Multistage Ignition Research Articles

Numerical Calculation of the Homogeneous Charge Compression Ignition Process by using an Elementary Reaction Model of Dimethyl Ether.

In this study, the numerical calculation of the Homogeneous Charge Compression Ignition (hereafter HCCI) process is carried out by using Curran et al.'s DME elementary reaction model for DME fueled HCCI engjne. On condition simulating shock tube, the validity of the DME elementary reaction model is investigated. And main elementary reactions releasing heat are investigated in case of multi-stage ignition. As a result, numerical calculation using Curran et al.'s elementary reaction model can qualitatively predict the influence of equivalence ratio, temperature and pressure on the appearance timings of low temperature reaction and high temperature reaction under the pressure and the temperature in HCCI engine.

Detailed numerical simulations of the multistage self-ignition process of n-heptane, isolated droplets and their verification by comparison with microgravity experiments

Detailed understanding of the basic physical and chemical processes of self-ignition phenomena for technical fuel sprays is required for many technical combustion applications. Because single droplets as the basic elements of fuel sprays allow the study of fundamental ignition behavior, this work focuses on the multistage self-ignition behavior of Φ 0.7 mm single n-heptane droplets in air. The investigated ambient conditions are temperatures from 580 to 1000 K and pressures from 0.3 to 1 MPa. A substantial physical and chemical model is developed for the detailed numerical simulation. The chemical reaction mechanism consists of a 62-step kinetic model with special consideration of the low-temperature reaction branch. The program was validated by comparison with results from microgravity experiments at Drop Tower Bremen, which allow clarification of the ignition process free from natural convection. Cool flame and hot flame appearances were obtained from non-intrusive interferometric measurement in a well-tested experimental setup. The calculated ignition delays resulting from temperature and temperature gradient criteria, respectively, were compared with these experimental results. Furthermore, the cool flame temperature was measured with a K-type thermocouple of Φ 25 μm at the place of its appearance and was also compared with the numerical simulations. A quantitative good agreement for first and total ignition delays as well as for the cool flame temperature could be achieved. With this detailed numerical model, the multistage ignition behavior was analyzed, and the ignition criteria employed in interferometric measurement, which are temperature and temperature gradient, respectively, were confirmed.

JSFR combustion processes of n-heptane and isooctane

Combustion processes of n -heptane and isooctane have been studied in a Jet Stirred Flow Reactor (JSFR), which operates according to the Continuous Stirred Tank ideal model of reactor. The range of experimental conditions spanned from 202 to 1212 kPa and from 573 to 840 K. The feed consisted of stoichiometric fuel/air mixtures and flow rates were such to establish a 0.4 s residence time. The analysis of dynamic behavior of combustion processes showed that the same variety of dynamic events is exhibited by both fuels, namely: slow combustion, low temperature multistage ignition, damped and periodic cool flames, “jumps” and high temperature ignitions. Ignition diagrams have been plotted on the temperature/pressure plane. Heat release rates have been measured experimentally and their dependence on temperature clearly shows the region of existence of negative temperature coefficient of the reaction rate. The heat release rate curve provides evidence of hysteresis due to the non linearity of the process. The analyses of reaction intermediates indicate that fuel molecules are not heavily degraded at the sampling conditions which are located in the negative temperature coefficient region. The major differences between the combustion processes of the two fuels are in the pressure limit of the region of low temperature multistage ignition and in the presence of acetaldehyde among the reaction intermediates detected only for the n -heptane/air system. On this basis, the hypothesis is formulated that the different knock resistance of the two fuels could well be caused by the differences of the low temperature mechanisms and by the ability of producing acetaldehyde at an early stage of the combustion process.

Behavior of Band Spectra in Diesel Combustion Flames: A Study on Chemical Reactions in Diesel Combustion

When a precombustion chamber type Diesel engine is operated under light loads and at low speeds, it is observed that the ignition flames in its main combustion chamber are generated in several stages under specific conditions of fuel injection. The authors, by observing the multistage ignition flames through an optical fiber probe inserted into the chamber, confirmed a number of band spectra emitted from some substances and CH radicals, existing in the flames, and have made clear some aspects of the chemical reactions taking place in Diesel combustion.

The behaviour of the band spectra in diesel combustion flames. (A study of the chemical reaction in diesel combustion flames).

The Authors inserted an optical fiber into the main chamber of a pre-combustion chamber engine. This fiber has excellent characteristics of transmittance in ultraviolet ranges. They found that the diesel combustion flames which originated from the multi-stage ignition phenomina in a main combustion chamber contained the band spectrum emitted from a radical of CH. The above investigation has given a glimpse of the chemical reaction in a diesel combustion.

Rhenomena of multi-stage ignition in the pre-combustion chamber type diesel engine in low speed ranges. (Investigation into the cause of the phenomena and measurement of the spectra emitted in the combustion chamber)

When a precombustion chamber type Diesel engine was operated under light loads and at low speeds, ignition flames were observed generating in the main combustion chamber in several stages (during each injection period). The authors, by means of inserting an optical fiber probe into the combustion chamber, investigated the real state and cause of the phenomena, and confirmed a number of spectra existing in those flames.

Multistage ignition in hydrocarbon combustion: Temperature effects and theories of nonisothermal combustion

In a study of self-heating during the low-temperature oxidation of propane, attention is given to direct measurement of temperature in the gas reacted under both unstirred and well-stirred conditions in a static reactor. During slow reaction, unstirred, the spatial temperature profile that develops is intermediate between that predicted for either purely conductive or strongly convective heat transfer. With stirring, the temperature and concentration distributions become nearly uniform in space. The P-T 0 ignition diagram has been mapped in the unstirred reactor: apart from the familiar slow reaction, cool-flame oscillations two-stage ignition, lobe and single-stage ignition, a region of multistage ignitions is characterized (cool flames precede the final ignition). Temperature-time profiles for multiple oscillations (≤5) and complex ignitions are given. Often, the cool flame propagates with a feeble front from the hottest zone of the reaction, and this leads to complex temperature-time curves. In the stirred reactor, each cool flame propagates homogeneously. Consequently, temperature-time traces are modified and become more easily interpreted. Thus, inhomogeneity during reaction perturbs the nonisothermal behavior, and this may vitiate conclusions that do not take the effect into account. Res u lts are discussed in terms of unified chain-thermal theory invoking two time-dependent variables (temperature and concentration of one intermediate x ). With the exception of multistage ignitions, all physical aspects are explained completely. The former are consistent only with a theory invoking three dependent variables. The new variable may be either another important chemical intermediate or a parameter of the system, such as fuel consumption, or a property controlling the rate of heat loss.

Auto-ignition and knock characteristics of benzene- n-heptane-air mixtures

Pre-ignition and knock properties of fuels, prepared by mixing benzene and n -heptane in various proportions, have been investigated by three parallel experimental techniques. They were auto-ignition in a tubular reactor and in a motored engine, and knock resistance in a firing engine. Recordings were made in the forms of auto-ignition limits, band structure of spectra of the luminous pre-ignition reactions, pressure rise characteristics of cylinder pressure diagrams and compression ratios for knock. The investigation revealed a sudden change of reaction mechanisms, characterized respectively by pure benzene and pure n -heptane in air, at the fuel with approximately ten per cent n -heptane concentration by volume. This work is a contribution to the understanding of the roles of single- and multi-stage ignition phenomena in both high and low temperature reaction mechanisms and their relations to the knock properties of engine fuels.