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The effects of changes in the cetane number of diesel liquid pilot fuels on the ignition delay period in dual fuel engines were investigated experimentally. Different pilot fuel quantities were employed with commercially pure methane, propane and low heating value gaseous fuel mixtures of methane with nitrogen or carbon dioxide over a range of engine load. The ignition delay variation with increased gaseous fuel admission showed a strong dependance on both the quantity and the quality of the pilot fuel used. It was found that the use of high cetane number pilot liquid fuels permitted smaller pilot quantities to be used satisfactorily. Engine operation on propane and low heating value gaseous fuels improved in comparison with dual fuel engine operation employing common diesel fuels.

Hydrogen is normally produced through the steam reforming of fossil fuels, notably natural gas or their partial oxidation in oxygenated air. The products of these processes would normally produce the H2 in the presence of a variety of concentrations of CO, CO2, H2O and N2. There is increasing interest in employing such mixtures whether on their own or in mixtures with traditional liquid or gaseous fuels in S.I. engine applications so as to improve the combustion process and engine performance. The combustion characteristics in S.I. engines of gas mixtures that contain H2 and CO need to be established to provide key operational information, such as the variations in the combustion duration and the knock limits. This paper presents experimental data obtained in a single cylinder, variable compression ratio, S.I., CFR engine when operated in turn on CH4, H2, CO and their binary mixtures.

An approach for simulating cyclic variations in spark ignition engines is described. It is based on a stochastic modeling coupled to a comprehensive model developed for predicting engine performance, mainly for gas-fueled engine applications. Such an approach is shown capable of generating cycle to cycle variations of pressure-time development records that are in good agreement with experiment. An account of the corresponding extent of cyclic variation in major performance parameters can be also established. It is demonstrated that the probability of the incidence of knock can be determined for any set of operating and design conditions while using this approach with sufficiently comprehensive detailed chemical kinetics. Examples involving mainly methane operation are shown.

A model is described for the prediction of the onset of autoignition and knock in compression ignition engines of the dual fuel type. The associated variations with time of performance parameters such as the energy release rate, cylinder pressure and charge temperature, power output and species concentrations can also be obtained. This is achieved through modelling in detail the chemical reaction rates of the gaseous fuel during compression and subsequently during diesel fuel pilot ignition and combustion. A comprehensive reaction scheme involving 105 reaction steps with 31 chemical species is employed for the purpose. The results are based mainly on methane or propane as the gaseous fuel while accounting for the contribution of pilot diesel fuel injection. Calculated data showed good general agreement with the corresponding experimental values.

The present contribution describes an analytical approach for predicting the highest limit for acceptable power or efficiency for any spark ignition engine while ensuring knock free operation. A deterministic gradient based model combined with a simple genetic algorithm were used in association with a two-zone engine combustion model to predict analytically the necessary changes in specified operating parameters to produce optimum performance. Various examples involving mainly spark ignition engine operation with methane-hydrogen fuel mixtures are presented and discussed.

The present contribution combines the consideration of the chemical reaction activity of the end gas and engine operating conditions to predict the onset of knock and associated performance in a spark ignition engine fuelled with methane. A two-zone predictive combustion model was developed based on an estimate of the effective duration of the combustion period and the mass burning rate for any set of operating conditions. The unburned end gas preignition chemical reaction activity is described by a detailed chemical reaction kinetic scheme for methane and air. The variation with time of the value of a formulated dimensionless knock parameter based on the value of the cumulative energy released due to preignition reaction activity of the end gas per unit volume relative to the total energy release per unit cylinder swept volume is calculated It is shown that whenever knocking is encountered, the value of builds up to a sufficiently high value that exceeds a critical value.

Light load operation of dual fuel engines, associated with the use of very lean gaseous fuel-air mixtures produces relatively significant exhaust concentrations of unconverted methane and carbon monoxide, especially when small pilot liquid fuel injection is involved. The nature of the processes that bring about such exhaust emissions and measures for their control are discussed.

A two-zone predictive model for the performance of a spark ignited gas engine is described. In this model, an effective mass burning rate and energy release pattern based on an estimate of the combustion duration are developed. For any given engine and set of operating conditions the pressure-time and temperatures-time histories, and hence performance parameters such as indicated power output, peak pressure, optimum spark timing, etc. are predicted. Through monitoring the chemical reaction activity, while employing detailed chemical kinetics of the end gas within the unburnt zone, the incidence of autoignition and knock can also be predicted. A dimensionless knock criterion that compares the specific energy release due to end gas preignition reaction activity to the specific energy release due to combustion of the fuel is developed and used to test for the incidence of knock and its severity.

The presence of carbon dioxide with methane is often encountered to varying proportions in numerous natural, industrial and bio-gases. The paper discusses how such a presence modifies significantly the thermodynamic, kinetic and combustion characteristics of methane in air. Experimental results are presented showing how the performance of engines, both of the spark ignition and compression ignition dual fuel types is adversely affected by the increasing presence of carbon dioxide with the methane. The bases for these trends are discussed and some guidelines towards alleviating the adverse effects of the presence of carbon dioxide in such fuel mixtures are made.

