The problem I am going to address in this article is related to
re-commission of hydraulic systems, powered by
diesel engines with mechanical fuel injection pumps.
Sadly, mechanical fuel injectors are becoming history,
each day giving more floor to the modern electronics,
but there are still quite a few of those diesels
toiling in the field, and if you are a hydraulic technician, there is
a good chance that at some point of your career you will deal with
hydraulics of such an "old-timer-mobile", and, consequently,
can get involved in the following re-commission. This post is,
therefore, a "heads-up" to those who don't know it yet - old
diesel engines can be much easier to start then to stop!
There is no doubt that electronic injection is more
efficient in transforming fuel into useful work, but the main advantage
of mechanical fuel injection - its quality of being able to
withstand all sorts of stresses and harsh environments - made these
engines everlasting. I've seen ones with as much as 30,000
hours on their shoulders and still working! Being purely mechanical
devices, they require no electricity to operate, and
there lies their strength and their weakness - because in some rare
cases, mostly due to malfunctions in stopping mechanisms, these engines
can be very hard to stop once they've started turning, which can be
a pickle when you have a large hydraulic pump coupled directly to
the output shaft and you are the one responsible for putting the
hydraulics back in service.
When you reinstall hydraulic components, especially
in large and complex hydraulic systems, with so many oil lines
being reconnected, it is not uncommon to exist oil leaks, loose ends or even bad
connections, which can cause all sorts of troubles and even unwanted movements as soon as the engine
is started. Even during well planned start-ups there are hundreds of
things that can go wrong and require immediate engine stop - that is why
it is extremely important to make sure that you know how to STOP the
diesel before you start it. I am telling you this because stopping a mechanically injected diesel engine can get much trickier than turning a key or pushing a button.
Naturally, in case
of a more or less regularly maintained equipment, not being able to
stop the engine you've just started will hardly be an issue, but I have
seen enough to warn you about a possibility of such situations, which can have disastrous consequences.
Imagine - the engine has started, and it turns out that
a pressure hose was left loose at one end by mistake. The oil
is streaming all over the machine, and since the machine is an
antique, the windows shielding the operator's space are missing and
the operator gets a jet of hydraulic oil directly in the face and runs
away screaming "My eyes!!! My beautiful eyes!!!!" There's only you
left, alone with the diesel beast... You rush to turn the ignition key,
but the engine doesn't stop... Oups... Now where's that damned
decompressor cord? Trying to cover yourself from the oil rain with one
hand, you pull on the first cord your hand can grab, which turns out to
be an outdated yet extremely powerful pneumatic horn... There are no
windows to protect you from the blast and you feel every one of those
160 decibels try to set your hair on fire... Despite being dazzled by
the horn and blinded by the hydraulic oil, you are still heroically
trying to stop the mechanical monster and pull something else real
hard, ending up with a broken lever in your hand... The sound of the
engine revving up tells you that you pulled the throttle... As you try
to bail out of the operator's cabin, you slip on the oily floor and
stop yourself from falling by grabbing on another closest thing within your
reach - the joystick (with one hand, as you are still holding the
broken lever in the other hand) setting something heavy in motion, most
probably the boom rotation. As the boom rotates you are thrown out of
the cabin by the centrifugal force. While you are flying through the
oil mist, you hear the boom hit something and the operator's cries come
to a sudden stop...
The situation I fantasized above is based on a REAL LIFE ACCIDENT, and is only slightly exaggerated...
Just a week ago I was asked to assist a start up of
a rebuilt hydrostatic transmission, which was mounted on a very
old front loader, powered by a two cylinder air
cooled diesel engine, which once started, only stopped when the
injector line connections were untightened. The transmission had a badly
adjusted null, and the loader began creeping, which caused a
couple of extra agitated moments, as all the attempts to stop the engine
"by the book" failed. (By the way, a small video of this machine will be
used as an example of correct null adjustment technique and will be
included in the second part of the "Finding Zero" series, which is due this month).
Allow me to add another industrial tale. A client of
ours had the diesel engine of his O&K excavator completely rebuilt.
The motor stopping system was not original and consisted of a small
pneumatic cylinder that closed the fuel valve. The valve did have a manual
override, but it "wasn't there"... The first time the engine started after the rebild, one
of the two screw-on engine oil filters was blown away by the oil
pressure (later it turned out that the pressure limiting valve had been
assembled incorrectly) and the engine lost oil pressure (and the oil as well). The
pneumatic "invention" didn't work as there was no air pressure in the
system yet, so the engine worked DRY for around five minutes before the
operator came up with an "alternative" stopping solution - all too
late. The motor had to be rebuilt one more time... Since then a piece
of wire was attached to the valve rod, with the other end hanging loose
from one of the sides. Although doubtfully aesthetic, it was a
functional secondary engine stopping solution, which is in service till
today...
When dealing with putting back in service old
machinery, powered by mechanically injected diesel engines, you should
always take some extra care to check if the engine stopping system is
in working condition. Even when there is an operator present, it
wouldn't hurt to take a minute to inspect the engine and define where
the fuel injecting pump is and if there is an alternative to manually
cut the fuel supply or pull the decompressor cord/rod/whatever... Don't
start these engines unless you are sure you can stop them.
Here's a couple of hints:
Diesel engines can be stopped by
a - cutting fuel supply
b - cutting air supply (or, alternatively, directing a CO2 fire extinguisher into the air inlet to smother the engine)
c - operating decompressor (when equipped with one)
d - stalling the engine (on systems equipped with fixed displacement
pumps it is often possible to stop the engine at idle by extending one
of the cylinders till it reaches the end of stroke)
Re-commission of hydraulic equipment often brings surprises that aren't
directly hydraulics related, like the
described diesel stop problem. Being prepared for them is
what differs a great hydraulics technician from a good one.
An Add-on to this post, featuring another amusing intustrial tale, was published on 29 June, 2011
