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Technical Paper

Knock and Combustion Characteristics of CH4, CO, H2 and Their Binary Mixtures

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.
Technical Paper


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.
Technical Paper

An Analytical Approach for the Optimization of a SI Engine Performance Including the Consideration of Knock

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.
Technical Paper

An Analytical Examination of the Effects of Exhaust Gas Recirculation on the Compression Ignition Process of Engines Fuelled with Gaseous Fuels

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.
Technical Paper

An Examination of the Role of Residual Gases in the Combustion Processes of Motored Engines Fuelled with Gaseous Fuels

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.
Technical Paper

A Predictive Model for the Combustion Process in Dual Fuel Engines

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.
Technical Paper

An Analysis of Fuel Droplets Ignition and Combustion within Homogeneous Mixtures of Fuel and Air

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.
Technical Paper

Prediction of the Performance of Spark Ignition Gas Engines Including Knock

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.
Technical Paper

A Predictive Model for Knock in Spark Ignition Engines

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.
Technical Paper

A Predictive Model for Knock in Dual Fuel Engines

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.
Technical Paper

Methane-Carbon Dioxide Mixtures as a Fuel

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.
Technical Paper

An Examination of the Combustion Processes of a Methane Fuelled Engine When Employing Plasma Jet Ignition

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.
Technical Paper

Examination of the Combustion of a Fuel Jet in a Homogeneously Premixed Lean Fuel-Air Stream

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.
Technical Paper

A Diagnostic Two-Zone Combustion Model for Spark-Ignition Engines Based on Pressure-Time Data

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.
Technical Paper

Some Considerations of Cyclic Variations in Spark Ignition Engines Fuelled with Gaseous Fuels

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.
Technical Paper

Comparative Studies of Methane and Propane as Fuels for Spark Ignition and Compression Ignition Engines

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.
Technical Paper

The Dual Fuel Engine of the Compression Ignition Type - Prospects, Problems and Solutions - A Review

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.
Technical Paper

Determination of the Performance of a Dual Fuel Diesel Engine with the Addition of Various Liquid Fuels to the Intake Charge

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.
Technical Paper

Examination of the Performance of a Dual Fuel Diesel Engine with ParticuIar Reference to the Presence of Some Inert Diluents in the Engine Intake Charge

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.
Technical Paper

Consideration of Ignition Lag and Combustion Time in a Spark Ignition Engine Using a Data Acquisition System

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.