Running in higher revs or not to charge

[Adam1991]

Hi, we have a barrus 40hp with 125amp alternator. We run the engine in idle with no revs to charge the batteries when we aren’t moving. 
 

Should we run at higher revs (in neutral) to charge the batteries or is simply running the engine idle with no revs enough to charge? Batteries are 4 Trojan T105 wired in 12v giving 430ah. 

 

cheers! 

[GUMPY]

It will take longer to charge at tickover and depending on the engine/alternator*  combination could wreck the crankshaft.

Personally I would measure the current going into the batteries and increase the revs until a maximum is reached. Usually on a modern engine somewhere about 11/1200 rpm

 

*Kubota based engines with dual alternators.

[Adam1991]

Thanks GUMPY will measure on the next charge and try to work it out 👍

[Tony Brooks]

8 minutes ago, GUMPY said:

It will take longer to charge at tickover and depending on the engine/alternator*  combination could wreck the crankshaft.

Personally I would measure the current going into the batteries and increase the revs until a maximum is reached. Usually on a modern engine somewhere about 11/1200 rpm

 

*Kubota based engines with dual alternators.

 

Exactly, but you can reduce the revs as the charging current falls, but still to maintain maximum charging revs.

 

This procedure maximises the load on the engine, which is always better for diesels.

2 minutes ago, Adam Mc Gowan said:

Thanks GUMPY will measure on the next charge and try to work it out 👍

 

Much better to use a per meant ammeter, because that way you get to know when the batteries are as fully charged as practical. If you don't keep them fully charges as much as possible you will gradually lose capacity caused by sulphation.

[blackrose]

3 hours ago, Adam Mc Gowan said:

Hi, we have a barrus 40hp with 125amp alternator. We run the engine in idle with no revs to charge the batteries when we aren’t moving. 
 

Should we run at higher revs (in neutral) to charge the batteries or is simply running the engine idle with no revs enough to charge? Batteries are 4 Trojan T105 wired in 12v giving 430ah. 

 

cheers! 

 

I think you mean low revs, not no revs.

 

If the engine is running then the crankshaft is revolving even at idling speeds. Your engine is probably idling at around 600 rpm? If there are no revs then the engine isn't running. 

 

Edited January 22 by blackrose

[Adam1991]

Is it much sorer on fuel to run at Higher revs? 

[GUMPY]

Running at the correct charging revs means the engine will need to run for less time so overall the fuel use will be about the same 

[Paul C]

3 hours ago, Adam Mc Gowan said:

Is it much sorer on fuel to run at Higher revs? 

 

Its best to think of load, not simply revs. True, when there's no other loads on the engine and its simply used to propel the boat, there is a clear relationship between load and revs. But when in neutral and spinning an alternator charging batteries at full power, most of the load is due to having to turn the alternator....which varies with revs too, but peaks at around 1200rpm. There will be a proportion of load which still relates to engine revs (for example frictional losses, water pump, pumping losses of the engine itself) but its not the full picture.

[Tony Brooks]

3 hours ago, Adam Mc Gowan said:

Is it much sorer on fuel to run at Higher revs? 

 

It may well use less fuel because an engine's consumption tends to be slightly higher at low revs, then falls as the revs build up, and finally starts to rise again. If you have an engine manual, look at the specific fuel consumption graph. However, with a diesel that is not so marked as with petrol and you will also be running way below full power at whatever revs you use. I would be far more concerned about wear and bore glazing by running it at idle than the fuel  used. I would also have concerns about the alternator trying to deliver a lot of current at low revs where the cooling fan is not pulling as much cooling air through the alternator.

[dmr]

4 hours ago, Adam Mc Gowan said:

Is it much sorer on fuel to run at Higher revs? 

 

Running at tickover is, maybe surprisingly, the least efficient use of fuel.

An engine uses quite a lot of fuel to do nothing, it has to rotate all its parts, drive its oil and water pumps, and overcome the friction of the piston rings etc. Increasing the revs gives more output from the alternators, maybe much more, for just a tiny increase in fuel consumption.

A desel engine is actually most efficient when working hard.

[Paul C]

Efficiency =/= fuel flowrate though. Its less efficient at idle, but because its turning slower, the fuel consumption is less.

 

And........it depends how "efficiency" is defined. A canal boat engine is effectively a CHP plant, there's 3 services which it can provide: heat, electrical power (via the alternator), propulsion.

[IanD]

If you're running the engine just for charging, more load (higher alternator current) and higher rpm means higher engine efficiency -- see attached plot for a Beta 43 with some actual numbers.

 

As has also been pointed out, some engines don't like running under heavy charging loads at low speed for extended periods, for example Beta say in their manual (and on warning stickers) that this should not be done below 1200rpm.

