Mogas in Aviation

Bill,

The supposition sounds reasonable, and the experiment would be interesting.  Challenge accepted.  Currently we have summer fuel here with an RVP of 8, so the results may not be as revealing as if I started with winter fuel. I’m thinking the summer fuel has a lot less butane to boil off, but let’s see what happens.  

Sean,

My aircraft is a low wing (wing tanks) with a 912iS, so vaporization is only likely on the suction side of the pump.  After the pump the fuel is at 43 psi and circulating at around 28 gallons/hour, so not likely to vaporize. I have experienced a loss of power on a hot day using a high RVP fuel after the plane had set in the sun for several hours. This was caused by the fuel vaporizing on the suction side of the pump, causing the pump to cavitate. The cavitation is audible as well.

I did an experiment with avgas in one tank and winter mogas in the other.  I left the aircraft in the sun for four hours then ran the fuel pumps on each tank (aircraft not running). The avgas tank provided 44 psi and quiet pump operation, while the winter fuel caused a noisy pump (light cavitation) and only 41 psi.  

Last year I installed a fuel temp sensor, which reads out on my G3X, and the results are quite interesting.  I have found that it takes only a couple hours on the ramp on a hot day to have fuel in the tanks over 110 deg f., and can easily reach 120 with a bit more time. The fuel quickly gets hotter than the outdoor temperature because of solar radiation heat through the aircraft skin.  On climb out, the cooling rate of the fuel is slow, initially less than 1 deg f per minute.  In full climb, I can reach 5000’ with the fuel only 5-8 degrees cooler than it started.  Once up in the cooler air the cooling rate increases. There is some heat contribution from the engine compartment as the fuel is returned to the tank, but on my aircraft that seems somewhat inconsequential as compared to solar heating of the fuel before takeoff.  Of course, vaporization in a carbureted aircraft (with its low pressure fuel system) has a very different set of factors than an injected aircraft. 

As far as fuel temps go, I always presumed that the fuel in the fuel rail of the injected engines never spent enough time near the engine to warm any measurable amount.
At 43 psi and 30 gal/min, the flow through the 1/4" fuel rails will have the fuel leaving the pumps back in the tank in less than 10 seconds.
Not enough time to collect much heat at all.
The fuel in the engine compartment may become heat soaked while parked, but those few ounces of fuel will be back in the fuel tank before the engine is even started.
The fuel tank's mass and surface area will be the temperature-controlling elements.

Now the older carb'd engines without a return line are a different story.
The only way out for the fuel is through the carbs.
Any hot fuel in the lines will have to wait its turn to slowly get to the carbs.
The plus side of Carbs is that they tend to run cold from the evaporating fuel.
It is a plus only to the point where Carb Heat is needed.
At least a Carb Vapor lock is not a thing.

- - -

When I built my plane, I temporarily used a clear hose for the return line to view the flow.
The fuel exiting the pressure regulator had thousands of tiny microbubbles similar to what you see when opening a soft drink bottle.
About a foot downstream, they merged into proper respectable bubbles on their way back to the fuel tank.

I have no experience with petrol fuel injection systems. The following is speculation.

My limited understanding of physics would suggest that raising the pressure of a liquid (petrol) will also raise the temperature.

I also understand that increasing the pressure, will increase the boiling point, reducing the chance of vaporisation (a good thing).

If this warmer/hot petrol is then further exposed to heat (engine) the fluid temperature will rise further.

The unused, hot, fuel will return to the tank, warming its contents. In so doing the pressure is reduced, to ambient and the boiling point reduced. increasing the chance of vaporisation.

It seems to me that any tendency for the tank fuel to be warmed should be avoided.

None of this presents as an issue when ambient temperatures are low, as the volume of petrol in the tank will likely be such that the rise in temperature is inconsequential and much of the fuel delivery system itself will tend to cool the petrol within..

Sean

The bubbles in the return line of the injected engines are do to the rapid decompression of the fuel as it leaves the high pressure side of the regulator at 43 psi and moves to the low pressure side at about 1 psi. This is expected, and the 912iS install manual instructs that the return line must enter the fuel tank such that these bubbles are not drawn back into the fuel supply line, and are instead vented out.

Watching the fuel temperature during flight, I have not seen any noticeable heating of the fuel in the tank from the recirculated fuel. The ambient conditions seem to be the determining factor.  

Jeff Blakeslee wrote:

Sean

The bubbles in the return line of the injected engines are do to the rapid decompression of the fuel as it leaves the high pressure side of the regulator at 43 psi and moves to the low pressure side at about 1 psi. This is expected, and the 912iS install manual instructs that the return line must enter the fuel tank such that these bubbles are not drawn back into the fuel supply line, and are instead vented out.

Watching the fuel temperature during flight, I have not seen any noticeable heating of the fuel in the tank from the recirculated fuel. The ambient conditions seem to be the determining factor.  

Sounds sort of reasonable to me Jeff.

I find it hard to believe, that there is no contribution, to fuel heating, from the FI system and that your "bubbles."are not in part due to heat as well as decompression.

I would also point out the "............bubbles are not drawn back into the fuel supply line, and are instead vented out" must have the effect (how much?) of reducing the concentration of volatile fractions within the fuel, therefore negatively impacting on its burn qualities.