A review of optical diagnostic platforms and techniques applied in internal combustion engines

The Internal Combustion engine (IC engine) is one of the most widely applied power machines in modern industry. Investigating the mechanisms of and developing control strategies for IC engines are of practical importance and give rise to interesting scientific issues, as the fuel penetration, evaporation and ignition inside the engine can tremendously affect the structure reliability, power efficiency and pollutant generation. In recent years, lots of efforts have been performed to achieve deeper understanding of the working processes of IC engines by applying experimental optical diagnostic techniques in engine-like laboratory platforms. This review starts with introducing the engine-like platforms (e.g. Constant Volume Combustion Bomb(CVCB), Rapid Compression Machine(RCM), optical engine, etc.) developed to experimentally simulate the practical working processes of practical IC engines. Moreover, multiple advanced optical diagnostic techniques are discussed, including their basic principles and particular applications for the study of detailed processes in IC engines. Specifically, two categories of optical diagnostic techniques are respectively discussed, including the traditional diagnostic techniques based on conventional optics (e.g. schlieren, Two Color Method, etc), and the laser-based diagnostic techniques (e.g. Particle Image Velocimetry, Laser Induced Fluorescence, etc). These techniques offer advantages to examine the spraying, evaporation and combustion processes of the IC engines by measuring the temperature, concentrations, droplet sizes and other valuable characteristics with multi-scale resolution. Furthermore, the diagnostic techniques enable deeper insights into the nature of the flow/combustion under high ambient pressure and temperature, which benefits us from understanding the physical and chemical mechanisms of engine processes in both macro and micro scales. This brief review is intended to be beneficial for both researchers and engineers to analyze the current shortcomings and limitations of the IC engines, and to design the state-of-the-art IC engines with better power performance, energy efficiency and pollutant reduction. Besides, the review paper is also intended to provide a guideline for researchers to conduct further fundamental experiments in IC engines to investigate the flow and combustion mechanisms.