نشریه سوخت و احتراق
سال سوم شماره 2 (پیاپی 6، پاییز و زمستان 1389)

In this study، the heat transfer of SI engine piston is calculated. Engine performance in two modes of CNG fuel and petrol has been studied. For thermal simulation، three different models have been used. In the first model، three approximate and constant temperatures are selected for piston، cylinder and cylinder head، and subsequently the temperature، pressure and convection coefficients of the in-cylinder gas are determined using two-zone combustion model. In the second model، for each of the walls (piston، cylinder and cylinder head)، the relevant heat transfer equations with two-zone model are solved considering three unknown temperature. In the third model، which is the most accurate method، 24 heat transfer equations coupled with the two-zone model are solved simultaneously based on resistor-capacitor thermal network model. Eventually، results of the three models are compared to study their effect on the piston thermal behavior. It has been shown that use of resistor-capacitor model with lower number of equations and consequently less solution time is an appropriate method for solving problems of engine heat transfer. The simulations were carried out by a MATLAB code and the results were validated with the experimental data of the EF7. TC engine.

In the present article، the effects of air injection into the combustion chamber and utilization of an insulated piston have been investigated on combustion process، emission formation and performance characteristics in a direct injection diesel engine. In the proposed method، the secondary chamber is created inside the piston which is connected to the main combustion chamber via throats. The obtained results indicate that fabrication of the air-cell inside the piston could be very effective on reduction of the exhaust soot emission and has a slight effect on NOx reduction in a diesel engine. Utilizing an insulated piston causes a sizeable decrease in specific fuel consumption and increases mean effective pressure. Also، the results show that use of an insulated piston increases NOx due to causing high temperature inside the combustion chamber as a result of reducing the heat loss from the piston walls. The obtained results are compared with the corresponding results in the literature and show a good agreement.

In the present study، numerical modeling of the combustion of a fully laminar stoichiometric premixed reactive flow of methane-air inside a 2D micro-combustion chamber was investigated. The aim of this study is the investigation of occurance، phenomenology and effective parameters such as flame stability on combustion process in MEMS devices for energy or propulsion generation on small scales for space exploration applications. The results show that flame stability in a micro- combustion chamber strongly depends on the combustion chamber wall thickness Lw، the combustion chamber wall thermal conductivity coefficient Kw، the combustion chamber width L، the outer wall convective heat transfer coefficient hout and reactive mixture velocity Vin.

The overall objective of present research is to provide insight into physical properties and atomization characteristics of the Chlamydomonas algal biofuel. Biofuels are produced mainly from oil crops، animal fat and waste cooking oil. Because of low productivity and efficiency، these sources are not suitable for continuous and large-scale production of biofuel. Microalgae appear to be the only source of renewable biodiesel that is capable of meeting the global fuel demand. According to a novel physiological-molecular research، the naturally isolated microalga Chlamydomonas sp. MCCS 026 can be used as a valuable feedstock for biofuel production in our country. This species has high growth rate and lipid content requiring just a simple and low-cost culture medium. However، these benefits do not guarantee the practical usage of this new green fuel in DI engines. In order to employ a biodiesel well، it is necessary to know its key properties، i. e. density، viscosity and surface tension. These properties are closely related to the molecular structures of its fatty acid components. This study presents a predictive analytical investigation of the effects of viscosity، density and surface tension on atomization characteristics of the new suggested green fuel. Results from statistical analysis showed that Chlamydomonas algal fuel had near-similar properties to the petroleum diesel fuel. The atomization analysis indicated that for the same operating conditions، the novel biofuel shows better spray characteristics than well-known biofuels، i. e. soybean and canola methyl esters. Also، the results showed that the quality of this new biofuel is higher than other alga-based biofuels.

An experimental study was conducted on the response of V-shaped and M-shaped premixed flames to external acoustic excitation. Low emission premixed combustors with anchored V-flames are susceptible to combustion instabilities. V-flames attach to an anchoring rod at the centre of the burner، whereas M-flames attach to the anchoring rod as well as the burner rim. This is in contrast to conical flames which are anchored only on the burner rim. As a consequence، V- and M-shaped flames are more sensitive to external flow perturbations. In the present study، a mixture of propane-air was used. The mixture equivalence ratio was changed to determine the stability range of V- and M-shaped premixed flames. The results indicated that M-shaped flames have a narrower stability range compared to that of V- flames. This limited the equivalence ratios range for acoustic excitation experiments. In order to study the flame response to external acoustic excitations، a loud speaker was placed upstream of the flame to generate acoustic waves in a certain range of frequencies. Pressure fluctuations caused by the acoustically excited flame were measured by a microphone placed downstream of the burner. Flame imaging was done with a CCD high speed camera during acoustic excitations. These images were used for flame response determination. Image processing was accomplished using the MATLAB software to obtain the amplitude and phase of the flame response. Variations of the flame shape and the intensity of light emitted by the flame were examined during a period of excitation for various values of equivalence ratios and flow rates. The results indicated that the vortex shedding and roll-up due to the velocity perturbations play an important role on the flame response. The phase response of flames evolves quasi-linearly with excitation frequency، indicating a certain time delay for the fluctuations to reach the flame surface. Also، in a certain range of excitation frequency، the flame gain response exhibits more sensitivity to variation of the equivalence ratio. Observations of the V- flame behavior during a period of excitation have led to determination of the effects of nonlinear parameters on the flame response.

The objective of this paper is to present a simulation model for controlling combustion phasing in order to avoid knock in turbocharged SI engines. An empirically based knock integral model was integrated in a quasi -dimensional simulation tool. The empirical knock model was optimized and validated against engine tests. This model can be used to optimize control strategy of combustion initiation in SI engine to reach the maximum berak turque. The model is able to predict knock onset with an accuracy of 2 crank angle degrees. With the established knock model، it is possible not only to investigate whether knock is observed by changing operating and design parameters، but also to evaluate their effects on knock intensity.

The aim of this paper is to investigate the mechanism of soot and NO emission reduction using split injection in a DI Diesel engine. In this research، split injection strategy was introduced as a useful way to reduce both emissions simultaneously. Investigations have been conducted on a DI Diesel engine manufactured by Motorsazan Tabriz Company. To evaluate the benefits of split injection strategy and to reveal the combustion mechanism، modified three-dimensional CFD code KIVA-3V was used. Results showed that use of split injection with appropriate dwell between injection pulses can be effective in simultaneous emissions reduction. Based on computation results، NO reduction mechanism is as single injection with retarded injection timing. Also it is shown that reduced soot formation is because of the fact that the soot-producing rich regions at the fuel spray head are not replenished by new fuel when the injection is stopped and then restarted. The results showed that by using split injection strategy the amount of soot and NO can be reduced by 33% and 11%، respectively.