Investigation of ignition and combustion processes on a single-stroke engine (RCEM)

For the fundamental investigation of ignition, combustion and knock processes in internal combustion engines, a so-called single-stroke engine (Rapid Compression-Expansion Machine, RCEM, Figure 1) has been available at the IVT since spring 2017. Due to the lack of a crank mechanism, this machine can be used to recreate individual combustion cycles with great flexibility in terms of piston stroke, compression ratio and composition of the working gas.

The piston is driven by compressed air at the rear and thus moved ballistically. The valve-free cylinder head allows many degrees of freedom when installing a wide variety of hardware (injection equipment, ignition system, sensors, etc.). In addition, the processes in the combustion chamber can be excellently visually recorded through various windows.

Figure 1: Structure of the single-stroke engine (left) and schematic representation of the combustion chamber (right).

Currently, the single-stroke engine is being used in projects in cooperation with LEC (www.lec.at) for fundamental investigations of ignition and combustion processes of premixed gas-air mixtures using electric spark ignition. On the one hand, these investigations serve to gain a deeper understanding of the complex processes in the combustion chamber and, on the other hand, offer the possibility to validate simulation models (e.g. in 3D-CFD simulation) by means of the generated measurement data.

Extensive measurement technology was applied at the RCEM for the investigations. Since July 2017, an oscilloscope from Yokogawa with sampling rates of up to 10 MS/s has been available for the temporally high-resolution recording of all relevant measured variables such as combustion chamber pressure, piston travel, and voltages and currents at the ignition system. For the optical recording of the processes in the combustion chamber, the already proven high-speed camera from Photron is used. Figure 2 shows the results of current investigations of laminar flame front propagation in CH4-air mixtures.

Figure 2: Broadband and streak recording of the flame front propagation (left) and evaluation of the image data using contour detection (right).

The comparison of broadband and schlieren images (Figure 2, left) has generally shown that optical detection of the flame front by recording the flame's own glow requires long exposure times due to the comparatively low light emission and thus results in low frame rates. When using the schlieren technique, on the other hand, short exposure times and high frame rates can be used due to the strong external light source. Moreover, the contour of the flame front is clearly recognizable throughout with this technique, which facilitates automatic contour recognition when evaluating the image data (Figure 2, right).