Experimental Investigation and Analysis of Homogeneous Charge Compression Ignition in a Two-Stroke Free-Piston Engine

Abstract

The present paper deals with the experimental investigation of homogeneous charge compression ignition in a free-oscillating two-stroke free-piston engine. The Free Piston Linear Generator (FPLG), which is being developed at the German Aerospace Center (DLR), is an innovative internal combustion engine for the generation of electrical power. This concept can for example be used in hybrid electric vehicles, as an auxiliary power unit or in combined heat and power units. The FPLG consists of three main components. In the two-stroke combustion unit, heat is released by burning a fuel-air mixture and a piston is accelerated. This energy is then converted into electric energy in the second component, the linear generator. This subsystem consists of electromagnetic coils as a stator and permanent magnets as a mover. The mover is rigidly coupled to the combustion piston. The third component, the gas spring, serves as an intermediate energy storage and reverses the piston motion by decelerating it. With this arrangement, the piston oscillates freely between the compression chamber of the combustion unit and the gas spring with no mechanical coupling like a crank shaft. Due to the free piston movement, the top and bottom dead centers and consequently for example the compression ratio are no longer fixed. This freedom and the use of a fully variable valve train provide the opportunity for a fundamental optimization of the internal combustion process. The homogeneous charge compression ignition (HCCI) is one advantageous combustion process which leads to low raw emissions and high efficiencies. As a basis to utilize a HCCI process, the paper firstly presents a strategy for the transition between the spark ignition mode and HCCI-mode in the FPLG using the high variability of the engine concept. The auto ignition of the lean fuel-air mixture is enabled by the adaption of the compression ratio and an internal exhaust gas recirculation. The latter is accomplished by an early intake valve opening and intermediate storage of combustion gases in the intake manifold. Additionally, the challenges that come with a freely oscillating piston will be discussed here. In the analysis of the experimental results, the pressure curve and free piston movement will be discussed in detail. Due to the free piston movement stochastic variations in the ignition can be equalized by the system without external control. This effect and the measured efficiencies show the promising potential for the HCCI process in free-piston engines.