Influence of Fuel Composition and Combustion Process on Thermodynamic Parameters of SI Engines 2012-01-1633

In the field of heavy-duty applications almost all engines apply the compression ignition principle, spark ignition is used only in the niche of CNG engines. The main reason for this is the high efficiency advantage of diesel engines over SI engines. Beside this drawback SI engines have some favorable properties like lower weight, simple exhaust gas aftertreatment in case of stoichiometric operation, high robustness, simple packaging and lower costs. The main objective of this fundamental research was to evaluate the limits of a SI engine for heavy-duty applications.

Considering heavy-duty SI engines fuel consumption under full load conditions has a high impact on CO₂ emissions. Therefore, downsizing is not a promising approach to improve fuel consumption and consequently the focus of this work lies on the enhancement of thermal efficiency in the complete engine map, intensively considering knocking issues.

Using a single-cylinder research engine, basic mechanisms to reduce knocking tendency have been evaluated. Subject of this part of the investigations was the influence of valve timing and cooled EGR on the knocking behavior of a stoichiometric SI engine. Generally cooled EGR and early or late inlet valve closing are reducing knocking tendency, due to benefits during gas exchange late intake valve closing has a greater potential to improve efficiency. A combination of both mechanisms is possible, leading to a reduction of specific fuel consumption of up to 18% in the investigated operation point, using RON95.

Another promising way to improve the CO₂ balance of the engine is the usage of ethanol as fuel, as it is possible to produce ethanol from regenerative sources. In most cases ethanol is blended with gasoline to reduce the fossil energy demand. This work focuses on four different blends E0, E25, E85 and E97. As a first step, investigations of the influence of the different ethanol blends on emissions, fuel consumption and general thermodynamic behavior have been accomplished. As a second step, the knocking behavior of the different blends has been analyzed more deeply. In order to determine the knock limited compression ratio of the different blends, the compression ratio was increased stepwise. For pure RON95 the knock limited compression ratio is in the region of 11, for E25 it is 13 and with E85 and E97 a compression ratio of 14.5 is realizable. With this compression ratio a break efficiency of over 41% could be demonstrated.