Expansion of the Operating Range of a Multi-fuel Single-Disk Rotary Engine Using a 2+2-Spark Plug Combustion Process

Abstract

Unmanned, autonomous aerial vehicle applications are an indispensable part of modern aviation. The propulsion of such aerial vehicles is often realized by Wankel rotary engine. They are particularly suitable for this application due to their high power-to-weight ratio and smooth operation. As a result of their properties and geometric characteristics, Wankel rotary engines are usually operated with highly volatile fuels like aviation gasoline (AvGas). In comparison, the existing aviation refueling infrastructure is oriented toward the most common aviation fuel, kerosene. This and other reasons, such as significantly lower prices and easier fire protection regulations, lead to the desire to be able to operate these propulsion units with kerosene. Opposed to reciprocating engines, the low compression ratio of rotary engines prevents the implementation of compression ignition (CI) combustion processes. In previous studies, the effects of an air-assisted injection system on operation with different fuels were examined on the basis of a spark-ignited combustion process. Operation with kerosene in particular requires appropriate mixture preparation in order to be able to ensure a stable combustion process over a wide operating range. The purpose of this paper is to investigate improvements for further optimization of the combustion process with regard to efficiency, stability, and power output based on the findings of the previous investigations. One of the most promising steps is a combustion process with the ignition of four spark plugs (2+2). Based on simulative investigations using computational fluid dynamics (CFD), a potential analysis regarding the optimal positioning of the additional spark plugs and the effects on the combustion process is carried out. It could be shown that a correspondingly "early"positioning of the trailing spark plugs should be aimed at. Based on the simulated results, the preferred spark plug configuration is implemented on the real engine. Subsequently, the influence of the use of four spark plugs on the combustion process is examined on the basis of experimental investigations. In particular, the effects on the possible operating range in kerosene operation are investigated in more detail.