A Study on Combustion Control by Using Internal and External EGR for HCCI Engines Fuelled With DME

摘要

The Homogeneous Charge Compression Ignition (HCCI) engine is possible to achieve high thermal efficiency and low emissions. One of the main challenges with HCCI engines is structuring the systems to control combustion phasing, crank angle of 50% heat release (CA50), for keeping high thermal efficiency and avoiding an excessive rate of pressure rise which causes knocking, when operating conditions vary. Though some HCCI combustion control systems, for example, Variable Valve Timing System and Variable Compression Ratio System, have been suggested, these control systems are complex and heavy. In this study, for the development of a lightweight and small-sized generator HCCI engine fuelled with Dimethyl Ether (DME) which is low-emission and easy to autoignite, a simple HCCI combustion control system is suggested, and the control system is evaluated experimentally. In the control system, the means of controlling gas temperature at inlet valve closure (IVC) is focused on to control CA50 as the specific autoignition temperature of a fuel is constant in HCCI combustion. The amount of premixture of air and fuel at stoichiometric ratio necessary to achieve load is fixed, and gas temperature at IVC is controlled by changing the mixing ratio of hot (internal) EGR gas and cold (external) EGR gas. And also cold EGR gas plays a role in the avoidance of a very rapid combustion reaction speed. The exhaust cam has two stages to use hot EGR gas, and exhaust gas is cooled through the heat exchanger to use cold EGR gas. The mixing ratio of three gases, stoichiometric premixture, hot EGR gas and cold EGR gas, is controlled by four throttles placed in the flow path of these gases. As input heat quantity per cycle greatly affects CA50, the change in input heat quantity per cycle needs to be considered to control CA50 by gas temperature at IVC. In the control system, the measured IMEP has slow response and vibrates to the IMEP set value. Meanwhile, the CA50 controller shows the good performance in aspects of steady and transient characteristics. Hereafter, the IMEP controller should be improved without reducing the performance of the CA50 controller.