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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 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.

Examination is made of the changes that take place in the major parameters of the combustion process and engine performance when using three different designs of plasma jet igniters of the open cavity type in a methane fuelled single cylinder engine. The characteristics of the combustion process were analysed employing a two-zone diagnostic model based on cylinder pressure-time development data. The use of plasma jet igniters with methane as a fuel enhanced the rates of burning in the initial stages of combustion, especially with very lean mixtures. The lean limit of engine operation was also extended. Their use for near stoichiometric fast burning mixtures tends in comparison to contribute little towards enhancing engine performance.

One of the main features of methane fueled spark ignition engines is their relatively slow flame propagation rates in comparison to liquid fuel applications which may lead to relatively lower power output and efficiency with increased emissions and cyclic variations. This is especially pronounced at operational equivalence ratios that are much leaner than the stoichiometric value. The addition of some hydrogen and oxygen to the methane may contribute towards speeding the combustion process and bring about significant improvements in performance and emissions. It has been suggested that the addition to the methane of products of water electrolysis generated in situ on board of a vehicle may produce such improvements.

The paper reviews the combustion characteristics of the two fuels and sets out to consider their respective performance in both spark ignition and compression ignition engines. Results of comparative tests involving spark ignition engines over a wide range of operating conditions are presented and discussed. Some of the performance characteristics considered are those relating to power output, efficiency, tendency to knock, cyclic variations, optimum spark requirements and exhaust emissions. Similarly, some of the performance characteristics in compression ignition engines considered include power output, efficiency, tendency towards knock and autoignition, exhaust emissions and low operational temperature problems. Finally, the relative operational safety aspects of the two fuels are evaluated. It is then suggested that in this regard, methane has some excellent physical, chemical and combustion characteristics that makes it a particularly safe fuel.

An examination of the engine performance and associated combustion is made for a dual fuel engine of the compression ignition type when additional auxiliary fuels were introduced in turn in the form of a spray into the main intake charge with methane being the main fuel. This was attempted with the view of modifying the dual fuel engine behaviour particularly at light load. Alcohols, gasoline, benzene or normal hexane were introduced in turn to various extents into the intake charge of the engine. Comparison with dual fuel operation on a range of gaseous fuels and with water spray injection was made. It is shown that gasoline, benzene or n-hexane intake addition reduced the overall ignition delay significantly and increased the power output of the dual fuel diesel engine at light load. This, however, was achieved at the cost of undermining the efficiency of fuel utilization at higher loads.

There are distinct operational mixture limits beyond which satisfactory spark ignition engine performance can not be maintained. The values of these limit mixtures which depend on the mode of their determination, are affected by numerous operational and design factors that include the type of engine and fuel used. Simple approximate methods are presented for predicting these limits. Good agreement is shown to exist between the calculated and the corresponding experimental values over a range of operating conditions while operating on the gaseous fuels: methane, propane and hydrogen. The experimentally observed operational limits deviate very substantially from the corresponding accepted flammability limit values for quiescent conditions evaluated at the average temperature and pressure prevailing at the instant of the spark passage.

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.

The paper describes the computational and experimental approach of deriving gross chemical kinetic data of the combustion of common fuels using a motored engine. The approach is based on the accurate calculation of the rate of heat release due to autoignition reactions. Kinetic data of the combustion of n-heptane, propane, and methane in air together with the role of the presence of a higher hydrocarbon vapour such as n-heptane with methane and propane are presented and discussed mainly in relation to dual-fuel engine phenomena.

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.

Cyclic variations in engines have been the subject of much investigation and there are some excellent reviews of this research. However, there is still a need to examine in an integrated manner the cyclic variation in performance parameters such as indicated power output, efficiency and cylinder pressure development in relation to the cyclic variation in some important combustion parameters notably those of the ignition lag, which is the time requirements to initiate a flame kernel following the passage of a spark and the duration to complete the combustion process particularly when gaseous fuels, notably methane are used. The paper describes the results of an investigation with these objectives using a single cylinder, variable compression ratio, spark ignition, CFR engine, run at constant speed, operating mainly on natural gas.

The total number of vehicles fuelled with compressed natural gas, CNG, is relatively very small in comparison to gasoline fuelled vehicles. Accordingly, because of the lack of statistics of accidents involving CNG fuelled vehicles, their safety aspects are evaluated in comparison to automobiles fuelled with gasoline or some other alternative fuels such as propane, hydrogen, LNG or LH2. It is suggested that methane, CNG, has some excellent physical, chemical and combustion characteristics that make it a safe automotive fuel. These characteristics are reviewed and the superior relative safety of methane in automotive applications in comparison to applications involving the other fuels is demonstrated where well designed conversion systems and operations are employed.

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 effects of charge non-uniformity on autoignition of methane/air mixtures in a motored engine are investigated analytically using a varying global kinetic data model derived from the results of a detailed chemical kinetic scheme under similar conditions in a simple adiabatic constant volume reactor. These derived varying global kinetic data model was implemented in the CFD KIVA-3 code. The relative contribution of fluid motion generated by piston motion, heat transfer, chemical reactivity of the cylinder charge and swirl movement to the inhomogeneities in the properties of the cylinder charge and their consequent effects on the evolution of the autoignition process are presented and discussed.

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.