Bruce in Bangkok wrote:
On Sat, 24 Nov 2007 23:49:00 -0500, Wayne.B
wrote:

On Sat, 24 Nov 2007 22:12:41 -0600, Brian Whatcott
wrote:

Diesels don't have this problem, and so are less wasteful at low
revs.
That's true but they have maintenance issues if run at low speed/low
load for extended periods of time.

Yes there "seems" to be that but mention of "it is bad to run the
engine at low poser" seems confined to the boating world. I've worked
many construction jobs where diesel engines were started in the
morning and shut down at the end of the day. During smoke breaks or
lunch they just sat there and idled. Cranes are a perfect example,
that spend most of their life at very low power settings.

Generator sets - I've never seen a gen set operating manual that said
"run this engine at high power settings".

Not that I'm advocating idling your diesel for days and days but I do
wonder about the people who worry about letting the engine idle. I've
seen people that would hardly let the poor old thing cool down before
stop-cocking it, "because it is bad to let the engine run at low
load".

The Perkins I have in the sail boat has a continuous rating of 3,000
RPM and for years I ran it at 1500 - 1800. When I overhauled it I
could see no evidence of abnormal wear or carbon or any other evidence
that slow running harmed anything.

I have the feeling that someone once said "it's not a good idea to
idle a diesel for a long time" and as the message passed from dockie
to dockie it became an urban legend and now everyone is worried about
idling the engine.

The issue with my motor, the Yanmar 2QM15, is that it runs cold at low
RPM. I can verify this. It's amazing how long it stays cold with no
load. This would be bad for it, right?

Stephen

"Stephen Trapani" wrote in
message ...

The issue with my motor, the Yanmar 2QM15, is that it runs
cold at low
RPM. I can verify this. It's amazing how long it stays
cold with no
load. This would be bad for it, right?


Yes. That is the main reason to not let a diesel sit at idle
too long. Low temp AND lower oil pressure at idle. For
years truck diesels have had a 'high idle' setting ( either
by air or electric solenoid, manual cable, or lately via
computer) so they can run them at night while parked.


SBV

On Sun, 25 Nov 2007 07:20:49 -0500, "Roger Long"
wrote:

It simply has to take more horsepower so push a twin screw boat with a
single engine than with both. One prop is dragging and whether it is fixed
or freewheeling, it is going to increase the overall resistance. The thrust

Snipped

money?

A mechanic I talked to recently told me that he sees a significant
difference in interior condition and time between overhauls on sailboats
that live on moorings or have engine driven refrigeration compressors
because owners of those boats tend to idle for long periods to charge and
cool.

You are confirming my own experiences regarding running on one engine
to save fuel is something I have never been able to achieve. Assuming
normally sized engines.

And, I'm sure that essentially running a diesel engine at idle
continuously is probably bad for it but I have never been able to
detect the, so called, glazing of the cylinder walls that is a
supposed result. Every diesel engine I have taken apart had cylinder
walls that looked exactly the same as any other engine, a slick
polished surface.

But anyway, Thank you for the comprehensive comments.

Bruce-in-Bangkok
(Note:remove underscores
from address for reply)

On Sun, 25 Nov 2007 07:20:49 -0500, "Roger Long"
wrote:

Engines that are idled a lot simply get overhauled a bit sooner. If you
don't have good reasons to idle, why throw away your engine time which =
money?

That's the way I understand it also. Instead of getting maybe 5,000
hours between overhauls, you end up with 3 or 4,000 instead. With a
turbo it can be a lot worse than that.

I have some hard numbers for single vs low speed twins on my own boat,
based on a relatively small number of data points. Running both
engines slowly to achieve about 1.0 x SQRT(LWL), the best fuel economy
I've been able to achieve is 1.4 NMPG.

Running single engine with the other one freewheeling I have been able
to get 1.7 NMPG.

The boat has sight guages on the tanks so that I can measure fuel burn
to within 5 gallons accuracy. The boat is a heavily loaded,
semi-displacement GB49 with total weight in the of range 60 to 70,000
lbs. The engines are 2 stroke DD 6-71s, naturally aspirated, rated at
280 hp each. They will hit their rated max of 2400 RPM at WOT.
Props are 4 bladed 30 x 25, reduction gears are 2.5 to 1.

With both engines the boat will reach 1.0SQRT(LWL) at 1200 RPM,
burning about 5 gph total (about 85 actual hp). Single engine at 1500
RPM, same speed, burns about 4 GPH (68 actual hp) .

