The removal of alkyl lead compounds certainly reduces the toxicity of exhaust gas emissions when used on engines with modern engine management systems and 3-way exhaust catalysts. If unleaded gasolines are not accompanied by the introduction of catalysts, some other toxic emissions may increase. Engines without catalysts will produce increased levels of toxic carbonyls such as formaldehyde and acrolein when using oxygenated fuels, and increased levels of toxic benzene when using highly aromatic fuels.
There is little doubt that gasoline is full of toxic chemicals, and should therefore be treated with respect. However the biggest danger remains the flammability, and the relative hazards should always be kept in perspective. The major toxic risk from gasolines comes from breathing the tailpipe, evaporative, and refuelling emissions, rather than occasional skin contact from spills. Breathing vapours and skin contact should always be minimised.
Unfortunately, the manufacturers of alkyl lead compounds have embarked on a worldwide misinformation campaign in countries considering emulating the lead-free US. The use of lead precludes the use of exhaust catalysts, thus the emissions of aromatics are only slightly diminished, as leaded fuels typically contain around 30-40% aromatics. Other toxins and pollutants that are usually reduced by exhaust catalysts will be emitted at significantly higher levels if leaded fuels are used .
The use of unleaded on modern vehicles with engine management systems and catalysts can reduce aromatic emissions to 10% of the level of vehicles without catalysts . Alkyl lead additives can only substitute for some of the aromatics in gasoline, consequently they do not eliminate aromatics, which will produce benzene emissions . Alkyl lead additives also require toxic organohalogen scavengers, which also react in the engine to form and emit other organohalogens, including highly toxic dioxin . Leaded fuels emit lead, organohalogens, and much higher levels of regulated toxins because they preclude the use of exhaust catalysts. In the USA the gasoline composition is being changed to reduce fuel toxins ( olefins, aromatics ) as well as emissions of specific toxins.
The major question about RFGs is not the toxicity of the emissions, but whether they actually meet their objective of reducing urban pollution. This is a more complex issue, and most experts agree the benefits will only be modest [18,19,20,61,62].
Cars currently use gasoline that is derived from fossil fuels, thus when gasoline is burned to completion, it produces additional CO2 that is added to the atmospheric burden. The effect of the additional CO2 on the global environment is not known, but the quantity of man-made emissions of fossil fuels must cause the system to move to a new equilibrium. Even if current research doubles the efficiency of the IC engine-gasoline combination, and reduces HC, CO, NOx, SOx, VOCs, particulates, and carbonyls, the amount of carbon dioxide from the use of fossil fuels may still cause global warming. More and more scientific evidence is accumulating that warming is occurring [64,65]. The issue is whether it is natural, or induced by human activities and and a large panel of scientific experts continues to review scientific data and models. Interested reader should seek out the various publications of the Intergovernmental Panel on Climate Change (IPCC). There are international agreements to limit CO2 emissions to 1990 levels, a target that will require more efficient, lighter, or appropriately-sized vehicles, - if we are to maintain the current usage. One option is to use "renewable" fuels in place of fossil fuels. Consider the amount of energy-related CO2 emissions for selected countries in 1990 .
CO2 Emissions ( tonnes/year/person ) USA 20.0 Canada 16.4 Australia 15.9 Germany 10.4 United Kingdom 8.6 Japan 7.7 New Zealand 7.6The number of new vehicles provides an indication of the magnitude of the problem. Although vehicle engines are becoming more efficient, the distance travelled is increasing, resulting in a gradual increase of gasoline consumption. The world production of vehicles (in thousands) over the last few years was ;-
Region 1990 1991 1992 1993 1994 ======================= ====== ====== ====== ====== ====== Africa 222 213 194 201 209 Asia-Pacific 12,064 12,112 11,869 11,463 11,020 Central & South America 800 888 1,158 1,523 1,727 Eastern Europe 2,466 984 1,726 1,837 1,547 Middle East 35 24 300 390 274 North America 7,762 7,230 7,470 8,172 8,661 Western Europe 13,688 13,286 13,097 11,141 12,851 Total World 37,039 34,739 35,815 34,721 36,289<>
Region 1990 1991 1992 1993 1994<> ======================= ====== ====== ====== ====== ====== Africa 133 123 108 101 116 Asia-Pacific 5,101 5,074 5,117 5,057 5,407 Central & South America 312 327 351 431 457 Eastern Europe 980 776 710 600 244 Middle East 36 28 100 128 76 North America 4,851 4,554 5,371 6,037 7,040 Western Europe 1,924 1,818 1,869 1,718 2,116 Total World 13,336 12,701 13,627 14,073 15,457<>To fuel all operating vehicles, considerable quantities of gasoline and diesel have to be consumed. Major consumption in 1993 of gasoline and middle distillates ( which may include some heating fuels, but not fuel oils ) in million tonnes.
Gasoline Middle Distillates USA 335.6 233.9 Canada 25.0 24.4 Western Europe 166.0 264.0 Japan 56.4 89.6 Total World 802.0 989.0<>The USA consumption of gasoline increased from 294.4 (1982) to 335.6 (1989) then dipped to 324.2 (1991), and has continued to rise since then to reach 335.6 million tonnes in 1993. In 1993 the total world production of crude oil was 3164.8 million tonnes, of which the USA consumed 787.5 million tonnes . Transport is a very significant user of crude oil products, thus improving the efficiency of utilisation, and minimising pollution from vehicles, can produce immediate reductions in emissions of CO2, HCs, VOCs, CO, NOx, carbonyls, and other chemicals.
Beginning with the 1975 model year, most new car models were equipped with catalytic exhaust treatment devices as one means of compliance with the 1975 legal restrictions in the U.S. on automobile emissions. The need for gasolines that would not adversely affect such catalytic devices has led to the large scale availability and growing use of unleaded gasolines, with all late-model cars requiring unleaded gasoline.".
