Bibek Baral
Performance and emissions of internal combustion engines are sensitive to the fuel used as they are designed to run on fuels with certain specifications. When an engine is run on a fuel with altered specifications, the performance and emissions of the engine may deteriorate and the durability and reliability of the engine components may be affected. Fuel adulteration is often a surreptitious operation in which a higher priced fuel, such as gasoline, is mixed with other cheaply available hydrocarbon fuels or solvents. This changes the composition and physical properties of the base fuel, and use of such fuels often results in reduced drivability of vehicles and increased tailpipe emissions. This problem is pervasive throughout South Asia and has also been reported in Greece, Brazil and African countries.
The basic reason for fuel adulteration is the financial benefit obtained from differential taxes imposed on different fuels. In many developing countries including Nepal gasoline is more expensive than diesel and kerosene because gasoline is taxed while diesel and kerosene are subsidised. Kerosene is used by moderate to low income people for cooking and lighting purposes and so, after subsidy, it is significantly cheaper compared with gasoline. The price difference between these two fuels is the main reason behind one of the most common forms of fuel adulteration, i.e. blending kerosene with gasoline. The other factor for this type of adulteration is the easy availability. Other types of gasoline adulteration prevalent in Nepal include mixing of gasoline with gasoline boiling range industrial solvents such as toluene, xylene and other aromatics, or light hydrocarbons such as pentanes and hexanes (rubber solution) which carry insignificant tax. Diesel adulteration, which includes mixture of kerosene and used lubricants with diesel is also equally ubiquitous. However, gasoline-kerosene adulteration would be more harmful in terms of emissions and damage caused to the spark ignition (SI) engines than the diesel adulteration would do to compression ignition (CI) engines and its emissions. Fuel adulteration is usually done by the operators of ‘for hire’ vehicles who do not own the vehicles, and, according to media reports, also by some of the public distribution system operators, and the fuel transporters.
Blending kerosene with gasoline will primarily result in a fuel with heavier hydrocarbon components contributed by the kerosene and thus a fuel with reduced volatility. It particularly elevates the middle and final evaporation temperatures by the introduction of heavier hydrocarbons in the kerosene. On the other hand, mixing small amounts of hydrocarbon solvents such as toluene and xylene with gasoline would not significantly affect the evaporation characteristics of the gasoline, because of the fact that they have boiling points in the range similar to gasoline components and some even occur in gasoline itself. However, these solvents do spike the fuel with an excessive amount of certain types of hydrocarbons. Both of these factors, i.e., change in gasoline volatility as well as the increase in certain class of hydrocarbon component, especially aromatic hydrocarbon, play key roles in the emission of spark ignition (SI) engines (gasoline type engines). It has been established from various studies that increase in molecular weight of the fuel, and hence decrease in volatility of the fuel, increases total hydrocarbons (THC), and particulate matter (PM) emissions from spark-ignition engines. Besides, increases in certain classes of hydrocarbon in the fuel have been found to be associated with an increase in THC and PM emissions as well as polycyclic aromatic hydrocarbons (PAH). These emissions, as shown by epidemiological studies, pose serious risk to public health and the environment. Some hydrocarbons, especially PAH, particularly are known carcinogens. Results from a series of experimental studies conducted by the author concluded that an adulterated blend containing 20% of kerosene produces 2 to 3 folds more THC, PM and PAH emission. The results are really alarming and if immediate action is not taken against the clandestine fuel adulteration operation, the general public will be exposed to a lot higher levels of such harmful pollutants. Situation is getting even worse by ever increasing number of vehicles in Kathmandu Valley and gasoline paucity which is promoting increased tendency of its adulteration by kerosene.
In addition to the emissions, the gasoline adulterated with kerosene would make the fuel more susceptible to knock, an abnormal combustion phenomenon in spark ignition engines. When knock is severe and persistent, it may damage the engine components including piston rings and lands, cylinder head gasket, and piston crown. Experimental studies showed that at certain engine condition when the engine fuelled with gasoline operates without knocking, a 10 percent kerosene blend would cause more than fifty percent of engine cycles knocking, and a 20 percent kerosene blend would cause almost all of the engine cycles knocking. Also at certain engine operating condition, the knock intensity resulting from 20 percent blend is more than 5 times greater than 10 percent kerosene blend whereas with 100 percent gasoline the knock intensity is zero. This indicates that prolonged running with adulterated fuel will seriously damage the engines. The consequences of fuel adulteration range from environmental to economic. The increased emissions resulting from the use of the adulterated fuel has a direct environmental consequence. However, there may be indirect consequences as well. Kerosene, which is the basic fuel for cooking and lighting intended for lower income people, is misused in the transport sector, thus depriving those people of their daily cooking and lighting fuel. This may compel people to use inefficient biomass stoves as an alternative causing higher level indoor pollution. The economic consequence of fuel adulteration is the loss in tax due to the large scale channelling of subsidised kerosene to the transport sector. As adulterated fuel may affect the durability of engine components, for example, due to knock, prompting the engine components to be replaced earlier than their usual operating life, this is also an economic loss.
To conclude, adulterated gasoline which has adverse consequences on environment and economy has to be limited by any means. For this, the government agencies including the Ministry of Supplies, the state-owned Nepal Oil Corporation and Nepal Bureau of Standards should consider the matter more seriously and come up with proper strategy to deal with the furtive fuel adulteration practices. The consumers should also be careful and choose more reliable fuel supplier. After all, it comes to their own well-being.