Simulation based A-posteriori search for an ICE microwave ignition system

TL Motor vehicles. Aeronautics. Astronautics;T Technology (General)

Sun, Fang ; Li, Yun

University:University of Glasgow

Department:School of Engineering

关键词: microwave ignition, ICE, petrol engine, evolutionary algorithm, A-posteriori, simulation;   

Others  :  http://theses.gla.ac.uk/2237/1/2010sunphd.pdf

来源: University of Glasgow

【 摘 要 】

Petrol internal combustion engines (ICEs) in automobiles use a high-voltage spark ignition system, which currently offers an energy efficiency of 25%-35% only and also produces excessive exhaust emissions. Recent political, economic, social, technical, legal and environmental drive has accelerated the worldwide research in ‘greener’ engines, such as the homogenous charge compression ignition (HCCI) engines, which focuses on total resource conservation and emission reduction per mile. However, its ignition timing needs real-time control of cylinder pressure and temperature in a closed loop, which is practically intractable to date. Leapfrogging HCCI and requiring no closed-loop control or modification to the engine, this thesis develops homogenous charge microwave ignition (HCMI) directly to replace the point-based spark ignition. Like HCCI, HCMI is volume based and is also applicable to diesel fuel. Through computer simulations, the thesis verifies the feasibility of the ICE radio frequency ignition concept first proposed by Ward in 1974. Building on the simulation-based design methodology of Boeing 777 aircraft, which required no hardware casting or prototyping at the design stage, this thesis employs intelligent search to evolve ‘designs of experiments’ by simulation means for vehicle-borne HCMI with potential to offer a step change in fuel efficiency and emission reduction.Investigation of this thesis into the effect of piston position confirms with graphical visualisation that the resonant frequency of the engine cylinder is very sensitive to the piston motion, because it can easily cause off-resonance and hence degraded field strength. It is revealed that this is the major factor that encumbers practical realisation of an HCMI system. This thesis shows that the natural frequency changes 0.015 GHz per 0.5 mm in average when the piston moves from 5 mm to 0.5 mm TDC and 0.0021 GHz per 0.05 mm when the piston moves from 0.5 mm to 0.05 mm to TDC. For the geometry of the given ICE cylinder, if the input microwave frequency is fixed, the resonance lasts for 7 s. Investigation on various diameters of cylinders that reveals the results on the effects of piston motion of a certain cylinder can be extended to other cylinders with different diameters. It is also shown that for different types of cylinders the frequency of input microwave can be very different. Therefore, the different microwave source of the HCMI systems has to be designed for different types of vehicles.Simulations reported in the thesis also reveal that a microwave based ignition takes 30 ns to 100 ns to break down the median of a permittivity and permeability that are the same as the chemically optimal 14.7:1 air-fuel mixture. This is much shorter than the duration of the microwave resonance and hence makes HCMI feasible in terms of duration. For a running engine, the variations of AFR can also cause off-resonance. It is found that the AFR does not affect the resonant frequency as much as piston motion does. The frequency only changes 38MHz when the AFR varies from 10:1 to 16:1. Properties and effects of microwave emitter and couplers are also studied and the results confirm with graphical visualisation that, for an emitter in the form of a probe antenna, the electric field intensity is dependant on the antenna length. For the given geometry of the Chryslor-Dorge ICE studied, a probe antenna of a length around 30% of wavelength shorter than the end of transmission line offers the best coupling efficiency in an HCMI system.To search for globally optimal designs, the Nelder-Mead simplex method and the ‘intelligent’ evolutionary algorithm (EA) are coupled with CAD simulations. These machine learning methods are shown efficient and reliable in dealing with multiple parameters.Under practical constraints, the best ignition timing and AFR combination is found, which for a 100 W input offers an electric field intensity of up to 9.8×106 V m-1, almost doubling the minimum requirement of 5.5×106 V m-1 for a plasma breakdown of the air-fuel mixture.In this work, six different geometric shapes of antennae are studied. Through the EA based global search, it is confirmed that the length and screen radius of the probe antenna do not affect the resonant frequency significantly. For the given ICE geometry, an antenna length of 14.3 mm offers the best efficiency and the least reflection regardless of the screen radius. The radius affects resonance the least among all the parameters searched, although it can contribute to enhancing electric field and reducing reflection of the coupling. For maximal electric field strength in the cylinder, the best combination of the antenna length and the screen radius is also searched and results are fully tabulated in this thesis.