I think he's considering the fact that the starter can't turn the engine
over makes it a hydralic lock. Similar to a hot car engine with the timing
too far advanced. In that case, the spark happens so early that the crank
stops or backs up before it reaches TDC. I wouldn't call that a hydralic
lock though, but I think that's what he is referring to.

The air in the cylinder gets hot and starts expanding as the piston is
trying to compress it as well, and with enough heat, you can get a
preignition from just the air fuel mixture too. Dieseling kind of. Another
example is too much ether in a diesel, a ether lock. Either way, the starter
is unable to overcome the compression of the engine while it is hot.
Quickest way I know around it is to have an assistant, hand-prop-assist the
starter. Safer overall though to just wait for it to cool.


"Peter Duniho" wrote in message
...
"Corky Scott" wrote in message
...
Pete, I'm assuming it's hydraulic lock because the starter cranked the
prop around half a turn and then jerked abruptly to a stop. [...]

I was an auto mechanic in a previous life and have seen hydraulic lock
before. I know no other way of describing it.

Well, like I said, I haven't flown the new C172's, so don't have any
personal experience with that particular engine installation. IMHO, if
that
engine IS experiencing hydraulic lock, that's a problem though.

Oil should not be draining into the cylinders. On the 540 engine in my
airplane, excess fuel that gets into the cylinders will drain out the
intake
manifold, and then out a valve that leads to the outside of the airplane.
I've made plenty of hot starts, including many that were really flooded
starts (per the technique described by Robert Gary ), and never had any
problem with the starter turning over the engine.

I've never experienced anything that could be considered hydraulic lock in
any of the fuel-injected horizontally opposed engines I've ever flown.
That's not a huge number of different airplane/engine combinations
(perhaps
a dozen or two over the years), but it's enough that I'd think I'd have
noticed the problem you're describing if it were common and normal.

I can't think of any mechanism by which you'd experience hydraulic lock on
the fuel-injected horizontally opposed engine that's in the new C172, and
I
would think that if you DID experience that, it would be worth discussing
with the mechanic who maintains the airplane.

Pete