What Is The Quickest Way To Dilute Leaded Fuel?


The Sunday Nation


04 December 2005


The Kenya Petroleum Refineries stopped producing leaded fuel on November 30 2005. As a result, from 1st December, the country will begin phasing out the dreaded lead additive. Many motorists have expressed concern over the effect of mixing leaded with unleaded fuel during the phase-out period.

Although mechanical engineers have explained that there will be no problem, mechanics are running a scare campaign advising all and sundry to “modernise” their vehicles. Consequently the question in many peoples minds is which is the quickest way to dilute away the old fuel. Let’s find out.

Suppose you have been using leaded fuel and that you consume a full tank of 50 litres every ten days, that is five litres per day. Assume further that on 30th November you had just filled your tank with the leaded petrol. Then, by the evening of December 10, your car will have a reserve of, say, one litre remaining in the tank

When you fill up with unleaded fuel, your tank will have a mixture comprising about 2 percent leaded fuel. That is, the one litre remaining divided by the 50L capacity of the tank. This will last another ten days to December 20 when again there will be about one litre in the tank.

Now, two per cent of this one litre is leaded, that is, there is only 20 millilitres of leaded petrol in the tank (four teaspoons!). When you fill up  on December 20, you will have a total of 50L of which 20ml will be leaded. That is 0.04 percent - 20ml divided by 50L.

In another ten days (December 30), you again have about one litre remaining in the tank. 0.04 percent of this will be leaded, that is 0.4ml (less than a drop). When you fill up again, the concentration of leaded fuel will be 0.0008 percent – 8 parts per million.

Thus in one month, you will have diluted the leaded fuel to an insignificant proportion (0.0008 percent) of the petrol in your tank. But what if, like many Kenyans, you cannot afford to fill up the tank in one go? How long the dilution process take?

Suppose that you still consume 5L every day and on the first day you have one litre of leaded fuel remaining in the car. On the second day you add 5L to make 6L in total. The mixture in the tank is therefore 17 percent leaded – one litre divided by 6L. By the end of that day, there is one litre remaining of which 170ml (17 percent of one litre) is leaded.

On third day you add another 5L to make 6L again and this time the mixture is now 2.8 percent leaded (170ml divided by 6L). Following this sequence, it easy to see that on fourth day the concentration of leaded fuel would be 0.5 percent, and at the end of one week it would be 0.0004 percent.

Thus it turns out that if add small amounts at a time, you will dilute away the leaded fuel faster than filling the whole tank at one go. The question that remains now is this: will all the leaded petrol (each and every molecule of it) ever get cleared from the system? The jury is still out on that one.

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