Between different sectors, GHG emissions released by automotive one in 2010 were 4.5 GtCO2, the 14% of the total (32 GtCO2). Moreover, transport sector depends by more than 93% on oil, to be refined into gasoline and diesel fuel. Natural gas demand in transport sector has clearly increased in the last decade considering the lowest CO2 emissions per units of energy produced among different fossil fuels but it will be used mostly in the next future. Among different sectors, the 21 % of the energy demand is indeed supplied by NG, due to lower price and reduced GHG emissions. Storage type (compressed natural gas or liquefied natural gas) and vehicle type (road transport, marine transport, etc.) mainly discriminate natural gas engine layouts. Spark-ignition natural gas engine with different configurations will be indeed taken into consideration in this research project. Today, vehicles for the road transport fueled with compressed natural gas are mainly bi-fuel ones with both gasoline and natural gas feeding system with a manual or automatic switch. To mitigate knock event, engine layout is designed up to gasoline characteristics and engine performances with natural gas are not fully exploited. Mono-fuel configuration is capable to totally exploit the potential of natural gas. Therefore, this thesis will focus on the development of mono-fuel natural gas engines and improvements in injection and combustion strategies have to be reached by implementing new combustion chamber shape, improved ignition management and improved injection systems.
A detailed analysis of the natural gas injection system will be hence carried out. Different injection system layouts will be analyzed: single-point, multi-point and direct injection systems, focusing on pressure reducing valve dynamic. As a matter of fact, its behavior affects the dynamic response of the injection system: mismatch between estimated injected fuel and real one could be appreciated. Typically, average rail pressure evaluated by ECU differs from mean value during injection window. Therefore, detailed analysis will be carried out on experimental data and a 0D-1D numerical model will be
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developed to enhance the problem understanding. The research activity has been carried out in order to reproduce properly all the components of the pressure reducing valve which affects the dynamic response of the injections system. The numerical model will give useful explanation of the fuel mass injected mismatch.
Then, a heavy-duty spark ignition compressed natural gas engine provided with two different injection systems will be examined. A standardized single-point injection system and a prototypal multi-point one will be evaluated so as to evaluate the possibility for performance enhancement. Cyclic variation and combustion efficiency for each configuration will be analyzed, proving the highest combustion efficiency of the prototypal configuration. Moreover, possible improvements with new engine control strategies will be investigated by adopting a 0D-1D numerical model. Single-point injection system modelling will prove the impossibility for efficiency improvement whereas multi-point injection system can be optimized by adopting enhanced strategies. As a matter of fact, fire-skipping mode will be simulated. Feasible reductions of fuel consumption under partial load conditions will be shown: decrease in fuel consumption up to 12% will be proved.
Finally, a new methodology for combustion, cyclic variation and knock onset modelling will be presented. Indeed, high-efficiency natural gas engines could in turn lead to knock conditions due to higher CR and different combustion chamber shape. Experimental analysis at test bench could be carried out to calibrate appropriate ECU control strategies for knock mitigation, but an experimental campaign under knock condition is dangerous and costly due to possible failure of mechanical parts of the engine. Numerical models for auto-ignition prediction could hence overcome this problem. Therefore, a predictive fractal combustion tool will be calibrated: it will be able to perform a correct mass fraction burned evolution estimation for different operating conditions (speeds, loads, relative air-to-fuel ratio, etc.). Then, knock onset estimation based on auto-integral (its usage is satisfactory considering the high natural gas chemical stability) coupled with a new method for cyclic variability simulation will be adopted; these two phenomena are indeed strictly correlated. A correct estimation of the percentage of knocking cycles will be shown. This new methodology will be carried out and verified on two light-duty spark ignition engines with different characteristics so as to verify its goodness
