine optimization requires a good understanding of the in-cylinder heat transfer since it affects the power output, engine efficiency and emissions of the engine. However little is known about the convective heat transfer inside the combustion chamber due to its complexity. To aid the understanding of the heat transfer phenomena in a Spark Ignition (SI) engine, accurate measurements of the local instantaneous heat flux are wanted. An improved understanding will lead to better heat transfer modelling, which will improve the accuracy of current simulation software. In this research, prototype thin film gauge (TFG) heat flux sensors are used to capture the transient in-cylinder heat flux within a Cooperative Fuel Research (CFR) engine. A two-zone temperature model is linked with the heat flux data. This allows the distinction between the convection coefficient in the unburned and burned zone. The experimental time-resolved convection coefficient is then calculated by deriving the moment of flame arrival. The convection coefficient contains all the information of the driving force of the convective heat transfer. The work focusses on the effect of the flame passage on the convective heat transfer.

Evaluation of wall heat flux models for full cycle CFD simulation of internal combustion engines under motoring operation

The present work details a study of the heat flux through the walls of an internal combustion engine. The determination of this heat flux is an important aspect in engine optimization, as it influences the power, efficiency and the emissions of the engine. Therefore, a set of simulation tools in the OpenFOAM® software has been developed, that allows the calculation of the heat transfer through engine walls for ICEs. Normal practice in these types of engine simulations is to apply a wall function model to calculate the heat flux, rather than resolving the complete thermo-viscous boundary layer, and perform simulations of the closed engine cycle. When dealing with a complex engine, this methodology will reduce the overall computational cost. It however increases the need to rely on assumptions on both the initial flow field and the behavior in the near-wall region. As the engine studied in the present work, a Cooperative Fuel Research (CFR) engine, is a simple single cylinder pancake engine, it was possible to implement more complex and numerically demanding methodologies, while still maintaining an acceptable computation time. Both closed and full cycle simulations were therefore performed, for which the heat flux was calculated by both implementing various wall function models and by resolving the complete thermo-viscous boundary layer. The results obtained from the different kind of simulations were then compared to experimental heat flux data, which was measured using a thermopile type heat flux sensor in different locations in the CFR engine. By comparing the results from the different types of simulations, a performance evaluation of the used methodology could be carried out. It was found that the heat flux obtained by resolving the thermo-viscous layer was accurate compared to experiments, while the wall functions were not able to correctly capture the heat flux. Full cycle simulations resulted in a slightly improved result, especially when resolving the boundary layer, but due to the increased computational cost, this method does not seem beneficial.