Leaded versus unleaded gasoline

-Hari Lal Rajbahak

Amidst the rising din around the environmental concerns and vehicular pollution and in the ensuing confusion generated by accusation and counter-accusations, sometimes, totally innocent are put in the same pedestal with real culprits, swiftly condemned and quietly convicted. If excessive smoke is the direct results of the misdeed of the bad vehicles, but for the presence of lead, in the vehicular emissions, vehicles, owners, operators and repair personnel are totally innocent and should not be blamed at all. Then who are the real culprits for the deadly and hazardous lead in the tail pipe emission? In fact, the culprits are none other than our rentless pursuit of efficiency from a motor vehicle. We want more fuel-efficient vehicles with higher power outputs from the same engine as well as gasoline at cheaper prices.

Lead pollutant in vehicular emission

Vehicles emit deadly lead components through its tail pipes but only if we fill gasoline tanks with gasoline that have been mixed with lead compound. The simple fact is, we feed the lead to vehicle engines and that engine gives out the lead in the form of tail pipe emission. The quantity of lead so omitted is almost equal to the quantity of lead put into gasoline tanks. The vehicle engines neither digest nor create the lead, therefore the lead polluter is not the vehicles but gasoline that is fed into the engines. If we could remove or stop adding the lead in gasoline, the problem of lead pollution from the vehicles will simply disappear without any fuss. One thing that is sure is that the lead compounds are added not in terms of commercial adulteration by the unscrupulous trader but deliberately mixed at refinery as an additive in order to enhance the octane rating of gasoline so that compression ratios of an engine could be raised. All the thermo-dynamic theories say that higher the compression ratios better is thermal efficiency of an engine.

Implications of higher compression ratios

If one is going to own an automobile, one should go after the engine with higher compression ratios. If fuel economy is the sole criteria, it is imperative to choose diesel fuel engine, in place of gasoline engines, because the diesel engine by design itself should have higher range of compression ratios as the fuel has to be self ignited by the heat generated by compression of air (compression engine) unlike gasoline engines in which the spark plugs are used to ignite the fuel inside the combustion chamber.  For running diesel engines, power from batteries is not required for ignition purposes.  The direct inference is that diesel fuelled vehicles are more economical to operate than gasoline engines.  It is to be noted that the ex-refinery prices of gasoline and diesel are almost the same. But the present disparities of prices are the result of cross subsidy to make the diesel artificially cheaper for a consumer. If cross subsidies are removed, then the prices of diesel and gasoline will almost be the same as in many other countries.

Knocking or detonation and lead in gasoline

The prices we are paying for fuel efficiency in gasoline engines is the very undesirable phenomenon of knocking or detonation. Practically, knocking can be observed in a high compression gasoline engine under certain circumstances such as when a driver tries to accelerate in low gear, or octane rating of gasoline is below par. In such cases an audible metallic sound of a knock or detonation is heard. In reality, it is a spontaneous and uncontrolled burning of gasoline without the help of sparks from spark plugs.  It is not the normal burning of fuel but an explosion that occurs inside the engine, giving rise to shock waves similar to those generated by supersonic aircraft or radical vibration of gaseous products producing an audible knock or detonation. It is accompanied by dense smoke, high thermal load on piston - cylinder group, and if allowed to continue for longer period, may damage piston and cylinder and break them up as well.

In order to gain efficiency and fuel economy, the compression ratio is increased, but the “natural gasoline” can not cope with the ensuing phenomena of knocking or detonation.

It has been found that the straight rum gasoline has octane number between 65-85 and that of cracking gasoline has octane number between 55-70. During the refining of the crude oil by fractional distillation - the most common method of obtaining gasoline, the top fraction is gasoline and it is called straight rum gasoline where as cracking is another method of obtaining gasoline by breaking down large and complex molecules of hydrocarbon into lighter and simpler compound with lower boiling points. Reforming is yet another method of obtaining gasoline from higher compounds of hydrocarbon and the cracking gasoline the reformed gasoline and straight rum gasoline have different octane numbers but none of those or their combination can meet the demand in octane rating required by modern engines.

Why was lead added to gasoline?

In order to suppress the knocking tendency, there are many options and compounds to choose, but from non oxygenate groups, the tetraethyl lead was found to be relatively inexpensive and simple compound as an anti knocking agent and was widely used giving rise to ‘leaded gasoline’. Evidently, in order to obtain the given octane rating of gasoline, different quantity of lead additives were mixed with ‘pure gasoline’ depending upon the ‘source’ of gasoline obtained.

Why unleaded gasoline?

It would not be prudent to search a motor vehicle which does not emit the smoke at all. The best what one can strive is to ensure that the smoke does not contain the excessive quantity of harmful gases and the pollutants like carbon particles, carbonmonooxide, hydrocarbons. Probably, a universal measure, that can be adopted and that are being adopted in many other countries, to mitigate the pollution problems from a motor vehicles, is to fit a catalytic converter between the exhaust manifold and the tailpipe of a vehicles.

Some two years back, after IOC in Delhi, NOC also started to provide unleaded gasoline from one gasoline pump in Kathmandu, thus introducing unleaded gasoline in Nepal for the first time. NOC authorities explain that octane rating of the new gasoline was enhanced by MTBE.

The perils of unleaded gasoline

Especially in the area of vehicular emission pollution, there seems to be much noise than real talk, more rumor in circulation than the facts. The astute watcher must be aware of the fact that no serious effort has been made in Nepal to study the pollution problem from motor vehicles by government or non-governmental organization except one UNDP funded Pilot Project (Kathmandu Valley Vehicular Emission Control Project) some 5 years back, when some base line data were generated and the preliminary assessment was made about the pollution problem from vehicular emission in Kathmandu valley.

The catalytic converter is the most reliable and surest device invented so far to solve the pollution problem in motor vehicle. But it is not without any weakness either as it cannot tolerate the lead compound. If normal working life of a good converter is 100000-200000 km, the use of leaded gasoline will cut short it working life abruptly to a few hundred kilometres. Even in the best design and ideally constructed motor engine in which the gasoline-hydrocarbon compound is burned to produce mechanical power, the best one can expect to get as unavoidable ‘smoke’ is carbon dioxide and water vapour.

One of the serious stakeholders in vehicular pollution in Nepal is NOC - simply jumped into the fray. While announcing the introduction of lead-free gasoline in Kathmandu, it issued a stern statement that the new avatar of gasoline should be used only in those motor vehicles, which are fitted with catalytic converters. Instead, the statement, at that time should have informed, the general public that with newly introduced lead-free gasoline, those willing to fit a catalytic converter in order to reduce the pollutant like CO and HC. Now, the NOC is distributing unleaded gasoline in Nepal without mentioning anything about catalytic converters.

The leadless gasoline presently distributed in Nepal has double benefit of directly reducing the lead contamination of air and soil and allows to control effectively other vehicular pollutant like CO and HC by using catalytic converters. The valve seat recession problem is not a serious concern with very limited impact. The concern regarding aromatics like Benzene has never been substantiated and is not widespread requiring serious attention. Despite a few concern from some quarters wide spread use of unleaded fuel, even in vehicles without catylyctic converters, is to be popularised.