EARLY FLAME CHARACTERISTIC STUDY IN A LEAN CNG DI COMBUSTION

Since the invention of the first successful four-stroke Otto cycle engine, engineers and scientists have done a lot to improve engine performance by thorough investigation on air-fuel mixture preparation, creation of turbulence in the cylinder, improving lean combustion, investigation on combustion and emission performance of different kinds of fuels and vary design feature of engine geometries. In regard to this, the study of flame initiation and development has acquired the attentions of many researchers due to its strong dependence to the in-cylinder flows and its capability to influence flame propagation behavior in spark ignition (SI) engines. The objective of this study also focused on this area of flame characterization.
The present study was carried out in two separate experimental setups. The first experiment was the investigation of flow characteristics of the induction process performed on the engine cylinder head using Particle Image Velocimetry (PIV) system. The other experimental work was conducted in a single cylinder direct injection (DI) engine to demonstrate the effect of induction swirl variations on the nature of early flames in a lean-stratified and lean-homogenous compressed natural gas (CNG) combustion at variable engine speeds. Flame images were captured using endoscope and intensified charge-coupled device (CCD) camera. Computer codes were prepared to identify induction flow quantities and flame characteristics.
The study showed that coherent and organized swirling cores were created and observed on the swirl plane of medium and high swirl angle inductions. Swirl intensity was increasing with swirl angle adjustment, whereas velocity RMS values declined with increasing swirl angle. Regarding flame development, the growth rate of the early flame was found to be high in homogeneous combustion than in stratified one. And medium tumble intake showed a better growth rate in stratified combustion, whereas high swirl intake was the best in homogeneous combustion. The medium swirl induction with the strong and compact swirl core resulted in a lower rate of flame development for both stratified and homogeneous combustion cases.