FUEL FLOW AND CAPACITY
Now that the decision to change to fuel other than petrol has been made, the
first thing to look at is the fuel tank. If of fiberglass, bear in mind the
new fuels act as solvents and most petrol resisting paints, shellac, varnish,
ethyl cellulose, cellulose nitrate and soft Bakelite suffer in contact, not
forgetting sealing compounds such as Bostic, Hermatite, Osotite and similar
leak stoppers.
If the tank resists on test, do bear in mind that if, at a later date, you
propose using Nitro Methane you will have to test still further as this acts
as a solvent on many resins, polyvinyl acetate, acetylchloride, chlorinated
rubber and low boiling hydrocarbons.
The obvious way to test is to deposit a little of the fuel on the tank surface
and see if it reacts, bearing in mind it may attack the paintwork and not
the actual material of the tank itself, so do not get misleading results.
If the tank is of metal construction, particularly of aluminum, it should
be anodised, thus stopping chemical reaction causing a white deposit to form
tending to clog fuel lines and carburetor parts that come in contact.
If of steel and tin plated, the fuel will tend to take off the tinplate and
form a deposit on other metal parts in the fuel system.
Washing out the entire system is sometimes carried out with petrol to stop
this deposit building up too much.
Bear in mind alcohol will descale material unaffected by petrol and it is
advisable to wash out and clean the whole tank first with a small amount of
fuel, to make sure you start clean, and to frequently inspect it to keep in
such condition.
In passing it might also be worth consideration that at least one well known
carburetor uses a plastic float that gets more than a little upset with fuel
and another uses plastic cut-off valves in the float chamber which also object.
FUEL CAPACITY
At this stage it is necessary to work out how much fuel you need to carry
and at what flow rate it will have to leave the tank.
There is no point in carrying more fuel than required, since, apart from the
weight, you are just increasing the fire risk.
The rate of flow will establish the diameter of the outlet pipe or pipes,
and a point often missed, the diameter of the breather hole, usually incorporated
in the cap. This last point does not apply if the tank is pressurised.
Having decided on the amount and the rate of flow, you have to consider the
cut-off valve and the fuel lines themselves.
in the case of a small engine the minimum bore diameter anywhere in the system
should be 6mm., and for the rest 13mm. and the fuel lines should have, at
worst, that diameter, preferably up to twice the diameter to reduce friction,
and be made of Polythene, Neoprene or other alcohol resisting material.
Do not fall into the trap of using fuel lines of these diameters and use,
at their termination's, unions which restrict the actual effective diameters
to much less. The ideal set-up is where the internal diameter right throughout
the whole system to the carburetors is of the same diameter, providing that
diameter is large enough to reduce friction to a sensible minimum.
FUEL FLOW
Now check the actual flow right down to the fuel line that supplies the carburetor
or fuel block.
If gravity feed is used this is simple to do, but remember to check with the
tank at the same height as used, and time the flow and quantity.
If the tank is pressurized, for your own interest, check with the cap open,
then under pressure and the difference will surprise you, also how quickly
the air pressure drops, more so if the air space over the fuel be smaller.
If the tank supplies some form of fuel pump, remember the pump diaphragm will
have to be of Neoprene or it will dissolve.
Electrically driven pumps are easy to check, but those driven by the engine
itself present a problem.
If the makers' figures are available all is well, but if not you will have
to establish the actual quantity of fuel pumped per stroke, then from the
rate at which the pump is operated, calculate the actual flow rate.
In all cases the rate should be at least twice the estimated maximum demand
of the engine at peak requirement.
The major obstruction will usually be found in the cutoff valve of the carburetor
float chamber and although some manufacturers can supply valves modified to
increase the flow at this point, they usually do not allow enough and you
will have to fabricate your own.
Remember here by increasing the diameter of the orifice by 40 per cent you
will double the flow of fuel.
Do check however the flow rate through the valve and make certain it is enough
. . . so many fall into this simple trap.
The figure of twice the required maximum demand rate may sound excessive,
but bear in mind apart from the sudden demand, you have to force the fuel
against the actual acceleration of the car or bike.
One final comment before leaving the fuel lines and means of getting the fuel
to the engine itself.
Do check that the fuel cut-off valve, when in the open position, is in fact
fully open, and having done that check, if the tank has a breather, that fuel
cannot spill out and possibly be blown back onto or into the engine, or for
that matter the driver.
TWICE AS MUCH
The actual amount of fuel required by the engine will be double the amount
it consumed when on petrol, but if you are starting from scratch so to speak,
the a mount is determined by the amount of air consumed by the engine, which
in effect is directly related to its capacity at full throttle.
With regard to a supercharged engine the amount required will be dependent
on the boost pressure, so that for every 15 lb. approximately of boost, that
is the amount over atmospheric pressure, as would be indicated on the gauge,
you will have to consider the actual effective capacity increased by the same
amount as the basic engine capacity.
For example a 1000 cc. engine running at a boost pressure of 15 lb. would
be regarded as of 2000 cc. and the same engine running at some 30 lb. pressure
would work out at 3000 cc. and proportionally for other boost pressures.
