نشریه تحقیقات موتور
پیاپی 33 (زمستان 1392)

This article presents a completed analysis for the material investigation, the mechanism, the vibration behavior, the stress and fatigue properties in valve springs. In the first stage, the failure analysis of a broken spring is performed based on material investigations. Then, by changing the spring design from one-step to two-step, a suggestion is proposed to improve the performance of the spring. In order to verify this suggestion, the spring mechanism is analyzed based on the velocity and the acceleration. In addition, stress and fatigue analyses are conducted.

The main purpose of this paper is the development of a multi-zone model for the prediction of combustion, performance and emissions characteristics of HCCI engines. The developed model contained four various zones that included the core zone, the boundary layer zone, outer zones, which were between the core and the boundary layer, and the crevice zone. The core zone was the innermost zone and the boundary layer was the nearest zone to the wall. The volume of the crevice zone was constant and equaled to 3% of the volume of TDC, but the volume of other zones changed by the time. There was the conductive heat transfer between neighborhood zones. The mass transfer was considered between zones, too. There was the convective heat transfer between boundary layer gases and combustion chamber walls. Suitable chemical kinetics mechanisms were used for the accurate combustion simulation. To calculate the in-cylinder temperature, the pressure and the composition at intake valve closing, a single-zone model was used for the simulation of the gas exchange process. The model was validated using experimental data of the HCCI combustion of n-heptane and methane fuels. Results showed that the model can predict HCCI combustion, performance and emissions characteristics, accurately. The maximum error of model results for the CO prediction was equal to 1.7% and for the UHC prediction was equal to 12.86%.

To improve the spark ignition (SI) engine performance, the study of the cycle-to-cycle variation is one the main concerns. The cyclic variations dependent on the fuel type, the equivalence ratio, the speed, the engine load and geometrical properties of the engine. The variations may cause fluctuations of the output power and the unburned hydrocarbon. Previous works presented effects of the fuel type, the equivalence ratio, the speed and the engine load on the cyclic variation, but no study has been reported yet on the effect of the cycle-to-cycle variation on unburned hydrocarbons. In this paper, an experimental study on a single-cylinder SI engine has been conducted to collect consecutive cycle variations and their statistical analyses. To reduce experimental errors, all controlling parameters were continuously monitored to prevent changes in engine operating conditions and that caused the inconsistent data collection. Results showed that although the cyclic variation due to the reduction of the output power was not a good phenomenon, but an increase in the cyclic variation reduced unburned hydrocarbons.

In this study, a six-degree-of-freedom dynamic model for the national engine on mounts is presented and is validated by experimental tests. Dynamic equations of the motion were derived using Lagrange’s and Newton-Euler equations. In addition, a thirteen-degree-of-freedom dynamic model for the engine and the vehicle was employed to consider dynamics of the vehicle body, tires and suspension systems in the vibration behaviour of the engine. Simulations were performed using data related to the national engine and the Samand vehicle. Experimental tests were performed on the Samand vehicle with the national engine using the chassis dynamometer. Finally, the validity of the presented mathematical model and the consideration of dynamics of the vehicle body, tires and suspension systems on the vibration behaviour of the engine were investigated.

Digital image processing is one of efficient methods that is adopted today for the determination of fuel spray characteristics in internal combustion engines. The imaging method and the images quality are two important factors that influence the accuracy of results. In this paper, an edge finding method was employed for processing of the Schlieren image of the gaseous jet, and sensitivity analysis has been conducted to investigate effects of the edge finding method threshold, the experiments scale factor and the level of the blur and the noise of images on image processing results. A method based on the Otsu method has been proposed also for finding the dynamic threshold for Schlieren jet images. Results showed that the sensitivity of the jet tip velocity to the edge finding method threshold was low (in the valid range of the threshold), but in regions that the jet tip velocity was low, this sensitivity was as high as 14%. The sensitivity of the jet tip penetration to the blur level of images was less than 2%, while the tip velocity was more sensitive to the blur content of images, and the blur with the standard deviation of 4 could led to 28% error in the tip velocity.

A nano-emulsion fuel consists of homorganic or heterogenic phases with 2-100 nm diameter droplets of the water. These droplets are dispersed in the fuel and they can change the fuel composition to improve the combustion and in consequence, they can reduce engine exhaust emissions. In this paper, the diesel fuel with 10-15% water nano-emulsification techniques by using various surfactants is investigated. The best formulation was obtained with long chain ethoxylated alcohols (C12-C14) that were environmentally friendly and easily available. In an engine test (model: OM314), the engine performance with this new fuel was studied. Results showed that the nano-emulsion fuel had lower engine exhaust emissions such as the smoke to 53.7%, in comparison to the diesel fuel. The nano-emulsion fuel decreased the engine power, but improved the brake thermal efficiency. In addition, the corrosion and the effect of this fuel on metals in fuel systems were studied.