GASOLINE 1. What factors influence your mileage? The main factor that influences mileage is the variability of the driving cycle. Mileage can vary because of several factors such as weather, driving speed, driving style, traffic conditions and road conditions. For example, EPA has estimated that poor road condition (gravel, curves, slush, snow, etc.) can reduce mileage by 4.3%, congested traffic can reduce mileage by 10.6%, acceleration rate can reduce mileage 11.8% and there are other factors that can combine to reduce mileage. Thus taking mileage measurements based on one tank of gas can give somewhat misleading results if that tank was used mostly for one type of driving (a highway trip, for example) as opposed to another type (like city driving). Extra load on the engine can reduce mileage. Fuel economy decreases approximately 2.8 mpg (15%) for all cars when running the air conditioner. Towing a boat or trailer, loading with equipment and luggage and transporting a full load of passengers can lower mileage. Poor maintenance, improper tire inflation or poor tire condition can also affect mileage. If several of these negative factors combine, the reduction in mileage can be quite noticeable. The energy content and therefore mileage of gasoline can vary somewhat, but this is a smaller factor that those mentioned above. If a customer uses conventional gasoline without oxygenates such as ethanol or ethers (i.e. MTBE) and switches to an oxygenated fuel, the energy content and mileage may be reduced by 2 to 3% depending on the amount of oxygenate in the gasoline. This amount of change would be difficult to measure tank to tank. Energy content can also vary from summer to winter blends by a small amount, about 1%. Butane is blended in gasoline to help cold starts during the winter. Addition of butane lowers the density and the energy content. 2. Why do we blend oxygenates in gasoline? In 1979 as lead in gasoline was being phased out oxygenates like methyl tertiary-butyl ether (MTBE) and ethanol have been added as octane enhancers. In 1990 the Clean Air Act Amendments (CAA) were enacted. One of its provisions was to require the use of oxygenates in the wintertime in cities that have high levels of carbon monoxide pollution. Oxygenates help to complete the combustion process in the engine to form carbon dioxide. Older engines, in particular, needed the additional oxygen to run leaner to reduce the amount of carbon monoxide that they produce. The CAA also required reformulated gasoline (RFG) beginning in 1995 for cities that had substantial ozone pollution. The ozone is formed when pollutants react chemically in the presence of sunlight. Reformulated gasoline is designed to lower the emissions from automobiles that can add to the ozone formation. RFG is effective in reducing ozone, but the addition of oxygen does not appear to be essential for maintaining this benefit. Low sulfur levels and low vapor pressure are much more effective gasoline parameters for reducing ozone. Ethanol and MTBE are the most widely used oxygenates. Ethanol may be blended up to 10 volume percent, and MTBE may be blended up to 15 volume percent. Beginning in 1996 the automobiles sold in the United States had On Board Diagnostics-II (OBD-II). This technology is used to manage and monitor the operation of the engine, transmission, and emissions control components. Rather than monitor what is coming out of the tailpipe, the OBD-II reduces emissions caused by emission related malfunctions, by monitoring virtually every component and system that can affect emissions during normal driving. It controls the amount of gasoline that is added to the engine cylinders to optimize the air/fuel ratio for maximum engine efficiency and to reduce emissions. This optimum air/fuel ratio can readily be reached without the addition of oxygenates.
What is MTBE? MTBE is methyl tertiary butyl ether. It is blended in gasoline because it is an octane enhancer and provides additional oxygen to help complete the combustion process. MTBE is banned in California, Connecticut and New York because of ground water contamination issues. Why reduce the sulfur content of gasoline to 30 ppm? The EPA traditionally divided automotive emissions into four broad areas: hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter (PM). In gasoline engines, PM is relatively small, and the other three classes of emissions are reduced by use of catalytic converters, called (logically enough) “three-way catalysts” (TWC). Such catalysts work by: 1. Oxidizing carbon monoxide to carbon dioxide, 2. Oxidizing hydrocarbons to carbon dioxide and water, 3. Reducing nitrogen oxides to nitrogen and carbon dioxide and/or water. As might be expected, the chemistry occurring within a catalytic converter can be quite complex. The simultaneous requirements for both an oxidizing and reducing atmosphere narrows the air/fuel ratio in which the engine operates, so that practically the best engine operation as far as emissions is concerned is when the air and fuel are present at stoichiometric quantities, or when there is just enough oxygen to combust the fuel, with neither an excess of fuel nor oxygen. Any air/fuel ratio outside this narrow band causes one or more of the three emissions to increase. In order to control the air/fuel ratio to this narrow band, cars built since the early 1980’s have been equipped with on-board diagnostics (OBD), which include sensors such as oxygen sensors and the associated electronics. Vehicles since 1996 have been equipped with the more advance OBD II systems. One of the effects of OBD ability to operate at correct stoichiometry has been the lessening of concern over CO emissions after the catalyst, as CO traditionally was formed at stoichiometry on the rich side (low air/fuel ratio). Accordingly, current automotive emissions regulations are chiefly concerned with three classes of emissions: NOx, Volatile Organic Compounds (VOC), and Toxics, which include such substances as benzene and various aldehydes. Sulfur has a negative effect on several aspects of vehicle emissions. 1. Direct emissions, typically sulfur dioxide but also other oxides of sulfur or hydrogen sulfide, depending on conditions. 2. Catalyst effects, involving reaction of the combustion products to form sulfates and sulfides, which bond to the catalyst itself. These catalyst poisons physically block catalytic sites and reduce efficiency. Such poisoning, however, is a reversible phenomenon, so that typically a catalyst will be operate at steady state as the car goes through normal driving cycles. 3. OBD effects, wherein catalyst sensor poisoning can lead to OBD reaction delay time, further increasing emissions by causing a mismatch between what the OBD sees as the proper air/fuel ratio and the actual operating conditions. Further, as the OBD systems are calibrated using fuels with a particular sulfur range, operation of the vehicle with fuels outside of that sulfur range may lead to false positive warnings from the OBD system. Quantitatively, the effect of sulfur levels in fuel was studied in two wide-ranging programs during the mid-1990s: the U.S. Auto/Oil program and the European Program on Emissions, Fuels and Engine Technologies (EPEFE). Within the Auto/Oil program, a decrease of fuel sulfur from 466 to 49 ppm gave the following emissions changes: HC decreased 13.9 % CO decreased 11.4 % NOx decreased 8.3 % The EPEFE obtained similar results, whereby a decrease of fuel sulfur from 382 to 18 ppm gave the following emissions changes: HC decreased 8.4 % CO decreased 8.6 % NOx decreased 10.1 % In the U.S., regulatory activity has been designed to bring the fuel sulfur levels down in a stepwise fashion from a current maximum of 1000 ppm (500 ppm for Reformulated Gasoline) to a level of 30 ppm by the year 2006. Why are octane ratings lower at high altitude areas? Lower octane number is required for carbureted engines at high altitudes because the lower air density results in lower combustion pressures and temperatures, the fuel/air ratio becomes richer due to the lower air density, and the spark advance is less due to lower manifold vacuum. Knock sensors and altitude compensators in fuel-injected engines have lowered the octane requirement reduction at increasing altitude. Studies show an average altitude difference of 0.2 and 0.5 (R+M)/2 per 1000 ft (300 m). Consumers may experience slight power and acceleration reductions.
