Effects of valve events on the engine efficiency in a homogeneous charge compression ignition engine fueled by dimethyl ether

Combustion characteristics of a homogeneous charge compression ignition (HCCI) engine were investigated with regard to the residual gas, i.e. internal exhaust gas recirculation (IEGR), by changing the intake and exhaust maximum opening points (MOP) and the exhaust cam lifts. Three different exhaust camshafts were used and had 2.5 mm, 4.0 mm and 8.4 mm exhaust valve lift. In-cylinder gas was sampled at the intake valve immediately before ignition to measure the IEGR rate. The heat release, fuel conversion efficiency and combustion efficiency were calculated using the in-cylinder pressure and composition of exhaust gases to examine the combustion features of the HCCI engine. The negative valve overlap (NVO) was increased as exhaust valve lift was reduced. Longer NVO made an increased IEGR through exhaust gas trapping. The IEGR rate was increased as the exhaust valve timing advanced while it was affected more by exhaust valve timing than by intake valve timing. Combustion phase was advanced by lower exhaust valve lift and early exhaust and intake MOP. It was because of higher amount of IEGR gas and effective compression ratio. The fuel conversion efficiency with higher exhaust valve lift was higher than that with lower exhaust valve lift. The late exhaust and intake MOP made the fuel conversion efficiency improve.     

A fundamental study on the control of the HCCI combustion and emissions by fuel design concept combined with controllable EGR. Part 2. Effect of operating conditions and EGR on HCCI combustion

In Part 1, the effects of octane number of primary reference fuels and equivalence ration on combustion characteristics of a single-cylinder HCCI engine were studied. In this part, the influence of exhaust gas recirculation (EGR) rate, intake charge temperature, coolant temperature, and engine speed on the HCCI combustion characteristics and its emissions were evaluated. The experimental results indicate that the ignition timing of the first-stage combustion and second-stage combustion retard, and the combustion duration prolongs with the introduction of cooled EGR. At the same time, the HCCI combustion using high cetane number fuels can tolerate with a higher EGR rate, but only 45% EGR rate for RON75 at 1800 rpm. Furthermore, there is a moderate effect of EGR rate on CO and UHC emissions for HCCI combustion engines fueled with n-heptane and RON25, but a distinct effect on emissions for higher octane number fuels. Moreover, the combustion phase advances, and the combustion duration shorten with the increase of intake charge temperature and the coolant out temperature, and the decrease of the engine speed. At last, it can be found that the intake charge temperature gives the most sensitive influence on the HCCI combustion characteristics.     

The relationship of knock during controlled autoignition to temperature inhomogeneities and fuel reactivity

Experiments have been performed in a rapid compression machine (RCM), to investigate the conditions for and the origins of ‘knock’ in controlled autoignition (CAI), or homogeneous charge compression ignition (HCCI). Ignition in an RCM is the closest approach to that in a CAI engine without engendering the full complexity of reciprocating motion and fuel+air charge induction. As a representative fuel of intermediate reactivity, the combustion of n-pentane in air was studied at the compositions φ=1.0, 0.75 and 0.6 at end-of-compression pressures of 0.80-0.86 MPa (7.9-8.4 bar) and 1.4-1.5 MPa (13.8-14.8 bar), respectively, over the compressed gas temperature range 690-820 K. Autoignition is characterised by a two-stage development in these ranges of conditions, a ‘cool flame’ being followed by hot stage combustion.Filtered Rayleigh scattering from a planar laser sheet was used to characterise the temperature field that developed in the combustion chamber following rapid compression. High resolution pressure records, combined with image intensified, natural light output originating from chemiluminescence, were used to characterise the transition from non-knocking to knocking reaction and the evolution of the spatial development of chemical activity in this temperature field. It appears that knock originates from a localised development of the incandescent hot stage of ignition. Even though non-homogeneities prevail in the non-knocking reaction, it is associated with a relatively benign development, in which the cool flame is followed by a second stage, blue flame rather than the normal incandescent hot flame. The kinetics that may contribute to this distinction are discussed.     

