Our "gascolator" is a combination fuel strainer/sediment bowl, and I
have heard that the screen in it is of a size that will pass gasoline
but not water. Presuming this true, If all water is not removed from
the fuel system before flight and the gascolator bowl becomes filled
with water fuel will cease to flow to the carburetor.
Such in-flight power loss would NOT be "vapor lock", but the pilot
tasked with suddenly having to select a suitable place to land really
doesn't care. From the standpoint of risk management, however, the
distinction is worthwhile.
The EAA's "Firewall Forward" book states that "Gascolators are
susceptible to the effects of heat from the co-located engine and
exhaust components, which can cause fuel vapourization in the
gascolator and vapour lock problems. This usually manifests itself as a
partial power loss in cruise flight." Our "gascolator" is a
combination fuel strainer/sediment bowl. When it is mounted on or
adjacent to the carburetor there is no "high point" where vapor lock
can occur.
When it is mounted low on the firewall, as is the case with F1A, Alon
and M10 fuel systems, the carburetor float bowl becomes a "high point"
in the fuel system. Fuel vapourization in the gascolator could
ultimately cause the float chamber in the carburetor to partially or
completely drain, causing fuel starvation in the engine. Even
temporary disruption of fuel supply into the float chamber is not
ideal; most carburetors are designed to run at a fixed level of fuel in
the float bowl and reducing the level will reduce the fuel to air
mixture delivered to the engine.
Wikipedia has, in part, this to say:
"Gravity feed fuel systems are not immune to vapor lock...if vapor
forms in the fuel line, its lower density reduces the pressure
developed by the weight of the fuel. This pressure is what normally
moves fuel from the tank to the carburetor, so fuel supply will be
disrupted until the vapor is removed, either by the remaining fuel
pressure forcing it into the float bowl...or by allowing the vapor to
cool and re-condense.
Vapor lock has been the cause of many[quantify] forced landings in
aircraft. That is why aviation fuel (AVGAS) is manufactured to far
lower vapor pressure than automotive gasoline (petrol). In addition
aircraft are far more susceptible because of their ability to change
altitude and associated ambient pressure rapidly. Liquids boil at lower
temperatures when in lower pressure environments."
If fuel vapor entering the carburetor float bowl is vented via the
intake system, the mixture is, in effect, enriched; creating a mixture
control issue.
Wikipedia also, in part, had this to say:
"Vapor lock was...common in older gasoline fuel systems incorporating a
low-pressure mechanical fuel pump driven by the engine, located in the
engine compartment... Such pumps were typically located higher than the
fuel tank, were directly heated by the engine... Fuel was drawn under
negative pressure from the feed line, increasing the risk of a vapor
lock developing between the tank and pump..."
In our systems, such "vapor lock" would not interrupt gravity flow of
fuel to the carburetor from the fuselage tank, but would only
intermittently disrupt the fuel pressure long enough to briefly
decrease the rate of replenishment of the fuel level in the fuselage
tank by the fuel pump. Our fuel systems allow such fuel vapor
delivered into the fuselage tank to condense back to the liquid phase,
either in the fuselage tank or in the process of excess fuel entering
the return standpipe and returning back to the wing tank(s). "Vapor
lock" of this sort would not affect our engine's operation.
Perhaps others can shed more light on this subject?
Regards,
William R. Bayne
.____|-(o)-|____.
(Copyright 2010)
--
On Aug 12, 2010, at 23:24, Wayne Woollard wrote:
Oftentimes with the F1's,A2's, and models with the larger gascolator
mounted lower on the fire wall, a vapor lock is misinterpreted as the
culprit when just plain old air gets into the system after carburetor
work, fuel leak repairs and running the header tank out of fuel. Air
can be purged from the fuel system by bleeding fuel at the input to
the carburetor.
I have had it happen on more than 1 occasion so I know it to be true.
WW
D. Wayne Woollard CPBE
o--iii--(
