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Optical and Thermodynamic Investigations of Reference Fuels for Future Combustion Systems
Abstract:
The finite nature and instability of fossil fuel supply has led to an increasing and enduring investigation demand of alternative and regenerative fuels. An investigation program is carried out to explore the potential of tailor made fuels to reduce engine-out emissions while maintaining engine efficiency and an acceptable noise level. In this paper, fundamental results of the Diesel engine relevant combustion are presented. To enable optimum engine performance a range of different reference fuels have been investigated. The fundamental effects of different physical and chemical properties on emission formation and engine performance are investigated using a thermodynamic diesel single cylinder research engine and an optically-accessible combustion vessel.
Depending on the chain length and molecular structure, fuel compounds vary in cetane number, boiling temperature etc. Therefore, different hydrocarbons including n-heptane, n-dodecane, and l-decanol were investigated. To gain knowledge about the emission formation process with future fuels, engine experiments are performed at four part load conditions and full load. Investigations show that an increased boiling temperature leads to significantly higher soot formation.
In order to understand the effects of emission formation (in particular the higher soot formation of n-dodecane) an equivalent common-rail Diesel injection system is installed in a combustion vessel, which provides nearly quiescent high-pressure and high-temperature conditions. The combustion and soot formation are analyzed using two different optical measurement techniques. The hot reaction zone is visualized using OH* chemiluminescence imaging. Additionally, images of the integral natural soot luminosity of the flame are acquired. The ignition delay, the flame penetration length and the unsteady lift-off length are determined. In particular, the results indicate that soot formation depends strongly on the fuel cetane number under the investigated part-load conditions.
In this integrated approach, investigations in a thermodynamic engine and an optically accessible combustion vessel offer the potential to analyze the effects of different fuel properties on the emission behavior in detail. The dependences of emissions formation on different fuel properties in a Diesel combustion system are identified and discussed. In particular, the impact of the cetane number, boiling temperature and oxygen content of the fuels is investigated. The relatively high soot formation of n-dodecane is reproduced in the optical vessel. The emission behavior can be explained by the overlap of the established flame and the liquid-phase fuel.