An experimental investigation on a DI diesel engine using waste plastic oil with exhaust gas recirculation

Environmental degradation and depleting oil reserves are matters of great concern around the globe. Developing countries like India depend heavily on oil import of about 125 Mt per annum (7:1 diesel/gasoline). Diesel being the main transport fuel in India, finding a suitable alternative to diesel is an urgent need. In this context, waste plastic solid is currently receiving renewed interest. Waste plastic oil is suitable for compression ignition engines and more attention is focused in India because of its potential to generate large-scale employment and relatively low environmental degradation. The present investigation was to study the effect of cooled exhaust gas recirculation (EGR) on four stroke, single cylinder, direct injection (DI) diesel engine using 100% waste plastic oil. Experimental results showed higher oxides of nitrogen emissions when fueled with waste plastic oil without EGR. NO_x emissions were reduced when the engine was operated with cooled EGR. The EGR level was optimized as 20% based on significant reduction in NO_x emissions, minimum possible smoke, CO, HC emissions and comparable brake thermal efficiency. Smoke emissions of waste plastic oil were higher at all loads. Combustion parameters were found to be comparable with and without EGR. Compression ignition engines run on waste plastic oil are found to emit higher oxides of nitrogen.     

Effects of internal exhaust gas recirculation on controlled auto-ignition in a methane engine combustion

The effects of internal exhaust gas recirculation (IEGR) on controlled auto-ignition were evaluated with a single cycle simulator consisting of a rapid intake compression and expansion machine (RICEM) using methane as the fuel. The fuel-air mixture and simulated residual gas were introduced to the combustion chamber through the spool-type valves. Simulated residual gas representing the internal exhaust gas recirculation (IEGR) was generated by burning the fuel-air mixture in the IEGR chamber during the intake stroke. Various supply timings, homogeneities, and equivalence ratios of simulated residual gas were tested to investigate their effects on the auto-ignition of the fuel-air mixture. Multi-point ignitions and faster combustion were observed along with realized controlled auto-ignition combustion. The supply timing of simulated residual gas correlates with its temperature which subsequently affects the auto-ignition timing and burning duration. Stratification between the fuel-air mixture and simulated residual gas can maintain locally high temperatures of the simulated residual gas and enhance the auto-ignition of the fuel-air mixture. The auto-ignition temperature under the stratified mixing condition was more than 100 K lower than that under homogeneous mixing conditions. Relatively lean mixtures had more difficulty with auto-ignition and frequently showed misfire even at high temperatures.     

An investigation of the effects of spray angle and injection strategy on dimethyl ether (DME) combustion and exhaust emission characteristics in a common-rail diesel engine

An experimental investigation was performed on the effects of spray angle and injection strategies (single and multiple) on the combustion characteristics, concentrations of exhaust emissions, and the particle size distribution in a direct-injection (DI) compression ignition engine fueled with dimethyl ether (DME) fuel. In this study, two types of narrow spray angle injectors (θ_spray = 70° and 60°) were examined and its results were compared with the results of conventional spray angle (θ_spray = 156°). In addition, to investigate the optimal operating conditions, early single-injection and multiple-injection strategies were employed to reduce cylinder wall-wetting of the injected fuels and to promote the ignition of premixed charge. The engine test was performed at 1400 rpm, and the injection timings were varied from TDC to BTDC 40° of the crank angle.The experimental results showed that the combustion pressure from single combustion for narrow-angle injectors (θ_spray = 70° and 60°) is increased, as compared to the results of the wide-angle injector (θ_spray = 156°) with advanced injection timing of BTDC 35°. In addition, two peaks of the rate of heat release (ROHR) are generated by the combustion of air-fuel premixed mixtures. DME combustion for all test injectors indicated low levels of soot emissions at all injection timings. The NO_x emissions for narrow-angle injectors simultaneously increased in proportion to the advance in injection timing up to BTDC 25°, whereas BTDC 20° for the wide-angle injector. For multiple injections, the combustion pressure and ROHR of the first injection with narrow-angle injectors are combusted more actively, and the ignition delay of the second injected fuel is shorter than with the wide-angle injector. However, the second combustion pressure and ROHR were lower than during the first injection, and combustion durations are prolonged, as compared to the wide-angle injector. With advanced timing of the first injection, narrow-angle injectors with multiple injections could achieve low NO_x levels and soot levels similar to single-injection cases.     

