Remarkable progresses in laser technologies such as micro-chip and VCSEL lasers which enable high durability and stability with low cost have been made in recent years. It is expected that these innovations will open up new applications for combustion research. The characteristics of laser ignition are electrodeless, free choice of ignition position, and ignitability at high pressure. In this paper, the application of laser ignition are introduced and new possibilities are discussed.
Highly brightness giant-pulse laser induced breakdown ignitions promise to enhance the performance of internal combustion engine. It makes possible the ignition at the optimal spatial point apart from the cylinder “cold wall” with multipoints and multi-pulses. Giant-pulse lasers can fire leaner or high-pressure mixtures that are difficult to be ignited by a conventional spark plug and thus the fuel is used efficiently, while low CO2 and harmful pollutant emissions are expected. Unfortunately, the giant-pulse laser ignition has been limited the basic researches until recent progress of micro solid-state photonics. In this review, we'd like to discuss the progress of laser ignition system based on the micro solid-state photonics progress, and further giant micro-photonics for innovative ignition.
An 808nm-range high-power fiber-coupled VCSEL module was developed for laser ignition system. The fabricated VCSEL module was very compact as about 100 mm×100 mm including a cooling unit, and stable operation is possible by air cooling. The light power of VCSEL array and the fiber-coupled module were 311 W and 204 W under the quasi continuous wave operation (pulse width 500 μs, pulse cycle 20 Hz), respectively. This output power is the largest one that was reported to date as a fiber-coupled VCSEL module. The oscillation spectrum width of module was as narrow as 1.7 nm in full width at half maximum at the above output, and the temperature dependence of wavelength was 0.05 nm/K. By exciting Nd3+: YAG / Cr4+: YAG composite ceramics using this VCSEL module, Q switched laser output of 2.5 mJ×4 pulse which was required for engine operation was obtained.
For understanding the characteristics of the ignition of a combustible gas mixture by laser-induced gaseous breakdown, fundamental physical processes in laser-induced gaseous breakdown and comparison between laser-spark ignition and electrode-spark ignition were described. Regarding the laser-induced gaseous breakdown, the threshold laserpower density, the plasma development during the laser pulse, and the fluid motion induced just after the end of the laser pulse were discussed. On the comparison between laser-spark ignition and electrode-spark ignition, the ignition ability and processes were compared between the two ignition schemes at the same deposited energy near the lean-fuel ignitable limit in quiescent gas mixtures. The ignition ability of the laser-induced spark was superior to that of the electrode-induced spark, due not only to the lack of heat loss to the electrodes but also to the augmentation of the flame-kernel energy by rapid heat release from the combustible gas mixture sucked into the flame kernel by the laser-spark-induced non-spherical gas flow.
The lean combustion is one of the key techniques for the advanced internal combustion systems due to the requirement of the higher thermal efficiency. Since the successful ignition must be guaranteed even in the lean combustion, advanced ignition systems have been developed in this decade. Laser ignition is one of the advanced ignition systems which have the profits of the flexibility in the position and the timing of ignition. To develop this ignition system for the actual combustion system, it is required to reveal the underlying physics of the laser ignition. The effects of the initial conditions of the gas mixture and of the spatial and temporal laser profiles on ignition of a homogeneous lean methane/air mixture were examined using a constant volume combustion chamber (CVCC). Results show that the laser ignition has a benefit to ignite leaner mixture under high pressure condition compared to the ignition with conventional spark plug. However, the high incident energy for ignition in lean and high pressure condition was required for maintaining flame kernel and sustainable flame propagation. In addition, the results imply that the large scale and the long duration high temperature region is required for achieving self-sustainable flame propagation.
Laser-induced ignition offers the potential benefit of improved internal-combustion engine performance compared with conventional spark ignition. Since the ignition point can be chosen arbitrarily, laser ignition is non-intrusive, and heat loss and radical quenching at the wall can be reduced. In this report, the results of laser-ignition experiments on gasolineengine performance under lean-burn and high-dilution conditions are introduced, along with the results of multi-point laserignition experiments.
The vehicle emission regulations have been driving forces for advancing the technologies of vehicle engines. Recently, gasoline vehicles, particularly, Gasoline Direct Injection (GDI) vehicles were subjected to regulations of PM (Particulate Matter) emissions in addition to diesel vehicles. In order to develop the engine of lower PM emissions, a computational fluid dynamics (CFD) simulation becomes powerful tool with developing high-performance computers. A PM model is required for CFD to predict PM emissions in an engine cylinder. Particularly, soot particle model is important because the major component of PM is found to be Elemental Carbon (EC). In this article, we review the soot particle calculation by using the method of moment as one of typical way of soot calculations. The nucleation, coagulation and surface reactions of soot particle are explained. The example of the soot calculation is introduced. The PAH growth mechanism for C1–C4 gaseous fuels (KAUST PAH Mechanism 2, KM2) was tested for the recent experimental data of the soot formation of C3H8/air mixture in a shock tube. The mechanism includes molecular growth up to coronene and 36 of nucleation reactions. The effect of nucleation reactions on computed soot generation characteristics was investigated using Chemkin-Pro with the method of moment for soot particle calculations. The calculated results were in good agreement with experimental data. Coronene and 4 ring PAHs were the major soot precursors at high temperature (~2000K) and low/middle temperature range (below 2000K), respectively.
Deflagration tests in a closed chamber were performed to investigate the temperature influence on the dynamic behavior and flame acceleration of spherically expanding hydrogen flames. The dynamic behavior of hydrogen flames was observed by using high speed Schlieren photography, and the flame radius and propagation velocity were measured by analyzing Schlieren images. When flame radius was small, smooth flames were found, where flame stretch affected propagation velocity strongly. We obtained the propagation velocity of a flat flame and the Markstein length through the correlation between flame stretch and propagation velocity of a spherical flame. When flame radius was large, cellular flames were found and flame acceleration appeared. We obtained the increment coefficient of propagation velocity and the critical flame radius corresponding to the onset of flame acceleration. As the initial temperature became higher, the dynamic behavior of hydrogen flames weakened, and then the increment coefficient of propagation-velocity ratio decreased. This was because of the weakness of intrinsic instability. In addition, the proposed model on flame acceleration, taking account of the initial temperature, was good agreement with the experimental results.