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Old 01-01-2013, 15:21   #1
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Diesel Fuel Burn: Idle vs. 3/4 RPM?

We are sitting at anchor with Yanmar 30hp diesel idling for the evening battery charge. We have been tracking run time (with RPM for each hour) to get accurate fuel consumption updated rate.
But, not sure how to calculate combined rates (idle vs. cruise)?
How much fuel (a %?) does a diesel use when at idle? Is it 10% of cruise speed RPM rate? 20%? 5%?
Note: I once had a couple of diesel cars, and was told they burned very little fuel when idiling at stop lights, etc. which was one reason diesel cars got such great mileage?
Any guidance/counsel for us?
Thanks in advance for your knowledge.
Happy New Year
JEdward
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Old 01-01-2013, 15:56   #2
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Re: Diesel Fuel Burn: Idle vs. 3/4 RPM?

You should be able to look up the fuel burn curves for your Yanmar on the Yanmar site.
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Old 01-01-2013, 16:20   #3
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Re: Diesel Fuel Burn: Idle vs. 3/4 RPM?

Hate to pop your balloon but one should not just let a diesel "sit and idle" unless you like doing ring jobs on the motor.
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Old 01-01-2013, 16:58   #4
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Re: Diesel Fuel Burn: Idle vs. 3/4 RPM?

Quote:
Originally Posted by JEdward View Post
We are sitting at anchor with Yanmar 30hp diesel idling for the evening battery charge. We have been tracking run time (with RPM for each hour) to get accurate fuel consumption updated rate.
But, not sure how to calculate combined rates (idle vs. cruise)?
How much fuel (a %?) does a diesel use when at idle? Is it 10% of cruise speed RPM rate? 20%? 5%?
Note: I once had a couple of diesel cars, and was told they burned very little fuel when idiling at stop lights, etc. which was one reason diesel cars got such great mileage?
Any guidance/counsel for us?
Thanks in advance for your knowledge.
Happy New Year
JEdward
Take a look at this link:

http://www.yanmarmarine.com/theme/ya.../PDF/3YM30.pdf

It looks like about 1/3 of a gallon at idle and almost 2 gallons per hour at max rpm. But, yeah, you should never run a diesel at idle for long periods of time - only to warm it up. Put a load on it, like let out a bunch of scope, shift it into reverse and throttle up a bit.

Happy New Year!
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Old 01-01-2013, 17:39   #5
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Re: Diesel Fuel Burn: Idle vs. 3/4 RPM?

Quote:
Originally Posted by JEdward View Post
We are sitting at anchor with Yanmar 30hp diesel idling for the evening battery charge. We have been tracking run time (with RPM for each hour) to get accurate fuel consumption updated rate.
But, not sure how to calculate combined rates (idle vs. cruise)?
How much fuel (a %?) does a diesel use when at idle? Is it 10% of cruise speed RPM rate? 20%? 5%?
Note: I once had a couple of diesel cars, and was told they burned very little fuel when idiling at stop lights, etc. which was one reason diesel cars got such great mileage?
Any guidance/counsel for us?
Thanks in advance for your knowledge.
Happy New Year
JEdward
First, you haven't specified the engine you have (is it 3YM30?), your idle rpm (should be about 900 rpm for 3YM30) or what you consider cruising rpm (2600?) Even with this information, the fuel consumption when cruising will depend on the amount of power consumed by the propeller, and in turn that will depend on your boat, the prop, sea state, etc.

If you really want to know, you should measure it. Get a 1 quart measuring cup, fill it with fuel, stick your fuel pick up and return lines in, measure how much is consumed in 10 minutes, and multiply by 6.

If you just want a guestimate, I would guess 0.05 gal/ hr when idling vs 0.6 gal/hr when cruising, but really, my guess is as good as yours.
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Old 03-01-2013, 04:54   #6
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Re: Diesel Fuel Burn: Idle vs. 3/4 RPM?

Quote:
Originally Posted by JEdward View Post
We are sitting at anchor with Yanmar 30hp diesel idling for the evening battery charge. We have been tracking run time (with RPM for each hour) to get accurate fuel consumption updated rate.
But, not sure how to calculate combined rates (idle vs. cruise)?
How much fuel (a %?) does a diesel use when at idle? Is it 10% of cruise speed RPM rate? 20%? 5%?
Note: I once had a couple of diesel cars, and was told they burned very little fuel when idiling at stop lights, etc. which was one reason diesel cars got such great mileage?
Any guidance/counsel for us?
Thanks in advance for your knowledge.
Happy New Year
JEdward
How heavy is your alternator? And how much does it charge when idling.
I put a size able alternator on my engine to charge batteries while idling.
But i can no longer consider it idling since the alternator charges 80-100 amps at 12V thus 1200Watt on my 10hp engine thats 8% of full load. However the engine runs low rpm so its loaded even more. If i look at the fuel rack lever its at 50% of max injection for idle rpm.
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Old 03-01-2013, 05:16   #7
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Re: Diesel Fuel Burn: Idle vs. 3/4 RPM?

