The addition of alcohol to conventional hydrocarbon fuels for a
spark-ignition engine can increase the fuel octane rating and the
power for a given engine displacement and compression ratio. In
this work, the influence of butanol addition to gasoline was
investigated. The experiments were performed in an optical ported
fuel injection single-cylinder SI engine with an external boosting
device. The engine was equipped with the head of a commercial SI
turbocharged engine having the same geometrical specifications
(bore, stroke and compression ratio). The effect of a blend of 20%
of n-butanol and 80% of gasoline (BU20) on in-cylinder combustion
process was investigated by cycle-resolved visualization. The
engine worked at low speed, medium boosting and wide open throttle.
Changes in spark timing and fuel injection phasing were considered.
Comparisons between the flame luminosity and the combustion
pressure data were performed. The fuel was injected both at closed
intake valve (CV) and open intake valve (OV). The spark timing was
changed to identify the maximum brake torque and the knocking
limit. Butanol blend allowed working in more advanced spark timing
without occurrence of abnormal combustion. For the blend BU20, the
duration of injection (DOI) was increased to obtain a
stoichiometric mixture. For both fuels, at stoichiometric
conditions, DOI in OV was maintained shorter than that in CV, due
to the lower amount of fuel deposited on intake port and on piston
surface.
The combustion images showed a different combustion behavior
injecting the fuel in CV and in OV conditions. In OV condition,
several light spots were observed in the flame front region due to
the condensed fuel attached on the optical piston wall. In CV
condition, the light spots are bigger but less in number than in OV
condition. For BU20 the previous phenomena were less evident due to
the chemical composition of the blend that reduces the amount of
fuel attached to the piston wall. For both fuels and at all engine
conditions, a low variation of IMEP was observed (≺5%). For BU20
injected at CV condition, the lowest IMEP value was measured for
all spark timings.