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 (a, rpm. EGR, spark timing, and coolant temperature) in a 2.3-liter four-cylinder engine. Three fuels were used i n 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 i n 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 0<0.95. Their contribution rises to 15-25 percent at 0=1.2 depending on the fuel used. The likely cause for this increase is incomplete combustion in the bulk gas o f the cylinder. These results highlight the importance o f chemical as well as physical effects in the production of hydrocarbon emissions during combustion. The exhaust hydrocarbon species distribution is also very sensitive lo 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 o f EGR increases all hydrocarbon concentrations and changes the dependence of hydrocarbon emission on spark timing. With an EGR rate equal to 213 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.

Speciated Hydrocarbon Emissions from the Combustion of Single Component Fuels. I. Effect of Fuel Structure

“…The percent distribution of the major hydrocarbon emissions from the single cylinder engine are given in Table IX. In the isooctane exhaust, methane and acetylene increase significantly in going from the fuel-lean setting to the fuel-rich setting, consistent with earlier results from a multicylinder SI engine. 9 Although ethene becomes more important at the higher fuel/air setting, propene and 2-methylpropene diminish in importance; the net result is that as a group, olefins represent a smaller fraction of the HC emissions under fuel-rich conditions as shown in Table X. As a fraction of total HC emissions, the contribution from unburned fuel is the same for the two fuel /air settings (the same is true for toluene as a fuel).…”

“…Under fuel-rich conditions, methane and acetylene (ethyne) are also generated in the bulk gas within the combustion chamber. 9 In contrast to the SI engine, sources of HC emissions from the PFC are not well defined. However, since oil absorption effects are not present, and since HCs trapped in crevices near the injector must still pass through the combustion furnace prior to exiting the PFC, the most likely source of (Figure 2) and in a multi-cylinder engine (Figure 3; taken from Reference 9) support this contention.…”

Speciated hydrocarbon emissions from a gas-fuelled spark-ignition engine with various operating parameters

“…The exhaust gas temperatures were increased about 25± 50 8C and the stoichiometric condition showed a higher temperature than lean mixtures. This means that, with higher b.m.e.p., the increase of¯ame propagation speed, the improvement of¯ow and mixing in the combustion chamber and exhaust manifold and the increase of exhaust gas temperature strongly aect the late-cycle burn-up process, resulting in a decrease of HC emissions [8]. On the other hand, HC emissions could be increased with higher combustion pressure.…”

“…Hence, the increased strength of the crevice mechanism and the reduced rate of post-combustion oxidation both potentially contribute to the observed increase in NMHC emission with advanced spark timing [8,9]. Figure 14 shows fuel components, SR and BSR for various spark timings.…”

“…Weaver et al [6] compared speciated emissions from a gasoline vehicle with those from a methanol vehicle. Hirita et al [7] tested gasoline and M85 fuel using a¯exible fuel vehicle (FFV) and Kaiser et al [8] compared the HC species of gasoline, propane and isooctane. Poulsen and Wallace [9] also tested natural gas fuel.…”