Winter & Piston Engines

[GyvAir]

Well, I didn't fret too much about even though it really was quite galling at the time.

[pelmet]

OK, so overpriming can cause a hydraulic lock. If so, you are supposed to either pull the prop through each compression stroke but this is with spark pugs removed(have people really done that for a horizontally opposed engine).

So now the last questions. When should you suspect that you have primed so much that there could be a hydraulic lock?

Probably you have been cranking the engine over each time you primed. Wouldn't it take a very large amount of priming to actually have a lock?

[Colonel Sanders]

You can smell it, and see it running all over the pavement
underneath the airplane. Fuel hydraulicking occurs when
priming with the prop stopped. By definition, there must
be enough fuel in the cylinder to fill the combustion chamber.

[pelmet]

OK. Good info. But I would think that it is quite possible for fuel to be coming out and not noticed in some cases.

Also, I do remember on the 337 sometimes priming(fuel injected IO-360) what didn't seem like a lot of prime but discovering once that fuel was coming out almost immediately onto the ground. Which is concerning.

[iflyforpie]

This is where it is good to know your engine and how things all fit together. Most pilots have no idea about how the priming system works, and there are several different configurations that affect what it will do to the engine.

First, no primer that I am aware of goes right into the cylinder. Cars are just coming out with Direct Injection because it must withstand the very high pressures of compression and combustion. Most fuel injection and primers are port-injection... they are in the intake manifold or intake port on the other side of the intake valve. You cannot fill a cylinder from over priming alone on just any engine.

Next is the configuration of the intake manifold. On a parallel valve and non-turbocharged angle valve Lycoming, the intake manifold is underneat the cylinders. You can prime until the cows come home and you might not get it started, but you won't hydraulic it (unless perhaps it is a vertical installation in a helicopter). The O-200 and O-300 usually have a single primer nozzle at the bottom of one side of the intake manifold (so it will go to three cylinders).... again, pretty hard to hydraulic this engine, if it is possible at all.... the fuel just drains down into the carburetor and intake box.

But many engines have the intake manifold on the top. The TCM IO-360, for example, on the 337 and Maule M-5, has the intake manifold above the cylinders.... so fuel can run down through the open intake valves and into the cylinders. Same with the angle valve Lycoming TIO-540s. On a radial engine, the top cylinders are now a concern for fuel hydraulic lock.

Now, most of these engines have a manifold drain to allow the fuel to escape (which is why the 337 starts spitting fuel right from the get-go) but lots of these drains are neglected and gummed up with old fuel or debris.

[Colonel Sanders]

many engines have the intake manifold on the top

Right - this is why the Continental IO-520 has
such a problem with this. It has a cross-flow
cylinder head.

[iflyforpie]

Right - this is why the Continental IO-520 has
such a problem with this. It has a cross-flow
cylinder head.

Speak for yourself.. mine doesn't.

[photofly]

back of the envelope calculation:

IO470F 470 cu.in. with a compression ratio of 8.5:1.
Cylinder volume at TDC therefore 470/(8.5*231) = 0.24 gallons aggregated over 6 cylinders.

Fuel flow on priming about 10 gallons/hr as remembered by me from my fuel flow gauge.

Approximate time to fill one of the combustion chamber volumes = 60 * 0.24 / 10 = 1.4 minutes.

Assuming equal fuel distribution to all cylinders, and all fuel enters and remains in the cylinders, I'm in danger of a hydraulic lock somewhere after the first full minute of priming.

I don't think I've ever had to prime for more than 15 seconds in aggregate over two or three starting attempts.

[GyvAir]

back of the envelope calculation: ....... danger of a hydraulic lock somewhere after the first full minute of priming. I don't think I've ever had to prime for more than 15 seconds in aggregate over two or three starting attempts.

My thoughts on the subject as well, photofly; I'm almost not sure how it's achievable under even very loosely defined "normal circumstances". Although, I am aware of the human capacity for screwing something up with the determined application of enough ignorance.
I certainly don't have anywhere near universal experience with all the engine variations out there, but as a rule, I've found you get faster, cleaner starts if you can prime while cranking, keeping pre-crank primiing to a minimum or none at all. In doing so, the whole issue of possible fuel hydrolocking becomes a non-issue.