There was a further reason why alkyl lead compounds were subsequently reduced, and that was the growing recognition of the highly toxic nature of the emissions from a leaded-gasoline fuelled engine. Not only were toxic lead emissions produced, but the added toxic lead scavengers ( ethylene dibromide and ethylene dichloride ) could react with hydrocarbons to produce highly toxic organohalogen emissions such as dioxin. Even if catalysts were removed, or lead-tolerant catalysts discovered, alkyl lead compounds would remain banned because of their toxicity and toxic emissions [70,71].
The health risks to service station workers, who are continuously exposed to refuelling emissions remain a concern . Vehicles will soon be required to trap the refuelling emissions in larger carbon canisters, as well as the normal evaporative emissions that they already capture. This recent decision went in favour of the oil companies, who were opposed by the auto companies. The automobile manufacturers felt the service station should trap the emissions. The activated carbon canisters adsorb organic vapours, and these are subsequently desorbed from the canister and burnt in the engine during normal operation, once certain vehicle speeds and coolant temperatures are reached. A few activated carbons used in older vehicles do not function efficiently with oxygenates, but carbon cannister systems can reduce evaporative emissions by 95% from uncontrolled levels.
Tuning a carburetted engine can only have a marginal effect on pollutant levels, and there still had to be some frequent, but long-term, assessment of the state of tuning. Exhaust catalysts offered a post-engine solution that could ensure pollutants were converted to more benign compounds. As engine management systems and fuel injection systems have developed, the volatility properties of the gasoline have been tuned to minimise evaporative emissions, and yet maintain low exhaust emissions.
The design of the engine can have very significant effects on the type and quantity of pollutants, eg unburned hydrocarbons in the exhaust originate mainly from combustion chamber crevices, such as the gap between the piston and cylinder wall, where the combustion flame can not completely use the HCs. The type and amount of unburnt hydrocarbon emissions are related to the fuel composition (volatility, olefins, aromatics, final boiling point), as well as state of tune, engine condition, and condition of the engine lubricating oil . Particulate emissions, especially the size fraction smaller than ten micrometres, are a serious health concern. The current major source is from compression ignition ( diesel ) engines, and the modern SI engine system has no problem meeting regulatory requirements.
The ability of reformulated gasolines to actually reduce smog has not yet been confirmed. The composition changes will reduce some compounds, and increase others, making predictions of environmental consequences extremely difficult. Planned future changes, such as the CAA 1/1/1998 Complex model specifications, that are based on several major ongoing government/industry gasoline and emission research programmes, are more likely to provide unambiguous environmental improvements. One of the major problems is the nature of the ozone-forming reactions, which require several components ( VOC, NOx, UV ) to be present. Vehicles can produce the first two, but the their ratio is important, and can be affected by production from other natural ( VOC = terpenes from conifers ) or manmade ( NOx from power stations ) sources [62,63]. The regulations for tailpipe emissions will continue to become more stringent as countries try to minimise local problems ( smog, toxins etc.) and global problems ( CO2 ). Reformulation does not always lower all emissions, as evidenced by the following aldehydes from an engine with an adaptive learning management system .
FTP-weighted emission rates (mg/mi) Gasoline Reformulated Formaldehyde 4.87 8.43 Acetaldehyde 3.07 4.71<>The type of exhaust catalyst and management system can have significant effects on the emissions .
FTP-weighted emission rates. (mg/mi) Total Aromatics Total Carbonyls Gasoline Reformulated Gasoline Reformulated Noncatalyst 1292.45 1141.82 174.50 198.73 Oxidation Catalyst 168.60 150.79 67.08 76.94 3-way Catalyst 132.70 93.37 23.93 23.07 Adaptive Learning 111.69 105.96 17.31 22.35<>If we take some compounds listed as toxics under the Clean Air Act, then the beneficial effects of catalysts are obvious. Note that hexane and iso-octane are the only alkanes listed as toxics, but benzene, toluene, ethyl benzene, o-xylene, m-xylene, and p-xylene are aromatics that are listed. The latter four are combined as C8 Aromatics below .
Aromatics FTP-weighted emission rates. (mg/mi) Benzene Toluene C8 Aromatics Gas Reform Gas Reform Gas Reform Noncatalyst 156.18 138.48 338.36 314.14 425.84 380.44 Oxidation Cat. 27.57 25.01 51.00 44.13 52.27 47.07 3-way Catalyst 19.39 15.69 36.62 26.14 42.38 29.03 Adaptive Learn. 19.77 20.39 29.98 29.67 35.01 32.40<> Aldehydes FTP-weighted emission rates. (mg/mi) Formaldehyde Acrolein Acetaldehyde Gas Reform Gas Reform Gas Reform Noncatalyst 73.25 85.24 11.62 13.20 19.74 21.72 Oxidation Cat. 28.50 35.83 3.74 3.75 11.15 11.76 3-way Catalyst 7.27 7.61 1.11 0.74 4.43 3.64 Adaptive Learn. 4.87 8.43 0.81 1.16 3.07 4.71<> Others 1,3 Butadiene MTBE Gas Reform Gas Reform Noncatalyst 2.96 1.81 10.50 130.30 Oxidation Cat. 0.02 0.33 2.43 11.83 3-way Catalyst 0.07 0.05 1.42 4.59 Adaptive Learn. 0.00 0.14 0.84 3.16<>The author reports analytical problems with the 1,3 Butadiene, and only Noncatalyst values are considered reliable. Other studies from the Auto/Oil research program indicate that lowering aromatics and olefins reduce benzene but increase formaldehyde and acetaldehyde