Walt wrote:
Because of the lower vapor pressure of auto gas, (sometimes assisted with chemicals for winter blends), the fuel can boil in the fuel lines. This boil is known as vapor lock. This is bad news for an airplane since it cuts off the fuel supply to the engine. One method to increase the vapor point temperature is to increase the pressure of the fuel. The mechanical fuel pump at the engine is sucking (lowering pressure) to get fuel into the cowl up to the pump. By putting an electric pump at the tank to supply the mechanical pump means positive pressure on the fuel entering the cowl and less chance for vapor lock. Another measure used on these planes is to return unused fuel to the tank so there is constant fuel circulating and the fuel inside the cowl doesn’t get heat soaked. AV gas, 100LL has a much higher boiling point and is also susceptible to vapor lock.
I have experienced vapor lock in flight, best described as a sag in power. What I am having is felt like a bump, or multiple bumps.
Hi Walt,
I think you may be barking up the wrong fuel vaporization tree😀.
Fuel vaporisation in Rotax 9 engines is;
Primarily a ground related problem, after flight, engine shutdown, and is experienced when you have difficulty getting your hot engine to start and run smoothly- continue running - taxi - sometimes in the TO role / early climb out , there will be a hesitation/loss of power, occasionally engine failure.
Conceivably it could also occur, on a very hot day, during a prolonged taxi/hold.
It is caused by convective & radiant heat, from the recently run/shutdown engine/exhaust system, heating the fuel lines, above the engine, to the point where the fuel boils/vaporises within.
It is very unlikely (still possible) for it to happen in flight - the fuel & air flow, is usually sufficient to cool the system.
To my understanding the following procedures are the only effective prevention;
Fit a fuel return line, located as close to carburettors as possible. This usually involves a 4–5 point distribution manifold (or similar). This will allow the bulk of vapour, to be purged from the fuel system and maintain & continuous flow of fuel, that will cool itself (See Rotax recommendations).
Fitting insulating/fire resistant sleeves, to your fuel lines, will help by slowing the penetration of the heat/boiling the fuel.
In conditions that are likely to cause fuel vaporisation;
Park your aircraft facing into wind, to try & maintain an, under cowl, air flow.
Open cowling doors / vents, to encourage air flow.
Use your Boost pump before attempting to start the engine, until a positive / acceptable, fuel pressure is registered (you may hear the pump "rattle" noise change). This will assist in purging vapour from the system.
You may find that having the Boost pump on during starting/cranking, helps to get the engine going. Continued use of the Boost pump, while on the ground will assist in circulating cooling fuel and purging vapour. I would not switch off the Boost pump until Cruise is achieved.
The alternative to the above, is to allow the engine (& fuel system) to cool to near ambient.
If fuel vaporization is suspected – conduct an extra-long pre TO run up, to allow cool fuel to enter the system and purge vapour. Have an “action plan” should your engine loose power at any stage of the TO/Climb-Out.
"One method to increase the vapor point temperature is to increase the pressure of the fuel." Correct in theory, unlikely to have much effect in practise. The pumps, usually used in small aircraft, will not deliver sufficient pressure, to increase the boiling point of the fuel to the point where it will not vaporise, from the heat of the engine.
In most installations, the fuel lines that are vulnerable to vaporisation, are those that go from the mechanical pump to the fuel distributor, then to each carburettor.
The pressure in these lines is principally from the mechanical pump. You can prove this by switching on/off the Boost pump and observing only a small rise/fall in pressure.
“The mechanical fuel pump at the engine is sucking (lowering pressure) to get fuel into the cowl up to the pump. By putting an electric pump at the tank to supply the mechanical pump means positive pressure on the fuel entering the cowl and less chance for vapor lock”
In my experience, the fuel delivery line, from tank to mechanical pump, only rises to pass over a short section of the engine – it is much less subject to the heat generated by the recently running engine. There may be a small benefit in using your Boost pump to pressurise it, however the proceeding suggestions have a far greater chance of success.
I have experienced fuel vaporisation/lock on the ground, never in the air. On one occasion I had to abort my TO role, taxi back & have another go.
Note: I fly in Australia - except for some relativly small areas, we do not have winter/summer fuel blends that further complicate the issue - so do not suffer the problems that our friend in the northern hemisphere complain of. We do however get summer temperatures into the mid 40C range.😉
