RESEARCH

Prevention Measures

AUTOMOBILE RELATED LEAD EMISSIONS

Professor B. S. Murthy

Introduction

In this international gathering of Scientists from multi- disciplinary areas and Policy makers/ Administrators, a part of the conference time is understandably devoted to automobile related problems of lead poisoning. Though, in terms of quantity, the total lead from automotive emissions to the environment may be only second in comparison to the lead output from other sources, qualitatively lead emission from automobile exhaust is causing more concern.

This presentation gives a background of lead additive to gasoline, its effects and methods of elimination from gasoline in developed countries. Unfortunately, in bulk of developing countries, leaded gasoline is still the preferred practice. Compounded with this practice, uncontrolled population of two- and three- wheeled vehicles powered by two-stroke cycle engines is causing alarming problems as they directly short-circuit raw gasoline to exhaust together with the unburned fuel in the cylinder. This aspect is emphasized in this lecture. Urgent steps, which are to be brought to the notice of policy makers, are highlighted in the concluding part of this lecture.

Birth and Death of Lead as Fuel Additive in Developed Countries

Credit of discovery of lead as a fuel additive, if one is permitted to use this word in a complementary manner, goes to the American scientist Thomas Midgley in 1921! Those were the days when gasoline formulation was such that the engine would not tolerate compression beyond a certain limit. On sparking the mixture the engine would start screaming if the mixture was squeezed beyond a limit during compression stroke. In technical parlance, this was called knocking due to auto-ignition of the unburned mixture without control. The engines had to work on low compression-ratios resulting in loss of power and excessive gasoline consumption.

Thomas Midgley tried various additives to decrease the onset of knocking and permit higher compression ratios by trying different additives including red paint. At this stage he chanced to discover that a heavy metal like lead implanted in two molecules of ethane resulted in a heavy liquid, greenish in color with hardly perceptible odor of fresh fruit-- indeed the forbidden fruit today! This compound is called tetra ethyl lead (TEL). Added in small quantities, knock resistance increased surprisingly and this was designated by an index called octane number (ON). Substantial increase in compression ratios was possible with savings of millions of gallons of gasoline.

This started a rat race. Engine designers aimed at higher and higher compression ratios with refineries mandated to produce higher octane gasoline by increase in TEL content. Now the trouble started. People were at that time not aware of the health of humans due to lead poisoning. On the contrary, engine users noticed build up of lead deposits on the walls of the combustion chambers resulting in the deterioration of the health of the engines. These deposits would glow at high temperatures and again start surface ignition with knocking. A viscious circle indeed! This called for further increase in ON using more additive, called octane rating improvers (ORI). Further additives were required to get rid of deposits and these were called scavengers. Phosphorus (called antogonist) was also added to modify the deposits. In the meanwhile, the smog problem in Los Angeles gave awareness of automotive exhaust pollution to the environment. This marked the beginning of formulation of emission control laws and subsequent enforcement. Mandatory fitment of exhaust catalytic converters around 1975 in USA sounded the death-knell to the use of lead additives to gasoline. With deposition of lead in the converter poisoned the catalyst surface and made it ineffective, it became essential that lead is removed from gasoline.

Alternatives to Lead as Antiknock Additive

Discovery of TEL opened up the field of organo metallic chemistry applicable to engine combustion. After all, what trick does the lead do to silence the knocking combustion? Combustion chemists found that dispersion of fine metals like lead offered millions of sites to suppress the formation of free radicals (OH) and atomic species (H), which are responsible for uncontrolled auto-ignition. Can other metals less harmful do the same trick? Manganese or iron? How to implant them in organic molecules?

Following these inquiries, in 1951 the English scientists Pauson and Kealy succeeded in synthesizing an organo-iron compound by sandwiching iron between two atoms of cyclopentadienels. Consequent to this success in metallo-organic chemistry, scientists synthesized many compounds of which manganese implant between cyclopentadienel and tri-carbonyl radical (MMT of CMT) became a significant compound as it proved to be a good anti-knock additive. This could replace lead but in its place manganese would be discharged to atmosphere. So one had to think of a technology independent of metallic additives.

