Diesel Fuel Burn: Idle vs. 3/4 RPM?

[Ziggy]

Originally Posted by DotDun

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.

I agree.

However, I'm not actually sure what is the OP's question. Is it fuel consumption at idle? Fuel consumption at cruising speed? Or the ratio between the two?

[Bash]

Originally Posted by Jim Cate

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.

Not true. Look at post #2, which recommends looking up the burn rate on the Yanmar website. Question answered.

(Burn rates are also in the owner's manuals.)

((BTW, the concerns expressed about low-speed charging are spot on. Maybe those guys were helping the OP?))

[Jim Cate]

Originally Posted by Bash

Not true. Look at post #2, which recommends looking up the burn rate on the Yanmar website. Question answered.

(Burn rates are also in the owner's manuals.)

((BTW, the concerns expressed about low-speed charging are spot on. Maybe those guys were helping the OP?))

Bash, I believe that the Yanmar (and other published fuel consumption curves) are for engines loaded with a propeller. That figure is markedly different from the consumption at idle with no external load, and again from the consumption at low speeds with a large alternator being driven, but no propeller.

There are some rule-of-thumb relationships that calculate fuel consumed per hp-hour for generic diesel engines (can't offer a link, sorry) and they should give a reasonable figure for the alternator loading, but I'm still curious about the consumption when simply idling in neutral with minimal charging going on.

Re the practice of low speed charging: there have been endless discussions here on CF on this subject, and I have not seen any substantive proof that this practice is hugely detrimental. Lots of anecdotal stuff on both sides of the issue, but little fact supported by data. So, perhaps the free advice was good, perhaps not... who can really say?

Cheers,

Jim

[Dockhead]

Originally Posted by Jim Cate

Bash, I believe that the Yanmar (and other published fuel consumption curves) are for engines loaded with a propeller. That figure is markedly different from the consumption at idle with no external load, and again from the consumption at low speeds with a large alternator being driven, but no propeller.

There are some rule-of-thumb relationships that calculate fuel consumed per hp-hour for generic diesel engines (can't offer a link, sorry) and they should give a reasonable figure for the alternator loading, but I'm still curious about the consumption when simply idling in neutral with minimal charging going on.

Re the practice of low speed charging: there have been endless discussions here on CF on this subject, and I have not seen any substantive proof that this practice is hugely detrimental. Lots of anecdotal stuff on both sides of the issue, but little fact supported by data. So, perhaps the free advice was good, perhaps not... who can really say?

Cheers,

Jim

Exactly right. Fuel curves are for max load of a theoretically ideal prop. They are useless for answering the OP's (or nearly useless).

I don't think there is any good way to figure it out without actually measuring it. You could try to calculate the max alternator load, but then you don't know how much is being spent overcoming internal friction.

Some people will say you don't need to micromanage fuel burn in a sailboat, but I think it's extremely useful to know. We motor enough and use diesel fuel for enough other purposes that it's useful, I think, to be able to understand and optimize consumption. If you have an N2K network, you can install a fuel flow metering system for only about a grand, something I'll be doing this very weekend.

[donradcliffe]

Originally Posted by Dockhead

If you have an N2K network, you can install a fuel flow metering system for only about a grand, something I'll be doing this very weekend.

Now you are obligated to answer the question with some real world data--hopefully idling with and without the alternator loaded and with and without the prop in gear.

[ccgarnaal]

Originally Posted by JEdward

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

I don't know any engine manufactures that give consumption rates for less then 20% load. Its very difficult to predict.
If you really want to know then measure it. Just let it suck diesel out of a 1l cup and measure how long it takes to empty half the cup. Make sure you don't let it suck air. This is actually the most accurate measurement.

Even on large cargo ships this method is used with a 200l vessel and a bypass line. You close the bypass and wait how long it takes to burn the 200l then you open the bypass again en calculate the consumption.

Other than that. Do you have an Ampere meter to see how fast you are charging? If you do calculate how long it takes to charge a death battery and if your not happy with the time. Then fix a bigger alternator like I did.

RPM wont make much of a difference, the engine is still at low load. I would keep it at low idle. Low load running is especially bad for turbocharged engines as the airflow isn't what its supposed to be and you get a bad combustion. With an atmospheric engine I wouldn't worry about it.