Effects of ethyl tert-butyl ether addition to diesel fuel on characteristics of combustion and exhaust emissions of diesel engines

The effects of ethyl tert-butyl ether (ETBE) addition to diesel fuel on the characteristics of combustion and exhaust emissions of a common rail direct injection diesel engine with high rates of cooled exhaust gas recirculation (EGR) were investigated. Test fuels were prepared by blending 0, 10, 20, 30 and 40 vol% ETBE to a commercial diesel fuel. Increasing ETBE fraction in the fuel helps to suppress the smoke emission increasing with EGR, but a too high fraction of ETBE leads to misfiring at higher EGR rates. While the combustion noise and NO_x emissions increase with increases in ETBE fraction at relatively low EGR rates, they can be suppressed to low levels by increasing EGR. Though there are no significant increases in THC and CO emissions due to ETBE addition to diesel fuel in a wide range of EGR rates, the ETBE blended fuel results in higher aldehyde emissions than the pure diesel fuel at relatively low EGR rates. With the 30% ETBE blended fuel, the operating load range of smokeless, ultra-low NO_x (<0.5 g/kWi h), and efficient diesel combustion with high rates of cooled EGR is extended to higher loads than with the pure diesel fuel.     

Study on the controlling strategies of homogeneous charge compression ignition combustion with fuel of dimethyl ether and methanol

The controlling strategies of homogeneous charge compression ignition (HCCI) fueled by dimethyl ether (DME) and methanol were investigated. The experimental work was carried out on a modified single-cylinder diesel engine, which was fitted with port injection of DME and methanol dual fuel. The results show that exhaust gas recirculation (EGR) rate and DME percentage are two important parameters to control the HCCI combustion process. The ignition timing and combustion duration can be regulated in a suitable range with high indicated thermal efficiency and low emissions by adjusting the DME percentage and EGR rate. EGR cannot extend the maximum indicate mean effective pressure (IMEP) of HCCI operation range with dual fuel, but can enlarge the DME percentage range in normal combustion. The combustion efficiency largely depends on DME percentage, and EGR can improve combustion efficiency. The results also show that HC emissions strongly depend upon DME percentage, and CO emissions have good coherence to the peak mean temperature in cylinder. In normal combustion, adopting large DME percentage and high EGR rate can attain an optimal HCCI combustion.     

The mechanical properties of poly(ether-ether-ketone) (PEEK) with emphasis on the large compressive strain response

The mechanical properties of PEEK 450G have been extensively investigated. The compressive properties were measured at strain rates between 1 × 10⁻⁴ and 3000 s⁻¹ and temperatures between −85 and 200 °C. The tensile properties were measured between the strain rates of 2.7 × 10⁻⁵ and 1.9 × 10⁻² s⁻¹ and at temperatures between −50 and 150 °C. The Taylor impact properties were investigated as a function of velocity and various large-strain compression tests were undertaken to explain the results. The fracture toughness was investigated as a function of temperature and compared with previous literature. Additionally, the fracture surfaces were studied by microscopy. As with all semi-crystalline polymers the mechanical response is a strong function of the strain rate and testing temperature. A previously reported phenomenon of darkening observed in Taylor impacted samples is shown to be due to reduced crystallinity brought about by large compressive strain. For samples deformed to large compressive strains using a variety of techniques and strain-rates the measured Vickers hardness was found to decrease in accordance with reduced crystallinity measured by other techniques.     

Effect of dimethoxy-methane and exhaust gas recirculation on combustion and emission characteristics of a direct injection diesel engine

The effect of fuel constituents and exhaust gas recirculation (EGR) on combustion characteristics, fuel efficiency and emissions of a direct injection diesel engine fueled with diesel-dimethoxymethane (DMM) blends was investigated experimentally. Three diesel-DMM blended fuels containing 20%, 30% and 50% by volume fraction of DMM, corresponding to 8.5%, 12.7% and 21.1% by mass of oxygen in the blends, were used. By the use of DMM, it is observed that CO and smoke emissions as well as the total number and mass concentration of particulate reduce significantly, while HC emissions and particulate number with lower geometric mean diameters (D_i < 0.039 μm) increase slightly. For each fuel, there is an increase of ignition delay whereas a decrease of cylinder pressure and heat release rate in the premixed combustion phase when the diesel engine was operated with EGR system. The brake thermal efficiency fluctuates at small EGR ratio, while decreases with the further increase of EGR ratio. With an increase of EGR ratio, NO_x emission is reduced at the cost of increased smoke, HC and CO emissions as well as the total number and mass of particulates for each fuel.     

