Exhaust emissions from gasoline- and LPG-powered vehicles operating at thealtitude of Mexico City

Unburned hydrocarbons (HCs), carbon monoxide (CO), and oxides of nitrogen ( NOx) are the compounds regulated as pollutants by an environmental standard in the Metropolitan Area of Mexico City (MAMC). The main fuel used in vehi cular transportation is gasoline, and the use of liquefied petroleum gas (L PG) is now an alternative as low emission technology to decrease the enviro nmental impact of transportation operations. The environmental impact of co mmercial gasoline consumption in the Valley of Mexico was estimated by on-r oad and FTP-75 testing of three formulations of gasoline (one leaded [octan e 81] and two unleaded [one octane 87 and one octane 93]). A fleet of 30 ve hicles was used: 10 were chosen that had pre-1990 technology while 12 were 1991-1996 vehicles equipped with fuel injection, catalytic converters, and air/fuel ratio control technology. The remaining eight vehicles were high-p erformance new model vehicles (1995-1996) equipped with the newest technolo gy available for pollution control. Fifteen vehicles in the fleet were also tested for the effect of changing from leaded to unleaded gasoline. Three different LPG formulations were tested using three vehicles representative of the LPG-powered fleet in the MAMC. Two gasoline-to-LPG conversion certif ied commercial systems were evaluated following the BAR-90 and the HOT-505 procedures. Emissions corresponding to the high-octane (premium) gasoline showed a 15% higher contribution to HCs with a 6% lower reactivity than the 87 octane ga soline; the HCs in the exhaust for premium gasoline are mainly isoparaffins . When the vehicles were tested on the road at high speeds, an average 3% i ncrease in mileage was obtained when vehicles were switched from leaded to unleaded gasoline, while a 5% increase in mileage was observed when vehicle s were switched from 87 octane to premium gasoline. The tests of LPG formulations indicated that a change in composition from 6 0% vol of propane to 85.5% vol reduces levels of HCs and CO emissions; such is not the case for the NOx emissions. The higher the concentration of pro pane, the higher the levels of NOx that reached values above the maximum li mits set by the environmental standard. A value of 70% vol of propane in th e LPG mixture, with variations no greater than 4%, seems to be the best met hod for reducing pollutant emissions in Mexico City.