20 Dec 2007
In a bid to develop cleaner diesel engines, researchers are using lasers to pinpoint the exact location where soot is formed.
A technique known as laser induced incandescence is helping researchers in the Netherlands locate exactly where soot is generated. The aim is to understand optimal engine conditions and in turn reduce soot emission.
"Our technique enables the in situ investigation of soot particles during the combustion process at very short time scales," Hans ter Meulen, a researcher at Radboud University, told optics.org. "As far as we know, there are no other techniques to measure soot particles during the combustion process and without disturbing the process itself."
Measuring soot formation in a diesel engine is difficult due to the turbulent and high-pressure environment within the combustion cylinder. "The soot we are trying to measure causes the largest problem because it strongly attenuates both the laser beam and the signal," commented ter Meulen.
In an attempt to perform the experiment in realistic conditions, the team made one of the engine's optically-accessible by mounting windows in its wall and head as well as the piston crown. "By elongating the cylinder, we were able to study the combustion process from below, through the piston window," commented ter Meulen. "This technique enabled us to investigate the soot particle size during the engine cycle, without needing to sample the soot. This avoids changing the soot structure or disturbing the combustion process itself."
The team fired 10 ns 1064 nm pulses with an energy density of 0.25 J/cm2 down through the engine. "The particles are heated by the laser pulses to a temperature of about 4000 K," explained ter Meulen. "The time taken for the particles to cool down to ambient temperature is dependent on the size of the particles."
The particle size was found to increase at the early stage of the combustion cycle due to agglomeration. During the later stages of the cycle, particle size decreases due to oxidation. The team will use its results in collaboration with a group from Eindhoven University of Technology to optimize diesel combustion models. "We will also continue to improve our technique and work towards applying it to other engine types and for other fuels, such as biofuels," concluded ter Meulen.
Hans ter Meulen carried out this research in collaboration with colleague Bas Bougie.