On Sat, 05 Apr 2008 06:51:26 GMT, Jere Lull wrote:

On 2008-04-04 05:01:39 -0400, "Edgar" said:

"Lew Hodgett" wrote in message
news:iMjJj.10032$s27.7854@trnddc02...

"Bruce in Bangkok" wrote:

read a reference to one of your posts, quoted in part by Roger, that
sounded as though the RPM was only unstable at low RPM, I remember
something about 1,000 RPM.


Most diesels need to idle in the 1300-1500 RPM area in order to generate
enough waste heat to prevent
"wet-stacking" which will cause glazing of the cylinder walls, creating a
VERY expensive repair.

Much better to cut out prolonged idling and get some load on sooner.
Start up, and by the time you have checked the exhaust for cooling water,
gone forward and cast off the mooring ropes the engine is ready to go.
In other words, just long enough to spread some oil around inside the
engine.
If you cannot get load on for some time then stop the engine until you are
ready.

Yours is actually off the original topic (though entirely appropriate
for the newsgroup), but exactly describes my techniques over the past
15 or so seasons.

It's almost disappointing that those techniques have resulted in zero
required adjustments or repairs in over 1000 hours' operation. I do the
tests, the tests say "no problem".

The problem is that " Idle" is a relative term. I just finished
overhauling the governor for a Gardner engine 6 cylinder engine that
turned 1150 RPM at full throttle, and idled at 500 RPM. An 18 Ltr
engine producing 170 H.P. A little hard to get it up to the
recommended 1300 - 1500 RPM :-)

I also saw, but didn't work on, a single cylinder semi-diesel that ran
at 200 RPM. I asked the Motor-man how long it had been running and he
told me that he had been there for five years and it was running when
he got there and never stopped during his stay.

Somehow this dire warning not to idle a diesel doesn't seem to apply
to some engines. I think that if you substitute "lightly loaded" for
"idle" you might more accurately describe the condition.

As an aside, I have been fooling with these engines for some 50 or 60
years and the only people I have ever heard talk about not idling
diesels are boat people and primarily yacht people. Heavy trucks,
drilling rigs, heavy equipment, fishing boats, all frequently idle or
run at low power for long periods with no apparent problems.

Now I'm not trying to say that yachties don't know what they are
talking about but it does seem strange that they seem to be the only
people that talk about "not idling engines".

I suspect, but can't prove that yacht engines are seldom run at
maximum continuous horsepower settings for any length of time while
most commercial engines are. A bulldozer, for example will operate at
either idle or full throttle all its working life. Generator sets are
usually sized to work at about 90% of maximum continuous rating. I
installed a 1500 HP compressor engine that was sized to operate at 90%
of maximum. That engine, by the way, was overhauled after 5 years of
24 hour a day operation, some 40,000 hours of operation. And that
overhaul was done primarily because the plant had to be shut down to
replace some piping and the manager decided that as long as the plant
was down for a month they "might as well overhaul the engine".

My own suspicions are that if you run an engine for long periods at
light loads, as many people do when charging batteries you are asking
for problems. Particularly if you don't follow that low load period
with a period at nearly full power.

My own Perkins, overhauled 8 years ago is run at 2,000 RPM to charge
batteries and at the same RPM when motoring. Other then that I don't
pay any attention to whether it is idling or not. If I'm rigging the
mooring lines it idles. It doesn't burn any more oil today then it did
just after I overhauled it....


Bruce-in-Bangkok
(correct email address for reply)