Fuel Composition Effects on Hydrocarbon Emissions from a Spark-Ignited Engine - Is Fuel Absorption in Oil Significant?

Absorption of fuel in engine oil layers has been shown to be a possible source of hydrocarbon (HC) emissions from spark-ignited engines. However, the magnitude of this source in a normally operating engine has not been determined unambiguously. In these experiments, a series of n-alkanes of widely different solubility (n-hexane through undecane) was added (1.5 wt % each) to a Base gasoline (CA Phase 2). Steady-state experiments were carried out at two coolant temperatures (339 and 380 K) using a single-cylinder engine with the combustion chamber of a production V-8. Both total and speciated engine-out HC emissions were measured. The emissions indices of the heavier dopants did not increase relative to hexane at either coolant temperature. These results do not support oil absorption as a significant HC source in our engine at temperatures above 339 K, since a significant oil contribution would result in larger emissions indices for the higher molecular weight alkane additives, which are more soluble. In a second set of experiments, 28 wt % of either toluene or 1,2,4-trimethylbenzene (TMB) was added to the Base fuel. TMB is 6-7 times more soluble than toluene, but again no evidence for a significant absorption contribution to total HC emissions was observed. This latter experiment showed that decreasing the fuel volatility by adding TMB increases the emissions indices of all constituents in the gasoline except TMB, including those with high volatility such as isopentane. The possibility that fuel absorption in oil increases HC emissions at the lower coolant temperatures (<339 K) that may be encountered during cold start can not be excluded by these experiments.