Gasoline FAQ

Contents:


  4.14) Is gasoline toxic or carcinogenic?

There are several known toxins in gasoline, some of which are confirmed human carcinogens. The most famous of these toxins are lead and benzene, and both are regulated. The other aromatics and some toxic olefins are also controlled. Lead alkyls also require ethylene dibromide and/or ethylene dichloride scavengers to be added to the gasoline, both of which are suspected human carcinogens. In 1993 an International Symposium on the Health Effects of Gasoline was held [53]. Major review papers on the carcinogenic, neurotoxic, reproductive and developmental toxicity of gasoline, additives, and oxygenates were presented, and interested readers should obtain the proceedings. The oxygenates are also being evaluated for carcinogenicity, and even ethanol and ETBE may be carcinogens. The introduction of oxygenated gasoline to Alaska and some other areas of the USA resulted in a range of complaints. Recent research has been unable to identify additional toxicity, but has detected increased levels of offensive smell [54]. It should be noted that the oxygenated gasolines were not initially intended to reduce the toxicity of emissions. The reformulated gasolines will produce different emissions, and specific toxins must initially be reduced by 15% all year.

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.


  4.15) Is unleaded gasoline more toxic than leaded?

The short answer is no. However that answer is not global, as some countries have replaced the lead compound octane-improvers with aromatic or olefin octane-improvers without introducing exhaust catalysts. The aromatics contents may increase to around 40%, with high octane unleaded fuels reaching 50% in countries where oxygenates are not being used, and the producers have not reconfigured refineries to produce high octane paraffins. In general, aromatics are significantly more toxic than paraffins. Exhaust catalysts have a limited operational life, and will be immediately poisoned if misfuelled with leaded fuel. Catalyst failure can result in higher levels of toxic emissions if catalysts or engine management systems are not replaced or repaired when defective. Maximum benefit of the switch to unleaded are obtained when the introduction of unleaded is accompanied by the introduction of exhaust catalysts and sophisticated engine management systems.

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 [55].

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 [55]. Alkyl lead additives can only substitute for some of the aromatics in gasoline, consequently they do not eliminate aromatics, which will produce benzene emissions [56]. Alkyl lead additives also require toxic organohalogen scavengers, which also react in the engine to form and emit other organohalogens, including highly toxic dioxin [57]. 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.


  4.16) Is reformulated gasoline more toxic than unleaded?

The evidence so far indicates that the components of reformulated gasolines ( RFGs ) are more benign than unleaded, and that the tailpipe emissions of hydrocarbons are significantly reduced for cars without catalysts, and slightly reduced for cars with catalysts and engine management systems. The emissions of toxic carbonyls such as formaldehyde, acetaldehyde and acrolein are increased slightly on all vehicles, and the emission of MTBE is increased about 10x on cars without catalysts and 4x on cars with catalysts [55]. When all the emissions ( evaporative and tailpipe ) are considered, RFGs significantly reduce emissions of hydrocarbons, however the emissions of carbonyls and MTBE may increase [55]. There has been an extensive series of reports on the emissions from RFGs, produced by the Auto/Oil Air Quality Improvement Research Program, who measured and calculated the likely effects of RFG [18,19,20,58,59,60,61]. More research is required before a definitive answer on toxicity is available.

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].


  4.17) Are all oxygenated gasolines also reformulated gasolines?

No. Oxygenates were initially introduced as alternative octane-enhancers in the 1930s, and are still used in some countries for that purpose. In the US the original "oxygenated gasolines" usually had a slightly- modified gasoline as the hydrocarbon fraction. The US EPA also mandated their use to reduce pollution, mainly via the "enleanment" effect on engines without sophisticated management systems, but also because of the "aromatics substitution" effect. As vehicles with fuel injection and sophisticated engine management systems became pervasive, reformulated gasolines could be introduced to further reduce pollution. The hydrocarbon component of RFGs is significantly different to the hydrocarbon fraction in earlier oxygenated gasolines, having lower aromatics contents, reduced vapour pressure, and a narrower boiling range. RFGs do contain oxygenates as the octane-enhancer, but have different hydrocarbon composition profiles [34,41,42,43,44].


Chapter 5) Why is Gasoline Composition Changing?


  5.1) Why pick on cars and gasoline?

Cars emit several pollutants as combustion products out the tailpipe, (tailpipe emissions), and as losses due to evaporation (evaporative emissions, refuelling emissions). The volatile organic carbon (VOC) emissions from these sources, along with nitrogen oxides (NOx) emissions from the tailpipe, will react in the presence of ultraviolet (UV) light (wavelengths of less than 430nm) to form ground-level (tropospheric) ozone, which is one of the major components of photochemical smog [63]. Smog has been a major pollution problem ever since coal-fired power stations were developed in urban areas, but their emissions are being cleaned up. Now it's the turn of the automobile.

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 [66].


                              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                        7.6 
The 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 [67];-


    5.1.1) Cars


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<>

    5.1.2) Trucks (including heavy trucks and buses)


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 &amp; 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 [68]. 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.


  5.2) Why are there seasonal changes?

Only gaseous hydrocarbons burn, consequently if the air is cold, then the fuel has to be very volatile. But when summer comes, a volatile fuel can boil and cause vapour lock, as well as producing high levels of evaporative emissions. The solution was to adjust the volatility of the fuel according to altitude and ambient temperature. This volatility change has been automatically performed for decades by the oil companies without informing the public of the changes. It is one reason why storage of gasoline through seasons is not a good idea. Gasoline volatility is being reduced as modern engines, with their fuel injection and management systems, can automatically compensate for some of the changes in ambient conditions - such as altitude and air temperature, resulting in acceptable driveability using less volatile fuel.


  5.3) Why were alkyl lead compounds removed?

" With the exception of one premium gasoline marketed on the east coast and southern areas of the US, all automotive gasolines from the mid-1920s until 1970 contained lead antiknock compounds to increase antiknock quality. Because lead antiknock compounds were found to be detrimental to the performance of catalytic emission control system then under development, U.S. passenger car manufacturers in 1971 began to build engines designed to operate satisfactorily on gasolines of nominal 91 Research Octane Number. Some of these engines were designed to operate on unleaded fuel while others required leaded fuel or the occasional use of leaded fuel. The 91 RON was chosen in the belief that unleaded gasoline at this level could be made available in quantities required using then current refinery processing equipment. Accordingly, unleaded and low-lead gasolines were introduced during 1970 to supplement the conventional gasolines already available.

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."[69].

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].


  5.4) Why are evaporative emissions a problem?

As tailpipe emissions are reduced due to improved exhaust emission control systems, the hydrocarbons produced by evaporation of the gasoline during distribution, vehicle refuelling, and from the vehicle, become more and more significant. A recent European study found that 40% of man-made volatile organic compounds came from vehicles [72]. Many of the problem hydrocarbons are the aromatics and olefins that have relatively high octane values. Any sensible strategy to reduce smog and toxic emissions will also attack evaporative and tailpipe emissions.

The health risks to service station workers, who are continuously exposed to refuelling emissions remain a concern [73]. 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.


  5.5) Why control tailpipe emissions?

Tailpipe emissions were responsible for the majority of pollutants in the late 1960s after the crankcase emissions had been controlled. Ozone levels in the Los Angeles basin reached 450-500ppb in the early 1970s, well above the typical background of 30-50ppb [74].

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 [75]. 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 [55].


                           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 [55].


                           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 [55].


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 [20]


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