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128 Adrian Sabău et al.
arguments have been used to evaluate novel engine concepts (Flynn et
al.,1984), to investigate the effect of operating parameters on efficiency
(Rakopoulos et al.,1993). The overall energy and availability balance during an
engine cycle are studied analytically in Refs. (Bedran&Beretta, 1985). This
article, present a generalization of the method developed in Ref. (Sabau et al.,
2001) for the second-law analysis of the operation with diesel fuel and use it to
analyze the operation with alternative fuels. Specifically, the cases of a light
gaseous fuel (CH4) and an oxygenated (CH3OH) fuel were studied.
2. Experimental Data
The engine used to study the operation with diesel fuel was a T684 (Euro
II) made by Tractorul Plant Braşov, direct-injection diesel engine. This was a
four-stroke, naturally-aspirated, water-cooled engine with a “bowl-piston”
combustion chamber having a bore of 102 mm, a stroke of 115 mm and rod
length 182 mm. The compression ratio was 17.5 and the nominal speed range
was between 800 and 2400 rpm. A five-hole injector nozzle (hole diameter of
240 µm) was used for diesel fuel injection. It was located near the combustion
chamber centre with an opening pressure of 210 bar. The full load conditions
correspond to equivalence ratio of φ=0.625, injection timing of 18°CA BTDC
and injection duration of 24°CA. Measurements were taken for equivalence
ratios 0.600 and 0.470 and injection timings of 16, 18 and 20°CA BTDC.
3. Modelling Approach
A single-zone model was used to simulate the engine operation. The most
important assumptions were the following:
i) The working medium was considered, in general, to be a mixture of 10
species (O2, N2, CO2, H2O, H2, OH, NO, CO, O, H) and fuel vapour.
ii) All 10 species were considered as ideal gases.
iii) Blow by was ignored.
The residual gas fraction was taken equal to 3% independent of
conditions of operation, which was a reasonable assumption for the operating
parameters described above (Rakopoulos et al.,1993).
Experimental data were available only for diesel fuel injection.
Computationally, the cases of methane (CH4) and methanol (CH3OH) injection
were also investigated. Proper processing of the experimentally acquired
indicator diagrams can yield the preparation and reaction rates of the fuel, as
described in detail in Refs. (Bedran & Beretta, 1985) and (Abraham et al., 1994).
The preparation stage involved vaporization of the fuel, its superheating to the
temperature of cylinder gases and mixing with the oxidizer so that a flammable
mixture forms in certain regions of the combustion chamber. The intuitive
assumption that preparation times for gaseous (CH4) injection must be much
shorter than the ones for liquid injection is not correct (Abraham et al., 1994).