[Big Pistons Forever]

From day 1, I have primed every IO 520 Continental engine (cold start scenario) by setting mixture rich, throttle full and holding the fuel pump on until I get the second rise in the fuel flow. I then crank the engine and it always starts.

Personally I think this whole "fuel hydraulic-ing " thing is an urban myth.

[pelmet]

But many engines have the intake manifold on the top. The TCM IO-360, for example, on the 337 and Maule M-5, has the intake manifold above the cylinders.... so fuel can run down through the open intake valves and into the cylinders.

Now, most of these engines have a manifold drain to allow the fuel to escape (which is why the 337 starts spitting fuel right from the get-go) but lots of these drains are neglected and gummed up with old fuel or debris.

Well, I used to prime the IO-360 for several seconds until 60 lbs/hr fuel flow was indicated and then cranked over. Hardly seems like there was too much risk. If it didn't start then I primed again or maybe if I thought it was flooded then started with the mixture lean and throttle wide open which really worked well. Either way, the engine had gone through several compression strokes so it was fuel going into just emptied cylinders.

But, perhaps you have made the key point. If the drain(s) is/are blocked on certain engines with certain manifold designs and you prime a lot then perhaps that is the final answer on how a hydraulic lock can happen. Perhaps TCM could have identified which of their engines were more vulnerable when making their statement about potential damage. I'm going up in a 150 soon and was getting very worried .

Took a lot of questions to get to this point but persistence pays off sometimes. Thanks for the answers guys.

[Spandau]

Has anybody here had the opportunity to try out one of these little portable heaters?

http://www.aircraftspruce.ca/catalog/ep ... ckkey=9140

I was thinking about picking one up if I get around to putting my machine on skis this year.

[Colonel Sanders]

IO470F 470 ...
Fuel flow on priming about 10 gallons/hr...
Approximate time to fill one of the combustion chamber volumes = 60 * 0.24 / 10 = 1.4 minutes.

It's possible to put considerably more fuel in your
engine than 10 gph. Your inline electric fuel pump
is capable of delivering more than enough fuel to
run your engine at wide open throttle after the
mechanical pump has failed.

With a 230hp IO-470 that's going to be at least
23 gph. Probably a bit more, in fact, but let's go
with a pessimistic 23 gph.

So if a pilot primed your engine with the mixture
full rich and the throttle all the way forward, he
would deliver enough fuel to hydraulic lock in:

60 * 0.24 / 23 = 0.63 minutes

or 38 seconds. I suspect it's less than that because
your electric fuel pump will have a margin - you can
look up the specs for it, if you want.

But using your numbers, there is a risk of hydraulic
locking the cylinders with fuel after 30 seconds of
priming, resulting in bent connecting rods.

Anyone here willing to state that no pilot has ever
primed for 30 seconds?

[photofly]

It's possible to put considerably more fuel in your
engine than 10 gph. Your inline electric fuel pump
is capable of delivering more than enough fuel to
run your engine at wide open throttle after the
mechanical pump has failed.

With a 230hp IO-470 that's going to be at least
23 gph. Probably a bit more, in fact, but let's go
with a pessimistic 23 gph.

There are two fuel pump switch positions, regular and high. High is for full throttle applications (sometimes operated by a micro switch on the throttle control, but not in my case) and I always prime on the regular setting. The fuel flow meter measures pressure after the electric pump, so I believe it gives me the same kind of accuracy as when the engine pump is turning, and doesn't read over 10gph on prime (typically it levels out at 8 gph). I'll check the digital fuel flow next time I prime but I'm confident 10gph is about right.

[Colonel Sanders]

The fuel pump switch on the TCM IO-360 (CR 8.5:1)
I fly has just "on/off". At 210hp it's flowing around
21 gph, and with smaller combustion chambers. You
run the numbers, it's probably around the 30 second
mark to hydraulic lock.

PS Here is an inline Weldon fuel pump. I have 3 of them:

http://www.aircraftspruce.com/catalog/e ... ckkey=3583

They flow 35 gph MINIMUM, according to the specs.

