Showing papers on "Spontaneous combustion published in 2001"

Analysis of the degree of constant volume and cooling loss in a spark ignition engine fuelled with hydrogen

Abstract: This study analyses factors influencing the thermal efficiency of a homogeneous charge spark ignition (SI) engine fuelled with hydrogen, focusing on the degree of constant volume and the cooling loss. The cooling loss from the burning gas to the cylinder walls in a homogeneous charge SI engine is quantitatively evaluated by analysing the cylinder pressure diagram and the exhaust gas composition. The degree of constant volume burning and the degree of constant volume cooling are also obtained by fitting the Wiebe function to the rate of heat release calculated using the cylinder pressure diagram. A comparison of combustion and cooling characteristics between hydrogen and methane combustion reveals that the cooling loss in hydrogen combustion is higher than that of methane combustion due to a thinner quenching distance and faster burning velocity for hydrogen combustion. It is also made clear that the higher cooling loss in hydrogen combustion results in lower thermal efficiency of hydrogen combusti...

Understanding of controlled autoignition combustion in a four-stroke gasoline engine

Abstract: Controlled autoignition (CAI) combustion has recently emerged as a viable alternative combustion process to the conventional spark ignition or compression ignition process for internal combustion engines, owing to its potential for high efficiency and extremely low NOx and particulate emissions. Since CAI combustion is a process dominated by chemical kinetics of the fuel-air mixture, an engine simulation model with detailed chemical kinetics has been developed and applied to a four-stroke gasoline engine fuelled with isooctane. After calibration and validation, the engine simulation model was used to study the effects of the intake temperature, exhaust gas recirculation (EGR), the air-fuel ratio, the compression ratio and the engine speed on CAI combustion in a four-stroke gasoline engine. The characteristics of CAI combustion investigated include the autoignition timing, the partial burning and knocking combustion and NO emission. Results show that CAI combustion could be achieved within a limited speed and load range. The lower end of the CAI combustion range was affected by partial burning, and the higher end of its operation was limited by knocking combustion. Among the engine parameters investigated, the intake charge temperature and EGR had the greatest effect on the CAI combustion process. The effect of EGR was further analysed in terms of its thermal (increase in heat capacity), dilution, chemical and charge heating effects by means of a series of simulation studies. It was found that the charge heating effect caused advanced ignition timing, faster heat release rate and moderate reduction in the CAI combustion duration. The thermal effect (increased heat capacity) retarded ignition, extended combustion duration and slowed down heat release rate. The dilution effect also resulted in longer combustion duration and slower burning but it did not affect the ignition timing. The chemical effect was found to accelerate the combustion process when the percentage of EGR was large.

Ignition and Combustion of Levitated Magnesium and Aluminum Particles in Carbon Dioxide

Abstract: This article considers ignition and combustion of single particles of magnesium and aluminum in carbon dioxide at pressures 0.1-2 MPa. An experimental setup with an electrodynamic levitator inside a high-pressure chamber was employed. The CO2-laser was used for heating to ignition of the particles. The results show that ignition mechanisms of Mg and Al in CO2 are different. Experiments with Mg indicate the existence of the critical partial pressure of CO2, whereas the ignition probability of Al particles in CO2 is low but independent on pressure. Analysis of flame images and combustion parameters shows that the mechanism of Mg particle burning in CO2 corresponds to conventional models of vapor-phase diffusion-controlled combustion, whereas in the case of Al exothermic processes on the particle surface or close to it play a leading part in the burning process.

Plasma Ignition and Combustion

Abstract: Electro-thermal-chemical (ETC) initiation and combustion offers the possibility to increase the performance of guns substantially as new propellant formulations and high loading densities (HLD) can be safely ignited and burnt in an augmented way. This paper reports investigations of burning phenomena in the low pressure region for JA2 and the effects of plasma interaction on ignition and study its in ̄uence on the burning rate. The comparison of transparent and opaque versions of the propellant is of special interest. Electrically produced plasma can strongly in ̄uence the ignition and combustion of solid propellants. Predominantly, plasma arcs in ̄uence strongly the burning of propellants by its radiation. The high intensity of the radiation initiates burning with short time delays in the ms-range and high conversion during exposure also in the case of a stable burning. Radiation can penetrate into the propellant interior and partially fragment at absorbing structures which could be arti®cially introduced or be inherently present as in the case of a JA2 propellant. Simpli®ed approaches based on the heat ̄ow equation and radiation absorption can explain these effects at least on a qualitative scale. Dynamic effects are understood by more sophisticated models.