 

And as Tony said, alternators also don't like providing lots of current/power while turning slowly, they get very hot. A 125A alternator probably won't provide full current for very long without overheating, at 100A (80% of rated) the alternator load on the engine would be about 2.5kW with the engine out of gear (12V alternators are only about 50% efficient).

 

The curves below show this is similar to prop load at 1200rpm for a Beta 43, and it turns out it's better to do charging while travelling if fuel consumption is the concern:

 

Charging out of gear at 1200rpm : 2.5kW load at 17% efficiency = 15kW from fuel

Cruising at 1200rpm : 2.5kW load at 17% efficiency = 15kW from fuel

Charging while cruising : 5kW load at 23% efficiency = 22kW from fuel ==> 7kW extra for charging ==> halved fuel consumption for charging!

-- even in this case engine output at 1200rpm is only about 30% of maximum, which is why the efficiency is low, peak efficiency is about 30% at 13kW output

 

If the OP ever switches to LFP batteries then the alternator problem gets a whole lot worse, both controlling it and stopping it cooking itself... 😉

 

Edited January 22 by IanD

[booke23]

Here's the chart from the Beta 43 manual. Notice the fuel consumption graph at the bottom....there is hardly any difference in hourly consumption between 800 and 1200 RPM, but likely a big difference in alternator output. 

 

 

 

 

[Adam1991]

Thanks folks. Enlightening responses, great to have an idea of how the charging process works. Will run at higher rpm when moored up from now on. 
 

 

[MtB]

1 hour ago, Adam Mc Gowan said:

Thanks folks. Enlightening responses, great to have an idea of how the charging process works. Will run at higher rpm when moored up from now on.

 

And NOT with the engine in gear, as an aside! 

 

 

 

[kevinl]

I ve always understood you shouldn,t rev a diesel engine when it,s not under load (ie propelling something that put a load on it).

At one time I make engines at Gardners for my sins, and bench test was always under  load.

K

[dmr]

9 minutes ago, kevinl said:

I ve always understood you shouldn,t rev a diesel engine when it,s not under load (ie propelling something that put a load on it).

At one time I make engines at Gardners for my sins, and bench test was always under  load.

K

 

There might be a bit of truth/theory to suport this.

The force needed to accelerate the piston comes through the con-rod and bearings and increase rapidly at high speed (square law). The combustion force (gas pressure above the piston) acts in the opposite direction so partly cancels this out. So, the highest bearing loads will be no load full speed. However the gas pressure in a diesel is high regardless of load (compression pressure) and engines have good safety margins so its probably no big deal, but its always nice to be kind to engines.

[Alan de Enfield]

15 minutes ago, kevinl said:

I ve always understood you shouldn,t rev a diesel engine when it,s not under load (ie propelling something that put a load on it).

At one time I make engines at Gardners for my sins, and bench test was always under  load.

K

 

A 100amp alternator puts quite a load on the engine.

 

I seem to remeber that a 150amp alternator load is somewhat similar to a prop at the same revs.

[ditchcrawler]

You are not talking maximum engine rev's and most engines in boats are set up to run well under their optimum idle speed by the mariniser. BTW have you ever seen a diesel car do an MOT 

[Tracy D'arth]

7 minutes ago, ditchcrawler said:

You are not talking maximum engine rev's and most engines in boats are set up to run well under their optimum idle speed by the mariniser. BTW have you ever seen a diesel car do an MOT 

And the damage that may then occur threshing a diesel up to its governor is the owners problem!

[kevinl]

Depends which takes the most load, a big fat alternator or a propellor pushing lots of feet of metal down the cut.

K

[booke23]

1 hour ago, Alan de Enfield said:

I seem to remeber that a 150amp alternator load is somewhat similar to a prop at the same revs.

 

150 amps at 13.8 volts is about 2kw or 2.7hp, so very similar to cruising at 3mph in most boats. 

 

Of course during charging 150 amps won't be generated for long, but it'll still be a significant load. 

Edited January 23 by booke23

[ditchcrawler]

6 minutes ago, booke23 said:

 

150 amps at 13.8 volts is about 2kw or 2.7hp, so very similar to cruising at 3mph in most boats. 

 

Of course during charging 150 amps won't be generated for long, but it'll still be a significant load. 

Thats at 100% efficiency, you can't buy alternators like that so the input will be between 3.75 and 5Hp

[booke23]

9 minutes ago, ditchcrawler said:

Thats at 100% efficiency, you can't buy alternators like that so the input will be between 3.75 and 5Hp

 

Good point.

[IanD]

4 hours ago, kevinl said:

Depends which takes the most load, a big fat alternator or a propellor pushing lots of feet of metal down the cut.

K

I gave actual numbers for this further up the thread -- an alternator charging at 100A and prop load at a 1200rpm cruise are similar...