General
The standard fuel system as directed by the cozy plans is relatively simple but,
in my opinion, suffers some minor annoyances. For this reason and because of the
need to modify the system to accommodate the electronic fuel injection system, I
set about redesigning the system.
Minor Problems & Irritations
- In the stock system the vent is arranged at the forward / top portion of
each strake fuel tank. In the parked position with the nose down and with fuel
in the tanks, the fuel immerses the end of the vent line. During the evening
and night periods the fuel tank cools and the air above the fuel contracts
drawing air into the tank through this vent point. when the morning comes the
temperature rises causing an increase in pressure which ultimately forces the
fuel out of the vent line since it is in a submerged state. To overcome this
problem pilots tend to leave their tanks in a semi-empty state until they
require fuel to fly, whereupon they add fuel. Most pilots are instructed, from
the onset of flight training, that the tanks should be kept full as possible
to prevent the ingress of water through the diurnal temperature cycling.
- The stock design uses a fuel arrangement in which the fuel is discharged
from the fuel tanks at one of the rearmost points, the fuel must then travel
forward through the cabin to a point between the pilot and co-pilot. At this
point there is a fuel valve followed by a long passage back through the
firewall, through the electric fuel pump and ultimately on to the mechanical
fuel pump where pressure is added to the system for feed to the carburettor.
The length of fuel line is significant and may result in vapour locks with low
vapour pressure fuels such as automotive fuel.
- There are numerous arguments for and against having a fuel selector valve
Vs an on off valve and there is no resolution nor will there ever be a
resolution for this debate. I learnt to fly in a Cessna 150 which had a single
on off valve which I liked very much. I then owned a Piper Warrior which had
the fuel selector valve with which I never felt completely happy. I would
never run a single tank dry just in case the valve would not switch over
correctly, so now I was always carrying extra unusable fuel. I never felt
comfortable switching tanks at less than 3000 feet, which, if you have ever
flown in England, is not always possible because of low cloud or the
restricted airspace. In general the single ON/OFF arrangement of the Cessna
150 was attractive to me.
The Vent System
The vent system incorporated in my aircraft is indicated in the diagrams below.
It is a little more complex than most systems but it is intended as a foolproof
system. The driving approach resulted from the need to accommodate the
automotive recirculation fuel injection system. The automotive industry now,
without exception, use electronic fuel injection systems of one form or another.
The systems use high pressure, typically 35 - 40 p.s.i. above manifold pressure,
fuel rails that are regulated by a controlled discharge of fuel back to the fuel
tank. If a fuel selector valve is used, then it must also provide for the excess
fuel return. These valves are available but they are expensive and do not solve
all of the problems.
My Aircraft is equipped with retractable landing gear which eats into the fuel
tank volume. I elected to recover some of the lost fuel by adding a sump tank in
the location normally used to support the fixed landing gear. This space yields
approximately six to eight gallons. This sump tank also provides a location for
submerged fuel pumps like those found in most automobiles. This position for the
fuel pump will also accept the fuel return. The fuel returning from the fuel
regulator is full of vapourised fuel as a result of the rapid change in pressure
at the regulator. It is necessary to exhaust this vapour.
In the configuration illustrated above the vapour moves to one or both of the
two 'ears' of the sump tank. A 3/8" vent line is connected to this point. The
two vent lines come together in a simple manifold which has a 1/2" vent pipe
connected to the vent box at the top of the firewall.
So let's describe what happens here:
The sump is full of fuel and the strake tanks are part filled and at nominally
the same level as each other because of the interconnectivity between the three
tanks. Fuel will also be partway up the 1/2" vent line going to the vent tank.
As vapour, resulting from the regulator valve, is dumped back to the sump, it
finds its way to one of the two sump "ears" The vapour will try to escape
through either the balance line or to one of the strake fuel tanks. In the ideal
level flight condition, the head of fuel on the side in which the vapour is
located, causes the vapour to move towards the TEE manifold through a
circulatory motion thereby balancing the pressures. As the bubbles pass by the
TEE manifold some of the vapour moves into the 1/2" vertical standpipe.
Experimentation has shown that vapour bubbles will then pass by the fuel in this
large diameter pipe allowing the vapour to escape to the vent box. (Note:
smaller 1/4" pipes will not allow the vapour to pass by the fuel and making the
vent ineffective.)
The two vents found in each fuel tank accommodate the venting needs for both the
nose down parked condition, and the nose high climbing condition. Any excess
fuel that is exhausted through the vent lines enter the vent box at the top of
the box. The liquids fall to the bottom of the box and are drained through the
1/2" stand pipe that feeds the sump.
One of the problems with the stock vent system so far aft is that with full fuel tanks
and a very high nose attitude, fuel can be discharged from the vent line. To
overcome this problem the final vent line is passed forward in the cabin then
folded backwards to through the firewall where it is fed down through the bottom
cowling. Using this configuration the fuel should never be exhausted overboard,
and in the unlikely eventuality that it is to be discharged overboard it will
not discharge on the top side of the aircraft. The figure below indicates this
final configuration.
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