Design and Simulation of Two Stroke Engines

Chapter 1 - Introduction to the Two-Stroke Engine
is toxic, the second is blamed for "smog" formation, and the last is regarded as a major
contributor to "acid rain." These and other facets of pollution are discussed more extensively
in Chapter 7.
As an example, consider a pollutant gas, PG, with molecular weight, Mg, and a volumetric
concentration in the exhaust gas of proportion, Vcg. In consequence, the numerical value
in ppm, Vppmg), would be 10 6 Vcg and as % by volume, V%g, it would be 100Vcg. The average
molecular weight of the exhaust gas is Mex. The power output is Wb and the fuel consumption
rate is mf. The total mass flow rate of exhaust gas is rhex:
mex = (1 + AFR0)mf
kg/s
(1 + AFRjihf (1.6.11)
= - ——-— kgmol/s
Mex
Pollutant gas flow rate = (M„VC„) ^ ^^ kg/s (1.6.12)
1 g g) Mex
Brake specific pollutant gas flow rate, bsPG, is then:
M„VCEr (1 + AFRn)riif ,.,,.,
bsPG = g cg x ^ 2i_L kg/Ws (1.6.13)
Wb
Mex
(M„Vcg) x (1 + AFR0)bsfc
bsPG = l g g) V - kg/Ws (1.6.14)
Mex
By quoting an actual example, this last equation is readily transferred into the usual units
for the reference of any exhaust pollutant. If bsfc is employed in the conventional units of kg/
kWh and the pollutant measurement of, say, carbon monoxide is in % by volume, the brake
specific carbon monoxide emission rate, bsCO, in g/kWh units is given by:
28
bsCO = 10(1 + AFR0) bsfc V%co — g/kWh (1.6.15)
29
where the average molecular weights of exhaust gas and carbon monoxide are assumed simplistically
and respectively to be 29 and 28.
The actual mass flow rate of carbon monoxide in the exhaust pipe is
mco=bsCOxWb g/h (1.6.16)
39