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Old 04-09-2015, 16:31   #1
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Use Main Engine as Generator-idling

Hi all

i was wondering, what if using the main engine as generator? i saw engine fitted with 2 or even 3 very big alternator to keep battery charged...

and if a smaller alternator is used, some one can please confirm me that diesel doesn't like idling for long time due the low oil pressure? excessive engine wearing and so on...

what would be acording to you a safe high idling speed? let's say 750-800 rpm normal idling...at least 1000-1100rpm would be enought?

thanks all
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Old 04-09-2015, 16:39   #2
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re: Use Main Engine as Generator-idling

I have a second alternator - 130 amps - on each of my engines and when I have to charge by running an engine, this is a huge benefit. But for me it's a charge source of last resort because it's not free (vs solars) and even at a fast idle, it's wear and tear on the engines. But I highly recommend this setup because when I have to motor, even for brief times entering or leaving an anchorage, I get extra charging. On days when we have to motor due to no wind, we can choose those times to run the breadmaker, make extra water, play the music REAL LOUD, or even wash undies in the "Faraday box".....

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Old 04-09-2015, 16:44   #3
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re: Use Main Engine as Generator-idling

I run a 4.5 kva 240 volt fanbelt driven generator off my main engine when motorsailing.,set the revs at about 1650 rpm,motoring at about 5 knots

runs the aircon,fridge etc
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Old 04-09-2015, 16:47   #4
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re: Use Main Engine as Generator-idling

If you've ever been to a rest stop that has big rigs, they are left idling for hours. In commercial fishing salmon trolling, my engine ran for 4-15 hours at about 700 rpm.
Although it usually ran at running speed some time every day. I overhauled it the 2nd year I had it, but it was 25 seasons old at that time. Low speed didn't seem to hurt.
What is tough on generator engines is running at speed with a light load. Usually glazes the cylinders.
Some alternators don't charge at low rpm.
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Old 04-09-2015, 16:56   #5
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re: Use Main Engine as Generator-idling

We used our Volvo MD2 engine to charge batteries, a couple of hours every other day or so for a couple years cruising. We owned the boat for 10 years and sold it on to a couple who tock the boat to SoPac twice. The engine lasted at least 10 more years before finally needing a rebuild.

Personally think the current thought that diesels not run at speed under load won't last. Wear them out not doing anything but satisfying the owners paranoia about low speed/load running, that's good for the engine mfg. Large alternators don't do much except cost a lot of money unless you have a large battery bank. Look at the amp output the next time you charge batteries. The charge controller cuts back right quick so the alternator is putting out way less amperage. Only if you have very large battery bank that you deeply discharge regularly will a big alternator be worth it.
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Old 04-09-2015, 17:13   #6
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re: Use Main Engine as Generator-idling

Idling is not harmful if the engine is at the right temperature. Mines an old engine -I don't idle when cold and under no load. Put into gear to keep a load on while it warms up.
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Old 04-09-2015, 18:27   #7
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re: Use Main Engine as Generator-idling

Quote:
Originally Posted by roverhi View Post
Large alternators don't do much except cost a lot of money unless you have a large battery bank. Look at the amp output the next time you charge batteries. The charge controller cuts back right quick so the alternator is putting out way less amperage. Only if you have very large battery bank that you deeply discharge regularly will a big alternator be worth it.
+1 Very true.
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Old 05-09-2015, 02:53   #8
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re: Use Main Engine as Generator-idling

Quote:
Originally Posted by Lepke View Post
If you've ever been to a rest stop that has big rigs, they are left idling for hours. In commercial fishing salmon trolling, my engine ran for 4-15 hours at about 700 rpm.
Although it usually ran at running speed some time every day. I overhauled it the 2nd year I had it, but it was 25 seasons old at that time. Low speed didn't seem to hurt.
What is tough on generator engines is running at speed with a light load. Usually glazes the cylinders.
Some alternators don't charge at low rpm.
25 season old means 25 years old?😃

Thanks all for the info...

So pratically idling a well warmed diesel engine is not harmfull...and save lots of money in stead of buying a generator 😃
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Old 05-09-2015, 06:52   #9
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re: Use Main Engine as Generator-idling

What are you trying to run?


If it's just topping up the battery bank, solar is the modern solution. Idling the engine for a few hours per day is a dying method from the days when solar was very expensive and not very efficient. Running a separate generator was never a popular solution for charging batteries.


