New combustion processes like LTC (Low Temperature Combustion) that includes HCCI (Homogeneous Charge Compression Ignition), PCCI (Premixed Charge Compression Ignition), PPCI (Partial Premixed Compression Ignition)… are promising ways to reduce simultaneously NOx and PM. Nevertheless, these combustion processes can be used only on a limited part of the engine load and speed map. Therefore, it appeared interesting to assess how the fuel, through its characteristics, could enhance the operating range in such combustion processes. That was the aim of an international consortium carried out by IFP and supported by numerous industrial companies.
First a specific procedure has been developed to compare the different fuels on a early injection HCCI single cylinder engine. Then, using this procedure, a matrix of fuels having different cetane numbers (CN = 40-63), volatilities and chemical compositions has been tested. Their propensity to increase the operating range limited by some criteria like smoke, noise… has been measured and compared to a conventional EN590 Diesel fuel.
Using this methodology, it has been shown that a low CN fuel leads to a long auto ignition delay, which lets more time for the fuel vaporization process and, consequently, gives a better homogenization of the fuel, air and exhaust gases mixture. A high fuel volatility also favors the homogenization process. Moreover, it has been shown that in addition to these classical physical properties, the fuel composition highly impacts the combustion behavior and, therefore, noise and smoke emissions. For instance, it has been observed that an appropriate fuel combining all these parameters could improve the operating range of more than 30%.
These results are a first step in the definition and the validation of the Combustion Driving Concept: the way to obtain the optimum combustion development through the fuel formulation.