Heading into winter, it's going to get harder to make your aircraft piston engine
start, when it's cold. That's because liquid gasoline does not burn - only gasoline
vapor, and 100LL evaporates really badly, to make it work at FL240. Mogas is
much, much better in the winter, if you can run it. There's a reason you can't
run mogas over 6,000 feet.
The whole idea of priming is bizarre. You jump into your car or truck or whatever,
and turn the key and off you go, every time. But with aircraft piston engines we
are back in the stone age, with manual mixture control and manual primer. Surprisingly,
there isn't a manual spark advance control, like old cars have - aircraft piston engines
are locked down to a set advance (except for starting, with impulse couplings etc).
Anyways. Prime, then wait. Don't crank immediately. Wait for the liquid gasoline
to evaporate, especially if your engine isn't toasty warm from a preheat or previous
run earlier that day. The colder it is, the longer you should wait.
I actually like to pull the prop through, after I prime, to distribute the fuel as best
I can, to all the cylinders and avoid hydraulic lock from fuel (see below). That gives
me really good starts.
I hate to see people cranking and cranking and cranking. You're overheating your
starter and running down your battery, and it's not working.
When an engine doesn't start - assuming you have spark - it means that you have
too little or too much fuel vapor in the cylinder - it's outside the possible burn range.
This is the really tricky thing - figuring out if you need to prime some more, or if you
need to keep cranking to get rid of some of the fuel already in the cylinder, and pump
it out the exhaust.
Annoyingly, fire OUTSIDE the engine is a real problem, which is pretty fucking annoying,
because it won't burn INSIDE the engine and start. Augh.
Where does the excess fuel go with your engine, when you over-prime? With my TCM
IO-360 Maule, the fuel overflow tubes all dump the fuel on the ground underneath the
engine (421 was the same way) and it's possible to light off the fuel on the ground.
This is not a big deal, even if people outside start to run around excitedly waving their
arms. Just keep the prop turning and blow it out like a candle. Remember, it's burning
outside the airplane. Ok, underneath it, but prop slipstream cures that.
Lycoming are different (and I will talk about why, below). They like to run fuel out
the intake, which is really dangerous. When this starts burning, you can do a lot of
damage. Again, keep cranking and suck it into the engine.
So, when you over-prime, fuel can go out your exhaust and burn (which is pretty
harmless if you keep it running), fuel can go out onto the ground and burn (which
is pretty harmless if you keep it running) and fuel can go out the intake, which is
kind of scary when it lights up. Again, keep the engine running to suck it in.
I remember my kid, starting the Pitts, would get flame at least a foot shooting out
forward out of the air intake. I was kind of worried he was going to toast the prop,
but I never saw any damage. If you have an air filter, you want to look carefully for
any damage, and replace it. I am not a big fan of air and oil filters. Both are causes
of engine failure. Pitts has neither air filter, nor an oil filter, so neither can fail
catastrophically.
Here's a fascinating and terribly obscure detail: TCM heads are crossflow, and
Lycomings are not. You're a pilot, and you say, why should I give a shit? Well,
bent connecting rods is why. Ask Seneca College about landing on the highway
in Vermont.
So. What is a crossflow head? Well, that's when you have the intake port on one
side of the cylinder, and the exhaust port on the other. If you look at your fuel injected
Continental engine, you will note the intake is on the TOP of the engine, and the exhaust
is on the bottom. This is supposed to make more power, but it has consequences.
Lycoming, on the other hand, have both the intake and exhaust on the BOTTOM of
the cylinders. This is why fuel from over-priming a Lycoming fuel-injected engine
runs down, out the intake.
But when you over-prime a flat TCM, the intake port on the top can collect the fuel, if
the intake valve happens to be open for a cylinder. Hm. Hydraulic lock, anyone?
If you have enough incompressible fuel in a cylinder, you can actually quietly bend the
connecting rod, which will fail later in flight. As I said, this is more of a flat TCM problem
than a flat Lycoming one. Certainly a problem with oil accumulation in the bottom
jugs of a radial, but nobody flies them any more. This is all about horizontally opposed
engines.
This begs the question: how much do have to prime a flat TCM before I can possibly
bend a connecting rod, which may fail and punch a hole in the crankcase later?
Well, electric prime on my Lycoming AEIO-540 is 6 gph with the throttle and mixture
full forward. That's 1 gph going into each cylinder, because there is no return line.
540 cubic inches is nine liters, that that means each cylinder is 9/6 or 1.5 liters.
With a 10:1 compression ratio (close enough for back of the envelope) that's
1500cc/10 or 150cc of fuel that I need in that cylinder before I have trouble.
150cc is 150/3780cc = 0.04 gallons. At one GPH flow rate per cylinder that's
0.04 x 60 minutes/hour or 2.4 minutes of continuous priming.
So. If I prime for 2.4 minutes, it is possible to hydraulic lock my Lycoming. And
actually it isn't, because I don't have a crossflow head - the intake port is below
the cylinder, and the liquid fuel is just going to run down, out the intake, and start
a fire there.
But if I have a TCM with the intake valve OPEN, I start risking hydraulic locking after
two and a half minutes or so, assuming the same relative numbers.
That's a long, goddamned time. And begs the question - how on earth did people
actually manage to bend their connecting rods, over-priming? We know it happens,
so people are out there, doing it .. and later, when the conrod fails and punches a
hole in the crankcase, people are puzzled. Except for the manufacturer, who knows
exactly why.
Which brings me back to my old habit of pulling the prop blades through X times
after I prime, to ensure that there is no hydraulic lock. And, I get a great start.
Anyways. Sorry for bombarding y'all with yet another wall of text. This shit is what's
going through my brain, all the time. I remember someone asking Stephen King why
he wrote such weird shit. He replied, "Why do you think I have a choice?"
I don't want a pickle. I just want to ride on my motor-sickle. I think I'll do that, now.
No choke or priming the Monkey. Electronic fuel injection, works first time, every time,
hot or cold.