Advancement in alcohol fuel technology lead the scientists to use oxygenates as blends to suppress knocking. Eventually this practice could completely eliminate lead additives. Tertiary butyl alcohol (TBA), Methyl tertiary butyl ether (MTBE) and ethyl tertiary butyl ether became some of the successful blending compounds. Though the vapor pressure of oxygenates may lead to increase in evaporative emissions and tendencies for vapor-locking problems, suitable formulations eliminated this trouble. MTBE in particular became a good choice as blending agent.

Lead was thus phased out of gasoline in US and Europe in mid 70s. Other industrialized countries followed suit and MTBE came to rescue as a successful blending agent as this can be manufactured by using methanol or petroleum feed stocks. Very recently, however, there was a scare in California from reports of MTBE leakage from storage tanks, and due to its high solubility in water, it could mix with ground water sources. So care is to be exercised in building leak-proof storage tanks. Lead is no doubt phased out but in its place some other evil seems to sneak in.

Special Problems Associated with Lead Additive in India

Presently all new passenger cars plying in major cities in India are mandatorily fitted with catalytic converters. So partially lead is phased out for new cars in big cities as long as they use unleaded gasoline. But old-model passenger cars and two- and three-wheeled vehicles (most of them powered by two-stroke engines) are running on leaded gasoline.

Of immediate concern is the consumption of the fuel by two and three wheelers. It is estimated that 70% of the total consumption of petrol is by two- and three- wheelers, majority of which are fitted with two-stroke engines. As catalytic converters are not yet a mature technology for two-stroke engines, it is unlikely that catalytic converters are feasible or effective to these classes of engines. It should be remembered that nearly 25-30% of raw gasoline is lost directly to exhaust. The TEL associated with this portion is directly discharged to atmosphere. The remainder will go through the combustion phase with deposits in the chamber walls and the rest to atmosphere as oxide of lead. The combustion is dirty in these engines as lubricating oil is mixed with the fuel. All these problems, compounded with the extra cost of catalytic converters of doubtful efficiency and life, are to be addressed before fitting converters to two and three wheeled vehicles.

The adulteration of petrol with kerosene is a major source of threat to environment. If this is done for unleaded fuel, the catalytic converter will be poisoned. Finally, the congested traffic, with lack of proper road infra structure and traffic regulation, contributes to stop and go driving which is associated with high discharge of unburned gasoline with associated lead.

Crucial Recommendations and Priorities

For developing countries like India the lead poisoning of atmosphere is associated with control of automotive exhaust emissions. The following priorities are suggested:

(a) Refineries to be modernized so as to phase out lead from gasoline as early as possible.

(b) There should be no two opinions regarding early phasing-out of two stroke engines concurrent to the efforts to phase-out lead from gasoline. Our two stroke engine manufacturers are doing excellent research and development work to minimize short circuiting of raw fuel and getting at performance figures as close to those of four stroke counterparts. So during transition from two to four stroke engines, these R & D efforts are commendable inputs. Moreover many leading industries are manufacturing four stroke two-wheelers. This effort must be speeded up.

© No developed country (to the best of the knowledge of the writer) has introduced catalytic converters to their motor cycles, excepting offering to manufacture them for developing countries. Unless these technologies are tested, proved and accepted for their domestic consumption, care must be exercised in importing this technology only for the developing countries.

(d) Four stroke engines are costlier, heavier and have less throttle response for acceleration and high speeds. But high speed is not required for city driving under the Indian driving conditions for reasons of safety. High cost can be compensated by better fuel economy of the four stroke engines.

(e) Regarding the higher cost, an incentive in the form of lower taxes can be given to the buyer of four-stroke powered scooter or two/three wheelers. Society has to be rewarded by the incentive for keeping the environment clean.

(d) Improving road infrastructure and management of traffic to reduce congestion will also reduce excess burning of gasoline.

(e) Rigorous steps are needed for the enforcement of pollution control. However, these controls and laws are of no use unless the above steps are fulfilled. It is only after this, that careful norms suitable to India are to be formulated and enforced. There seems to be competition in making stringent emission laws at par with highly industrialized countries. Unfortunately, there is no competition whatsoever in fulfilling the crucial factors that limit emissions as listed in (a) to (d).

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