Search Results

The dual-fuel engine represents in principle a simple flexible approach to employing gaseous fuels in conventional diesel engines. Compared to the use of hydrogen in spark ignition engines, there is relatively limited information about its effect when present as a supplementary fuel in suitably modified conventional compression ignition engines. This is especially for engines of the IDI type and when employing only low concentrations of hydrogen in the intake air while retaining the injection of large diesel fuel quantities. In the present contribution, a 3D-CFD model based on KIVA 3, developed with a “reduced” detailed chemical kinetics of 294 elementary reaction steps with 79 chemical species for diesel fuel combustion which includes 20 steps for the oxidation of hydrogen, is outlined.

Dual-fuel engines represent a simple and flexible approach to employing gaseous fuels including hydrogen in conventional diesel engines. This paper reports on the investigation into the effects on performance, emissions and combustion characteristics of introducing low volumetric concentrations of hydrogen into the intake air of a multi-cylinder IDI diesel engine. It is shown that increasing the admission of hydrogen up to a certain low concentration increased power output, whereas further increases in hydrogen admission began to decrease power output. For the same total fuel energy input, the engine brake power and thermal efficiency were less with hydrogen admission as compared to the corresponding pure diesel operation. The exhaust gas opacity and the emissions of oxides of nitrogen and carbon monoxide showed correspondingly relatively significant reductions.

A 3D-CFD model with detailed chemical kinetics was developed to investigate the combustion characteristics of HCCI engines, especially those fueled with hydrogen and n-heptane. The effects of changes in some of the key important variables that included compression ratio and chamber surface temperature on the combustion processes were investigated. Particular attention was given, while using a finer 3-D mesh, to the development of combustion within the chamber crevices between the piston top-land and cylinder wall. It is shown that changes in the combustion chamber wall surface temperature values influence greatly the autoignition timing and location of its first occurrence within the chamber. With high chamber wall temperatures, autoignition takes place first at regions near the cylinder wall while with low surface temperatures; autoignition takes place closer to the central region of the mixture charge.

In order to pursue further improvements to the performance and emissions of small size dual fuel engines with a swirl chamber, a 3KC1 ISUZU engine setup was rebuilt, and a 3D-CFD model based on KIVA was developed. The simulation incorporates a reduced detailed chemical kinetics for the diesel fuel but with detailed chemical kinetics for the gaseous fuel component, mainly methane while considering the turbulence function to simulate the combustion processes and associated performance of a dual fuel engine with a swirl chamber. The effects of changes in the quantities of the liquid fuel pilot and gaseous fuels on the combustion processes, engine performance, cyclic variations, and emissions were investigated both experimentally and numerically.

The operation of S.I. engines on lean or diluents containing gaseous fuel-air mixtures is attractive in principle since it can provide improved fuel economy, reduced tendency to knock and low NOx emissions combined with a possible improvement to the operational life of the engine. However, the overall flame propagation rates then tend to drop sharply as the operational mixture is excessively leaned or diluted with CO2 or N2. The paper presents experimental data obtained in a single cylinder, variable compression ratio, S.I., CFR engine when operated on a number of gaseous fuels and some of their mixtures. A gradual leaning of the operating mixture can affect adversely in turn, emissions of CO and unburned fuel and cyclic variation. The extent of deterioration in these operating parameters is shown to correlate well with the corresponding values of the combustion period, a key combustion indicator. Similar effects were observed when adding diluents to stoichiometric CH4-air mixtures.

Carbon monoxide (CO), carbon dioxide (CO2) and unburned hydrocarbon (UHC) are present in the exhaust gases of S.I. engines operated on pure hydrogen. These carbon-bearing species result from the oxidation of the lubricating oil and can be considered conveniently as natural tracers for indicating the lubricating oil consumption by combustion. Accordingly, such a novel approach can be employed to examine factors that affect engine oil consumption without the need to resort to more complex approaches. This contribution presents experimental results of oil combustion in a variable compression ratio single cylinder CFR engine when fueled with pure hydrogen established by determining the concentrations of CO and CO2 in the exhaust gas. The effects of changes in key operating variables that include equivalence and compression ratios, spark timing and the onset of knock on oil combustion are examined.

An approach for predicting the onset of knock and estimating its intensity in spark ignition engines is described. It is based on evaluating a dimensionless energy functional group, Kn, formulated to provide a numerical criterion to test continually, while using predictive models of the performance of spark ignition engines, for the onset of knock and its relative intensity at any instant during the combustion process. The basis for the derivation of this knock criterion and its significance are described. Examples involving gaseous fuels and their mixtures under different operating conditions show how the criterion can be employed for the prediction of the onset of knock and the associated knock- limited performance. Experimental results validating this predictive approach are also included.

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.

An engine is described that operates exclusively on stoichiometric H2-O2 mixtures with significant amounts of excess hydrogen circulated to effect controlled combustion of excessively rich mixtures within the engine cylinder combined with exhaust water condensation and removal. A two-zone quasi-dimensional model for predicting the performance and the likelihood of the onset of knock developed earlier for CH4-H2-air operation has been extended suitably to predict the approximate performance of this H2-O2 engine arrangement. A detailed chemical kinetic scheme for the oxidation reactions of H2-O2 mixtures is adopted in the knock prediction of this model. A prior knowledge of the variation of the combustion period needed for this predictive model was estimated through processing corresponding data for H2-air mixtures. The limited experimental results reported by Furuhama, et al., were used to validate the corresponding predicted values.

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.

A comprehensive modeling is made of the transient transport processes taking place within nearly-empty axisymmetrical cylindrical liquid fuel tanks. The resulting likelihood of the formation or dissipation of flammable mixtures within the tank with time as a consequence of the transient vaporization processes of the fuel are established. Some aspects of the predicted results were validated against our corresponding experimental results involving mainly pure liquid fuels in open cylindrical containers that were nearly-empty. The effects of fuel properties, geometry and ambient conditions are considered in relation to the possibility of having the atmosphere within the tanks supporting flame propagation, in the event of the presence of an ignition source somewhere within the tank or a flame just outside the tank. The effects of changes in some operating conditions on the potential for fire spread inside liquid fuel tanks are discussed.

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.

Autoignition for most fuel-air mixtures in engines is preceded by relatively slow chemical changes. These changes are the main area of interest in this contribution, since a better control of the autoignition process in homogeneously charged motored engines may become potentially more viable through a better understanding of the reactions that lead to autoignition. An approach to modify the partial oxidation process is by changing the composition of the charge through a deliberate recirculation of some of the exhaust gases back into the cylinder. These recycled gases, when not fully cooled, can influence the autoignition process thermally. They may also contain small concentrations of active chemical species that could influence kinetically the partial oxidation processes of the engine.

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.