Since the introduction of turbo charging and direct-injection, passenger car diesel engines show a tendency towards lower compression ratios. Up to now the reduction of compression ratio has mainly been used to restrict the maximum combustion pressure, and to compensate increased boost pressures and higher load densities. The reduction of compression ratio below 16:1 to improve emissions is a new tendency, enabled by the progress in injection equipment development. This leads to problems with cold-starting and cold-idling. Under severe coldstart conditions the compression end temperature and pressure is very low, which has a strongly negative effect on the evaporation and self-ignition process. The consequence is an insufficient starting behaviour and white smoke emissions, which are not acceptable with respect to customer requirements and emission legislation. Aim of this research project is the investigation of the cold-starting behaviour of diesel engines with a reduced compression ratio. New strategies to improve cold-starting and cold-idling are investigated at a high pressure chamber and a single-cylinder diesel engine with optical access. Beneath an improved understanding of the air fuel mixture and the self-ignition process at extremely low temperatures down to -20 degree celsius, fundamental information about optimized injection timing and glow plug strategies are gathered. By analysing the soot luminescence at varying fuel quantity, it became apparent that small fuel quantities burn faster, but also show a distinctly larger spread/standard deviation than larger fuel quantities, a phenomenon hinting at ignition problems. Injecting the fuel tangentially onto the glow plug a reaction zone at the glow plug is formed, which is suitable to initiate the further combustion as soon as the injection process is completed. Misfire probability increases with larger distances between the fuel jet and the glow plug as well as with smaller glow plug protrusions. The engine investigations show that not only the relative position between glow plug and the spray jets has a significant effect on the combustion process, but also wall impingement and turbulent swirl motion. Correlating the results from the thermodynamic analysis with those of the corresponding optical combustion analysis, it can be demonstrated that there is no linear relationship between the mapping pairs under consideration. All investigations performed in the combustion chamber and in the optical engine have demonstrated that under cold start and cold idle conditions, any interaction between the fuel jet and the piston bowl can result in a deterioration of the combustion process. The occurrence of swirl, in combination with decreasing blow-by and wall heat losses at increasing engine speeds, by contrast, is able to stabilise the combustion.


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    Title :

    Stable running of diesel engines at very low ambient temperatures


    Contributors:


    Publication date :

    2008


    Size :

    15 Seiten, 11 Bilder, 11 Quellen


    Type of media :

    Conference paper


    Type of material :

    Storage medium


    Language :

    English




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