piston engine hydraulicing

[downwind]

What does the term hydraulicing mean in regards to a piston engine?

[kenparry]

AFAIK it refers to having so much liquid in the cylinder that, on the compression stroke, the whole of the supposed combustion space is filled by that liquid - typically either excess fuel, or lube oil. Not a good thing, usually leading to a bent con-rod or worse.

[Exaviator]

It refers to the piston coming on to TDC (Top Dead Center) with both inlet and exaust valves closed and any liquid, usually oil or water in the cylinder. The liquid not being compressible causes the piston to hammer and can cause sever engine damage.

Most common in radial engines where the lower inverted cylinders can collect oil. On the DC3 for example it was necessary to turn nine blades before starting to prevent this from happening.:ok:

[west lakes]

Entry of fluid (liquid fuel, oil, coolant) into the cylinders as a liquid is generally incompressable as the piston rises in it's compression stroke major engine damage can occur.

Examples (non aviation)

Steam engines were equipped with cylininder drain valves to release water, failure to do so could result in cylinder heads? being blown off

We had a V16 (4 banks of 4 cylinders) diesel generator in use a few years ago ran it up on day 1 to trst it, shut down. Overnight, owing to a fuel system fault, diesel leaked into one of the banks of cylinders. When we started it the next day it cracked the cylinder head and 2 pistons - needed a major rebuild

[MMEMatty]

I once hydraulic-ed a diesel (car) engine, fording a stream that was a bit deeper than i thought...

... Basically water was sucked through the turbo, into the pistons itself, where, when the pistons reached Top Dead Centre, as the water couldn't be compressed in the same way air can, caused 2 bent con-rods and a £1500 engine rebuild. Not to mention the Turbo that went 2 weeks after I got the car back, due to the bearing failing. But that is another story.

The engine would still turn over with a bent con-rod, enough to catch before the engine management system (far more intelligent than I) switched it off. And yes, I do still have the banana shaped con-rod as a reminder not to be so daft!

Matty

[SNS3Guppy]

Downwind,

Liquid Lock or Hydraulic Lock most often occurs on radial engines, and commonly with the lower cylinders. Fluids drain downward with gravity, and when fluids such as oil accumulate in the combustion chambers of the lower cylinders, they become "locked." This simply means that the engine can't turn, as the piston can't move toward the top of it's stroke in the cylinder, when the cylinder is full of oil. If one tries to move the cylinder, there will be some resistance, but it's very possible to bend or break a connecting rod or damage a cylinder in the process. In very large engines, there may be little feeling of resistance, and use of the starter motor or the considerable leverage provided by pulling or pushing on the propeller can quickly damage an engine.

Hydraulic lock needn't be oil. Oil does run down and enter cylinders from several different sources (depending on the engine and it's configuration), but fuel can do the same thing. In large radial engines, priming fuel is dumped right into the supercharger on the back of the engine, and from there delivered to the cylinders in a very crude, but effective manner. Overpriming can also quickly flood lower cylinders during an engine start, and they may become hydraulically locked.

Sometimes, if one is lucky, the liquid which is locking the cylinder may drain out if a valve is partially open. However, most of the time one must remove the spark plugs on the lower cylinder to drain them, and when oil is the culprit, clean the spark plugs before reinstalling.

Hydraulic lock isn't limited to large round radials. Most radial engines are susceptible, and even horizontally opposed engines may be liquid locked under the right circumstances (a rare, but possible occurrence).

[hawker750]

Hydraulic lock
Had it yesterday starting my Chipmunk for the 1st time in a while.
Procedure is to turn 8 blades first to check for oil in cylinders, then prime with the mechanical pump until fuel drips from the overfow pipe, then 8 more blades to "suck in"
I thought the priming took a hell of a long time because when I came to "suck in" 2 cylinders had completely flooded with fuel presumable from a sticking something in the carb. If I had not done the full procedure properly then I guess I would be advertising here to see if any had a spare Gypsy Engine! As it happens the engine started fine after the hydraulic locks had been cleared and a good few more blades to be on the safe side

[411A]

.... and even horizontally opposed engines may be liquid locked under the right circumstances (a rare, but possible occurrence).



Opposed engines are prone to hydraulic locking if they are fuel injected, and the pilot then primes prior to starting, excessively.
The starter will not cause the possible damage, it is the engine immediately firing with one or more cylinders experiencing a hydraulic lock that does the deed.

Can be an expensive proposition.

[SNS3Guppy]

Can be an expensive proposition.


Indeed!

Interestingly on radials, some operators, and some powerplants, prefer that the engine be pulled through by hand, others by the starter. In some cases, the risk of damage by hydraulic lock is minimized when pulling the engine through with the use of the starter, due to slippage in the starter clutch. In other cases, the starter can cause damage, whereas pulling the propeller through by hand is the protection.

Usually on an opposed engine one is looking at a fire hazard long before a hydraulic lock issue.

[stevef]

Hydraulic lock - happened to me a long time ago on an An2. I turned the engine (Asz 621R, basically a metric version of the Wright Cyclone, I believe) by hand in the direction of rotation for however many blades were required, jumped in and fired it up. It ran for a few seconds and stopped dead, which shook me (and the airframe) considerably. A load of oil ran out when the bottom plugs were removed. All I can think of is that the supercharger oil seal blew, dumping oil into the induction. Seems unlikely though.
Result - one engine change.