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.