Loss of Power on Takeoff, believe Vapor Lock

[erikpmort]

After my engine out I went back to av gas. Not worth the risk.

[jmbrwn]

Jon- I was at the Vidalia Airshow Sunday and saw your landing. Good job on getting her down!! I also have a carbed RV6. If you were taking off on 100% mogas, IMO, you definitely experienced vapor lock. Easy to duplicate the same conditions, except this time don't takeoff...just run up the power and see what happens. Then switch to other tank with 100LL and after a few minutes to let that fuel enter carb, run up power again and see what happens.
I experienced the same thing while on a XC and had to get Mogas because that was all that was available. On next fuel stop, engine stumbled right after I got the tail up. Luckily, was able to abort. Taxied back, and after a few minutes to let 100LL enter carb, did full power run-up. No problem. Took off successfully and no problems since. Just my opinion.

[DanH]

How can a standard engine driven fuel pump provide pressure above that provided by its mainspring? Can someone please explain the mechanics?

Jon, where is your fuel pressure transducer located? The usual tee off the engine pump outlet fitting?

[caryr]

I have to agree with Dan. The Mechanical FP is only a lever sliding on an eccentric lobe that pushes a on a diaphram and chamber with check valves.

It can't over pressure

[Mike H]

Originally Posted by DanH

How can a standard engine driven fuel pump provide pressure above that provided by its mainspring? Can someone please explain the mechanics?

Jon, where is your fuel pressure transducer located? The usual tee off the engine pump outlet fitting?

Good question Dan. I have not personally seen this failure mode with a mechanical pump on an aircraft, but the local FBO told me that had this scenario on a Cherokee 140 once where a new pump fixed the issue. I have read of similar reports in the past as well.

I have experienced multiple AC Delco mechanical fuel pump failures on small block Chevy engines in my old truck. One of these failed pumps would cause intermittent erratic pressure causing fuel starvation most of the time, but did cause the float bowls to overflow and cause power loss and run so rich that the exhaust looked like an over fueled diesel truck.

[petehowell]

Todd has forgotten more on this topic than I will ever know.....

Pete:

I read through the comments on the website your link led me to, it? pneumatic lock, no question about it. The trailing black smoke confirms that the engine was running too rich. I cannot exactly explain the mechanics of what is happening with the float and needle other than to say they are overpowered by the foam much as the float would be overpowered by a fuel pump that gave too much pressure.

In a pneumatic lock the fuel boils when it hits the carb, it foams up and pegs out the pressure gage, and the engine dies because it?s too rich. Fuel at 85F is at the most crucial temp for vapor and/or pneumatic lock. The better an airplane performs the more critical these issues are and with the RV?s performance I cannot say I am surprised.

In an untested non-STC?d airplane like the RV, if one is going to use automotive gasoline then the autogas should be in one tank for cruise and another tank with avgas in it should be used for takeoff and landing.

Feel free to post this letter on the Vans Air Force forum.

Regards,

Todd L. Petersen
Petersen Aviation, Inc.
984 K Road
Minden, NE 68959
308-832-2200
autofuelstc.com

[DanH]

Ahhh, but how can a pump be the problem?

IIRC, the mechanical force of the pump lever enlarges the diaphragm chamber, and fuel enters the pump. Squeezing the diaphragm chamber is a function of a spring, not lever force. The size of the spring sets fuel pressure.

I can only think of one way to generate pressure higher than provided by the pump spring, and that would be vapor pressure. However, seeing 11 psi or more on a fuel pressure gauge tapped into the pump-to-carb line would require a blocked carb inlet (the fuel pump's inlet check valve would prevent reverse flow). The blockage could be the result of trash, or in this case, because the float bowl was full enough to firmly close the float needle valve. My conclusion is that the high fuel pressure was probably an after-the-event item, not a pump issue.

[petehowell]

Gary Z. had some interesting thoughts here that seem to apply and were very helpful to me with hot starts:

(Text copied from linked post)

Not all hot starts are the same. I check the fuel pressure before starting to decide whether I'll use plan B or plan C for starting. Plan A is for cold starts.

Plan B: Check fuel pressure and if below 5 psi a normal warm start ensues; Mixture to about the middle of travel, throttle cracked about 1/2 inch, left mag on, press the starter button. Starts every time, right mag now on.

Plan C: Check fuel pressure and if 5 or above a hot start ensues; Mixture to ICO, throttle cracked about 1/2 inch, left mag on, press starter button. After several blade swings the engine will start. As soon as the engine starts the right mag is switched on. Keep the engine running with the mixture only, Do NOT TOUCH the Throttle. After it starts push the mixture in to keep it running. It may try to die at this point so the mixture goes to ICO to keep it from quitting, advance the mixture when it catches. This is a mixture dance to keep it running until the fuel pressure drops below 5 psi and then the boost pump is turned on and the mixture set for taxi.

Fuel pressure is my clue. The fuel is boiling somewhere and causing a super rich mixture if the pressure is 5 or above. I have seen it as high as 7 or 8 psi and when that high it takes a longer dance with the mixture to keep it running. Once the pressure stabilizes to normal values the engine runs like it was a warm start.

I suspect that the fuel is boiling the carb bowl which floats the float to better seal the needle valve allowing the high pressure reading in the fuel line. The boiling fuel in the bowl may spill some into the carb throat coupled with the fuel vapor causes the rich mixture. Hence, the mixture at ICO for the hot start.

YMMV

[AZtailwind]

Originally Posted by Ken Harrill

PeteHowell has it right....As a result of many such experiences, I now use avgas only in the right tank, and I land and takeoff on that tank....

I second the above for another data point....

[Todd L. Petersen]

The fuel "boils" when it hits the carburetor. In the fuel lines it's under pressure from the pump, plus it's trying to boil off due to the fuel temp and vapor pressure, it can't do so until it gets to the carb, and the carb is so hot that the fuel goes to foam when it enters. Think of a warm can of coke, shake it and open it and when it hits atmospheric, it explodes. This is not unlike what happens when an airplane develops pneumatic lock. Fuel pressure pegs out and the engine either surges or dies because it's too rich. Constantly adjusting the mixture can help you limp to an airport. The critical thing here is the vapor pressure of the fuel which is less than in times past, but still higher than aviation gasoline. Spring is the time of year when it's most likely to be a problem. 85F and 110F are critical fuel temperatures, and 85F is more critical than 110F because the light ends start cooking off at that temp. By the time you've reached 110F most (but not all) of the ingredients that cause vapor lock have boiled off.