What Is The Quickest Way To Dilute Leaded Fuel?
By MUNGAI KIHANYA
The Sunday Nation
Nairobi,
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 phaseout 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.
