Chris Edwards

Our research into exergy management for simple cycle engines made clear that increasing compression ratio is the most direct way to vastly improve engine efficiency. To explore this, we have built in the lab a free-piston combustor capable of compression ratios in excess of 100:1. A 6 cm diameter piston is accelerated down a 2 meter cylinder to speeds in excess of 100 m/s; it then travels into a high-strength steel combustor section where it compresses the gas to 500 bar before expanding again. This clip shows a view through a large sapphire window mounted in the bottom of the combustor, looking up at the face of the piston. The blue color is from an argon-ion laser; by collimating the laser beam and reflecting it off a mirror mounted to the piston face we can achieve schlieren photography, in which light and dark variations in the image indicate gas density variation. A high-speed camera captures the video, in this case taking 20,000 frames per second. The video begins about 3 ms before the point where the piston turns around (called TDC). From the schlieren you can see a large degree of turbulence in the center and upper right portion of the cylinder. At about 1ms before TDC a high-pressure diesel injector in the side of the chamber opens. As the spray reaches the center of the chamber it is ignited by energy in the hot gas, and burns with a bright yellow color indicative of soot, and typical with diesel combustion. With video such as this we can investigate the nature of liquid spray combustion at the previously unexplored compression ratios that this device can achieve.