buletinul institutului politehnic din iaşi - Universitatea Tehnică ...

94 Alexandru Radu et al.
Spark timing is important because of ignition delay issues. Ethanol
actually has a slightly higher flame speed in smooth combustion than does
gasoline. However, it has a much longer delay between application of the spark
and a fully-formed flame (the “ignition delay”). This means that the spark
timing may need to be advanced.
Ethanol flammability range is much wider for air-ethanol mixtures
comparative to gasoline (0, 3...1, 56 versus 0, 4...1, 4) providing engine run
stability in the area of lean mixtures (Pană et al., 2007).
The enthalpy of evaporation of ethanol is 842-930 kJ/kg that is higher
than the value of 330-440 kJ/kg for gasoline. This property can contribute to the
increase of engine power and efficiency because of the resulting higher density
of the mixture, but this also causes ignition problems at low temperatures
(Cowart et al., 1996).
3. Modelling the Engine Cylinder Process
The main reason for the growth in engine modelling activities arises from
the economic benefits and to reduce the time. Real engine testing cannot be
replaced by models, but they are able to provide good estimates of performance
changes resulting from possible engine modifications.
There are now available comprehensive “commercial” models, which
have a wide purpose of use with refined inputs and outputs to facilitate their use
by engineers. Also, many universities have produced their own thermodynamic
models, of varying degrees of complexity, scope and ease of use.
Such software is AVL BOOST v.2011.6 with which we have developed a
model for the engine running simulation with gasoline and ethanol-gasoline
blends and with normal intake and supercharged admission. The engine
numerical model was validated by comparing the obtained values with the
experimental data when using gasoline and E20 as fuels, on a 1.5L DOHC
engine (76.5 mm bore, 81.5 mm stroke, 9.2 compression ratio).
The theoretical researches were carried at engine speed 2500 rpm, full
load and different dosage values (λ=0.8; 0.85; 0.9 for gasoline and λ=1.0; 1.1;
1.2 for blend) for two fuels: gasoline and E20 (80% gasoline blend with 20%
ethanol) and with different intake pressures for the normal admission engine
and supercharged engine with 1.2 bar. For E20 the tests were carried out for
dosages λ>1 in order to emphasize the main advantages of using ethanol in the
lean mixtures field.
The present investigation specifically concerns the engine performance
and reduction of pollutant emissions from the exhaust of engines using ethanol
fuel as a substitute for gasoline.
3.1. Results
Fig. 1 shows that peak cylinder pressure (pmax) and indicated mean
effective pressure (IMEP) increases for E20 gasoline-ethanol blend comparative