The paper describes an analytical approach that models the vaporization, ignition and combustion of liquid fuel droplets in a heated environment of homogeneously mixed gaseous fuel and air at constant pressure such as taking place in dual fuel engines of the compression ignition type. Results are presented typically for the ignition and combustion of n-heptane droplets initially introduced cold into a heated homogeneous surrounding of methane-air mixtures. Variations in various parameters in space and time, such as temperature, the concentrations of the two fuels, oxygen, and products of combustion, rates of energy release, etc. are presented and discussed.

The ignition delay period in dual fuel engines is examined, while employing the gaseous fuels methane, propane, ethylene and hydrogen. It is shown that the changes due to gaseous fuel admission in the temperature and pressure levels during the delay period, the extent of energy release due to preignition reaction processes, variations in the parameters of external heat transfer to the surroundings and the contribution of residual gases are the most important factors that determine the ignition delay characteristics of dual fuel engines. The consequences of these factors on the observed values of the ignition delay were evaluated while using detailed reaction kinetics for the oxidation of the gaseous fuel and employing an experimentally based formula for the ignition of the liquid pilot.

A multi-zone model has been developed for the prediction of the combustion processes in dual fuel engines and some of their performance features. The consequences of the interaction between the gaseous and the diesel fuels and the resulting modification to the combustion processes are considered. A reacting zone has been incorporated in the model to describe the partial oxidation of the gaseous fuel-air mixture while detailed kinetic schemes are employed to describe the oxidation of the gaseous fuel, right from the start of compression to the end of the expansion process. The associated formation and concentrations of exhaust emissions are correspondingly established. The model can predict the onset of knock as well as the operating features and emissions for the more demanding case of light load performance. Predicted values for methane operation show good agreement with corresponding experimental values.

The action of exhaust gas recirculation (EGR) is examined numerically to find out whether EGR can be used to enhance the preignition reactions of a cylinder charge in a motoring, compression ignition engine fuelled with a homogeneous gaseous fuel - air mixture. The changes to the concentrations and properties of the contents of the cylinder and the associated changes in the preignition reaction rates are followed over a number of consecutive, calculated working cycles at a constant engine speed to establish whether autoignition will take place and the number of cycles required for its occurrence. It is shown that controlled EGR can enhance the autoignition processes in gas-fuelled compression ignition engines by suitably ‘seeding’ the intake charge of the current cycle with the chemical species found in the exhaust gases of the previous cycle.

The effects of residual gases on the combustion process of engines are examined through analysing the cyclic variations of autoignition in a motored engine fuelled with homogeneous gaseous fuel-air mixtures. The changes in composition and temperature of residual gases as well as the associated rates of the preignition reactions are followed over a number of consecutive working cycles at a constant engine speed to establish whether autoignition will take place and how many cycles are need for its occurrence. It is in that the residual gases associated with partial oxidation reactions tend to have strong kinetic but hardly any thermal or diluting effects, while residual gases produced from the more complete combustion following autoignition tend to possess significant thermal, kinetic and diluting effects.

A preliminary investigation was made into the use of hydrogen-oxygen mixtures in spark ignition engines. This appeared to be attractive in view of the serious air pollution problem. Furthermore, hydrogen has been considered by others as a possible alternative fuel to replace depleting petroleum resources. Following a literature survey regarding the combustion characteristics of hydrogen, a computer program based on a constant-volume combustion engine cycle was used to evaluate the overall performance of an engine. Another program, which considered chemical reaction kinetics, was used to predict the onset of autoignition in mixtures undergoing compression in an engine. Results of the program indicated that an attractive and safe way to use hydrogen-oxygen mixtures in an engine involved the recycling of exhaust gases. Such a system would be fed with a stoichiometric mixture, while excess hydrogen would be circulated within to control combustion in the engine.

The rate of heat release analysis of combustion processes in a diesel engine, derived from a knowledge of cylinder pressure time records, has now developed to be an effective tool for considering and evaluating the progress of these processes for research and development purposes. This paper examines some of the main errors and assumptions normally associated with the calculation of apparent rates of combustion heat release in engines, and suggests ways to improve the accuracy of these calculations.

The paper describes some of the operational and exhaust emission characteristics of a single-cylinder direct-injection diesel engine that was operated warm under a wide range of intake air temperatures extending down to -45°F and occasionally -60°F. The operating range considered included the no-load and partially motored regions.

The present contribution is mainly concerned with an investigation of the characteristics of dual fuel operation under cold intake temperatures, primarily from the viewpoint of engine performance and exhaust emissions. The gaseous fuels employed were methane, propane, hydrogen and ethylene. The addition of the inerts carbon dioxide and nitrogen were also considered. Comparison with the corresponding normal diesel operation was made throughout.

The combustion processes in a compression-ignition engine of the direct-injection type are examined by the independent variation of the partial pressure of oxygen in the intake charge coupled with analyses of rates of heat release employing computer methods.

Dual fuel engines compress the air/gas fuel mixture to just below autoignition conditions and then ignite it by the injection of a small amount of liquid fuel. The use and performance of these engines, however, have been limited by knock. Single cylinder engine experiments show that this limitation is a readily defined autoignition phenomenon, and can be analyzed by a mathematical model that indicates the effects on performance imposed by fuel changes and operating conditions. Experimental findings confirm that these performance data correlate broadly with those obtained conventionally in standard spark ignited or motored engines.