Generators are generally used if you want to directly run high draw items like air/con.


Having an alternator on the propulsion engine is necessary and you will get some charging if you motor somewhere but I wouldn't spend a lot of money upgrading the alternator. A couple hours running with a basic 60amp alternator will still put a lot of amps-hrs back in the battery bank if your solar system goes down for some reason.
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Old 05-09-2015, 07:58   #10
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re: Use Main Engine as Generator-idling

Quote:
Originally Posted by roverhi View Post
...
Personally think the current thought that diesels not run at speed under load won't last. .......
I'm not sure if this is current thought. I have been hearing it for at least the last 45 years and I reckon the concept is much older than that.
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....
So pratically idling a well warmed diesel engine is not harmfull...and save lots of money in stead of buying a generator ��
IMO, the devil is in the detail. Idling a diesel is not harmfull as long as it also run periodically under load.

Someone previously gave the example of truck sitting and idling for long periods but remember, after such operation, they are back on the road for many hours operating at load.

I often use the engine just to charge the batteries but I prefer to do it a bit above idle and with some load so I pop it into gear while charging. Helps to keep the prop clean as well. In addition, I will take it up to full load for a few minutes every half hour or so.

I believe Yanmar recommends 10 minutes of full throttle every 2 hours of low RPM use.
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Old 05-09-2015, 09:11   #11
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re: Use Main Engine as Generator-idling

an alternators out put is rated on its RPM. at idle you'll only be putting out about 1/3 of it's rated out put.running at 1000 rpms in most cases will bring you to about half the rated out put. watch your amp meter and lower your rpms accordingly. going with a high out put alt. on a small amp/hr bank is a waste of money and fuel. an alt. out put should be just shy of half the amp/hrs of the bank. be sure not to add all the banks onto the load, at once, as that would over heat the alt. time and effort is the cost of having comfort and conserving the environment. get solar and wind, your great grandchildren will thank you.

running at idle won't hurt an engine. altho if there are no intervals of running under load at 2/3 or more throttle there will be a high carbon build up and cause excessive cylinder wear and sticking rings -resulting in lower compression and uneven wear in the cylinder. not to mention higher fuel and lube oil consumption/pollution ;-)

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Old 05-09-2015, 09:22   #12
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re: Use Main Engine as Generator-idling

In my experience living aboard and cruising Mexico for 6 months last year anytime you run the engine/generator it's annoying. It makes the cabin warmer and outside you can sometimes get the diesel smell. Our solution this year is adding solar to keep the batteries topped off and a small portable gas generator for emergency charging.
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Old 05-09-2015, 09:33   #13
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re: Use Main Engine as Generator-idling

Idling at 800 rpm won't give you much out of any alternator unless it has been specifically selected to have higher low rpm output than a "normal" alternator. They're out there, but few have them UNLESS they bought one knowing this and understood their electrical system completely.

I need 1200 to 1500 rpm to get anything worthwhile running the engine for out of my 100A alternator with an external Balmar MC-612 regulator.

So, I don't run my engine at idle, and I DO run it with a LOAD, at 1500 rpm with the load being the alternator! That answers the question. 2900 engine hours on a 29 year old engine.
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Old 05-09-2015, 15:10   #14
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re: Use Main Engine as Generator-idling

I realize that there is a lot of anecdotal knowledge on this issue. I've been told that there are several disadvantages to running a small marine diesel engine at low load, low rpms. One is the fact that there will be carbon buildup and varnish on the cylinders. The second is that the exhaust mixing elbow will clog a lot faster. I'm not sure that I understand the exhaust elbow question very well. But that's what I've been told...at least for my Yanmar 3 cyl.

I agree with Stu, when my batteries are low, my Balmar generator must pull 5 hp off my 27 hp diesel. I can sure hear it load down!

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Old 05-09-2015, 16:09   #15
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re: Use Main Engine as Generator-idling

Quote:
Originally Posted by Wotname View Post
IMO, the devil is in the detail. Idling a diesel is not harmfull as long as it also run periodically under load.

Someone previously gave the example of truck sitting and idling for long periods but remember, after such operation, they are back on the road for many hours operating at load.

I believe Yanmar recommends 10 minutes of full throttle every 2 hours of low RPM use.
However a truck is only under full load going up hills - most of the time its on quite low load just trucking along.
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