This paper describes a chassis-dynamometer study of sulfate formation on a catalyst-equipped automobile under steady-speed and cyclic operating conditions, with particular attention to the effects of preconditioning on sulfate emission rates.
The vehicle was a 1973 Ford Torino equipped with monolithic noble metal catalysts and secondary air injection to simulate a 1975 catalytic system. The fuel used was unleaded certification gasoline with a nominal 0.025 weight percent sulfur content. The experimental program consisted of (1) protracted steady-speed driving at five different road speeds up to 60 mi/hr and at idle, ordered in such a way as to show storage/release effects and stabilized sulfate and SO2 emission rates; and (2) a series of hot-start FTP-cycle tests initiated with a cold-start FTP test.
Sampling for sulfate and SO2 measurement was from a dilution tube in which the vehicle exhaust gases were diluted 10:1 with filtered air. Total mass, size distribution, and light scattering properties of the exhaust particulates were also determined.
Sulfate emissions increased with decreasing catalyst temperature down to ∼500 C, approaching 0.1 g/mi under stabilized conditions at the optimum speed, which on this system was 40 mi/hr. At this speed, sulfate comprised 75 to 80 percent of the emitted sulfur and ∼90 percent of the fuel sulfur consumed. Conversion closely approached the thermodynamic-equilibrium limit at the highest catalyst temperatures (≥600 C).
Sulfur storage was evident at idle following operation at 50 mi/hr, and also at 20 mi/hr following 60 mi/hr operation. Storage and release integrated over a 2-hour period were less than ±20 percent of fuel sulfur under most of the other changes from one steady speed to another. The storage/release transients were of the order of a few hours.
Storage was also evident in the series of 1972 FTP cycles (one cold-start and ten hot-starts in succession), which began after 60 mi/hr preconditioning. Sulfate emission was initially 0.027 g/mi and continued to climb for several cycles towards an asymptote of 0.08 g/mi or 50 percent of the fuel sulfur consumed.
There are a number of additional results concerning the acidity, water content, and size distribution of the emitted sulfate aerosol; noble-metal and nitrate emissions; and sampling and analysis methodology - all confirming previously-published information.