نشریه تحقیقات موتور
پیاپی 29 (زمستان 1391)

Heat transfer plays an important role in the conceptual and the detailed design of cooling systems in internal combustion (IC) engines and has considerable influences over its operational performance. The demand for higher possible heat transfer rates has led to use a controlled transition from a single-phase convection to a sub-cooled boiling flow in hot regions, through the cylinder head. In order to achieve controlled boiling over a wide range of operating conditions, detailed flow and heat transfer analysis are essential. The computational fluid dynamic (CFD) simulation, incorporating the boiling model, is an effective approach for the thermal analysis of IC engines. In this paper, the water jacket of the national turbo-charged engine was simulated with the AVL-FIRE software to consider effects of boiling phenomena in the water jacket. For the validation of calculated results was performed through a test validation in a horizontal heated duct with two models, Chen and BDL models. It showed that the Chen boiling model was in better satisfactory with experimental data than the BDL model under the same conditions and could be used for calculating the boiling heat transfer in cooling passages of the engine. Contours of the pressure, the velocity, the heat transfer coefficient were demonstrated with and without consideration of boiling. Results showed that when boiling occurs, the heat transfer coefficient was significantly enhanced.

Renewable and clean biofuels are known as the best replacement of fossil fuels. Biofuels have animal or vegetable bases and produce less pollution than fossil fuels during the combustion in an engine. Many researches performed on using blends of biodiesel and diesel fuels in diesel engines. In this study, blends of biodiesel, ethanol and diesel fuels were used in Massy Ferguson 399 Tractor and the engine power and the specific fuel consumption are survived. The response surface methodology was used to assess the impact of the biodiesel volume percent (5-15%), the ethanol volume percent (2-6%) and the engine speed (1520-1900 rpm) on the power and the specific fuel consumption to optimize the process. Based on results, obtained in this study, the engine speed was the most effective factor on the power. Also, the biodiesel volume percent and the engine speed were main factors on the specific fuel consumption. Optimum conditions for maximizing the power (51.48 kW) and minimizing the specific fuel consumption (209.45 ml/min) were obtained at 15% biodiesel, 3.53% ethanol at 1787.4 rpm engine speed.

Many researches have tried to find causes of engine and vehicle vibrations and reduction methods by considering that the engine unbalance condition and inputs from bumpy roads are vibration sources and would reduce the vehicles performance. However, there is a lot of unknowns in modeling and the simulation of vibrations. One of them is active mount cooling and side effects of cooling on its structure and performance. This paper defines a new active multi-compartment mount with a cooling capacity and explains reasons for the improvement of thermal conditions by considering this fact that the heat generates in active mounts and reduces their performance. Vibration modeling of the system is presented and effects of parameters on results of the new chamber have also been studied. As a result, using the middle duct and adding a new compartment are proposed as a solution to improve cooling conditions. The new vibration model and the control algorithm had no adverse effect on the mount. Finally, an appropriate control strategy is proposed by the control algorithm of the system.

In this work, an experimental study has been performed to evaluate the effect of the tube insert in an engine cooling system (the radiator of Peugeot 405). Experimental setups, as an automobile cooling system, was simulated to perform experiments for assessing the effect of the tube insert in the thermal performance of the radiator. A tube insert with a thin wire of a sinusoidal profile steel tube, as a turbulator, was used in the radiator. Results showed that when tube inserts were used, we reached 23% average value, 5% minimum value and 41% maximum value for the heat transfer coefficient enhancement. Also, experimental data exhibited a good agreement with correlations of other researchers.

The fracture problem in gears with cracks on the root of the gear teethes has been considered in this article. The problem was solved by the finite element method, using the ABAQUS software. Then, the stress intensity factor at the tip of cracks was calculated for different loading conditions. The stress intensity factor at the tip of cracks for steel gears and gears made of functionally graded materials were obtained and compared. The position of the indentation load, the geometry of the tooth root and the tooth flank load distribution on the stress intensity factor was investigated. Numerical results showed that by considering the size of the crack in the tooth root, the value of the stress intensity factor, mode I, was bigger than mode II and mode III.

In the direct gas injection engine, the mixture formation and the combustion rate are affected by the geometry of injector holes (the diameter, angles, K-factor) and discharge the coefficient of the exit nozzle. This paper presents a new method to determine the diameter and the nozzle exit angle, based on silicone molding of the injector exit nozzle, using image processing techniques. The actual fuel mass flow rate was measured through experiments, which were done with a designed experimental set-up. Tests have been done in an optical constant volume chamber, where the ambient gas was pressurized air, limited between 5 to 15 bar and the injection gas was compressed natural gas (CNG) at two different pressures, 30 and 40 bar. Discharge coefficients have been calculated for different ambient and injection pressures, considering values of actual and ideal mass flow rates. Results verified the present approach for determining the diameter and the nozzle exit angle. Moreover, results showed that there was a direct relationship between the actual mass flow rate and the discharge coefficient with ambient and the injection pressure.