Energy & Environment I NAME

Quiz #4

25 points

Closed book and notes

Answers in red
  1. The following numbers are associated with the study of global climate change, also known as the greenhouse gas problem. Briefly explain the meaning of each number.

280 ppm

Concentration (as parts per million by mole or volume) of CO2 in the earth’s atmosphere circa 1800.

1370 w/m2

Average solar energy flux (energy per second per unit area) at the edge of the earth’s atmosphere, with the earth appearing as a flat disk perpendicular to the sun’s rays.

342 w/m2

Average solar energy flux at the edge of the earth’s atmosphere –

with the earth as a sphere. [Note: prearth2x1370 = 4prearth2x342]

2.5 w/m2

Downward energy flux due to radiative forcing – caused by the increase in the greenhouse gas concentration of the atmosphere since the beginning of the industrial age.

31%

Albedo of the earth – the percentage of the incoming solar energy flux reflected back to space by the earth and its atmosphere.

255 K

Temperature of the earth and its atmosphere as seen from space: note s2554 = 342.

288 K

Average temperature of the earth’s surface.
  1. Write phrases – 1 per pollutant, 10 words maximum per pollutant – that best describe why the gasoline engine forms and emits significant amounts of the following pollutants:

CO

Two phrases are acceptable:

"Fuel-rich burning" – ie, there is insufficient air to fully oxidize the fuel.

"Dissociation of CO2 to CO" – ie, even if there is sufficient air [as in the modern automotive which runs chemically correct most of the time], because of the high temperature of the products of combustion in the engine, some of the CO2 is dissociated to CO. This CO does not completely recombine to CO2 as the burnt gases cool and are exhausted from the engine – there is insufficient time for the recombination to happen.

UHC

The key phrase is the following: "Pockets of fuel/air mixture hide in the cracks and crevices of the engine, thereby escaping the flame." An important crack/crevice is the piston ring land. Note: fuel that goes through a flame is oxidized at least to CO (and H2). Piston engines, compared to other types of combustion equipment, are especially susceptible to this problem.

NOx

The key phrase is: "The products of combustion in the engine cylinders have a high temperature." Note: if the products of combustion are hotter than about 1500 degrees C, as is true in gasoline engines, NO will rapidly form as some of the N2 is oxidized. The hotter the products, the greater the amount of NO formed. Nitrogen organically bound within the C-H structure of the fuel is another source of NO – the organically bound nitrogen is readily oxidized to NO. However, this is not much of an issue with the gasoline engine, since there is little organically bound nitrogen in gasoline.