C.D. Rakopoulos

Operational and Environmental Evaluation of Diesel Engines Burning Oxygen-Enriched Intake Air or Oxygen-Enriched Fuels: A Review

A method to curtail emissions of smoke and other pollutants from diesel engines is to enhance the oxygen supply to their combustion chamber. This can be accomplished by enriching either the intake air stream or the fuel stream with oxygen. Experimental studies concerning the oxygen-enrichment of intake air, have revealed large decrease of ignition delay, drastic decrease of soot emissions as well as reduction of CO and HC emissions while, brake specific fuel consumption (BSFC) remained unaffected and increasing of power output is feasible. However, this technique was accompanied by considerable increase of NO x emissions. Experimental and theoretical studies with oxygenated fuels have demonstrated large decrease of soot emissions, which correlated well with the fuel oxygen content. Reduction of CO and HC emissions with oxygenated fuels was also obtained. However, penalties in both BSFC and NO x emissions have been observed with oxygenation of diesel fuels. In both cases one has to weigh the tradeoffs in fuel economy, in power output and in the emissions of various pollutants. Moreover, fuel cost, availability and supply infrastructure, as well as equipment and operational costs, are among concerns that apply to these techniques. This manuscript presents a comparative evaluation of the two techniques regarding engine performance characteristics, environmental repercussions and economy of operation. The primary objective is to contrast the benefits and the drawbacks of the two techniques in view of economic, operational and environmental parameters. Results have shown that the overall economy of operation of the two techniques may be comparable, if the price of oxygenated fuel blends is similar to that of diesel fuel. Their impact on pollutant emissions may also be comparable, if the oxygen enrichment of either technique is limited to a low level (<23% by mass in the cylinder mixture). However, there are possibilities of increasing the power density of engines with oxygen enrichment of the intake air.

A fast algorithm for calculating the composition of diesel combustion products using 11 species chemical equilibrium scheme

Abstract The knowledge of the exact composition of the gases produced during combustion is very important for simulating the whole cycle of an internal combustion engine and especially for successfully predicting the exhaust pollutants emissions and mainly the nitric oxide concentration. For this purpose, the present paper produces a FORTRAN program for calculating the composition of the diesel combustion products, which is based on the development of a model using 11 species chemical equilibrium considerations. The 11 × 11 non-linear system, which is obtained from the seven non-linear equilibrium equations and the four linear atom balance equations, is converted to a 4 × 4 non-linear system. This system is solved using the Newton-Raphson method for non-linear systems, with no conversion problems and small computational cost. The program is applied successfully for n -dodecane fuel, which typically represents the diesel fuel, at conditions prevailing in diesel engine combustion chambers, revealing the influence on composition of pressure, temperature and equivalence ratio.

The combustion of n-butanol/diesel fuel blends and its cyclic variability in a direct injection diesel engine

An experimental study is conducted to evaluate the effects of using blends of diesel fuel with n-butanol (normal butanol) up to 24 per cent (by volume), which is a promising fuel that can be produced from biomass (bio-butanol), on the combustion behaviour of a standard, high-speed, direct injection (DI), ‘Hydra’ diesel engine located at the authors’ laboratory. Combustion chamber and fuel injection pressure diagrams are obtained at four different loads using a developed, high-speed, data acquisition, and processing system. A heat release analysis of the experimentally obtained cylinder pressure diagrams is developed and used. Plots of histories in the combustion chamber of the gross heat release rate and other related parameters reveal some interesting features, which shed light on the combustion mechanism when using these blends. These results, combined with the differing physical and chemical properties of the n-butanol against those for the diesel fuel, aid the correct interpretation of the observed engine behaviour performance based on and emissions. Moreover, given the concern for the rather low cetane number of the n-butanol that may promote cyclic (combustion) variability, its strength is also examined as reflected in the pressure indicator diagrams, by analysing for the maximum pressure and its rate, dynamic injection timing and ignition delay, by using stochastic analysis for averages, standard deviations, probability density functions, autocorrelation, power spectra, and cross-correlation coefficients.

Experimental instantaneous heat fluxes in the cylinder head and exhaust manifold of an air-cooled diesel engine

Abstract An experimental analysis is performed to study the instantaneous heat fluxes, during the engine cycle, in the combustion chamber walls of a direct injection (DI), air cooled, four stroke, Diesel engine located at the authors laboratory. For this purpose, a novel experimental installation has been developed, which separates the engine transient temperature signals into two parts, namely the ‘long’ and the ‘short’ term response ones, followed by their discrete processing in two independent data acquisition systems. Furthermore, a new pre-amplification unit for fast response thermocouples, appropriate heat flux sensors and an innovative, object oriented, control code for fast data acquisition have been designed and developed for the needs of the study. One dimensional heat conduction with Fourier analysis of the raw temperature data are implemented in order to calculate the instantaneous engine cylinder and exhaust pipe heat fluxes. Analysis of the experimental results reveal many interesting aspects of transient engine heat transfer. The effect of engine speed on cylinder head and exhaust manifold heat losses is presented. The simultaneous presentation of heat fluxes on the cylinder head and exhaust manifold, together with the engine indicator diagram, sheds light into the mechanisms governing the transient heat transfer. This is very important, since especially for air cooled Diesel engines, limited information seems to exist in the relevant literature.