Since it seems reasonable to assume that the overhead of running a
large engine slowly is greater than a small engine, it is entirely
possible that a different outcome would be obtained with smaller
engines running closer to their rated power output.

On Sun, 25 Nov 2007 08:11:30 -0800, Stephen Trapani
wrote:

Bruce in Bangkok wrote:
On Sat, 24 Nov 2007 23:49:00 -0500, Wayne.B
wrote:

On Sat, 24 Nov 2007 22:12:41 -0600, Brian Whatcott
wrote:

snipped

The Perkins I have in the sail boat has a continuous rating of 3,000
RPM and for years I ran it at 1500 - 1800. When I overhauled it I
could see no evidence of abnormal wear or carbon or any other evidence
that slow running harmed anything.

I have the feeling that someone once said "it's not a good idea to
idle a diesel for a long time" and as the message passed from dockie
to dockie it became an urban legend and now everyone is worried about
idling the engine.

The issue with my motor, the Yanmar 2QM15, is that it runs cold at low
RPM. I can verify this. It's amazing how long it stays cold with no
load. This would be bad for it, right?

Stephen

If your engine runs cold put a thermostat in it. Your Yanmar should
warm up like any engine and stay the same temperature from then on.
Running an engine at a lower then designed temperature is not the way
to go.

Bruce-in-Bangkok
(Note:remove underscores
from address for reply)

On Sat, 24 Nov 2007 23:49:00 -0500, Wayne.B
wrote:


On Sat, 24 Nov 2007 22:12:41 -0600, Brian Whatcott
wrote:


Diesels don't have this problem, and so are less wasteful at low
revs.


That's true but they have maintenance issues if run at low speed/low
load for extended periods of time.

Bruce said:
Yes there "seems" to be that but mention of "it is bad to run the
engine at low poser" seems confined to the boating world. I've worked
many construction jobs where diesel engines were started in the
morning and shut down at the end of the day. During smoke breaks or
lunch they just sat there and idled. Cranes are a perfect example,
that spend most of their life at very low power settings.

Generator sets - I've never seen a gen set operating manual that said
"run this engine at high power settings".

Not that I'm advocating idling your diesel for days and days but I do
wonder about the people who worry about letting the engine idle. I've
seen people that would hardly let the poor old thing cool down before
stop-cocking it, "because it is bad to let the engine run at low
load".

The Perkins I have in the sail boat has a continuous rating of 3,000
RPM and for years I ran it at 1500 - 1800. When I overhauled it I
could see no evidence of abnormal wear or carbon or any other evidence
that slow running harmed anything.

have the feeling that someone once said "it's not a good idea to
idle a diesel for a long time" and as the message passed from dockie
to dockie it became an urban legend and now everyone is worried about
idling the engine.

But what do I know?

Bruce-in-Bangkok

Bruce, I suspect that all this came about as there are different types
of diesels out there, and *some of them* surely cannot be safely idled
for long periods. I knew a few tow truck operators that drove ford
diesels that told me that if the engines were not equipped with
high-idle switches the engines consistently did not last that long (tow
trucks spend a lot of time at idle). On the other hand, guys that drove
some other brands of tow trucks said they didn't need the fast idle, the
engines lasted just fine. My Dodge/Cummins pickup truck owner's manual
says to avoid idle any longer than 3-5 minutes - kind of a bitch since I
spend half the day in traffic.
In boats we have some (mostly older now) slow-turning diesels that
can be run all year at low speeds without any harm. But I suspect that
most of the newer, lighter, fast turning diesels of recent vintage are
better off above idle.
One more thing... *most* people I've observed around here driving
their boats into the dockage area are idling anyway. By the time they
are docked the engine is already sufficiently cooled and needs no more
idling at the dock, yet they usually spend another five or ten minutes
wasting fuel.
Red

Roger Long wrote:
The bigger the diesel, the more of a factor this is
which is why railroad engines are seldom shut down for maintenance.

