Evangelos G. Giakoumis

Review of Thermodynamic Diesel Engine Simulations under Transient Operating Conditions

Study and modeling of transient operation is an important scientific objective. This is due to the fact that the majority of daily vehicle driving conditions involve transient operation, with non-linear situations experienced during engine transients. Thus, proper interconnection is needed between engine, governor, fuel pump, turbocharger and load. This paper surveys the publications available in the open literature concerning diesel engine simulations under transient operating conditions. Only those models that include both full engine thermodynamic calculations and dynamic powertrain modeling are taken into account, excluding those that focus on control design and optimization. Most of the attention is concentrated to the simulations that follow the filling and emptying modeling approach. A historical overview is given covering, in more detail, research groups with continuous and consistent study of transient operation. One of the main purposes of this paper is to summarize basic equations and modeling aspects concerning in-cylinder calculations, friction, turbocharger, engine dynamics, governor, fuel pump operation, and exhaust emissions during transients. The various limitations of the models are discussed together with the main aspects of transient operation (e.g. turbocharger lag, combustion and friction deterioration), which diversify it from the steady-state. Some of the most important findings in the field during the last 30 years are presented and discussed. The survey extends to special cases of transient diesel engine simulation, such as second-law analysis, response when the turbocharger compressor experiences surge, and whole vehicle performance. Several methods of improving transient response are also mentioned, based on the various simulations. An easy-to-read tabulation of all research groups dealing with the subject, that includes details about each model developed and engines/parameters studied, is also provided at the end of the paper.

Simulation and exergy analysis of transient diesel-engine operation

A computer analysis has been developed for studying the energy and exergy performance of an indirect-injection, naturally-aspirated diesel engine operating under transient load or speed conditions and covering the operating profile of both industrial and automotive engines. The model is validated at steady-state operation and incorporates many novel features for simulating the transient response and analyzing all of the engine availability terms. The analysis reveals via multiple diagrams how the exergy properties of the diesel-engine subsystems vary according to the engine cycles for various speed and load changes. The diagrams also show the current-speed response. In addition, the effects of operating parameters such as the intensity of the applied change or heat loss to the walls are described from first- and second-law perspectives.

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.

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.

Study of the performance and exhaust emissions of a spark-ignited engine operating on syngas fuel

To resolve the problem of depletion of petroleum based liquid fuels, various solutions have been proposed. One of them is the use of gaseous fuels that are generated from the gasification of woods, namely syngas or wood-gas fuels, as full supplement fuels in spark ignition internal combustion (IC) engines. This fuel consists of nearly 40% combustible gases, mainly hydrogen and carbon monoxide (CO), while the rest is non-combustible gases. In the present work, a comparison between experimental and computed results is presented for a conventional natural gas, spark-ignited engine, fuelled with syngas instead of natural gas fuel. For the theoretical investigation, a computer model is developed that simulates the syngas combustion processes in a conventional natural gas, spark-ignited engine. The combustion model is a two-zone one, where the combustion rate of syngas fuel depends on the velocity of the flame front that forms around the area of the burning zone and then spreads inside the combustion chamber. The flame front development takes into account the history of pressure and temperature inside the chamber and the local composition, in order to estimate the flame velocity. An equilibrium model is used to determine the concentration of the chemical species involved, the extended Zeldovich mechanism is used to determine the concentration of nitric oxide (NO) and a CO kinetics scheme is used to estimate the CO emission. To validate the predictive ability of the model, experimental measurements are used from the operation of a multi-cylinder, four-stroke, turbocharged, spark-ignited engine fuelled with syngas fuel, with the measurements corresponding to various values of the air to fuel ratio (load). The experimental results are found to be in good agreement with the respective computed ones obtained from the computer model. Comparing the computed results when operating the engine with natural gas as against syngas fuel, a serious effect of the syngas operation on the cylinder pressure diagrams and the engine brake efficiency is revealed, for all test cases examined. Moreover, as far as pollutant emissions are concerned, the use of natural gas instead of syngas has a positive effect on both NO and CO emissions (reduction).

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.

Experimental Assessment of Turbocharged Diesel Engine Transient Emissions during Acceleration, Load Change and Starting

The control of transient emissions from turbocharged diesel engines remains an important objective to manufacturers, since newly produced engines must meet the stringent criteria concerning exhaust emissions levels as dictated by the legislated Transient Cycles. In the present work, experimental tests are conducted on a medium-duty, turbocharged and after-cooled diesel engine in order to investigate the behavior and formation mechanism of nitric oxide (NO), smoke and combustion noise emissions under various transient operating schedules including acceleration, load change and starting. To this aim, a fully instrumented test bed was set up in order to record and research key engine and turbocharger variables during the transient events. The main parameters measured are nitric oxide concentration and smoke opacity (both using ultra-fast response analyzers) as well as combustion noise. Various other variables were monitored, such as in-cylinder pressure, engine speed, fuel pump rack position, boost pressure and turbocharger speed. The main focus of the experimental investigation was devoted to engine acceleration tests representative of automotive and truck applications, commencing from various engine speeds and loads. The experimental test pattern also included load increases and (cold and hot) startings. Analytical diagrams are provided to explain the behavior of exhaust emissions development in conjunction with turbocharger and governor/fuel pump response. Turbocharger lag was found to be the main cause for the emissions peak values observed during all transient events. During starting, the lack of air and its mismatch with fueling caused excessive black smoke, identified by the extremely high values of exhaust gas opacity.

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.