This paper describes a method for accurately measuring sulfur oxides in automotive exhaust. In this method, the exhaust from a car is diluted with ambient air, then introduced into a large bag filled with clean dry air. The temperature, pressure, and humidity of the diluted exhaust are measured, along with the concentrations of hydrocarbons, carbon monoxide, carbon dioxide, SO2, and sulfates. Bag concentrations are related to the exhaust by using the sulfur/carbon ratio of the fuel. Established instrumental methods are used for the carbon compounds. The sulfur dioxide in the diluted exhaust gas is measured by the West-Gaeke method, which involves collecting a gaseous sample in a scrubber containing potassium tetrachloromecurate. The sulfates are collected on a particulate filter and measured by a new colorimetric method.
The techniques we have developed have been applied to both non-catalyst and catalyst-equipped cars. These studies have shown that:
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Non-catalyst cars emit very low sulfates (less than 1% of fuel sulfur). This conclusion applies to non-catalyst production cars burning unleaded fuel, leaded fuel, or fuel containing manganese, and also to prototype cars equipped with advanced emission systems (for example, Ethyl's Lean Reactor car and cars equipped with Ethyl's Turbulent Flow System).
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Under some driving conditions, monolithic catalysts convert a major portion of the fuel sulfur to sulfates at concentrations close to thermodynamic equilibrium. At 40 mph, as high as 86% of the fuel sulfur was emitted as sulfates in one 1975 car.
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Monolithic catalysts can store significant amounts of sulfates, and this storage seems to be temperature related.
Driving on a durability route discharges sulfur from the catalyst bed. Thus, low sulfate emissions are found for the next 150 miles or so while the catalyst stores sulfates.