Roger, I am under the impression that railroad diesels are run at pretty
high "idle". In fact when waiting for the various commuter trains to get
out of the way around here I get the impression of a pretty fast running
engine when they are stopped. They do not sound anywhere near idle. Is
this the case just when they are dropping off and picking up and/or is
there another time when they actually idle slower?
Red

Bruce in Bangkok wrote:
On Sun, 25 Nov 2007 08:11:30 -0800, Stephen Trapani
wrote:

Bruce in Bangkok wrote:
On Sat, 24 Nov 2007 23:49:00 -0500, Wayne.B
wrote:

On Sat, 24 Nov 2007 22:12:41 -0600, Brian Whatcott
wrote:

snipped
The Perkins I have in the sail boat has a continuous rating of 3,000
RPM and for years I ran it at 1500 - 1800. When I overhauled it I
could see no evidence of abnormal wear or carbon or any other evidence
that slow running harmed anything.

I have the feeling that someone once said "it's not a good idea to
idle a diesel for a long time" and as the message passed from dockie
to dockie it became an urban legend and now everyone is worried about
idling the engine.
The issue with my motor, the Yanmar 2QM15, is that it runs cold at low
RPM. I can verify this. It's amazing how long it stays cold with no
load. This would be bad for it, right?

Stephen

If your engine runs cold put a thermostat in it. Your Yanmar should
warm up like any engine and stay the same temperature from then on.
Running an engine at a lower then designed temperature is not the way
to go.

Why didn't Yanmar put a thermostat in this engine? Is there any
disadvantage to having the thermostat?

Stephen

On Sun, 25 Nov 2007 17:49:19 -0800, Stephen Trapani
wrote:

Bruce in Bangkok wrote:
On Sun, 25 Nov 2007 08:11:30 -0800, Stephen Trapani
wrote:

Bruce in Bangkok wrote:
On Sat, 24 Nov 2007 23:49:00 -0500, Wayne.B
wrote:

On Sat, 24 Nov 2007 22:12:41 -0600, Brian Whatcott
wrote:

snipped
The Perkins I have in the sail boat has a continuous rating of 3,000
RPM and for years I ran it at 1500 - 1800. When I overhauled it I
could see no evidence of abnormal wear or carbon or any other evidence
that slow running harmed anything.

I have the feeling that someone once said "it's not a good idea to
idle a diesel for a long time" and as the message passed from dockie
to dockie it became an urban legend and now everyone is worried about
idling the engine.
The issue with my motor, the Yanmar 2QM15, is that it runs cold at low
RPM. I can verify this. It's amazing how long it stays cold with no
load. This would be bad for it, right?

Stephen

If your engine runs cold put a thermostat in it. Your Yanmar should
warm up like any engine and stay the same temperature from then on.
Running an engine at a lower then designed temperature is not the way
to go.

Why didn't Yanmar put a thermostat in this engine? Is there any
disadvantage to having the thermostat?

Stephen

Well, I think that they did. At least the 2GM20 I owned had a
thermostat and the parts book shows two, a normal temperature one for
freshwater cooling and a low temperature one for salt water cooling.


Bruce-in-Bangkok
(Note:remove underscores
from address for reply)

On Sun, 25 Nov 2007 21:37:55 -0500, "Roger Long"
wrote:

My 2QM20 has a thermostat. It is also a very cool running engine. I have
heard several places that they are among the coolest running diesels in
common use and mine seems quite happy running that way. I have also heard,
but not substantiated, that this engine runs just as well, or better, with
the thermostat removed. I don't recommend it or plan to do it though.

The 2QM20 has a precombustion chamber which I don't think is common in small
diesels. This might account for different operating characteristics. It
might also account for their reputation for lasting a long time with poor to
non-existent maintenance. With a precombustion chamber, a clogged injector
maybe isn't consentrating the combustion into hot spots on the piston head.
I'm speculating a bit and would like to know more however.

Yanmar's factory parts books lists two thermostats and two overheat
sensors for the small engines. One for salt water cooling and a higher
temp one for freshwater.

Also all the small Yanmar engines and I assume the larger engines use
pre-combustion chambers as do Caterpillars and many other engines. The
idea of the pre-combustion chamber is that the fire starts there and a
gout of burning fuel is injected into the combustion chamber rather
then simply spraying a jet of fuel into the combustion chamber. I
believe it is intended to provide better flame propagation and
improved combustion.

I would hesitate to recommend that anyone remove the thermostat unless
the engines is equipped with a temp. gauge as in some cases removing
the thermostat actually increases cylinder head temperatures.

It seems contrary to logic but on some engines - there was a Ford V-8
I remember - that actually run hotter without the thermostat as the
thermostat in addition to controlling temperature acted as a orifice
and restricted the speed of coolant flow through the block and head.


Bruce-in-Bangkok
(Note:remove underscores
from address for reply)