Kevin Robinson

Thermal modelling of modern engines: A review of empirical correlations to estimate the in-cylinder heat transfer coefficient

Abstract Over the last 40 years, several empirical correlations have been developed to estimate heat fluxes from the combustion chambers of internal combustion engines. Some of these expressions are based on correlations to compute the Nusselt number for forced convection in turbulent flow inside circular tubes. The fundamental suitability of this kind of empirical model in representing the highly complex processes of in-cylinder heat transfer is questionable, but in practice the models have steadily improved owing to contributions from numerous investigators. Other correlations have a less theoretical basis than those of the Nusselt number form. Formulae of this type have been obtained from the application of simple statistical techniques to large datasets, taking into account several engine operational parameters and engine types. The resulting correlations provide reasonable estimates but perform poorly when extrapolated or applied to novel concepts. In this paper, the most important correlations are reviewed against the features of a modern diesel engine, and research requirements for future modelling developments are identified and discussed.

Experimental and modelling aspects of flow boiling heat transfer for application to internal combustion engines

Abstract A detailed programme of work has been undertaken to quantify the suitability of predictive methods for accurate determination of the levels of boiling heat transfer within an internal combustion (IC) engine cooling gallery simulator. An extensive array of experimental data has been obtained as the basis for the predictive validation. Working on the principle of superposition, the convective component of heat transfer has been represented by the established Dittus-Boelter correlation which has been extensively modified to account for developing boundary layers, surface roughness and nearwall viscous effects. The boiling component has been represented by the Chen model, modified for binary fluids and subcooling. For the IC engine cooling application it is concluded that the application of the Chen approach must be complemented by a convective heat transfer model that accurately represents the complex thermo-fluid situation being experienced within a developing flow.

Convective coolant heat transfer in internal combustion engines

Abstract Simple heat transfer correlations are known to underpredict the single-phase convective heat transfer coefficient when applied to internal combustion (IC) engine cooling passages. The reasons for such underprediction were investigated using a specially designed test rig which was operated under a wide variety of test conditions relevant to IC engine operation. Data from this rig study identified that undeveloped flow (fluid dynamically and thermally), surface roughness and fluid viscosity variation with temperature were the physical reasons responsible for the mismatch. Simple empirical heat transfer models have subsequently been extended to take account of these factors and are shown to give much improved correlation with rig data, and data from an engine study. The implications of this work for predicting engine heat transfer in a three-dimensional computational fluid dynamics environment are discussed.

Predictions for Nucleate Boiling - Results From a Thermal Bench Marking Exercise Under Low Flow Conditions

Two predictive methods have been applied to an IC engine cooling gallery simulator to provide benchmarking heat transfer information. The object of this work was to assess the suitability and accuracy of these methods for application to future on-engine heat transfer studies. Such studies are aimed at developing predictive tools to aid in the design of precision cooling systems. The modelling techniques of Rohsenow and Chen have been used, modified and validated. Compared against experimental data, the sub- cooled form of the Chen model has been found to be most representative for the cooling gallery simulator designed specifically to meet the requirements of this work.

Thermal profile of a modern passenger car diesel engine

In the last 15 years, diesel engines in passenger cars have evolved rapidly. The combination of performance, fuel consumption, emissions and refinement offered by a modern diesel engine makes it the preferred engine choice in many sectors of the market. The enormous progress made has resulted from technological improvements such as low swirl 4-valve per cylinder, direct injection combustion systems complemented by high pressure common rail fuel systems and high levels of turbocharger boost pressure. The durability and output potential of such engines is strongly linked to the operating temperature of certain key components. Accurate temperature predictions are an essential pre-requisite to the continuing evolution, thus placing emphasis on the need for high quality predictive tools. Despite this, existing methods for predicting heat flux and component operating temperatures in diesel engines rest mainly with methodology developed over 50 years ago, often updated in the light of more recent experimental data. It is questionable how well those methods represent modern diesel engines. In order to look at this issue, the authors have undertaken a wide-ranging experimental and analytical study using a highly instrumented modern diesel engine. The engine had 109 thermocouples implanted into the walls of the cylinders and cylinder head to reveal the spatial variation of temperature and heat flux over a wide range of operating conditions. In this paper some of the results of the study are discussed, together with comparisons of experimental data and the existing predictive models. Areas of agreement and discrepancy are highlighted in the context of the operating conditions and engine characteristics, and proposals for improved methodology are discussed.

