Appendix C
Combustion Chemistry of Alternative and Traditional Transportation Fuels

Hydrocarbon fuel combustion under ideal conditions would produce carbon dioxide and water as the only products.

where n and m are positive integers, p is a positive number, and delta-H_c^o is the heat of combustion, or “enthalpy,” under ideal conditions. The specific reactions for four alternative fuels under ideal conditions are given below [88]:

Compressed Natural Gas:

Liquefied Petroleum Gas:

Methanol:

Ethanol:

However, hydrocarbon fuels combustion under actual (non-ideal) combustion conditions produces several intermediate products in addition to carbon dioxide and water. This is the general combustion reaction for any hydrocarbon fuel under non-ideal conditions:

where C_kH_y = unburned or partially burned hydrocarbon fuel, CO₂ = carbon dioxide, H₂O = water, CO = carbon monoxide, NO_x = oxides of nitrogen (x = 1-3), N₂O = nitrous oxide, OH* = hydroxyl radical, O* = oxygen radical and/or singlet oxygen, H⁺ = hydrogen ion, H_c = heat of combustion [89] (or enthalpy) under actual (non-ideal) conditions, and the coefficients a, b, c, d, e, f, g, h, and j vary with fuels and operating conditions. The heat of combustion is also referred to either as the “lower heating value” (with “gaseous” water as one of the products) or as the “higher heating value” (with “liquid” water as one of the products).

Combustion in internal combustion engines is a very complex chain reaction system that leads to hundreds of intermediate products, byproducts, and end products [90]. In internal combustion engines, the products in the exhaust are determined by a large number of parameters, including fuel to air ratio, compression ratio, fuel composition, internal design, operating condition of the cylinder and combustion chamber, and exhaust inflation. For methane, propane, and methanol fuels, the number of identified reaction products in the exhaust are 25, 41, and 84, respectively.

Proceed to Appendix D