Strains under transient hygro-thermal states in concrete loaded in multiaxial compression and heated to 250 °C

Previous experimental research has shown that the compactive strains in concrete subjected to a load-then-heat regime exceed those measured in heat-then-load tests under compression. This excess in strain is known as transient thermal creep or load-induced thermal strain (LITS). All previous experimental research on LITS in mature concrete has been conducted in unsealed conditions, mainly under uniaxial compression (with a few biaxial compression tests, but no multiaxial tests) on specimens subjected to monotonic heating to high temperatures (> 500 °C). This paper presents the findings from a novel laboratory investigation of LITS under uniaxial, biaxial and hydrostatic compression in partially sealed conditions, at transient temperatures of up to 250 °C. The results from 49 experiments show that LITS in the sub-250 °C range is highly dependent on the moisture flux conditions and, consequently, on the relationship between heating and drying rates.     

The effects of thermomechanical history and strain rate on antiplasticization of PVC

Antiplasticization is mechanically characterized by an increase in the polymer stiffness and/or yield strength upon the incorporation of a small amount of a low-molecular weight diluent. It is attributed to hindrance of the local β-relaxation motions of the polymer. Here, we have studied the effects of thermal treatment, plastic deformation, and strain rate on the antiplasticization of the yield stress of a 95 wt% poly(vinyl chloride)/5 wt% dioctyl phthalate (PVC/5 wt% DOP) compound. Two thermal treatments were applied to the materials - cooling to room temperature from above T_g by a quench or by a slow oven-cool anneal. When compressed at low to moderate strain rates, antiplasticization was observed in the annealed (physically aged) PVC/5 wt% DOP but not in the quenched (unaged) PVC/5 wt% DOP. Load-unload-reload compression cycles revealed that antiplasticization can be erased by plastic strain; the anomalously high yield stress of PVC/5 wt% DOP observed in the first load cycle softens to a value lower than that of the neat PVC in subsequent cycles. The results indicate that disordered, high free volume microstructural states, obtained either from thermal quenching or from plastic straining, liberate the beta motions of the PVC molecule which, in turn, erase antiplasticization of the yield stress. Earlier work on the rate-dependence of yield has demonstrated that beta motions must be stress-activated in order to yield neat PVC when deformed at high strain rates (>100/s). Hence, we have characterized the rate-dependence of the antiplasticization of the yield stress by testing the annealed materials in uniaxial compression over a wide range of strain rates (10⁻⁴/s-3000/s). Antiplasticization was observed in PVC/5 wt% DOP in the low strain rate regime where beta motions are free in neat PVC but hindered in PVC/5 wt% DOP; however, the antiplasticization (elevation of yield stress) gradually diminished with increasing strain rate.     

Experimental investigation of prechamber autoignition in a natural gas engine for cogeneration

A novel ignition concept based on autoignition in an unscavanged prechamber is currently being developed at the Laboratory for Industrial Energy Systems (LENI). On a single cylinder test engine a series of experimental runs (CR = 8.5-14, λ = 1 − 1.6, RPM = 1150/1500 min⁻¹) have been realized with natural gas as fuel, comparing the new ignition concept to standard spark ignition. The comparison is based on fuel efficiency and exhaust emissions (CO, THC, NO_x). The feasibility of operating the engine in autoignition mode has been demonstrated, and the potential of prechamber autoignition, in particular in the lean combustion regime, is indicated by the trends in fuel efficiency and emission concentration. The resistive heating of the prechamber walls has been shown to be an effective mean to trigger ignition. The prechamber could clearly be identified as primary ignition location. A reduction of the cycle-by-cycle variations - due to mixture fluctuations - is necessary to exploit the full potential of this engine concept.     

Measurement of through-plane effective thermal conductivity and contact resistance in PEM fuel cell diffusion media

An experimental study was performed to determine the through-plane thermal conductivity of various gas diffusion layer materials and thermal contact resistance between the gas diffusion layer (GDL) materials and an electrolytic iron surface as a function of compression load and PTFE content at 70 °C. The effective thermal conductivity of commercially available SpectraCarb untreated GDL was found to vary from 0.26 to 0.7 W/(m °C) as the compression load was increased from 0.7 to 13.8 bar. The contact resistance was reduced from 2.4×10⁻⁴ m²°C/W at 0.7 bar to 0.6×10⁻⁴ m²°C/W at 13.8 bar. The PTFE coating seemed to enhance the effective thermal conductivity at low compression loads and degrade effective thermal conductivity at higher compression loads. The presence of microporous layer and PTFE on SolviCore diffusion material reduced the effective thermal conductivity and increased thermal contact resistance as compared with the pure carbon fibers. The effective thermal conductivity was measured to be 0.25 W/(m °C) and 0.52 W/(m °C) at 70 °C, respectively at 0.7 and 13.8 bar for 30%-coated SolviCore GDL with microporous layer. The corresponding thermal contact resistance reduced from 3.6×10⁻⁴ m²°C/W at 0.7 bar to 0.9×10⁻⁴ m²°C/W at 13.8 bar. All GDL materials studied showed non-linear deformation under compression loads. The thermal properties characterized should be useful to help modelers accurately predict the temperature distribution in a fuel cell.     