The Effect of Various Dynamic, Thermodynamic and Design Parameters on the Performance of a Turbocharged Diesel Engine Operating under Transient Load Conditions

Thermodynamic, dynamic and design parameters have a significant and often conflicting impact on the transient response of a compression ignition engine. Knowing the contribution of each parameter on transient operation could direct the designer to the appropriate measures for better engine performance. To this aim an explicit simulation program developed is used to study the performance of a turbocharged diesel engine operating under transient load conditions. The simulation developed, based on the filling and emptying approach, provides various innovations as follows: Detailed analysis of thermodynamic and dynamic differential equations, on a degree crank angle basis, accounting for the continuously changing nature of transient operation, analysis of transient mechanical friction, and also a detailed mathematical simulation of the fuel pump. Each equation in the model is solved separately for every cylinder of the 6-cylinder diesel engine considered. The model is validated against experimental data for various load changes. The effect of several dynamic, thermodynamic and design parameters is studied, i.e. load schedule (type, and duration of load applied), turbocharger mass moment of inertia, exhaust manifold volume and configuration, cylinder wall temperature, aftercooler effectiveness as well as an interesting case of a malfunctioning fuel pump. Explicit diagrams are given to show how, after an increase in load, each parameter examined affects the engine speed response, as well as other properties of the engine and turbocharger such as fuel pump rack position, boost pressure and turbocharger speed. It is shown that certain parameters, such as the type of connected loading, the turbocharger inertia, a damaged fuel pump and the exhaust manifold volume, can have a significant effect on the engine and turbocharger transient performance. However others, such as the cylinder wall temperature, the aftercooler effectiveness and the exhaust manifold configuration have a less important effect as regards transient response and final equilibrium conditions.

Study of the short-term cylinder wall temperature oscillations during transient operation of a turbo-charged diesel engine with various insulation schemes

Abstract This work investigates the phenomenon of short-term temperature (cyclic) oscillations in the combustion chamber walls of a turbocharged diesel engine during transient operation after a ramp increase in load. For this purpose, an experimentally validated simulation code of the thermodynamic cycle of the engine during transient conditions is used. This takes into account the transient operation of the fuel pump and the development of friction torque using a detailed per degree crank angle submodel, while the equations for each cylinder are solved individually and sequentially. The thermodynamic model of the engine is appropriately coupled to a wall periodic heat conduction model, which uses the gas temperature variation as boundary condition throughout the engine cycle after being treated by Fourier analysis techniques. Various insulation schemes are examined (plasma spray zirconia, silicon nitride) for load-increase transient operation. The evolution of many variables during transients is depicted, such as amplitude of oscillation, depth where the swing dies out, or gradient of temperature swing. The investigation reveals many interesting aspects of transient engine heat transfer, regarding the influence that the engine wall material properties have on the values of cyclic temperature swings.

Experimental and simulation analysis of the transient operation of a turbocharged multi‐cylinder IDI diesel engine

SUMMARY An experimental and theoretical analysis is carried out to study the response of a multi-cylinder, turbocharged, IDI (indirect injection) compression ignition engine, under transient operating conditions. To this aim, a comprehensive digital computer model is developed which solves the governing di⁄erential equations individually for each cylinder, providing thus increased accuracy over previous ‘single-cylinder’ simulations. Special attention has been paid for diversifying the transient operation from the steady-state one, providing improved or even new relations concerning combustion, heat transfer to the cylinder walls, friction, turbocharger and aftercooler operation, and dynamic analysis for the transient case. An extended steady state and transient experimental work is conducted on a specially developed engine test bed configuration, located at the authors’ laboratory, which is connected to a high-speed data acquisition and processing system. The steady-state measurements are used for the calibration of the individual submodel constants. The transient investigation includes both speed and load changes operating schedules. During each transient test four major measurements are continuously made, i.e. engine speed, fuel pump rack position, main chamber pressure and turbocharger compressor boost pressure. The hydraulic brake coupled to the engine possesses a high mass moment of inertia and long nonlinear load-change times, which together with the indirect injection nature of the engine are important challenges for the simulation code. Explicit multiple diagrams are given to describe the engine and turbocharger transient behaviour including smoke predictions. The agreement between experimental and predicted responses is satisfactory, for all the cases examined, proving the validity of the simulation process, while providing useful information for the engine response under various transient operations. ( 1998 John Wiley & Sons, Ltd.