[pelmet]

But using your numbers, there is a risk of hydraulic
locking the cylinders with fuel after 30 seconds of
priming, resulting in bent connecting rods.

Anyone here willing to state that no pilot has ever
primed for 30 seconds?

But would the earlier statement that this is a concern only if a drain is plugged correct(Of course how do you likely know if it is plugged).

[Colonel Sanders]

For each cylinder, when you are priming (into the
intake port) either the intake valve is open, or not,
if the engine is not cranking.

If the intake valve is open, the fuel can run into the
cylinder - whether the drain is plugged or not probably
won't make any difference. Additionally for this case
either the exhaust valve is open or closed. If the
exhaust valve is open, it provides a drain for the fuel
into the exhaust, which isn't a great place for the fuel,
but blowing off the exhaust is probably better than
bending a connecting rod. If the exhaust valve is
closed - and does not open again before the compression
stroke - then you have hydraulic lock, if the volume of
fuel in the cylinder is equal to or greater than the volume
of the combustion chamber, plus whatever dishing/notches
are in the piston.

If the intake valve is not open, then the drain matters.

[iflyforpie]

While all fuel pumps need to deliver a surplus fuel flow, most of it goes back to the fuel tank through the vapour return lines. You won't see the fuel pump's rated fuel flow going through the flow divider/manifold valve to the injectors.

[Colonel Sanders]

TCM fuel injection has return line. Lycoming does not.

[pelmet]

How do you like this wrap-up of various posts put together with a few amendments. Feel free to correct.

According to TCM "Over priming can cause a flooded intake resulting in a "hydraulic lock" event and subsequent engine malfunction or failure. If you over prime, or flood your engine, make certain that all fuel has drained from the intake manifold and/or cylinder prior to attempting engine starting."

Fuel hydraulicking occurs when priming with the prop stopped. By definition, there must
be enough fuel in the cylinder to fill the combustion chamber.

Most fuel injection and primers are port-injection. They send fuel to the intake manifold or intake port on the outside of the intake valve. For each cylinder, when the engine is not cranking, the associated intake valve will be in the open or closed position.

Next to consider is the configuration of the intake manifold. On a parallel valve and non-turbocharged angle valve Lycoming, the intake manifold is underneath the cylinders. You can prime as long as you like and while you might not get it started, but you won't hydraulic it (unless perhaps it is a vertical installation in a helicopter). The O-200 and O-300 usually have a single primer nozzle at the bottom of one side of the intake manifold (so it will go to three cylinders).... again, pretty hard to hydraulic this engine, if it is possible at all.... the fuel just drains down into the carburetor and intake box.

But many engines have the intake manifold on the top. The TCM IO-360, for example, on the 337 and Maule M-5, has the intake manifold above the cylinders.... so fuel can run down through any open intake valves and into the cylinders. Same with the angle valve Lycoming TIO-540s. On a radial engine, the top cylinders are now a concern for fuel hydraulic lock.

Most of these engines do have a manifold drain to allow the fuel to escape (which is why the 337 starts spitting fuel right from the get-go) but lots of these drains are neglected and gummed up with old fuel or debris.

If the intake valve is open, the fuel can run into the cylinder - whether the drain is plugged or not probably won't make any difference. Additionally for this case either the exhaust valve is open or closed. If the exhaust valve is open, it provides a drain for the fuel into the exhaust, which isn't a great place for the fuel, but blowing off the exhaust is probably better than bending a connecting rod. If the exhaust valve is closed - and does not open again before the compression stroke - then you have hydraulic lock, if the volume of fuel in the cylinder is equal to or greater than the volume of the combustion chamber, plus whatever dishing/notches are in the piston. For cylinders with the intake valve closed, the drain becomes important for removing excess fuel in the manifold.

So what is an indication that you could be at risk for a fuel hydraulic lock on an engine vulnerable to this condition?

Extended periods of priming between turning the engine over is the initial cause. You may be able to smell fuel and see it running all over the pavement underneath the aircraft.