Dynamic measurement of heat flux through the cylinder wall of a modern HSDI engine over a new European drive cycle

A modern high speed four cylinder Diesel engine equipped with high pressure common rail fuel injection equipment has been fitted with extensive instrumentation to allow the heat flux and coolant convective heat transfer coefficient through the cylinder walls to be estimated. The instrumentation was located around the circumference of the cylinder and longitudinally down the cylinder. The engine has been run through the new European drive cycle using a dynamic test stand. From the experimental results it was found that there was a strong correlation between the one dimensional heat flux through the cylinder wall and the engine speed. The changes in heat flux were found to be repeatable over the four repeated ECE sections of the drive cycle. It was also found that the magnitude of heat flux reduced down the length of the cylinder.

Actively controlled cooling jets

A proof-of-concept study has been undertaken to demonstrate the use and potential benefits of actively controlled coolant jets in an IC engine-cooling gallery simulator. Results have shown that substantial reductions in coolant volumes are possible and that the control of the liquid/metal surface temperature can be achieved within \mP 0.2\mDC in response to transient heat flux conditions.

Computational modelling of convective heat transfer in a simulated engine cooling gallery

Abstract Experimental data from internal combustion (IC) engines suggests that the use of proprietary computational fluid dynamics (CFD) codes for the prediction of coolant-side heat transfer within IC engine coolant jackets often results in underprediction of the convective heat transfer coefficient. An experimental and computational study, based on a coolant gallery simulator rig designed specifically to reproduce realistic IC engine operating conditions, has been conducted to explore this issue. It is shown that the standard ‘wall function’ approach normally used in CFD models to model near-wall conditions does not adequately represent some features of the flow that are relevant in convective heat transfer. Alternative modelling approaches are explored to account for these shortcomings and an empirical approach is shown to be successful; however, the methodology is not easily transferable to other situations.

Estimated Total Energy Transfer Over an NEDC Through Steady State Performance Evaluation

A steady state test procedure has been developed and implemented on an extensively instrumented production diesel engine to estimate the total energy transfer to coolant over a New European Drive Cycle. The test procedure involved segmenting the drive cycle into 26 operating conditions, each with a corresponding weighting factor. The test program consisted of both the steady state tests and a series of transient tests for comparison. The engine instrumentation consisted of a bespoke measurement device to calculate the rate of heat transfer through the combustion chamber walls. The sensors were located vertically down both the inlet and exhaust sides of one cylinder. It was found that the steady state approximation method estimated the total energy transfer to the coolant to within 10% of the transient tests. Differences in the idle speed condition were found to have the largest effect due to 21.7% of the drive cycle occurring at this condition. The steady state approximation method can therefore be used to sufficiently estimate the drive cycle performance for energy transfer if an exact condition is used for a region where the weighting factor is significant, i.e. greater than 15%. Subsequently it could also be used for other parameters, such as fuel consumption.

Thermal modelling of modern diesel engines: proposal of a new heat transfer coefficient correlation

Existing methods for predicting heat fluxes and temperatures in internal combustion engines, which take the form of correlations to estimate the heat transfer coefficient on the gas-side of the combustion chamber, are based on methodology developed over the past 50 years, often updated in view of more recent experimental data. The application of these methods to modern diesels engines is questionable because key technologies found in current engines did not exist or were not widely used when those methods were developed. Examples of such technologies include: high-pressure common rail and variable fuel injection strategies including retarded injection for nitrogen oxides emission control; exhaust gas re-circulation; high levels of intake boost pressure provided by a single- or double-stage turbocharger and inter-cooling; multiple valves per cylinder and lower swirl; and advanced engine management systems. This suggests a need for improved predicting tools of thermal conditions, specifically temperature and heat flux profiles in the engine block and cylinder head. In this paper a modified correlation to predict the gas-side heat transfer coefficient in diesel engines is presented. The equation proposed is a simple relationship between Nu and Re calibrated to predict the instantaneous spatially averaged heat transfer coefficient at several operating conditions using air as gas in the model. It was derived from the analysis of experimental data obtained in a modern diesel engine and is supported by a research methodology comprising the application of thermodynamic principles and fundamental equations of heat transfer. The results showed that the new correlation adequately predicted the instantaneous coefficient throughout the operating cycle of a high-speed diesel engine. It also estimated the corresponding cycle-averaged heat transfer coefficient within 10 per cent of the experimental value for the operating conditions considered in the analysis.