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Old 25-02-2010, 06:52   #1
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2GM20F Idling / Charging / Fuel Consumption

Being that I have now done some work on my 2GM20F I would like to care for it the best I can.
We spend many nights out and long days without hitting the dock. My battery bank is strong but after a night or long day they need a good charge. What should I run the engine at with no load to charge batteries and not harm the engine. I know long idles are frowned upon and no load RPM is also frowned upon. Is there a compromise?

Also I had a question about fuel consumption, on the Yanmar service manual is shows it using:
1400rpm - 240 g/HP-h
2800rpm - 208 g/HP-h
3600rpm - 215 g/HP-h
Why would it use more fuel at the lower RPM? The chart shows that I run it at the peak economy (2700rpm where it is the smoothest). I was just puzzled at how it could burn less at lower speeds.
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Old 25-02-2010, 07:34   #2
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The danger in idling the engine is that you are not producing enough heat and can wash the cylinder walls. So you need to produce more heat and on most engines, this simply means high idling. I would recommend starting out at 150% of idle rpm (1200rpm for an 800rpm idle) and watching your temp gauge. If the temp is not in the normal range, you need to put more load on and also if you get a cloud of blue smoke when you rev up quickly after a while, then you need to put more load on. Another thing to consider is whether you need to rev up more for your alternator to charge. Many alternators will require that you are closer to 1500rpm to put out significant charging current so make sure that you are revved up enough for that.

Regarding fuel consumption and rpm, the numbers that you posted appear to be WOT numbers which is how they are normally published. It is hard to correlate this to running your engine on your boat since you are not at WOT at all rpms. It is quite useful for figuring out gearing and propellers and is used extensively in training truck drivers as to where to shift. There are several reasons why this occurs but generally, the most efficient rpm is right near the torque peak. At lower rpm, you need to burn more fuel during the power stroke and you cannot move enough air for it and the engine runs rich. At higher rpm, you are putting a lot of energy into rotating the engine and you start to run into problems with filling and evacuating the cylinder fast enough. In addition, if your engine has static timing, it will generally be timed optimally for a mid range rpm and timing is quite important to fuel economy. If you were doing this under no load, it should be obvious that the most efficient (not necessarily best) rpm is idle and at wide open, it is usually near the torque peak.
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Old 25-02-2010, 07:44   #3
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You would be best to charge at a higher engine speed. Think greyhound bus, when parked for an extended period the engine is switched to high idle to keep the exh valves warm and prevent clogging.
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Old 25-02-2010, 08:17   #4
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My 2GM idles at 850 but is a little rough, I find that I usually idle at 1100-1200 even between shifts or long glides just to smooth things out.
My volt gauge holds at 14v even at an idle of 850 so I am sure my alternator is charging even though I have never speced it out to se what it calls for.

The motor goes threw smooth and rough rpm's, I am hoping to find the best medium to help the engine, charge batteries and not shake the boat.
Is my 1100-1200rpm to low for extended no load time? Would 1500rpm be better for the motor with no load for extended time?
Either way, should I also rev it up to blow out the system every so often?
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Old 25-02-2010, 08:20   #5
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Quote:
Originally Posted by cfoxcvg View Post
My 2GM idles at 850 but is a little rough, I find that I usually idle at 1100-1200 even between shifts or long glides just to smooth things out.
JUST TO CLARIFY MY LAST POST, I DO PULL THE THROTTLE DOWN TO AN IDLE BEFORE I SHIFT, I JUST RAISE IT TO 1100 BETWEEN SHIFTS OR WHILE WAITING FOR A BRIDGE.
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Old 25-02-2010, 09:34   #6
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I have the same (fine) engine. So what is the actual fuel consumption?
Thanks, John
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Old 25-02-2010, 09:46   #7
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I have the same (fine) engine. So what is the actual fuel consumption?
Thanks, John
In my reading it seams like the majority conservatively estimate .5 gal per hour.
I believe this is a safe estimate from my experience with this engine but I have never actually verified it.
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Old 25-02-2010, 10:40   #8
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Quote:
Originally Posted by cfoxcvg View Post
My 2GM idles at 850 but is a little rough, I find that I usually idle at 1100-1200 even between shifts or long glides just to smooth things out.
My volt gauge holds at 14v even at an idle of 850 so I am sure my alternator is charging even though I have never speced it out to se what it calls for.

The motor goes threw smooth and rough rpm's, I am hoping to find the best medium to help the engine, charge batteries and not shake the boat.
Is my 1100-1200rpm to low for extended no load time? Would 1500rpm be better for the motor with no load for extended time?
Either way, should I also rev it up to blow out the system every so often?
It is application specific. You are trying to prevent wet stacking which is temperature related so you need to figure out whether it is okay by looking at your temp gauge. Put it to a high idle that you like and keep an eye on the gauge. If it starts to drop out of its normal range, you need to rev up more.
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Old 27-02-2010, 05:46   #9
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Quote:
Originally Posted by cfoxcvg View Post
Also I had a question about fuel consumption, on the Yanmar service manual is shows it using:
1400rpm - 240 g/HP-h
2800rpm - 208 g/HP-h
3600rpm - 215 g/HP-h
Why would it use more fuel at the lower RPM? The chart shows that I run it at the peak economy (2700rpm where it is the smoothest). I was just puzzled at how it could burn less at lower speeds.
That's because your engine has its best thermal efficiency at 2800 rpm. Efficiency is lower at 1400 revs; so each Hp.h you have to produce results in more energy (heat) not being transferred into mechanical energy. Since at 1400 rpm the engine may produce just 5 Hp while at 2800 it may be 15 the absolute fuel consumption (in gallons per hour) will of course still be lower at 1400 rpm's. The trick is in the formula: a sustained amount of Hp in a time (like the kW versus kWh issue when comparing light bulbs).
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