Pyrolysis and combustion kinetics of Fosterton oil using thermogravimetric analysis

Thermogravimetric analysis (TGA) was used to examine the thermal behavior of Fosterton oil mixed with reservoir sand. TGA experiments were performed in nitrogen and air atmospheres at the heating rate of 10 °C/min up to 800 °C. In this study, four sets of TGA runs were performed to examine the thermal behavior of Fosterton whole oil, and the coke sample derived from the whole oil. Similar to previous studies in the literature, we also observed low-temperature oxidation (LTO), fuel deposition (FD), and high-temperature oxidation (HTO) in the non-isothermal combustion experiment. Higher activation energy values were obtained in reaction regions at higher temperatures. The mean activation energy for whole oil in nitrogen and air atmospheres was 33 and 126 kJ/mol, respectively. Fresh coke samples derived from whole oil were subjected to isothermal combustion at different temperatures from 375 to 500 °C. Arrhenius model was used to obtain the kinetic parameters from the TGA data. From the model, the Arrhenius parameters such as activation energy (E = 127 kJ/mol) and the pre-exponential factor (A = 1.6 × 10⁸/min) were determined for the coke combustion. The results showed a close agreement between the kinetic model and experimental data for different combustion temperatures. It was observed that the apparent order of combustion reaction for different temperatures approach unity.     

Triaxial strength and failure criterion of plain high-strength and high-performance concrete before and after high temperatures

Triaxial tests were performed on 100 mm × 100 mm × 100 mm cubic specimens of plain high-strength and high-performace concrete (HSHPC) at all kinds of stress ratios after exposure to normal and high temperatures of 20, 200, 300, 400, 500, and 600 °C, using a large static-dynamic true triaxial machine. Friction-reducing pads, using three layers of plastic membrane with glycerine were placed between the compressive loading plate and the specimens; the tensile loading planes of concrete samples were processed by an attrition machine, and then the samples were glued-up with the loading plate with structural glue. The failure mode characteristic of the specimens and the direction of the crack were observed and described. The three principally static strengths in the corresponding stress state were measured. The influence of the temperatures and stress ratios on the triaxial strengths of HSHPC after exposure to high temperatures was also analyzed. The experimental results showed that the uniaxial compressive strength of plain HSHPC after exposure to high temperatures does not decrease completely with the increase in temperature, the ratios of the triaxial to its uniaxial compressive strength are dependent on the brittleness-stiffness of HSHPC after different temperatures and the stress ratios. On this basis, a new failure criterion with the temperature parameters is proposed for plain HSHPC under multiaxial stress states. It provides the experimental and theoretical foundations for strength analysis of HSHPC structures subject to complex loads after subjected to a high temperature environment.     

Dielectric spectroscopy of poly(butylene succinate) films

Dielectric properties of poly(butylene succinate) crystallized under different conditions have been reported in the temperature range of 163-383 K and in the frequency range of 0.01-10⁵ Hz. Both the dipolar α and β processes have been identified at low temperatures: the α process is associated with the amorphous fraction while the β with the relaxations in both the amorphous and crystalline fractions. The space charge effect dominates the high temperature dielectric spectra. These spectra have been analyzed in the light of an equivalent circuit model. The Maxwell-Wagner-Sillars polarization, electrode polarization and free charge motion are well resolved. At 383 K, near the melting temperature (387 K), massive melting and subsequent recrystallization have been observed. The peculiar evolution of the spectra is also analyzed using the same equivalent circuit model. The relationship between the fitting parameters and the evolved microstructures is discussed.     