What rpm do i run at?
250-300RPM stationary.
1500 RPM is full power. But that is a 50 year old 1 cilinder 200kg 10hp SABB engine.

[Ziggy]

Originally Posted by Jim Cate

There are some rule-of-thumb relationships that calculate fuel consumed per hp-hour for generic diesel engines (can't offer a link, sorry) and they should give a reasonable figure for the alternator loading, but I'm still curious about the consumption when simply idling in neutral with minimal charging going on.

0.3 liters/hr/hp is a reasonable rule of thumb for a small diesel. You still need to know what is the load (propeller+alternator+gearbox+cutless bearing) in hp.

[CeesH]

IMO diesel consumption has no relation between hp/hr. The volume of injected diesel is calculated as a function off Actual Torque, RPM and time.

Thats how modern electronic controlled engines find the volume (read opening time of the injector) in a stored tabel in the ECU.

So this must also count for older mechanical controlled engines.

CeesH

[mrm]

Without knowledge of a BSFC map for this particular engine, all discussion will be more or less of an educated guess.

P.S. @ CeesH - HP/hr and Torque/RPM/hr is exactly the same relationship...

[MarkJ]

So of all the people who have posted here No one knows how much fuel they burn to charge their batteries?

Astounding!

So no one here knows who many weeks or months they could remain on the pick charging by engine?

So no one here knows their maximum sailing passage by fuel consumption?

I am amazingly astounded. For such a penny pinching mob of let's live on 2 cents per month we don't know how much the big noisy donk costs us?

In fact I might be the only person who knows their cruising speed consumption.... 4 Gerry cans per 48 hours at 1800 revs.

Someone here must have their own figures not just Yanmars (which I can't find for the 4jh3e)


Mark

[Cotemar]

I have Sails and Solar, so calculating fuel burn on my 30HP diesel at idle is not a priority as I do not really care to know. Sort of like the end of the world calculations. Not really usefull to me.

[CeesH]

Originally Posted by MarkJ

So of all the people who have posted here No one knows how much fuel they burn to charge their batteries?

Astounding!

So no one here knows who many weeks or months they could remain on the pick charging by engine?

So no one here knows their maximum sailing passage by fuel consumption?

I am amazingly astounded. For such a penny pinching mob of let's live on 2 cents per month we don't know how much the big noisy donk costs us?

In fact I might be the only person who knows their cruising speed consumption.... 4 Gerry cans per 48 hours at 1800 revs.

Someone here must have their own figures not just Yanmars (which I can't find for the 4jh3e)

Mark

Mark, first Gerry Cans or Jerry Cans?

Now serious who the hack wants to know how much his engine burns at Idle?

Second, if you want to charge your batteries at low RPM then mount a big Alternator and the pump of your watermaker on the engine. Now you are taking a amount of Torque from your engine and makes it sense. Still almost inpossible to know how much you burn in this new situation because the amount of Torque taken by the alternator veries a lot depending on the number of Amps it delivers.

My 2 Centavos

[Dockhead]

Originally Posted by CeesH

IMO diesel consumption has no relation between hp/hr. The volume of injected diesel is calculated as a function off Actual Torque, RPM and time.

Thats how modern electronic controlled engines find the volume (read opening time of the injector) in a stored tabel in the ECU.

So this must also count for older mechanical controlled engines.

CeesH

Umm, torque times RPM gives you horsepower. Ziggy is right! You're right, too, if you delete the first sentence.

[MarkJ]

Originally Posted by Cotemar

I have Sails and Solar, so calculating fuel burn on my 30HP diesel at idle is not a priority

Yeah, I just did 3 months in New York without ever charging by engine. And that's not the tropics.

[donradcliffe]

Originally Posted by ccgarnaal

I don't know any engine manufactures that give consumption rates for less then 20% load. Its very difficult to predict.
If you really want to know then measure it. Just let it suck diesel out of a 1l cup and measure how long it takes to empty half the cup. Make sure you don't let it suck air. This is actually the most accurate measurement.

Even on large cargo ships this method is used with a 200l vessel and a bypass line. You close the bypass and wait how long it takes to burn the 200l then you open the bypass again en calculate the consumption.

This may work on a big ship, but it won't work on a boat diesel unless you put another cup on the return line. Messy and a lot of work.