Reaction of nitrogen dioxide with hydrocarbons and its influence on spontaneous ignition. A computational study

Abstract: Estimates are made, by using BHandHLYP/6-311G** density functional molecular orbital theory, of the activation energies and frequency factors for the reaction of NO2 with methane, ethane, propane, isobutane, and benzene. For the aliphatic hydrocarbons, over the temperature range 600–1100 K, the rate of formation of a new isomer of nitrous acid, HNO2, is very similar to that for the formation of the common isomer, HONO. This complicates our description of the acceleration of spontaneous ignition of diesel fuels by organic nitrates. These rate data are used in a reduced kinetic model to examine the effect of NO2 upon the spontaneous ignition of some linear- and branched-chain aliphatic hydrocarbons. It is concluded that, under typical diesel engine operating conditions, the spontaneous ignition of linear-chain paraffins is accelerated by the presence of NO2, but may be retarded for heavily branched-chain isomers. An Appendix discusses the relative importance of tunnelling in hydrogen-transfer reactions.

Temperature fields during the development of autoignition in a rapid compression machine

Abstract: Temperature and concentration fields have been investigated in the cylindrical combustion chamber of a rapid compression machine (RCM) by schlieren photography, chemiluminescent imaging and planar laser induced fluorescence of acetone and of formaldehyde in a 2-dimensional sheet across the diameter. The timescale of particular interest was up to 10 ms after the piston has stopped. Experiments were performed in non-reactive and reactive conditions. Acetone was seeded in non-reactive mixtures. Combustion was studied first in a system containing di-tert-butyl peroxide vapour in the presence of oxygen. The decomposition of di-tert-butyl peroxide generates methyl radicals, which are then oxidised if oxygen is present. The overall reaction is exothermic and is characteristic of a conventional thermal ignition. In addition, chemiluminescence, resulting from CH2O*, accompanies the oxidation process. The combustion of n-pentane was then investigated at compressed gas temperatures that spanned the range in which there is a negative temperature dependence of the overall reaction rate, typically 750–850 K. The response to thermal feedback in this more complex thermokinetic system can be the opposite of the “thermal runaway” that accompanies di-tert-butyl peroxide combustion. The purpose of making comparisons between these two types of systems was to show how the temperature field generated in the RCM is modified in different ways by the interaction with the chemistry and to discuss the implications of this for the spatial development of spontaneous ignition. As the piston of the RCM moves it shears gas off the walls of the chamber. This probably creates a roll-up vortex, but more importantly it also collects gas from the walls and moves it across the cylinder head pushing it forward into a plug at the centre. Thus, soon after the end of compression there is an adiabatically heated gas which extends virtually to the wall, but this incorporates a plug of colder gas at its core. Diffusive transport will occur, but the timescale is relatively slow, and the effect hardly shows until at least 10 ms post-compression. The consequence of “thermal runaway” on a timescale that is compatible with the development of this temperature field is that the reaction rate in the adiabatically compressed toroidal region accelerates faster than in the core, and goes to completion first. A somewhat similar pattern emerges during n-pentane combustion when the initial condition is set at the lower end of the negative temperature dependent range. By contrast, at adiabatically compressed gas temperatures close to the upper end of the negative temperature dependent region, the reaction rate in the cooler core develops faster than that in the surrounding zone, and the temperature difference is rapidly smoothed out. This does not lead to spatial homogeneity in all respects, however, because different rates and extents of reaction generate different concentrations of intermediates. This stratification has implications for the eventual spatial evolution of spontaneous ignition.

Method of operating a diesel internal combustion engine

Abstract: In a method of operating a diesel internal combustion engine, wherein a homogeneous mixture being formed in the combustion space before a spontaneous ignition by means of fuel injected directly and separately supplied combustion air, the combustion air is admitted to the combustion space in a swirling flow about the longitudinal axis of the cylinder that can be adjusted as regards its swirl intensity by a control element. A control unit adjusts the swirl intensity in a manner coordinated with the injection parameters to provide a homogeneous mixture for highly effective engine operation with low emissions.

Spark ignition stratified combustion internal combustion engine

Abstract: An internal combustion engine, wherein fuel injection nozzles (6) and spark ignition plugs (7) are disposed in combustion chambers (5), self-ignitable premixed air-fuel mixture is formed in the combustion chambers in the stratified form so as to provide a spatial distribution to the density of the premixed air-fuel mixture in the combustion chambers (5), a part of the premixed air-fuel mixture formed in the combustion chambers (5) is ignited by the spark ignition plugs (7) so as to perform a flame propagation combustion before the remaining premixed air-fuel mixture is self-ignited and burned in order with a time difference, and an ignition timing is set so that the ratio of the premixed air-fuel mixture self-ignited and burned is equal to or more than a predetermined lower limit value and equal to or less than a knocking occurrence limit.

Method for operating a direct injection internal combustion engine

Abstract: The invention relates to a method of operating a direct injection internal combustion engine operated on both spark-ignitable and autoignitable fuel, more specifically on gasoline, wherein the operational range of the engine is allocated autoignition ranges and spark-ignition ranges, an at least nearly homogeneous fuel-air mixture being produced in the combustion chamber in autoignition ranges, a high compression ratio suitable for spontaneous ignition of the fuel being provided and the combustion in the autoignition range being mostly initiated by spontaneous ignition, and wherein, in the ranges of spark ignition, combustion is initiated by spark ignition of said air-fuel mixture and a spark ignition range is assigned to the full load range and an autoignition range assigned to at least part of the part load range and wherein the effective compression ratio is lowered in spark ignition ranges and combustion controlled by regulating the residual gas content in the ranges of spontaneous ignition. In order to improve the exhaust emission quality while keeping efficiency high, the operational range of the engine is allocated two-stroke and four-stroke operational ranges and the internal combustion engine is operated on a two-stroke cycle in the ranges operating in the two-stroke mode and on a four-stroke cycle in the ranges operating in the four-stroke mode.