Why use a 30hp engine to recharge batteries. Its an expensive engine and is not recommended to run at idle for any length of time.

A 2hp honda 2000watt will do a better job with less noise and vibrations
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Old 03-01-2013, 16:45   #8
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Re: Diesel Fuel Burn: Idle vs. 3/4 RPM?

Aw come on guys! The OP asked a reasonable question and all he has gotten in return is slagging his practice of low speed battery charging.

Does anyone have any data about the fuel consumption of typical small marine diesels at idle speed, either no load or small (alternator) loads?

I would like to know too!

Cheers,

Jim
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Old 03-01-2013, 17:04   #9
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Re: Diesel Fuel Burn: Idle vs. 3/4 RPM?

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Originally Posted by Jim Cate View Post
Aw come on guys! The OP asked a reasonable question and all he has gotten in return is slagging his practice of low speed battery charging.

Does anyone have any data about the fuel consumption of typical small marine diesels at idle speed, either no load or small (alternator) loads?

I would like to know too!

Cheers,

Jim
Jim,
The OP may have asked a reasonable question, but did not provide enough information about his engine, how he uses it, or his boat to answer it. I did provide a rough guess (can't really call it an estimate) and suggested a method for him to get a more accurate answer:
Quote:
Originally Posted by Ziggy View Post
First, you haven't specified the engine you have (is it 3YM30?), your idle rpm (should be about 900 rpm for 3YM30) or what you consider cruising rpm (2600?) Even with this information, the fuel consumption when cruising will depend on the amount of power consumed by the propeller, and in turn that will depend on your boat, the prop, sea state, etc.

If you really want to know, you should measure it. Get a 1 quart measuring cup, fill it with fuel, stick your fuel pick up and return lines in, measure how much is consumed in 10 minutes, and multiply by 6.

If you just want a guestimate, I would guess 0.05 gal/ hr when idling vs 0.6 gal/hr when cruising, but really, my guess is as good as yours.
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Old 03-01-2013, 17:05   #10
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My 8BTD Westerbeke burns .7 of a gallon about 1/2 load, I believe it's 14 HP @1800 rpm.
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Old 03-01-2013, 17:58   #11
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Re: Diesel Fuel Burn: Idle vs. 3/4 RPM?

Charging revs is one of my questions I ask all Yanmar mechanics and they all say different things. Many say don't worry idle is fine, but I run mine at 1,200 rpm when idle is 900.

I cruise at low revs... 1,800 to about 2,100 and faster cruise at 2,200 to 2,400
Max is about 3,300 rpm and I have only once run at that... And that was running! From pirates....

At 1,800 rpm I think mine uses about .40 gallon per hour. 4 Gerry cans per 48 hours.

It would be very interesting to determain the real fuel consumption charging. I really don't think it can be too much.

By the way, on the point of buying a little generator instead of using the big engine... Work it out, cruisers are on the pick for 90% of the time... So they could be using the engine less than an avid Saturday sailor! Damn thing might rust
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Old 03-01-2013, 18:45   #12
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Re: Diesel Fuel Burn: Idle vs. 3/4 RPM?

My experience was about 1/4 gallon per hour at idle for charging with a 100 amp alternator. That was for a Perkins 4108, so the Yannie might use a bit less.
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Old 03-01-2013, 18:57   #13
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Re: Diesel Fuel Burn: Idle vs. 3/4 RPM?

The published fuel consumption curves don't typically cover low rpms. Even if they did, they usual cover maximum power output for a specific rpm. You are probably not beginning to touch rated power at your chosen rpm, hence the governor will back off the fuel to meet demand.

The only way is to measure the consumption yourself.
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Old 03-01-2013, 19:18   #14
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Re: Diesel Fuel Burn: Idle vs. 3/4 RPM?

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The published fuel consumption curves don't typically cover low rpms. Even if they did, they usual cover maximum power output for a specific rpm. You are probably not beginning to touch rated power at your chosen rpm, hence the governor will back off the fuel to meet demand.

The only way is to measure the consumption yourself.
Actually, Yanmar gives the fuel consumption curve at the theoretical propeller load, so it might be a bit closer to reality. Still, I agree with you that the only way to get a reasonably accurate estimate is to measure it.
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Old 03-01-2013, 19:44   #15
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Re: Diesel Fuel Burn: Idle vs. 3/4 RPM?

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Actually, Yanmar gives the fuel consumption curve at the theoretical propeller load, so it might be a bit closer to reality. Still, I agree with you that the only way to get a reasonably accurate estimate is to measure it.
My point is that load determines fuel consumption, not (just) rpm. I would doubt a charging alternator can load the engine anywhere close to a propeller.
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