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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.

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.

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.

An approach is described, employing a data acquisition system that provides information regarding the statistical cycle variation in a range of performance parameters of a spark ignition engine fuelled with methane. The approach can provide a simultaneous record of the rate of work production as well as the time taken for a flame kernel to be developed following spark ignition and the subsequent time needed to complete the combustion process. Such information can be provided either continuously or randomly over a large number of cycles. Thus, cyclic variation in performance parameters is linked to important combustion parameters without recourse to high speed photography nor to the use of transparent heads or pistons. Some typical results involving a single cylinder variable compression ratio CFR engine are then presented.

Operating characteristics including ignition limits, cyclic variability, and exhaust emissions were studied in the combustion of natural gas in a spark ignition, single-cylinder, variable compression ratio engine, operated at intake mixture temperatures ranging between 120 and -60 F. The work confirmed in general the feasibility of using natural gas in a spark ignition engine operated under extremely cold intake temperature conditions. It was learned that both the maximum peak cylinder pressures and the mass of mixture inducted by the engine increased as the intake mixture temperature was lowered, and that the emissions of pollutants were not significantly increased. These findings are thought to be particularly relevant to the use of natural gas in spark ignition engines, either as LNG or under very cold wintry conditions.

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 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.

A simple diagnostic combustion model for spark-Ignition engines, based on pressure-time data, is described. It considers the charge to be made up of two zones, burnt products and unburnt reactants, each of which is undergoing a series of continuously varying yet distinctly different polytropic processes. The two zones exchange mass across the flame front, due to combustion. This simple approach which utilizes essentially no correlations or empirical formulae produces results such as the rate of burning of the reactants, the rate of change of volume of each of the two zones, as well as the mean temperature hirstories of each of the two zones throughout the combustion period.

The observed lean and rich operational limits in a spark ignition engine of the variable compression ratio type fuelled with either methane or propane are shown to be amenable to correlation in terms of a calculated mean mixture temperature at the time of passing the spark. Moreover, using a detailed chemical kinetics model for the oxidation of lean methane-air mixtures in a compression ignition engine, the autoignition of methane-air mixtures is examined. It is shown that these autoignition limits are also influenced by the mean charge temperature and the kinetic and thermal sensitizing of the charge through mixing with residual gases.

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.

A predictive procedure for establishing the performance parameters of spark ignition engines fueled with a range of gaseous fuels and their mixtures is described. The incidence of knock and its relative intensity are also accounted for. The two-zone model incorporates a procedure for deriving an estimate of the effective duration of combustion and the associated mass burning rate for various operating conditions and gaseous fuels. The preignition chemical reaction activity of the unburned end gas zone and its consequences on cylinder pressure development is evaluated while using detailed chemical kinetics. The onset of autoignition and knock is established via a parameter that monitors the incremental pressure increase solely due to the preignition reaction activity per unit of mean effective combustion pressure.

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.