The Effect of Fuel and Operating Variables on Hydrocarbon Species Distributions in the Exhaust from a Multicylinder Engine

Abstract

Measurements of the concentrations of individual exhaust hydrocarbon species have been made as a function of engine operating variables (φ, rpm, EGR, spark timing, and coolant temperature) in a 2·3-liter four-cylinder engine. Three fuels were used in these experiments: propane, isooctane (2,2,4-trimethylpentane), and an unleaded gasoline (indolene clear). The results show that a change in operating variable can change not only the total hydrocarbon concentration but also the distribution of species in the exhaust. All three fuels show similar trends when an operating variable is changed. Fuel-air equivalence ratio is a critical parameter in controlling exhaust hydrocarbon emissions. Beginning near stoichiometric, the total hydrocarbon concentration and the percentage contributions of methane and acetylene to the exhaust increase as the mixture becomes richer. These species contribute less than 2 percent to the total hydrocarbon emissions at <t><0.95. Their contribution rises to 15–25 percent at φ=1.2 depending on the fuel used. The likely cause for this increase is incomplete combustion in the bulk gas of the cylinder, These results highlight the importance of chemical as well as physical effects in the production of hydrocarbon emissions during combustion. The exhaust hydrocarbon species distribution is also very sensitive to the engine speed and spark timing in the absence of EGR. Increasing the engine speed or retarding the spark causes the hydrocarbon emissions to decrease. These parameters change principally the concentration of unburned fuel in the exhaust, while the concentrations of partial oxidation products remain approximately constant. Addition of EGR increases all hydrocarbon concentrations and changes the dependence of hydrocarbon emission on spark timing. With an EGR rate equal to 2/3 of the maximum tolerance of the engine, the hydrocarbon emissions increase as the spark is retarded. This increase consists of increased concentrations of partial oxidation products in the exhaust gases, while the concentration of fuel remains unchanged.