A 3D computational fluid dynamics model with a reduced detailed chemical kinetics of the combustion of diesel and methane fuels is developed while considering turbulence during combustion to simulate the mixture flow, formation, and combustion processes within diesel and diesel/methane dual fuel engines having swirl chambers. The combustion characteristics of the pilot injection into a small prechamber are also investigated. Modeled results were validated by a group of corresponding experimental data. The spatial and temporal distributions of the mixture temperature, pressure, and velocity under conditions with and without liquid fuel injection and combustion are compared. The effects of engine speed, injection timing, and the addition of carbon dioxide on the combustion process of dual fuel engines are investigated. It is found that in the absence of any fuel injection and combustion, the swirl center is initially formed at the bottom-left of the swirl chamber, and then moved up with continued compression in the top-right direction toward the highest point. The swirling motion within the swirl and main combustion chambers promotes the evaporation of the liquid pilot and the combustion processes of diesel and dual fuel engines. It was observed that an earlier autoignition can be obtained through injecting the pilot fuel into the small prechamber compared with the corresponding swirl chamber operation. It is to be shown that reduced engine emissions and improved thermal efficiency can be achieved by a two-stage homogenous charge compression ignition combustion.

In the dual fuel engine much of the energy release comes from the combustion of the gaseous fuel while only a small amount of diesel liquid fuel provides ignition through timed cylinder injection. Such operation with optimum conversion methods has the potential to provide operational characteristics that are comparable or superior to those of the corresponding diesel or spark ignition engines. These characteristics may be realized only if sufficiently effective measures can be ensured both for the avoidance of knock, usually at high loads, and incomplete gaseous fuel utilization at relatively light loads. An objective of this contribution is to demonstrate that the main effort needed to overcome the problems associated with the operation of gas fueled dual fuel engines is via a better control of the relatively complex processes of combustion. Both experimental and analytical modeling procedures for effecting optimum improvement to the combustion process are described.

The knock tendency in spark ignition engines of binary mixtures of hydrogen, ethane, propane and n-butane is examined in a CFR engine for a range of mixture composition, compression ratio, spark timing, and equivalence ratio. It is shown that changes in the knock characteristics of binary mixtures of hydrogen with methane are sufficiently different from those of the binary mixtures of the other gaseous fuels with methane that renders the use of the methane number of limited utility. However, binary mixtures of n-butane with methane may offer a better alternative. Small changes in the concentration of butane produce almost linearly significant changes in both the values of the knock limited compression ratio for fixed spark timing and the knock limited spark timing for a fixed compression ratio.

The reliable prediction of the processes leading to autoignition during the rapid compression of an initially homogeneous mixture of fuel and air requires the coupled modeling of multidimensional fluid dynamics and heat transfer together with a sufficiently detailed description of the chemical kinetics of the oxidation reactions. To satisfy fully such requirements tends at present to be unmanageable. The paper describes an improvised approach that combines multidimensional fluid dynamics modeling (CFD KIVA-3) with derived variable effective global chemical kinetic data. These were generated through a fitting procedure of the corresponding results obtained while using a detailed chemical kinetic scheme; albeit with uniform properties, at constant volume and an initial state similar to that existing during the ignition delay. It is shown while using such an approach that spatially nonuniform properties develop rapidly within the initially homogeneous charge due to piston motion, heat transfer and any preignition energy release activity. This leads autoignition to take place first within the hottest region and a reaction front progresses at a finite rate to consume the rest of the mixture. The present contribution examines the compression ignition of hydrogen-oxygen mixtures in the presence of argon as a diluent. Validation of the predicted results is made using a range of corresponding experimental values obtained in a single-shot pneumatically driven rapid compression apparatus. It is to be shown that the simulation which indicates the build up of temperature gradients during the compression stroke, predicts earlier autoignition than that obtained with a single-zone simulation. Good agreement between predicted and experimental results is achieved, especially for lean and stoichiometric mixtures under high compression ratio conditions. The CFD-based simulation results are found to be closer to the corresponding experimental results than those obtained with an assumed reactive system of uniform properties and using detailed reaction kinetics.

The influence of variations in the composition of natural gas on the ignition and combustion processes in engines is investigated. Particular attention is given to changes in the relatively small concentrations of high molar mass alkanes that may be present in the fuel. A detailed chemical kinetic scheme for the oxidation of the higher hydrocarbon components of up to n-heptane was used to investigate analytically the combustion reactions of different fuel mixtures under constant volume adiabatic conditions with initial states that are similar to those during the ignition delay of a typical internal combustion engine. These comprehensive simulation calculations require much computing capacity and time that would preclude their incorporation in full simulation models of engine processes. A simplification is introduced based on replacing artificially the small concentrations of any higher hydrocarbons that may be present in the natural gas by a kinetically equivalent amount of propane in the fuel mixture. This is done such that the resulting equivalent fuel has the same ignition delay as the original fuel under constant volume engine T.D.C. conditions. This “propane equivalent” concept was used in full engine simulation models while employing a relatively short scheme of 150 steps for the oxidation of fuel mixtures of propane, ethane, and methane in air.

The paper describes results of a parametric study obtained while using an analytical model described earlier (Bunama and Karim, 1997b) investigating the combined effects of mass, energy, and momentum transfer with variable transport and thermodynamic properties on the formation of fuel vapor-air mixtures above a stagnant liquid fuel surface within the confines, of a vertical cylindrical vessel. This was done mainly to examine the establishment of the formation of flammable mixtures and their changes in size and location with time within liquid fuel tanks that are partially empty. The effects of changes in the ambient and wall temperatures, presence of liquid on the walls and vessel geometry were considered. Moreover, the results of a corresponding experimental investigation are presented. Much of the data relates to the high volatility fuel n-pentane that represents the lighter fuel fractions in commercial fuels which through their early evaporation contribute much to the fire hazards in fuel tanks.