posted September 04, 2001 10:29 AM
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.
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.
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.