Effect of nucleation and plasticization on the crystallization of poly(lactic acid)

In this paper, different strategies to promote PLA crystallization were investigated with the objective of increasing the crystalline content under typical polymer processing conditions. The effect of heterogeneous nucleation was assessed by adding talc, sodium stearate and calcium lactate as potential nucleating agents. The PLA chain mobility was increased by adding up to 10 wt% acetyl triethyl citrate and polyethylene glycol as plasticizers. The crystallization kinetics were studied using DSC analysis under both isothermal and non-isothermal conditions. The isothermal data showed that talc is highly effective in nucleating the PLA in the 80-120 °C temperature range. In the non-isothermal DSC experiments, the crystallinity developed upon cooling was systematically studied at cooling rates of 10, 20, 40, and 80 °C/min. The non-isothermal data showed that the combination of nucleant and plasticizer is necessary to develop significant crystallinity at high cooling rates. The nucleated and/or plasticized PLA samples were injection molded and the effect of mold temperature on crystallinity was determined. It was possible to mold the PLA formulations using mold temperatures either below 40 °C or greater than 60 °C. At low temperature, the molded parts were nearly amorphous while at high mold temperatures, the PLA formulation with proper nucleation and plasticization was shown to achieve crystallinity levels up to 40%, close to the maximum crystalline content of the material. Tensile mechanical properties and temperature resistance of these amorphous and semi-crystalline materials were examined.     

Improvement of fuel economy of a direct-injection spark-ignition methanol engine under light loads

The effects of ignition system, compression ratio, and methanol injector configuration on the brake thermal efficiency (BTE) and combustion of a high-compression direct-injection spark-ignition methanol engine under light loads were investigated experimentally, and its BTE was compared with its diesel counterpart. The experimental results showed that these factors significantly affect the fuel economy under light load. The BTE of a methanol engine using a high-energy multi-spark-ignition system is on average 25% higher than that of one using a single-spark-ignition system at a brake mean effective pressures (BMEP) of 0.11-0.29 MPa and an engine speed of 1600 rpm. Decreasing the compression ratio of the methanol engine from 16:1 to 14:1 markedly increases the BTE under low loads and decreases the BTE at high loads. For the methanol engine, using an injector of a 10-hole × 0.30 mm nozzle decreases the ignition delay and improves the fuel economy compared to when an injector of a 7-hole × 0.45 mm nozzle is used. The combustion duration using an injector of a 7-hole × 0.45 mm nozzle is much longer than that with one of a 10-hole × 0.30 mm nozzle under light loads. As a result, the BTE for a methanol engine with optimal parameters is improved by 27% compared to that for a methanol engine without optimized parameters at a BMEP of 0.17 MPa and an engine speed of 1600 rpm, but the BTE of the optimized methanol engine is 20% lower than that of its diesel counterpart under these operating conditions.     

Spontaneous ignition of wood, char and RDF in a lab scale packed bed

Many municipal waste combustors use preheated primary air in the first zone to dry the waste. In most cases the preheat temperature does not exceed 140 °C. In previous experiments it is found that at temperatures around 200 °C, in some circumstances, self- or spontaneous ignition can be achieved. Using preheated air can be a powerful tool to control the ignition and combustion processes in a waste combustion plant. To use this tool effectively, the influence of the preheated air on the fuel bed needs to be well understood. The present work is done to investigate in a systematically way the spontaneous ignition behaviour of a packed bed heated with a preheated air stream. Experiments on a lab scale packed bed reactor are carried out for various fuel types. Because MSW is an highly inhomogeneous fuel, wood and char are used as model fuels. To include the inhomogeneous character of MWS, also experiments are carried out with RDF. Parameters such as primary air flow velocity and temperature, addition of inert material, moisture content of the fuel (wood chips) and particle size (char) have been changed to see their effect on the spontaneous ignition temperature and on the minimum air temperature needed for ignition. The spontaneous ignition temperature is defined as the bed temperature at which a transition takes place from a negligible or slow fuel reaction rate to a rapid oxidation of either the volatiles or the solid fuel without an external source such as a spark or a flame. The minimum or critical air temperature is defined as the lowest air temperature at which ignition can be obtained. It is found that the type of fuel has influence on the ignition temperatures. Besides both the critical air temperature needed for the spontaneous ignition and the spontaneous ignition temperature increase with an increase in the primary air velocity (between 0.1 and 0.5  m/s) and increasing the added inert fraction (between 0 and 40 wt%), irrespective of the fuel type. The effect of air flow velocity and temperature and also the effect of inert on both the critical air temperature and the spontaneous ignition temperature can be explained qualitatively by using Semenov’s analysis of thermal explosions. Semenov’s theory is quantitatively applied to predict the spontaneous ignition and the critical air temperatures for wood.