So what to do if you suspect a hydraulic lock. Gently pull the prop by hand to see if there is a greater than normal compression resistance. If this is encountered, do not force the prop through and do not move it backwards. The pull through is not to clear it, its to check for it. When it comes to a hydraulic lock, the only proper way to remove fluid is to remove spark plugs and drain. It is said in some articles that pulling through by hand can create enough force to damage a connecting rod. Rotating the prop backwards is not done as it just pushes the liquid into the intake where it will simply re-emerge when you try to start. Pulling bottom plugs and turning normal direction is the way to clear a hydraulic lock.

[Colonel Sanders]

One caveat: I would add that with the mixture and
throttle all the way in, the electric fuel pump has the
flow rate to hydraulic lock a cylinder in as little as 30
seconds, when priming with the prop stopped.

Obviously any priming before cranking should be of
shorter duration than that, to avoid bending connecting
rods. I find it extremely odd that you never see that
in a POH/AFM or engine manufacturer handling notes.
Then again, I find a banana-shaped connecting rod
extremely odd, too.

I wonder if it's possible to damage a piston when
hydraulic lock occurs? The connecting rods (and
wrist pin) are going to be extremely beefy steel
alloy - perhaps even a 4340. The piston, meanwhile
is probably some brittle forged aluminum. It would
seem that the wrist pin forces would be immense
on the poor piston, too.

I doubt that Lycoming and TCM are using this technology
for pistons yet:

http://en.wikipedia.org/wiki/Hypereutec ... ent_alloys

But I might be wrong. After all, the high school dropout
throttle jockeys here don't think much of my engineering
capabilities.

[photofly]

I actually checked today. The fuel pump fuel flow stabilizes at 10.1 gph, after 7 seconds, during a pre-start prime, and by the time the figure had stabilized the engine was already flooded.

So I figure I'm safe from any kind of hydraulic lock for my original estimate of about a minute and half. But if seven seconds floods the engine I think that's a good margin.

Everyone else can make their own estimates, as they please.

I did encounter another winter flying danger though: don't try to kick out stuck chocks when they're frozen to the ground. I have a very badly bruised big toe.

[Colonel Sanders]

PF: Out of curiousity, does your POH/AFM
say to prime only on the low boost pump
position? Presumably if you primed using the
high boost pump position, it would take only
half the time to prime?

The 421 is the same - fuel boost pumps off (center)
down (low) or high (up). I remember checking a
nice young fellow out on a 421 and I told him to
turn the electric boost pumps off on the ground,
set them to low for takeoff, and only use high
after an engine failure.

Well, we line up for takeoff, and he selects up
(high) on the boost pumps, and both engines
immediately quit. He was so angry with himself,
he got this thousand yard stare thing going. Didn't
say a word.

I couldn't help myself. I laughed so hard I almost
swallowed my chewing gum and told him, "You'll
never make that mistake again!"

Learning factor of effect applies, but because people
like to spit on instructors, that doesn't matter.

[photofly]

PF: Out of curiousity, does your POH/AFM
say to prime only on the low boost pump
position? Presumably if you primed using the
high boost pump position, it would take only
half the time to prime?

I believe it specifies to use the "low" position until the fuel flow rises to 8gph, then to shut off the pump. The engine is installed under an STC, and the AFM supplement is not exactly expansive on the subject. It does say the pump is NOT to be used, on either setting, for takeoff or during normal flight.

[Shiny Side Up]

The 421 is the same - fuel boost pumps off (center)
down (low) or high (up). I remember checking a
nice young fellow out on a 421 and I told him to
turn the electric boost pumps off on the ground,
set them to low for takeoff, and only use high
after an engine failure.

Well, we line up for takeoff, and he selects up
(high) on the boost pumps, and both engines
immediately quit. He was so angry with himself,
he got this thousand yard stare thing going. Didn't
say a word.

If I remember, TCM specifically warns about this, I think the POH for the TU206 does as well. One can't help but wonder if this was such a problem that later models moved to the split switch that the P210 and later model 206s and 185s have.

After all, the high school dropout
throttle jockeys here don't think much of my engineering
capabilities.

I'll have you know that I totally have a high school diploma.