Search Results

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.

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.

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.

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.

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.

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 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.

A review is made of some of the main problems associated with the use of natural gas, notably methane, in dual fuel engines of the compression ignition diesel type. It is shown that such applications represent in principle a very attractive mode for the utilization of the fuel for the production of power generally at relatively high efficiencies and outputs with good exhaust emissions characteristics. Some relevant solutions to the problems outlined are then discussed. Moreover, some further research and development needed in this general area is also outlined.

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.

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.

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.

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.

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 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 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.

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.

An examination of the combustion phenomena and associated engine performance is made for a dual fuel engine of the compression ignition type when additional auxiliary fuels and inerts were introduced with the main intake charge. This was considered to have direct relevance to operation of diesel engines on low heating value gaseous fuel mixtures. A single cylinder direct injection diesel engine was employed throughout. H2, CH4, CO2 and N? were the gas constituents used. Operation on gaseous fuel mixtures containing significant amounts of diluent inerts showed a marked deterioration of the already inefficient combustion of lean fuel-air mixtures. Of the two inert gases (CO2 and N2) introduced in turn, CO2 has a comparatively greater influence in narrowing the operating range of the engine. Both diesel and dual fuel operations were also considered when a spray of water was introduced into the intake of the engine.

There are numerous situations in a wide range of engineering applications involving combustion devices where the combustion of a fuel jet takes place in flowing streams containing varying proportions of a fuel homogeneously premixed with the surrounding air. Such applications can be found, for example, in dual fuel engines and in some gas turbine combustors. The paper describes some of the findings of an experimental investigation, supported by some analytical modeling, of the combustion of a circular gaseous fuel jet within lean homogeneous mixtures of various gaseous fuels and air. The nature of the combustion process of the pilot fuel jet, flame spread characteristics and limits within the surrounding moving atmosphere were considered in terms of the fuels used for the jet and the surrounding atmosphere and in terms of the jet discharge and surrounding stream flow characteristics.

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.