The transiency of in-cylinder flame development in an automotive-size diesel engine

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Copyright: Rusly, Alvin Mulianto
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Abstract
Global growth in the sales of light-duty diesel-powered vehicles is effectively driven by diesel engine s superior fuel economy though concerns implicating emission formations and usage of non-renewable fossil still persist. Such obstacles present a need for better understanding of the diesel combustion, which will help improve engine efficiency and reduce pollutant emissions. To address this issue, experimental study of in-cylinder combustion processes is conducted in a light-duty diesel engine with focus on flame development transience. A new experimental research facility has been designed and constructed to study transient behaviour of diesel flames during combustion. The facility houses a modified single-cylinder diesel engine that allows optical access to the combustion chamber at realistic engine environment and ambient conditions. Two distinctly different diesel combustion regimes are investigated: one with short injection duration and the other with long injection duration. The first of the combustion regimes consists of short injection duration and long ignition delay ultimately resulting in a positive ignition dwell (fuel injection completes prior to ignition). In this regime, the overall combustion is dominated by premixed burn phase whereby further improvement of efficiency is limited by a drastic increase in in-cylinder pressure. If the problem is severe, undesirable pressure ringing follows the initial pressure rise, which is called diesel knock. The first part of this thesis addresses this issue of knocking in a light-duty diesel engine. In the optical engine, high-speed natural hot soot luminosity imaging was performed to visualise flame behaviour during the knocking cycles. It is found that the diesel flame oscillates against the normal swirl direction and the oscillation frequency matches the frequency of in-cylinder pressure ringing, which is the first observation of such correspondence in a diesel engine. Experimentation with pilot injection showed a remedial effect through elimination of pressure ringing and dampening of flame oscillation. Such results are connected with a short ignition delay and less intense premixed combustion as shown through a lower pressure rise rate and negative ignition dwell (i.e. mixing-controlled combustion). The second regime investigated in this thesis presents long injection duration through a single-hole injector resulting in a negative ignition dwell (combustion starts prior to the end of injection). This regime is dominated by mixing-controlled combustion phase corresponding to high-load engine operating conditions. Opposed to the short-injection regime with positive ignition dwell, this long-injection regime is characterised by a lifted flame that is under the strong influence of jet-wall interaction during and after the fuel injection. Therefore, the focus of last half of this thesis is on the jet-wall interaction and its impact on lift-off length (i.e. distance between the nozzle to the first detectable flame base within a specified spatial range with respect to the jet trajectory) that is known to play an important role in pollutants formation. Interaction between the reacting jet and the wall was visualised through hot soot luminosity and hydroxyl radical (OH*) chemiluminescence imaging. An interesting finding from these imaging diagnostics is shortening of the lift-off length against the incoming jet momentum during the fuel injection. This trend is reversed (i.e. the lift-off length increases) only after the end of injection when the jet momentum diminishes. Detailed analysis of temporal evolution of the lift-off length and parametric studies of injection pressure and addition of neighbouring jet suggest that a potential cause for the shortening of the flame base is the redirection of hot combustion gases that are entrained back to the incoming jet, i.e. re-entrainment. The findings in this thesis help understand the current constraints in improving engine efficiency and reducing pollutant formation in light-duty diesel engines. It is suggested that at low-load operating conditions, engine developers should limit the pressure rise rate below a certain level so that problematic diesel knock can be avoided. By contrast, at high-load operating conditions, injection pulse width should be controlled to reduce jet-wall interaction as otherwise the pollutants formation would increase with decreasing lift-off length.
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Author(s)
Rusly, Alvin Mulianto
Supervisor(s)
Kook, Sanghoon
Hawkes, Evatt R.
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Publication Year
2013
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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