Design and Simulation of Two Stroke Engines

Chapter 4 - Combustion in Two-Stroke Engines
Note that the closer the AFR during the combustion is to the stoichiometric, the higher the
carbon dioxide mass concentration, which means that the most efficient extraction of heat has
been obtained from the fuel. This reinforces the empirical relationship, based on measurements,
given in Eq. 4.3.26 for the relative combustion efficiency with respect to fueling.
Hydrogen
This is probably the most complex reaction behavior of the series, for it can be seen in
Fig. A4.9 that it is dissimilar on either side of the stoichiometric fueling level, which is at an
AFR of 14.3 for this particular (unleaded gasoline) fuel. Close to, or richer than the stoichiometric
value, where X is unity, the hydrogen is more pronouncedly dissociated than at lean
mixture burning.
At rich mixtures, and at higher temperatures in the burn zone, more hydrogen is created
leaving more free oxygen. As the cylinder contents cool (if 1800°C can be described as "cooler")
the oxygen is able to combine with the hydrogen to form steam, and reduce the free hydrogen
content of the embryo exhaust gas. At an AFR of 12 the process is barely complete by exhaust
port opening at 107° atdc. The proportions involved are tiny, as a mere 0.15% of the cylinder
mass is free hydrogen, even at the richest mixture combustion. Nevertheless, it is upon such
complete chemistry that the computation depends for its accurate deduction of the oxygen
mass concentration, without which level of accuracy the prediction of nitric oxide (NO) formation
would become hopelessly incorrect.
General
As discussed in Appendix A4.1, it is evident that this combustion model, using both
equilibrium chemistry and reaction kinetics, is absolutely necessary to provide temperature
data, and mass concentration data for all of the relevant gas species, in the burned zone for the
prediction of the mass emissions of oxides of nitrogen and, ultimately, the properties of exhaust
gas.
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