Riassunto analitico
The increasingly more stringent emission legislation for engine homologation has underlined the need of new technologies for the reduction of engine-out emissions. The most promising, which overcome the limits of the conventional combustion methodologies, are the so called Low Temperature Combustions (LTC). Among these, Gasoline Compression Ignition (GCI) promotes a combustion from a gasoline-like fuel, combining high efficiency with low pollutant emissions. Previous research demonstrated that to properly control GCI combustion, a multiple injection patter is needed to reach the thermodynamic conditions able to auto-ignite gasoline and properly manage the heat release. One of the main problems related to the use of a multiple injection pattern are the high-pressure waves generated in the feeding duct of the injector. These pressure difference from the target leads to inject a different amount of fuel with respect to the request.
In this thesis, a model-based methodology for the reconstruction of the pressure fluctuations in the feeding ducts of high-pressure Common Rail injectors is presented. Moreover, a strategy that can be easily implemented in an Electronic Control Unit (ECU), to control the injected masses starting from these reconstructions, is proposed.
To properly study and characterize this phenomenon, a dedicated flushing bench has been developed. Once the measurement chain has been validated through the generation of a reference injector map, experimental activity has been carried out testing different injectors and fuels both for single and double injections.
Starting from the acquired data, mathematical model-based mass-spring-damper system has been developed. The natural frequency and damping ratio of the first and second carrier frequency of each wave have been mapped as function of the rail pressure and Energizing Time (ET, defined in μsec). The analysis of the double injection has shown how the wave after the second injection is almost unaffected by the first one. Exploiting this aspect, the modelling of the double injection strategy has been performed by overlapping two waves produced by the single injections.
Finally, a focus on a way to generalize the parameter maps for a whole family of injectors, for different fuels and to reduce them to the minimum possible number is presented.
Although in the discussed layout a backpressure facing the tip of the injector, similar to the end-of-compression pressure in the cylinder, is not present, the accuracy of this layout, except for very low ET and rail pressure, has been demonstrated.
The modelling of the single and double injections with the double frequency approach returned accurate results. It has been also demonstrated that the behaviour of pressure fluctuations in the feeding duct of an injector remains approximately the same for the whole family of injectors. In addition, the whole approach remains valid if a different fuel is used, adding a simple correction coefficient allows using the same maps.
Future development of this study should focus on testing the same procedure with a different geometry of the feeding duct. Moreover, it would be interesting to implement a temperature sensor on the rail, in order to evaluate the temperature effects on the pressure waves propagation. Lastly, to verify the potential of the wave reconstruction, a different injector type should be tested.
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Abstract
The increasingly more stringent emission legislation for engine homologation has underlined the need of new technologies for the reduction of engine-out emissions. The most promising, which overcome the limits of the conventional combustion methodologies, are the so called Low Temperature Combustions (LTC). Among these, Gasoline Compression Ignition (GCI) promotes a combustion from a gasoline-like fuel, combining high efficiency with low pollutant emissions. Previous research demonstrated that to properly control GCI combustion, a multiple injection patter is needed to reach the thermodynamic conditions able to auto-ignite gasoline and properly manage the heat release. One of the main problems related to the use of a multiple injection pattern are the high-pressure waves generated in the feeding duct of the injector. These pressure difference from the target leads to inject a different amount of fuel with respect to the request.
In this thesis, a model-based methodology for the reconstruction of the pressure fluctuations in the feeding ducts of high-pressure Common Rail injectors is presented. Moreover, a strategy that can be easily implemented in an Electronic Control Unit (ECU), to control the injected masses starting from these reconstructions, is proposed.
To properly study and characterize this phenomenon, a dedicated flushing bench has been developed. Once the measurement chain has been validated through the generation of a reference injector map, experimental activity has been carried out testing different injectors and fuels both for single and double injections.
Starting from the acquired data, mathematical model-based mass-spring-damper system has been developed. The natural frequency and damping ratio of the first and second carrier frequency of each wave have been mapped as function of the rail pressure and Energizing Time (ET, defined in μsec). The analysis of the double injection has shown how the wave after the second injection is almost unaffected by the first one. Exploiting this aspect, the modelling of the double injection strategy has been performed by overlapping two waves produced by the single injections.
Finally, a focus on a way to generalize the parameter maps for a whole family of injectors, for different fuels and to reduce them to the minimum possible number is presented.
Although in the discussed layout a backpressure facing the tip of the injector, similar to the end-of-compression pressure in the cylinder, is not present, the accuracy of this layout, except for very low ET and rail pressure, has been demonstrated.
The modelling of the single and double injections with the double frequency approach returned accurate results. It has been also demonstrated that the behaviour of pressure fluctuations in the feeding duct of an injector remains approximately the same for the whole family of injectors. In addition, the whole approach remains valid if a different fuel is used, adding a simple correction coefficient allows using the same maps.
Future development of this study should focus on testing the same procedure with a different geometry of the feeding duct. Moreover, it would be interesting to implement a temperature sensor on the rail, in order to evaluate the temperature effects on the pressure waves propagation. Lastly, to verify the potential of the wave reconstruction, a different injector type should be tested.
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