Design and Simulation of Two Stroke Engines

Chapter 4 - Combustion in Two-Stroke Engines
as declared earlier, the fuel for the spark-ignition engine is octane and dodecane for a diesel
unit. The real situation is more complex than that, as Table 4.1 shows. The properties are the
hydrogen to carbon molecular ratio, H/C (or n as seen in the chemical relationship CHn), the
lower calorific value, Cfl, the specific gravity, and the latent heat of vaporization of the fuel,
hvap-
FUEL
H/C ratio, n
Cf|, MJ/kg
Specific gravity
hvap, kJ/kg
Table 4.1 Properties of some fuels used in engines
C 8 H 18
2.25
44.79
0.70
400
C 12 H 26
2.17
43.5
0.75
425
SU Gasoline
1.65
43.0
0.76
420
Auto Diesel
1.81
43.3
0.83
450
The tabular values show the typical properties of iso-octane, CsHis, and dodecane, Ci2H26-
The values in the table for gasoline, labeled as SU or "super-unleaded" gasoline and for diesel
fuel, labeled as "auto" diesel, are reasonably representative of the typical properties of fuels
commercially available. The properties of such fuels are very dependent on the refining process
which will vary from country to country, from refinery to refinery, and from the constituents
and origins of the crude oil source for that particular fuel.
4.3.2 Properties of exhaust gas and combustion products
Sec. 2.1.6 shows the basic theory for the computation of the properties of a mixture of
gases. The example chosen is the stoichiometric combustion of octane. This continued in the
introduction given in Sec. 1.5.5. In both these examples, stoichiometric, or chemically and
ideally exact, combustion was used and so all carbon burned to carbon dioxide and carbon
monoxide was not formed. In all real combustion processes, dissociation takes place at elevated
temperatures and pressures so that, even at stoichiometry, free carbon monoxide and
free hydrogen will be created. There are two principal dissociation reactions involved and
they are:
C02 CO + -j-02
H20 + CO H2 + C02
(431)
(4.3.2)
The latter equation is often known as the "water-gas" reaction. The combustion equation
that includes all of these effects, and for all air-fuel ratios with a generic hydrocarbon fuel
CHn, is then completely stated as:
CHn + *.m(02 + kN2)
= XjCO + x2C02 + x3H20 + x402 + x5H2 + x6N2
297
(4.3.3)