Selected PhD Theses Abstracts:
Proposing a comprehensive algebraic Model for Spark Ignition Engine at Part Throttle
Dr. Amir Hasan Kakaee
Dr. Behrooz Mashhadi
A semi-empirical and generalizable model of the engine performance is proposed in this thesis whose inputs are engine control variables and ambient air conditions (12 variables), and its output is engine torque. Model development is based on the analysis of the variations of the engine behavior with the variations of its working variables. Furthermore the physical relations governing the engine behavior are also used where possible. Regarding the large number of input variables, and hence the large number of the required tests, the analysis of the engine behavior in different conditions is used by applying a physical comprehensive model of XU7/JPL3 engine.
In the first step, considering the large number of input variables, and in order to simplify the modeling process, the model is broken into several sub-models by use of physical relations, and each sub-model is then developed separately. The final model is produced by combining these sub-models in multiplicative manner. In the next step, in order to reduce the complexity of the proposed model, a systematic approach is used to simplify the model. The result is a model with 58 unknown coefficients that considers a large number engine working variables, needs a small amount of data for regression (determination of the unknown coefficients), and has good accuracy in prediction of unseen data. Furthermore, based on the physical relations, the proposed model is also applicable for prediction of the engine volumetric efficiency.
Finally the proposed model is verified using the numerical and experimental data of seven different engines. Results show that the prediction error is below 10% in all conditions. Furthermore the mean of training and test errors are 3% and 4% for volumetric efficiency prediction, and 3% and 5% for torque prediction, respectively. The accuracy of the proposed model for prediction of different engines behavior in different conditions is a proof of the its accuracy in different regions of engine performance space, and also its generalizability for different engines.
Keywords: Semi-empirical modeling, Spark ignition engine, Engine performance, Torque, Volumetric efficiency
Numerical Study of Combustion and Emissions Characteristics of Natural gas/Diesel Reactivity Controlled Compression Ignition (RCCI) Engine
Dr. Amir-Hasan Kakaee
Low temperature combustion (LTC) is an emerging engine technology that has ability to yield low NOx and soot emissions while maintaining high fuel efficiency. LTC strategy includes homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI), reactivity controlled compression ignition (RCCI) and partially premixed combustion (PPC). These LTC strategies use early fuel injections to allow sufficient time for air/fuel mixing before combustion. According to the literature, some LTC strategies are not promising strategies for future automotive and power generation applications due to difficulties in controlling the heat release rate (HRR) and the lack of a combustion phasing control mechanism. To mitigate these problems, the RCCI combustion concept was introduced. RCCI is a dual-fuel PPC concept which uses port fuel injection (PFI) of a low reactivity fuel (e.g., gasoline, natural gas and alcohol fuels) and direct injection (DI) of a high reactivity fuel (e.g., diesel and biodiesel) blending inside the combustion chamber to increase the combustion duration and to provide phasing control. Combustion phasing is controlled by the relative ratios of the two fuels and the combustion duration is controlled by spatial stratification between the two fuels.
In the first part of the present dissertation, the effects of diesel injection strategies, engine initial temperature and engine speed on the combustion and pollutant emissions characteristics of a modified heavy-duty reactivity controlled compression ignition engine fueled with natural gas/diesel are studied. Natural gas with low reactivity is assumed to be inducted into the engine through the intake port, while diesel fuel with high reactivity is directly injected into the engine using a double injection strategy. Several parameters were studied including the premixed natural gas amount, the first and second injection timings and the injected diesel mass split between the two injections. The results showed improved engine efficiency with reductions in soot and oxides of nitrogen emissions could be achieved with the injection strategies studied, but hydrocarbons and carbon monoxide emissions were deteriorated. Three factors, namely first start of injection timing, second start of injection timing and the diesel injection fuel fractions, had pronounced effects on reactivity controlled compression ignition engine combustion performance and emissions. To reduce soot and oxides of nitrogen emissions, increasing the natural gas percentage, advancing first and second starts of injection timing beyond a certain point and increasing fuel fraction in first start of injection timing are preferred, but they had an adverse effect on hydrocarbons and carbon monoxide emissions. Then, the effects of natural gas composition and engine speed on combustion and emissions characteristics of an RCCI engine are studied. It is shown that Wobbe number (WN) of gases and engine speed significantly affects RCCI engine combustion and emissions. The gas with higher WN displayed higher peak pressure, temperature and NOx emissions, and lower unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions.
Finally, the effects of piston bowl geometry showed that the optimum piston bowl design from peroformance and emissions point is the bathtub design. In addition, it was reported that the piston bowl depth and chamfer size can influence engine out UHC emissions.
Keywords: Reactivity controlled compression ignition (RCCI); combustion; efficiency; emissions; natural gas; piston bowl geometry
Analytical, Numerical and Experimental Study of Circular Structures’ Energy Absorption under Axial Impact Load with Rupture Path Orientation
Dr. Javad Marzbanrad
Dr. Behrooz Mashadi
This dissertation studies energy absorption of circular thin wall structures under axial impact load with rupture path orientation, analytically, numerically and experimentally. The aim is to rupture thin-walled aluminum tube simultaneously in some points during the impact load application in a controlled manner and to bend the provided petals by the deviator for more energy absorption during rupture. To do so, analytical solution of the cutting process is done at first by considering the new energy loss rates including bending of the provided petals outwards and creation of swarfs at the tool’s head and correcting previous energy loss rates by calculating cutting process sensitivity to the strain rate. Then, numerical modeling is done using ABAQUS software based on the Hooputra’s criterion. Different geometries of tube including length, diameter and thickness, and the cutting tool including number of blades, depth, thickness and cross section form of the cutting edge and different impact speeds are studies and compared. The results show that the tube’s length and diameter have less effect on the rupture process than the crushing; the force increases with increase of the tube thickness, number of blades, depth and thickness of the cutting tool. Then, a specific multi-blade cutting tool with a deviator is designed and made by using analytical and numerical results. The impact test device is prepared and equipped then and the experiments were performed to gather required data. The obtained results of numerical and analytical solutions are compared with experimental results and have proper conformity; the numerical model and the analytical equation were validated in this way. Cutting and crushing modes were compared with each other and effective parameters on energy absorption like mean and maximum axial force, specific energy absorption and the force efficiency were studied. The results show that the rupture process is predictable, controllable, repeatable, stable and continuous. Less maximum force, less sensitivity to the strain rate, higher force efficiency and force variations with less range are other specifications of the rupture with the cutting tool in comparison to the crushing between two flat plates. Moreover, a designer can easily improve the energy absorption behavior and the mean force level and can change the maximum value and form of displacement during impact by changing the tool’s geometry including number of blades, edges’ thickness, form of the cutting edges and the deviator radius.
Keywords: Impact; Energy absorption; Circular thin wall structure; Rupture; Multi-blade cutting tool