Abstract
A new concept of combustion which is using the characteristic of plasma jet ignition is proposed. This new combustion method has features of both premixed and diffusive combustion. In the procedure of plasma jet ignition, a high-voltage electrical discharge is generated from the tip of the central electrode to the edge of circular orifice and then the gas in the cavity is transformed to a plasma state. This high-temperature plasma jet spreads into the combustion chamber and can ignite a lean mixture. When the cavity of the plasma jet igniter is filled with a liquid fuel, the liquid fuel is evaporated by the high-temperature plasma immediately after the electrical discharge. When the air at room temperature and atmospheric pressure is charged in the combustion chamber, the plasma jet of the fuel vapor is mixed with air, and a diffusive combustion occurs. The plasma jet made from the fuel vapor includes a lot of radicals and is very high temperature, therefore the diffusive combustion will immediately occur without ignition delay. The plasma jet made from the fuel vapor issues into combustion chamber very rapidly, so that the combustion will be completed within very short duration. The lean combustion and high compression are also possible because of diffusion combustion. The constant volume vessel is used for the experiment. The igniter is consisted of three parts: cavity that is a cylindrically shaped hole, a circular orifice and a central electrode. The center electrode is made by tungsten. The electrical discharge occurs between the tip of central electrode and the edge of circular orifice. Liquid methanol of which volume was measured by micro syringe is charged in the cavity. The supplied methanol volume is defined by the excess air factor which is determined from the ratio of methanol volume to volume of standard air charged in the combustion chamber. The diffusive combustion processes were visualized by conventional Z shape schlieren system and were taken by high-speed camera. The igniter configuration and the ratio of methanol volume to cavity volume influence the diffusive combustion process. In cases of small orifice diameter, the diffusive combustion is not recognized. In the case where a little fuel is provided in the cavity, the enough methanol to initiate and to continue the diffusive combustion is not included in the methanol-air mixture jet and diffusive combustion does not occur. In the case where the most cavity is filled with methanol, the fuel mist is only ejected from the igniter. In the case where appropriate methanol is supplied, the diffusive combustion certainly occurs. The initial flame kernel inside the cavity is necessary to initiate the diffusive combustion, because the electrical spark does not exist already when the fuel-air mixture jet is ejected from the igniter.
Keywords:Igniter, plasma jet ignition, diffusive combustion, high-voltage electrical discharge, methanol fuel