Dual mode engine combustion process

Abstract: An engine employs a dual mode combustion process including a first combustion mode for light loads utilizing premixed charge forced auto ignition (PCFA) wherein a pulse jet of reacting fuel mixture from a prechamber mixes with an ultra dilute premixed fuel-air charge in a main chamber, causing rapidly expanding combustion that ignites the remaining ultra dilute mixture by compression ignition. This improves efficiency, allows combustion phasing control and reduced NOx emissions. For higher speeds and loads, a conventional second combustion mode is utilized wherein a strong premixed mixture is ignited conventionally with spark ignition and/or pulse jet ignition. Cylinder pressures which would result from compression ignition at the higher speeds and loads are thus reduced providing overall engine operation with reduced emissions, improved fuel economy, and reduced noise through improved control of the combustion process.

Diffusion-controlled Combustion

Abstract: We devote this brief review to some relevant aspects of diffusion-controlled combustion. After a survey of the conservation equations involved, we shall describe the Burke-Schumann limit, which is applicable when the reaction time at the flame is very short compared with the mixing time. Using as a protopypical example the flow downstream from a fuel injector in a combustor chamber, we next introduce some phenomena related to finite-rate kinetics. We shall see how the high temperature sensitivity typical of combustion reactions is responsible for the presence near the injector of chemically frozen regions of low temperature where the reactants mix without chemical reaction, these regions being separated by thin premixed flames, with rich and lean branches, from regions of near equilibrium flow, where the reactants coexist only in a thin trailing diffusion flame. The role of these triple flames in the ignition, anchoring, and lift-off processes of diffusion flames will be briefly discussed.

Simultaneous Observation of Liquid Phase Distribution and Flame Front Evolution during the Ignition Transient of a LOX/ GH_2-combustor

Abstract: Phenomena such as flame propagation, flame/spray interaction and flame stabilization during the transient ignition process in a cryogenic model rocket combustor are investigated on sub-millisecond time scale. Diagnostic techniques developed to characterize the stationary spray flame are applied to investigate the transient evolution of the LOX-spray and the flame front during the ignition process. Ignition is initiated by focusing a pulsed laser into the combustion chamber. Thus, ignition time as well as the position of ignition is well defined. This and the exact control of the delay between ignition and detection time allowed the observation of the evolution of the flame front. The distribution of the liquid oxygen phase and the velocity of LOX droplets and ligaments are determined by light sheet techniques using a double-pulsed laser system. Simultaneously the position of the flame front is measured by recording the spontaneous emission of the OH-radical. By varying the delay time t between ignition and detection in a series of test runs, the transient ignition phenomena has been investigated in the interval from 0 to 5 ms after ignition.

Overview of models of boron particle ignition in hot air

Abstract: The phenomenon of boron particle ignition in hot air was defined.Two different boron particle ignition mechanisms and supporting experiments with corresponding chemical reactions were explained.The velocity of oxide generation and consumption in different ignition mechanisms was compared,and factors that affected boron particle ignition were analyzed.Boron particle ignition mathematical models were established finally.

Multi-Dimensional Modeling of Natural Gas Autoignition using Detailed Chemical Kinetics

Abstract: The autoignition of natural gas injected into a combustion bomb at pressures and temperatures typical of top-dead-center conditions in compression ignition engines is studied by combining a detailed chemical kinetic mechanism, consisting of 22 species and 104 elementary reactions, with a multi-dimensional reactive flow code. The effect of natural gas composition, ambient density and temperature on the ignition process is studied by performing calculations for three different blends of natural gas on a three-dimensional computational grid. The predictions of ignition delay compare very well with measurements in a combustion bomb. Based on this work, it is established that a particular mass of fuel burned is a much better criterion to define the ignition delay period than a specified pressure rise. The effect of additives like ethane and hydrogen peroxide in increasing the fuel consumption rate as well as the influence of physical parameters like fuel injection rate and intake temperature is studie...

Mathematic model for calculating the shortest coal spontaneous combustion time

Abstract: In order to forecast coal spontaneous combustion time more accurately, based on formerly researches and the thermo balance equation of coal oxygen reaction, the calculating mathematic model of coal shortest spontaneous combustion time has been put forward, and a suit of experimental methods has been set up It has been carried through that forecast coal spontaneous combustion time of two working faces The forecast accuracy has reached 74%～90% It is shown that the established forecast model is correct, and the forecast method has directing effect to the safe production of coal mine