Analysis of combustion chamber insulation effects on the performance and exhaust emissions of a DI diesel engine using a multi-zone model

Abstract A comprehensive two-dimensional multi-zone model of a diesel engine cycle is presented in this study, in order to examine the influence of insulating the combustion chamber on the performance and exhaust pollutants emissions of a naturally-aspirated, direct injection (DI), four-stroke, water-cooled diesel engine. The heat insulation is taken into account by the corresponding rise of wall temperature, since this is the final result of insulation useful for the study. It is found that there is no remarkable improvement of engine efficiency, since the decrease of volumetric efficiency has a greater influence on it than the decrease of heat loss to the coolant, which is converted mainly to exhaust gas enthalpy (significant rise of the exhaust gas temperature). As far as the concentration of exhaust pollutant emissions is concerned, it is found that the rising heat insulation leads to a significant increase of the exhaust nitric oxide (NO) and to a moderate increase of the exhaust soot concentration. Plots of temperature, equivalence ratio, NO and soot distributions at various instants of time inside the combustion chamber, emanating from the application of the multi-zone model, aid the correct interpretation of the insulation effects gaining insight into the underlying mechanisms involved.

Parametric Study of Transient Turbocharged Diesel Engine Operation from the Second-Law Perspective

A computer analysis is developed for studying the energy and exergy performance of a turbocharged diesel engine, operating under transient load conditions. The model incorporates some novel features for the simulation of transient operation, such as detailed analysis of mechanical friction, separate consideration for the processes of each cylinder during a cycle (“multicylinder” model) and mathematical modelling of the fuel pump. The model is validated against experimental data taken from a turbocharged diesel engine, located at the authors’ laboratory, operated under transient load conditions. The availability terms for the diesel engine and its subsystems are analyzed, i.e. cylinder for both the open and closed parts of the cycle, inlet and exhaust manifolds, turbocharger and aftercooler. The effect of various dynamic, thermodynamic and design parameters on the second-law transient performance of the engine, manifolds and turbocharger is investigated, i.e. magnitude of applied load, type of connected loading, turbocharger mass moment of inertia, exhaust manifold volume, cylinder wall temperature and aftercooler effectiveness. Explicit diagrams are given to show how, after a ramp increase in load, each parameter examined affects the second-law properties of all subsystems such as cylinder, heat loss to the walls and exhaust gas availability as well as combustion, exhaust manifold and turbocharger irreversibilities. It is revealed from the analysis that the in-cylinder (mainly combustion) irreversibilities outweigh all other similar terms for every transient event but with decreasing magnitude when load increases, with the exhaust manifold processes being the second biggest irreversibilities producer with increasing magnitude when load increases. Design parameters such as cylinder wall insulation or aftercooler effectiveness can have a notable effect on the second-law properties of the engine, despite the fact that their effect on the (thermo)dynamic response is minimal.

Studying the effects of hydrogen addition on the second-law balance of a biogas-fuelled spark ignition engine by use of a quasi-dimensional multi-zone combustion model

Abstract Although a first-law analysis can show the improvement that hydrogen addition impacts on the performance of a biogas-fuelled spark-ignition (SI) engine, additional benefits can be revealed when the second law of thermodynamics is brought into perspective. It is theoretically expected that hydrogen enrichment in biogas can increase the second-law efficiency of engine operation by reducing the combustion-generated irreversibilities, because of the fundamental differences in the mechanism of entropy generation between hydrogen and traditional hydrocarbon combustion. In this study, an experimentally validated closed-cycle simulation code, incorporating a quasi-dimensional multi-zone combustion model that is based on the combination of turbulent entrainment theory and flame stretch concepts for the prediction of burning rates, is further extended to include second-law analysis for the purpose of quantifying the respective improvements. The analysis is applied for a single-cylinder homogeneous charge SI engine, fuelled with biogas—hydrogen blends, with up to 15 vol% hydrogen in the fuel mixture, when operated at 1500r/min, wide-open throttle, fuel-to-air equivalence ratio of 0.9, and ignition timing of 20° crank angle before top dead centre. Among the major findings derived from the second-law balance during the closed part of the engine cycle is the increase in the second-law efficiency from 40.85 per cent to 42.41 per cent with hydrogen addition, accompanied by a simultaneous decrease in the combustion irreversibilities from 18.25 per cent to 17.18 per cent of the total availability of the charge at inlet valve closing. It is also illustrated how both the increase in the combustion temperatures and the decrease in the combustion duration with increasing hydrogen content result in a reduction in the combustion irreversibilities. The degree of thermodynamic perfection of the combustion process from the second-law point of view is quantified by using two (differently defined) combustion exergetic efficiencies, whose maximum values during the combustion process increase with hydrogen enrichment from 49.70 per cent to 53.45 per cent and from 86.01 per cent to 87.33 per cent, respectively.